A Review of Riparian Mesquite and Crop

Water Use

Norm “Mick” Meader Lower San Pedro Watershed Alliance Cascabel Conservation Association

February 2015

Lower San Pedro Watershed Alliance, P.O. Box 576, Mammoth, AZ 85618, http://lowersanpedro.org, (520) 487-1903, lowersanpedro@gmail.com.

Cascabel Conservation Association, 6146 N. Cascabel Road, Benson, AZ 85602, http://cascabel conservation.org, (520) 323-0092, nmeader@cox.net.

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TABLE OF CONTENTS EXECUTIVE SUMMARY .......................................................................................................... 1 INTRODUCTION......................................................................................................................... 2 ARIZONA YEARLY CROP WATER USE ............................................................................... 3 

Calculating Alfalfa Water Use Using a Reference Evapotranspiration and Crop Coefficients ............................................................................................................................. 5 

Water Use by Other Crops and Riparian Vegetation ........................................................... 6 Irrigation Efficiency and Crop Water Budgets ....................................................................... 8 Double Cropping .................................................................................................................... 9 Deficit Irrigation and Water Duty .......................................................................................... 9 

QUANTIFYING RIPARIAN MESQUITE WATER USAGE ............................................... 10 Early Studies ........................................................................................................................... 10 

Gila River, Safford, Arizona 1943–1944 .............................................................................. 10 Walnut Gulch Experimental Watershed, Tombstone, Arizona 1965–1967 .......................... 11 San Pedro River, 1990 .......................................................................................................... 12 Santa Cruz River, Nogales, Arizona, 1995 ........................................................................... 13 San Pedro River, Lewis Springs (Sierra Vista), Arizona, 1997 ............................................ 14 

New Studies ............................................................................................................................. 14 Comparison of Mesquite and Sacaton Grassland Data Sets .............................................. 18 

Data Set 1 .............................................................................................................................. 18 Data Set 2 .............................................................................................................................. 19 Data Set 3 .............................................................................................................................. 20 

Other Studies .......................................................................................................................... 20 Comparing Mesquite Woodland Evapotranspiration Throughout the Year ................... 22 Reasons for Lower Mesquite Water Usage .......................................................................... 23

Mesquite dormancy ...............................................................................................................23 Hydraulic redistribution and storage of precipitation..........................................................25 Limited canopy ground coverage..........................................................................................26 Arid-adapted leaf structure and physiology .........................................................................26

DISCUSSION .............................................................................................................................. 27 CONCLUSIONS ......................................................................................................................... 28 ALFALFA AND CROP-RELATED REFERENCES ............................................................. 31 MESQUITE AND RIPARIAN REFERENCES ...................................................................... 34 APPENDIX A: Mesquite evapotranspiration and groundwater use calculated in early

studies ...................................................................................................................................... 38 APPENDIX B: Detailed Review of Qashu (1966), Qashu and Evans (1967) and Tromble

(1972) ....................................................................................................................................... 39 

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LIST OF FIGURES

Main Text Figures

Figure 1. Seasonal use of water by Arizona alfalfa .....................................................................5 Figure 2. Average yearly crop coefficient for 2010 for alfalfa grown in the Imperial Valley ....6 Figure 3. Location of the Charleston and Lewis Springs study sites .........................................15 Figure 4. Location of major vegetation communities with respect to the river channel and

groundwater ................................................................................................................16 Figure 5. Charleston riparian mesquite woodland .....................................................................16 Figure 6. Lewis Springs riparian mesquite shrubland ...............................................................17 Figure 7. Lewis Springs sacaton grassland ................................................................................17 Figure 8. Location of the Mojave River north of the San Bernardino Mountains, southern

California ....................................................................................................................21 Figure 9. Comparison of annual evapotranspiration for mesquite and sacaton with the annual

standard crop reference evapotranspiration, ETo .......................................................23 Figure 10. Figure 2b from Nagler et al. (2013) showing the evapotranspiration for the three San

Pedro River communities (ETa) .................................................................................24 Figure 11. Evapotranspiration of the three communities through the year for 2003 ...................24 Figure 12. A. Downward transfer (hydraulic redistribution) of precipitation via the mesquite

tap root at night during the summer monsoon season ................................................25 Figure 13. Hydraulic redistribution (downward transfer) of winter precipitation by mesquite

during the dormant winter and early spring months ...................................................26 Box Figures

Figure B1. Evapotranspiration ......................................................................................................4 Figure B2. Calculating the evapotranspiration of a crop (ETc) from the reference

evapotranspiration (ETo) ............................................................................................4 Appendix Figures

Figure A1. Google Earth view of the Qashu and Tromble site on Walnut Gulch ......................39 Figure A2. Diurnal water-level changes for 1965 in a well associated with the mesquite

woodland ...................................................................................................................40 Figure A4. Transpiration values calculated for the mesquite woodland from diurnal changes in

water well levels .......................................................................................................41 Figure A4. Water level fluctuations for 1964–1965 in the water well associated with the

Walnut Gulch mesquite site ......................................................................................42 Figure A5. Variation in measurements of specific yield with time for Nebraska alluvium .......44

LIST OF TABLES

Table 1. Arizona annual alfalfa water use ....................................................................................3 Table 2. Comparison of Arizona crop water use with riparian vegetation water use ....................... 7 Table 3. Field crop applied water budgets for 1999–2000 for Arizona desert lowland counties 8 Table 4. Comparison of mesquite and sacaton water-use data sets ...........................................18 Table 5. Mesquite, cottonwood–willow, and saltcedar riparian evapotranspiration along the

Mojave River, Southern California, averaged for 2007 and 2010 ...............................22 Table A1. Mesquite evapotranspiration and groundwater use calculated from all studies ..........38

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EXECUTIVE SUMMARY Mesquite water use has generated increasing concern in recent years in the West because of dwindling water supplies. Some in the agricultural community have even claimed that it uses more water than alfalfa, our highest water use crop. To address these concerns and claims this paper reviews mesquite water use in riparian areas where it is highest and compares it with agricultural use. The most recent studies of riparian mesquite in Arizona show that average evapotranspiration (ET) for a mature woodland with 70%–74% ground cover is between 27.2" and 28.6" (692 and 727 mm) per year. For a less-mature woodland (“shrubland”) with 50%–55% ground cover, it is between 22.2" and 26.0" (564 and 660 mm) per year. These values are for groundwater-dependent mesquite, which accesses and uses several times the water that upland mesquite does. When precipitation is removed from these values to obtain groundwater use, mature mesquite woodland is found to use between 17.7" and 19.3" (449 and 490 mm) per year, while mesquite shrubland uses between 11.0” and 15.0” (279 and 381 mm) per year. If the mesquite canopy is scaled to cover 100% of the ground area, groundwater use for a mature woodland would be 25.2"–26.0" (641–660 mm) per year. In contrast, average annual Arizona alfalfa evapotranspiration is about 72" (1829 mm) per year. This value does not reflect total applied water, which can range from 85+" (2160+ mm) per year with center pivot irrigation to more than 100" (2540 mm) per year with flood irrigation. The amount of alfalfa evapotranspiration derived from groundwater would be 60" or greater given Arizona’s annual average precipitation. Most of Arizona’s other major crops also use more water than mesquite woodland except barley and wheat, which use similar or slightly less amounts. These data show that mesquite does not use vastly more water than agricultural crops as some believe. Removing mesquite from riparian areas and replacing it with cultivated crops or irrigated pasture will thus not reduce water loss or improve stream-flow conditions. While allowing mesquite to reclaim abandoned agricultural fields will gradually result in increased groundwater use if the water table is sufficiently shallow, that use will not exceed preceding agricultural use unless fields were fallowed for part of the time. For forage crops such as irrigated pasture and alfalfa, fields would have to be fallowed for up to two-thirds or more of the time to reduce groundwater use to that of the average mesquite woodland. Even at full reestablishment, mesquite woodland would use substantially less than half the groundwater that irrigated forage crops do. Mesquite uses less water for four reasons: (1) mesquite is dormant for approximately half the year, whereas forage crops such as alfalfa are grown for nine months of the year or more, (2) mesquite harvests rainwater and stores it in deeper soil layers for use in drier times, reducing groundwater use, (3) in contrast to most crops, the mesquite canopy usually covers significantly less than 100% of the ground surface, reducing total evapotranspiration, and (4) native mesquite is physiologically adapted to an arid environment whereas most cultivated crops are not. Just as with other riparian vegetation, mesquite woodland becomes stressed with falling groundwater levels and will begin to die back in response.

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INTRODUCTION

Mesquite has a widespread reputation for water use, which has generated concern in Texas and the Southwest where water supplies are limited. Mesquite is known for its ability to access and exploit groundwater in arid riparian areas and to outcompete grass for water on uplands. This reputation has led some to consider all mesquite to be detrimental to water availability in a watershed, most importantly where it is replacing existing vegetation or reclaiming agricultural land. Some in the agricultural community have claimed that mesquite uses almost twice as much water as alfalfa at all times, implying that mesquite’s water consumption greatly exceeds even that of our most water-intensive crop.

In response to the concern about mesquite’s water use, the Arizona State legislature passed a bill in 2014 prohibiting the use of Water Protection Fund money to protect and restore riparian mesquite while allowing funds to be used to remove mesquite in all environments throughout a watershed. The act classifies mesquites as “high water use trees that consume water to a degree that is detrimental to water conservation efforts.” The presumption is that any mesquite removal benefits the watershed and riparian areas. Given the impact that perceptions of mesquite have had on policy, it is important to quantify mesquite’s actual water use, particularly in riparian areas where its use is greatest. This review thus summarizes the literature on this use and compares it with crop use, most importantly for alfalfa, a standard reference crop.

Very little work had been done on riparian mesquite water use until the late 1990s when the Agricultural Research Service (ARS) of the U.S. Department of Agriculture began studies along the San Pedro River in Arizona. This was in part a response to the establishment of the San Pedro Riparian National Conservation Area and the need to determine the full water budget for the watershed to protect river flow. The most extensive work on mesquite riparian water use is that of Russell Scott of the ARS and his colleagues in the U.S. Geological Survey (Scott et al., 2006b; Scott et al., 2008b). They focused on both dense mesquite woodland (74% ground cover) and less-dense mesquite shrubland (55% ground cover). These studies have become the benchmark for assessing mesquite riparian water use and are the basis of the conclusions presented in this review. While earlier studies attempted to address the issue, none approaches the thoroughness and sophistication of these studies.

Regarding crop water use, Arizona agriculture depends almost wholly on external water sources or groundwater, and crop water use has been extensively measured and quantified to optimize water delivery. Cultivation of alfalfa for forage is one of Arizona’s most important crops and occupies more than 25% of Arizona’s irrigated acreage. Numerous studies show that annual alfalfa evapotranspiration in southern Arizona is between 70" and 80" (Table 1). The most complete available summary of general Arizona crop water use is that by Erie et al. (1982), which this report uses. Data from newer studies are used whenever possible, however.

This review does not address mesquite water use in upland regions away from riparian areas. That topic is left for another time. Studies have shown that mesquite readily accesses deeper soil moisture in these environments and can outcompete grass for water. Mesquite’s overall effect on the hydrology of arid grasslands is not fully understood and remains controversial, which has led to considerable debate over whether and how to manage mesquite in that environment. This review also does not compare the riparian habitat value of native mesquite with that of irrigated crops or other groundcovers, another potentially contentious issue.

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ARIZONA YEARLY CROP WATER USE Alfalfa is a standard reference crop whose water use has been compared with that of mesquite to emphasize mesquite’s water consumption. Table 1 summarizes the available literature on alfalfa water use to accurately compare the two, and Figure 1 shows Arizona alfalfa water use throughout the year. Primary water use by plants is measured in terms of evapotranspiration, which is a combination of water evaporated from the stomata of plant leaves and plant tissues and water evaporated from the soil beneath the plants. Evapotranspiration is measured in millimeters or inches of water over a given period of time. Box 1 explains evapotranspiration along with reference crop evapotranspiration and crop coefficients. Farmers must at times apply much more water than all plants in a field can use to ensure that all parts of a field have sufficient water. Table 1. Arizona annual alfalfa water use.

Source Evapotranspiration Comments ADWR (1999) 56.3"–74.3" (1829 mm) Second value is adapted from

Erie et al. (1982); origin of first value is unknown

Brown (2008) 72" (1829 mm) For low-desert production Erie et al. (1982) 74.3"–80"

(1880–2032 mm) From University of Arizona farms in Mesa and Tempe

Hanson et al. (2011) 56"–65" (1422–1651 mm)

Calculated from eddy covariance data for Imperial Valley; lower than historical value of 76"

Hunsaker et al. (2002) 76" (1930 mm) Confirmed by personal communication

Martin et al. (1994) 72" (1829 mm) Western Arizona alfalfa Martin et al. (2004) up to 84" (2134 mm) California average is 60"-66";

use is up to 7 ft (84") in southern CA and AZ

Ottman (2010) 80.5" (2045 mm) Applied water, two irrigations/ cutting, controlled conditions. UAMaricopa Agricultural Center.

Putnam and Ottman (2002) 72"to >84" (1829–2134 mm)

72" is for active management areas; argued to be too low for optimum crop production.

Snyder and Bali (2008) 90" (2286 mm) For Indio, California, 8 cuttings UA Cooperative Extension (n.d.) 74" (1880 mm) Phoenix area (presumably from

Erie et al., 1982)

Yearly alfalfa water use/evapotranspiration values cluster around 72" (1829 mm). Growers often exceed this amount in what they apply, however, to ensure that the crop is not stressed, as noted above. Values for applied (irrigation) water are higher than evapotranspiration values due to the inefficiency of water distribution and use. Efficiency varies from ~70%–90% (UA Extension Service, 2001; Brown, 2008; Martin, 2011).

Alfalfa water use is typically tied to cuttings per year, with a given number of inches applied per cutting. Eight to ten cuttings per year are common in Arizona (UA Cooperative Extension, n.d.), with an average time between cuttings of 30 days (Brown, 2008). Sundance Farms in Coolidge,

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BOX 1: DEFINITIONS

Evapotranspiration

Evapotranspiration (ET) is the loss of water from a vegetated surface from plant transpiration and soil evaporation (Figure B1) and is frequently considered consumptive use. Transpiration is the loss of water from plants by evaporation mostly through the stomata of the leaves.

Important environmental factors that affect ET include solar radiation, temperature, atmospheric dryness (vapor pressure deficit or humidity), wind and soil moisture. Biological factors that affect ET include vegetation type, foliage geometry and foliage density. Reference Evapotranspiration

Reference evapotranspiration (ETo) is an estimate of the water used by a well-watered, full-cover grass surface 12 cm (5") in height (the reference crop) and is used to calculate crop water needs. Cool-season grass (e.g., rye grass) and alfalfa have been used as reference surfaces for ET estimation for several decades. ETo is computed from meteorological parameters (temperature, wind speed, humidity, etc.) and varies throughout the day and year. Daily values are summed together to provide total yearly water use. Crop Coefficients

Because evapotranspiration accounts for most of the water lost from the soil during crop growth, estimates of evapotranspiration rates are important in scheduling irrigation. To determine how much water a specific crop needs, a correction factor or crop coefficient (Kc) is required to convert ETo to ETc, the crop evapotranspiration (Figure B2).

ETo × Kc = ETc Figure B2. Calculating the evapotranspiration of a crop (ETc) from the reference evapotranspiration (ETo). Modified from Brown and Kopec (2000).

A well-accepted crop coefficient for Arizona alfalfa is 0.95. To determine how much water would be required to grow alfalfa year round at the Charleston mesquite site, one would multiply this coefficient times the yearly ETo for the site, which for 2001–2003 was 1774 mm or 69.84" (Scott et al., 2006a). Alfalfa would thus need 1685 mm or 66.34" of water per year here.

Figure B1. Evapotranspiration. From Salinity Management (2007).

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Figure 1. Seasonal use of water by Arizona alfalfa. Alfalfa is relatively dormant from December through mid-February, and no water use is allocated for those months. Data are from University of Arizona farms in Mesa and Tempe, Arizona. From Erie et al. (1982). Arizona, typically leaves 28–30 days between cuttings and cuts nine times each year (Blake, 2009). The Heaven Sent Ranch in the lower San Pedro Valley near Cascabel gets 8–9 cuttings each year (Rogers’ Heaven Sent Ranch, n.d.). Arizona alfalfa fields are commonly dormant for a few months in the winter. Calculating Alfalfa Water Use Using a Reference Evapotranspiration and Crop Coefficients Alfalfa yearly evapotranspiration values can be calculated for a given location using an Arizona Meteorological (AZMET) evapotranspiration value (ETo) determined for a reference crop at a specific site, typically a cool-season short-crop grass such as rye or fescue. This value is then multiplied by a crop coefficient for alfalfa to obtain water use. Box 1 explains the procedure. Using this approach and the ETo values of the mesquite woodland and shrubland sites in Scott et al. (2006b) provides the most accurate comparison of water use.

Brown (2008) gives an Arizona crop coefficient for alfalfa of 0.95, which is also the standard value given for Imperial Valley by Doorenbos and Pruitt (1977). Hanson (n.d.) provides a graph of daily crop coefficients for alfalfa for Imperial Valley, with a yearly average value of 0.977 (Figure 2, calculated from the figure). Data in Hansen et al. (2011) give an average value of 0.94 (excluding one outlier) for the years 2007–2010. Thus 0.95 is used for calculations here.

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Figure 2. Average yearly crop coefficient for 2010 for alfalfa grown in the Imperial Valley. Crop coefficients vary hourly and daily with the weather and need to be averaged to calculate total yearly water use. The fluctuations in values indicate that the alfalfa was cut nine times during the year. The crop coefficient drops sharply when the alfalfa is cut because of the greatly reduced leaf area. Modified from Hansen (n.d.). Scott et al. (2006b) give a reference evapotranspiration ETo of 1774 mm (69.84") for the Charleston mesquite woodland site and 1778 mm (70.00") for the Lewis Springs mesquite shrubland site. These values were calculated by the Penman–Monteith method and must be converted to AZMET (Arizona Meteorological Network) values to use with this crop coefficient. Using the AZMET–Penman ratio from the Safford meteorological site (Brown, 2005) increases these values to 1890 mm (74.41) and 1894 mm (74.57), respectively. The actual ratio of the two ETo values for the site is unknown, however, and the accuracy of this is questionable. Using these two values with the alfalfa crop coefficient of 0.95 gives an annual alfalfa evapotranspiration at these sites of 1796 mm (70.69") and 1799 mm (70.84"), respectively. These values provide a conservative basis for comparing mesquite water use. Water Use by Other Crops and Riparian Vegetation To more fully compare mesquite and agricultural water use, Table 2 lists the water use for San Pedro River riparian plant communities from Scott et al. (2006b) and (2008b) and water use by common Arizona crops grown in southern Arizona. This table excludes vegetable crops. Many of the crop water values are taken from Erie et al. (1982) and were determined by the soil moisture depletion method using gravimetric analysis. The data were derived from irrigation treatments that resulted in near-maximum crop production. Drainage was not considered, which may make some of the values high. This study is still considered to provide the best overall Arizona crop water use data for many crops, however (M. Ottman, personal communication, 2014).

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Table 2. Comparison of Arizona crop water use with riparian vegetation water use.

Crop or Vegetation

YearlyEvapotranspiration Source or Notes

Alfalfa 74.3"–80"(1887–2032 mm)

Erie et al. (1982)

Giant bermuda grass 60"–72"1

(1524–1829 mm)Desert Sun Marketing Company (2014),Phoenix, Arizona

Permanent pasture mix 68.0" (1727 mm)1 ADWR (1999), Phoenix Active Manage-ment Area

Bermuda/rye pasture 54"–66"(1372–1676 mm)

P. Brown, pers. comm. (2014); adaptedfrom Brown and Kopec (2000) and Brown et al. (2001) for San Pedro Valley

Mixed-grass pasture 48"–72"1

(1219–1816 mm)Young et al. (1984), 4,000–6,000 ft elevation

Pecans 54"–70"(1372–1778 mm)

ADWR (1999), Phoenix Active Manage-ment Area

Pecans 39"–57.5"(1000–1460 mm)

Skaggs et al. (2008), New Mexico; applied water up to 76" (1940 mm)

Double-cropped forage sorghum

54.2" (1377 mm) Erie et al. (1982), wheat often double-cropped instead of sorghum

Blue panic grass 52.3" (1328 mm) Erie et al. (1982)Grapefruit 47.9" (1217 mm) Erie et al. (1982)Safflower 45.5" (1156 mm) Erie et al. (1982)Bermuda lawn grass 43.5" (1104 mm) Erie et al. (1982), May-October seasonSugar beets 42.8" (1087 mm) Erie et al. (1982)Cotton 41.2" (1046 mm) Erie et al. (1982)Cotton 38"–42"

(965–1067 mm)Hunsaker et al. (2005a), 42"–48" applied water, highly controlled conditions

Naval oranges 38.1" (968 mm) Erie et al. (1982)Cottonwood–willow 38.0" (966 mm) Scott et al. (2006b), Lewis Springs site,

San Pedro River, perennial water Flax 31.3" (795 mm) Erie et al. (1982)Saltcedar (tamarisk) 29.5" (750 mm) Dahm et al. (2002), Rio Grande River,

unflooded stand Mesquite woodland 28.6” (727 mm) Scott et al. (2008b), Charleston site, San

Pedro River Cotton (short season) 28.5" (724 mm) Watson and Sheedy (1992), 1991 valueMesquite riparian shrubland

26.0" (661 mm) Scott et al. (2008b), Lewis Springs site, San Pedro River

Wheat 23.3"–25.6"(591–624 mm)

Hunsaker et al. (2005b)

Single-cropped grain sorghum

25.4" (645 mm) Erie et al. (1982)

Sacaton grassland 25.3" (643 mm) Scott et al. (2008b), Lewis Springs site, San Pedro River

Barley 25.0” (635 mm) Erie et al. (1982)Cottonwood–willow intermittent water

19.1 (484 mm) Scott et al. (2006b), Boquillas site, San Pedro River

1Applied water assumed; evapotranspiration not available.

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Irrigation Efficiency and Crop Water Budgets Yearly crop evapotranspiration values do not consider additional water consumption resulting from inefficiency factors associated with each irrigation method (flood, sprinkler and drip). Table 3 gives crop water budgets for several Arizona counties that demonstrate this. These were compiled by the University of Arizona’s Cooperative Extension Service for 1999–2000 (Teegerstrom and Clark, 1999). These are the last set that the University of Arizona compiled, and how these may have changed as irrigation practices have changed is unknown. All values are for furrow (flood) irrigation on selected farms in each Arizona county. Presumably these farms have been laser-leveled, which increases irrigation efficiency. Table 3. Field crop applied water budgets for 1999–2000 for Arizona desert lowland counties.

County/Area

Alfalfa

Barley

Wheat

Corn

Grain Sorghum (Single Crop)

Pima Cotton

Upland Cotton

Cochise County, Kansas Settlement

68" * 40" 48" 42" 40" 37"

Graham County, Safford Valley

84" 44" 44" 48" 44" 60" 54"

Greenlee County, Duncan Valley

78" * 46" 36" 48" 48"

La Paz County, Parker and Salome†

79" P * 35" P 42" S

43" P 36" P 55" P 50" P 62" S

Maricopa County, Salt River Project

90" 32" 34" * 36" 72" 61"

Mohave County, Mohave Valley

85" 35" * * 60"

Pima County, Marana

72" 36" 36" * 30" 48" 42"

Pinal County 79" 32" 32" 43" 44" 60" 49" Yuma County, Yuma Valley N/S

85" 35" 39" * 36" 60" 42"

*Crop not grown on surveyed farm. † P = Parker; S = Salome. As noted previously, efficiency can vary from approximately 70%–90% (UA Extension Service, 2001; Brown, 2008; Martin, 2011). This is to say, if evapotranspiration for healthy alfalfa is 72"/year (1829 mm/year), to achieve this with 70% efficiency, common for flood irrigation, one must apply 103" (2616 mm) of water per year. This is reflected in the amounts of water used to grow alfalfa by Sundance Farms near Coolidge, Arizona, which applies 8"/cutting for drip irrigation vs. 12"/cutting for flood irrigation (Blake, 2009). Sundance Farms cuts alfalfa 9 times/year, resulting in an annual water use of 72"–108" depending on the irrigation method. Water applied through drip irrigation essentially matches the consumptive water use of alfalfa, demonstrating the method’s much greater efficiency. Other crops also use additional water. While evapotranspiration for well-watered Arizona wheat is given as 25.8" (655 mm) per year, the amount of water needed to achieve this is 42.0" (1067 mm) with flood irrigation and 34.8" (884 mm) with a sprinkler system (Moore et al., 1990).

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Husman and Ottman (2004) also note the need to apply 36"–42" (914–1016 mm) of water by flood irrigation to meet the consumptive needs of wheat and barley, which they estimated to be approximately 24" (610 mm). To achieve the evapotranspiration of 41.2"/year (1046 mm/year, Erie et al. 1982 value) for cotton requires 57.6" (1463 mm) of water with flood irrigation and 46.8" (1189 mm) with a sprinkler system (Moore et al., 1990). These values represent flood and sprinkler irrigation efficiencies for wheat of 61% and 71%, respectively, and for cotton of 74% and 90%, respectively. While some of this additional water may be recovered for reuse, not all of it is, and it is eventually lost through deep soil drainage, evaporation or runoff. This contrasts with how mesquite uses water. While riparian mesquite does use significantly more water than non-riparian arid-land plants, it uses that water efficiently. It does not waste it as irrigated crops can. In addition, mesquite routinely harvests and stores excess precipitation in deeper soil layers for later use, which cultivated crops do not do. (See the “Reasons for Lower Mesquite Water Usage” section.) Double Cropping Two crops per year may be grown on the same acreage, known as double-cropping. This practice makes the most efficient use of farm land. Wheat is commonly double-cropped with sorghum to make full use of the growing season. Wheat is planted in early winter and then harvested in late May or June. Sorghum is then planted in June and harvested in September. The yearly evapotranspiration and is the sum of both crops, which in this case would be 25.8" + 25.4" = 51.2" (1300 mm). Applied water must be summed also, which would be up to 80" (2032 mm). Double-cropping is also commonly done with bermuda grass and a cool-season grass such as rye to maintain year-long irrigated pastures or lawns. For forage, bermuda grass is often grown from late May through the end of September, and then rye or another cool-season grass is seeded over this to provide forage through the late fall, winter and early spring. The combined yearly evapotranspiration for this grass mixture in the lower San Pedro Valley would be 60" (1524 mm) 10% to compensate for grazing intensity (P. Brown personal communication, 2014; adapted from Brown and Kopec, 2000; Brown, 2001; and Brown et al., 2001). Total applied water could be up to 20" (508 mm) greater to compensate for irrigation inefficiency. Deficit Irrigation and Water Duty A practice that reduces irrigation water consumption is deficit irrigation, when less water is applied to a crop than is needed to meet its full evapotranspiration needs. While increasing irrigation efficiency and saving water, this practice reduces crop yields. The amount of applied water may still exceed the full evapotranspiration needs of the plant because of irrigation inefficiency, noted above. Deficit irrigation can cut water requirements by up to 30% or more, depending on how great a reduction in yield a farmer or rancher is willing to accept. In its 1991 report on the hydrology of the San Pedro River for the Gila River Adjudication (ADWR, 1991), the Arizona Department of Water Resources estimated that 12.4% of the acreage under irrigation in the San Pedro basin was deficit irrigated based on field examinations.

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Another important concept is water duty, the amount of water required to be applied in a given agricultural area above precipitation to meet the full water requirements of all crops. This is computed from a combination of crop evapotranspiration requirements and irrigation efficiency. ADWR (1991) calculated average water duties for the five subwatersheds of the San Pedro River basin that ranged from 4.68 acre-feet (56.16" acre-inches) in the Sierra Vista subwatershed to 5.71 acre-feet (68.5" acre-inches) in the Redington subwatershed. The much higher duty for the Redington subwatershed stems from lower overall irrigation efficiency and the much higher percentage of pasture grass grown there.

QUANTIFYING RIPARIAN MESQUITE WATER USAGE Early Studies Only four mesquite water-use field studies predate those by Scott and colleagues, three of them focusing on groundwater fluctuations in wells within woodland areas (Gatewood et al., 1950, Qashu, 1966, and Tromble, 1972). The Arizona Department of Water Resources (ADWR, 1991) subsequently used Gatewood’s determinations to extrapolate mesquite water use along the San Pedro River as part of proceedings for the Gila River Adjudication. Unland et al. (1998) was the first investigator to use advanced meteorological instrumentation and the Bowen-ratio method to attempt to measure mesquite ET. She did this work in 1995. Other than these studies, no other work had been done on water use by riparian mesquite prior to Scott’s work. In 1997 Scott and colleagues established their initial mesquite study site on the San Pedro River at Lewis Springs east of Sierra Vista, Arizona. They, too, used the Bowen-ratio method, and provided initial results on a fairly sparse mesquite shrubland site. Scott later abandoned this site for better-located ones and switched to eddy covariance instrumentation to undertake the main studies examined in this report. Appendix A summarizes mesquite evapotranspiration and groundwater use determined by these early studies. Gila River, Safford, Arizona 1943–1944 Gatewood et al. (1950) were given the task of determining the water use by phreatophytes along the Gila River downstream from Safford, Arizona, between Calva and Ft. Thomas, originally to salvage water for mining. While they attempted to determine total vegetation water use by six methods, they evaluated evapotranspiration of specific vegetation types by only two methods, (1) measurement of plant water use by plants grown in tanks, and (2) measurement of daily fluctuations in the water table due to plant transpiration in the natural environment (transpiration-well method). Mesquite was the only plant for which evapotranspiration was measured only by the transpiration-well method, and then only one well was used. This method is questionable because of the difficulty in determining specific yield (sediment porosity principally) and the areal extent of vegetation contributing to the effect. In addition, the results could not be replicated for the two years for which measurements were made. With this method Gatewood and colleagues determined an average annual mesquite groundwater use of 826 mm (32.52") at 100% canopy coverage. They added 180 mm (7.08") of precipitation to this average value to obtain a yearly evapotranspiration rate of 39.6" (1006 mm). These data

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are questionable, however, because they are derived by averaging highly discordant data for July through October for 1943 and 1944. The 1944 values for these four months declined by 45% from 1943. No declines of this magnitude occurred for six other data sets for other vegetation from other transpiration wells, four of which were located near the mesquite well. Transpiration values for those four sites for July through October for 1943 and 1944 were similar and consistent. If the anomalous 1943 mesquite values are excluded and the full-year 1944 values are used alone, then yearly groundwater use at 100% canopy coverage would be 26.3" (668 mm). Adding 7.08" of precipitation would result in 33.38" (848 mm) total evapotranspiration. While groundwater use was scaled for 100% canopy coverage, the actual canopy coverage for the 984 acres of mesquite in the study area was only 47.54%. Using the original averaged data would yield an annual woodland groundwater use over this area of 15.46" (393 mm). If only the 1944 data were used, this would yield an annual mesquite groundwater usage of 12.50" (317.5 mm). If one assumes that most precipitation evaporates or is transpired and contributes to ET regardless of the percentage of ground cover by vegetation, mesquite annual ET would be 22.54" (573 mm) and 19.58 (497 mm), respectively. While using the transpiration-well method to obtain these values is questionable, they are similar to those that Scott et al. (2006b) obtained for the Lewis Springs mesquite shrubland (immature woodland) on the San Pedro River. That site had 50%–55% canopy coverage, similar to the Gila River site, with total annual ET being 26.0" (661 mm) and mesquite groundwater use being 15.0" (381 mm). While this correspondence may be fortuitous, it is consistent. Walnut Gulch Experimental Watershed, Tombstone, Arizona 1965–1967 Qashu (1966) and Qashu and Evans (1966) attempted to measure the evapotranspiration of a small, confined mesquite woodland in the Walnut Gulch Experimental Watershed of southeastern Arizona with the transpiration-well method using White’s (1932) approach. Tromble (1972) then expanded this year-long study with two more years of data. The studies obtained growing-season water-use data for May and June only and did not collect values for the four remaining months (July to October). Once the summer monsoon season began, daily groundwater fluctuations ceased because of recharge, and no further measurements were possible. For a complete discussion of the Qashu and Tromble studies, see Appendix B. Tromble (1972) states that the woodland had a canopy coverage of 80% and was growing above a perched water table, which was at a depth of ~3–4 m (10–13 ft) during the period of measurement. Granodiorite bedrock was as shallow as 1.5 m (5 ft), with pockets of deeper sediment within it. The water-bearing alluvial layer was less than 2 m (6.5 ft) thick in places, with the underlying bedrock acting as a barrier to deeper percolation. The study first measured the rate of groundwater decline through subsurface seepage into bedrock during the winter. This value was then subtracted from summer groundwater fluctuations to calculate actual transpiration use. During the peak water-use period in June, the primary well used to measure groundwater levels recorded daily groundwater fluctuations of up to 20 cm compared to 3–4 cm measured by Qashu. Tromble converted these fluctuations to a daily transpiration rate by following Troxell’s (1936)

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modification of White’s (1932) method. Tromble incorporated Qashu’s (1966) results and determined an average daily transpiration rate for the first three weeks of June of 10.7 mm (0.421") and a total ET for April 1 to June 23 of 383 mm (15.08"). This value was exceedingly high when compared with Gatewood’s results. Extrapolating the canopy coverage to 100% would give a mesquite daily evapotranspiration rate of 13.4 mm/day for June, twice that determined by Gatewood et al. (1950) for mesquite along the Gila River, calculated at 6.7 mm/day. Tromble’s daily value of 10.7 mm is also greater than the approximate reference evapotranspiration for the area, ~8.8 mm/day (AZMET method), which is not physically feasible. While Tromble cautioned that advection of hotter, drier air from nearby desert scrub areas would increase the ET rate, the rate should not exceed the reference ET. The most likely source of error is an overestimation of aquifer porosity and specific yield, which would result in higher calculated transpiration values. An aerial review of the site shows that in some areas mesquite is growing directly on exposed granodiorite bedrock and must be obtaining its water from fractures within it. Mesquite would also be doing this in areas of shallow sediment thickness. The groundwater aquifer is thus effectively a combination of bedrock and local sand and gravel, which would greatly reduce the aquifer’s effective porosity and result in a disproportional drawdown of water in the alluvium when transpiration occurred. Erroneous assumptions in the calculation methods could also contribute to the error. San Pedro River, 1990 In 1990 the Arizona Department of Water Resources (ADWR) attempted to determine water use by mesquite in the San Pedro River basin as part of proceedings for the Gila River Adjudication. ADWR did not measure these values but calculated them from the annual mesquite consumptive use given in the Gatewood et al. (1950). Thus the ADWR study does not actually provide additional measurements of mesquite water use. ADWR determined an annual water-use coefficient (or crop coefficient) for mesquite from Gatewood’s ET value by calculating a reference evapotranspiration for the Safford area using the Blaney-Criddle method. ADWR then multiplied that coefficient, determined to be 0.622, by reference evapotranspiration values (ETo) calculated from meteorological data for four stretches of the San Pedro River. While the Blaney-Criddle method is considered less reliable than Arizona’s AZMET or the American Society of Civil Engineers (ASCE or Penman-Monteith) method, it is commonly used in legal proceedings. ADWR defined two types of San Pedro River riparian mesquite: (1) dense mesquite composed of 80% canopy and 20% bare ground, and (2) medium-dense mesquite composed of 40% canopy and 60% bare ground. From the method described above, ADWR calculated total consumptive use (evapotranspiration) of dense mesquite to be 884 mm (34.81") and of medium-dense mesquite to be 613 mm (24.12") (ADWR, 1991). ADWR assumed that these amounts contained 127 mm (5") of effective precipitation, giving a groundwater use of 757 mm (29.81") and 486 mm (19.12"), respectively. If ADWR had used just Gatewood’s 1944 full-year data, excluding the questionable 1943 data, these ET values would be reduced to 744 mm (29.28") and 516 mm (20.29"), respectively, with groundwater use being 624 mm (24.58") and 388 mm (15.29"), respectively.

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Using such a small value for effective precipitation in an arid environment is questionable and inconsistent with later methods by Scott and coworkers for calculating groundwater use. Scott et al. (2006b, 2008a, 2014) assumed that effective precipitation was 100% in their studies, which would be approximately 6" greater in this area. Removing 11" (279 mm) of precipitation from ADWR’s values would give mesquite groundwater use of 23.81" (65 mm) and 13.12" (333 mm), respectively. Subtracting this revised precipitation value from the adjusted ADWR values gives groundwater use of 464 mm 236 mm (18.28") and (9.29"), respectively. These values are similar to those determined by Scott et al (2006b) for the years 2003–2005 for the Charleston and Lewis Springs mesquite sites, discussed in the “New Studies” section. Scott’s groundwater value for dense mesquite (woodland) was 490 mm (19.28"), and his value for medium-dense mesquite (shrubland) was 381 mm (15.00"). Groundwater use by mesquite shrubland at a prior Lewis Springs site with 32% canopy coverage was 157 mm (6.18") (Scott et al., 2006b). Santa Cruz River, Nogales, Arizona, 1995 Unland et al. (1998) attempted to determine total annual evapotranspiration values for all classes of vegetation, both agricultural and natural, along a one-mile-long losing stretch of the Santa Cruz River north of Nogales, Arizona. This study was undertaken in January 1995 using a micrometeorological station and the energy balance–Bowen ratio method. One of the five difference classes of vegetation was primarily dense mesquite bosque, in which one of two meteorological stations was placed. While the paper did not directly give the evapotranspiration rate for mesquite (“tall riparian vegetation”), it did give the total annual volume of water calculated to have been used, 2.77 106 m3, and it gave the total acreage of mesquite, 326.69 hectares (807 acres). Dividing water volume by this acreage gives an annual ET of 848 mm (33.39"). While only 46% of the Bowen ratio measurement data acquired for the mesquite bosque was considered valid, Unland and coworkers felt that in many cases it was possible to substitute a reliable evaporation measurement for unusable values using the aerodynamic method. The study gives the precipitation for the year as 714 mm (28.11"), with 11.2” coming in the month of August. However, this is 280 mm (11") more than the average annual rainfall for Nogales. A check of the precipitation records for the Nogales 6 N weather station, located at the Nogales wastewater treatment plant 4.8 km (3.0 mi) southeast of the study site, shows that the area received 365 mm (14.36") of rain in 1995. Tucson 70 km to the north received somewhat less 250 mm (10"), and Patagonia 24 km (15 mi) to the east received 335 mm (13.2"). The Nogales 6 N weather station did receive 712 mm (28.04") of rainfall in 1993, two years prior to the study. Thus the value given in the paper is erroneous. In an arid environment most precipitation is returned to the atmosphere via evapotranspiration irrespective of the vegetation type and density. Scott et al. (2006b, 2008b) used this relationship to determine approximate mesquite groundwater use by subtracting precipitation from evapotranspiration. Doing so in this case would give a groundwater use by the Nogales bosque of 483 mm (19.01"). This compares with 490 mm (19.28") determined by Scott et al. (2006b) for the Charleston mesquite woodland site on the San Pedro River. The canopy coverage of the two areas is approximately the same, 74% vs ~80% (estimated by Scott et al., 2000). The Nogales site received 128 mm (5.03") more rainfall than the Charleston site during the year of

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measurement. This difference in precipitation reconciles the difference in evapotranspiration rates for the two sites, 848 mm (33.39") vs. 727 mm (28.62 mm), respectively. San Pedro River, Lewis Springs (Sierra Vista), Arizona, 1997 The next significant study of mesquite water use was undertaken by Scott and colleagues at the Lewis Springs site east of Sierra Vista in 1997 (Scott et al., 1998; Scott et al., 2000; Goodrich et al., 2000) using the energy balance–Bowen ratio method, which Unland and colleagues used. The depth to groundwater at this site was 10 m (32 ft) with a canopy coverage of 32% (revised downward from an initial assessment of 50%). Three separate total ET measurements are given for mesquite for the Lewis Springs site: (1) 402 mm (15.83"), April 11, 1997–October 21, 1997 (Goodrich et al., 2000) (2) 375 mm (14.76"), March 1997–March 1998 (Scott et al., 2000) (3) 330 mm (13.00"), May 1, 1997– November 27, 1997 (Scott et al., 2006b) The disparity with the value in case (1) is apparently related to its being the initial estimate, which was later refined downward somewhat. For case (2), annual precipitation was 247 mm (9.72”). When this is removed from ET, it gives an annual groundwater use of 128 mm (5.04”). For case (3), growing-season precipitation minus the change in soil moisture was 157 mm (6.18"). When this is removed from ET, it gives a seasonal groundwater use of 173 mm (6.82"). For 100% foliage coverage, this value would scale up to 491 mm (19.33”) of groundwater use, closer to results of later studies. This study location initially suggested that groundwater use by riparian mesquite shrubland and sacaton grassland was minimal or nonexistent, leading Scott et al. (1998) to conclude, “This leads us to wonder if the dominant, and perhaps only significant, groundwater consumptive use in the Upper San Pedro Basin is determined solely by the cottonwood/willow gallery that stands adjacent to the river.” This notion was reinforced because the 1997 annual precipitation of nearby Tombstone was 343 mm (13.50") (Scott et al., 2000). The intensive data gathering from 2001 to 2005 at new and better-located sites with eddy covariance instrumentation showed these early results to be low and confirmed that both riparian mesquite and sacaton readily access groundwater. New Studies In 2001 a study site was added at Charleston (mesquite riparian woodland), and in 2003 new sites with eddy covariance instrumentation were established at Lewis Springs (mesquite riparian shrubland and sacaton grassland) on the east side of the river. Russell Scott of the ARS in Tucson has been the lead scientist on most of this work. Important papers published related to these studies include Scott et al. (2003), Scott et al. (2004a,b), Leenhouts (2006), Leenhouts et al. (2006), Scott et al. (2006a,b), Scott et al. (2008b), Williams and Scott (2009), and Scott et al. (2014). Several other papers have also used these data for other analyses. The eddy covariance method measures and calculates turbulent fluxes or movements of gases within the atmospheric boundary layer surrounding vegetation and is used to estimate heat, water and CO2 exchange. The instrumentation measures these variables at frequent intervals and is considered the most effective way of determining evapotranspiration without directly measuring

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changes in soil moisture. It is often use in conjunction with the Bowen ratio method, which is used to check and adjust calculated evapotranspiration values. The Bowen ratio method measures latent and sensible heat transfer to determine evapotranspiration. The three established sites (Figure 3) have the characteristics given below, taken from Scott et al. (2006a) and Scott et al. (2014). Figure 4 shows the relationship of the three community types with respect to the floodplain and channel. What Scott calls “shrubland” is actually immature riparian woodland and is distinct from upland shrubland, what many consider the term to mean. Riparian woodland and shrubland grade into one another, and we would tend to group them together. Both occur on the first terrace above the floodplain (Figure 4). Upland shrubland is entirely precipitation dependent and uses a fraction of the water that riparian mesquite does. Charleston Mesquite Woodland Site: Tree canopy cover is 70%–74%; mean canopy height is 7 m (23.0 ft); depth to groundwater is 10 m (32.8 ft).

Lewis Springs Mesquite Shrubland Site: Mesquite cover is 51%–55%; mean tree height is 3.7 m (12.0 ft); sacaton grasses and various smaller shrubs are abundant in scattered patches in tree interspaces; mean depth to groundwater is 5.4 m (17.7 ft).

Lewis Springs Sacaton Grassland Site: Lush growth of sacaton grass, grass cover is 65%; canopy height is 1 m (3.3 ft); mean depth to groundwater is 2.4 m (7.9 ft).

Figure 3. Location of the Charleston and Lewis Springs study sites. The evapotranspiration data cited herein are from the eddy covariance sites denoted by the small gray circles with red outlines on the east side of the river. Modified from Scott et al. (2006b).

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Figure 4. Location of major vegetation communities with respect to the river channel and groundwater. Modified from Williams and Scott (2009). In perennial reaches, groundwater would be higher, with flow in the channel.

Figures 5, 6, and 7 give aerial views of these sites and the sacaton grassland site. How much groundwater mesquite uses depends on tree size and the percent of the ground area covered by canopy. The difference in ET values between the Charleston and Lewis Springs mesquite sites reflects this. To determine water use at other mesquite sites, one would need to extrapolate these values based upon the percentage of ground cover. Water use is partitioned principally between ground-water and precipitation. Runoff is so slight that precipitation is removed directly from total evapotranspiration to calculate groundwater use.

Figure 5. Charleston riparian mesquite woodland.

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Figure 6. Lewis Springs riparian mesquite shrubland.

Figure 7. Lewis Springs sacaton grassland.

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Comparison of Mesquite and Sacaton Grassland Data Sets Table 4 summarizes the three mesquite and sacaton data sets that Scott and colleagues have published on. These studies present three sets of eddy covariance data that quantify San Pedro mesquite water use for three overlapping periods: 2001–2003, 2003–2005, and 2003–2007. For the first three years (2001–2003), measurements were made from only May 1–November 27 to cover the period when mesquite is actively transpiring and using water. Measurements for subsequent periods were made for the entire year. No measurements were made in 2001 and 2002 for the mesquite shrubland or sacaton grassland. Scott calculated evapotranspiration values for the first two periods, 2001–2003 and 2003–2005, by adjusting measured heat values to fully balance heat energies (“forced closure”). This increases evapotranspiration values and places an upper bound on them, making them conservatively high (Russell Scott, personal communication, 2014). For the third period, 2003–2007, forced closure of the energy balance was not done, which results in lower ET values. Actual ET values presumably lie between these two extremes. Data Set 1 For comparison, early alfalfa evapotranspiration calculated for this site would be 1796 mm or 70.71" (0.95 × ETo = 0.95 × 1890 mm; ETo value modified from Scott et al., 2006b to Table 4. Comparison of mesquite and sacaton water-use data sets.

1Values are from Scott et al. (2006b) for May 1–November 27. Measurements for mesquite shrubland and sacaton grassland are for 2003 only. Precipitation in this table includes slight changes in soil moisture also.

2Values are from Scott et al. (2008b). Precipitation in this table includes slight changes in soil moisture also. The average annual water use is slightly larger than the seasonal use given in Table 3 because it includes additional soil moisture evaporation for the months of December through April.

3Values are from Scott et al. (2014). Precipitation in this table does not include changes in soil moisture.

Evapotranspiration Mesquite Woodland Mesquite Shrubland Sacaton Grassland

Data Set 1: Average 2001–2003 seasonal water use with forced closure of the energy balance1

Evapotranspiration 669 mm (26.34") 565 mm (22.24") 554 mm (21.81") Precipitation 205 mm (8.07") 185 mm (7.28") 180 mm (7.09") Groundwater Use 464 mm (18.27") 380 mm (14.96") 374 mm (14.7")

Data Set 2: Average 2003–2005 annual water use with forced closure of the energy balance2

Evapotranspiration 727 mm (28.62") 661 mm (26.02") 643 mm (25.33") Precipitation 237 mm (9.33") 281 mm (11.06") 276 mm (10.87") Groundwater Use 490 mm (19.28”) 381 mm (15.00") 367 mm (14.45")

Data Set 3: Average 2003–2007 annual water use without forced closure of the energy balance3

Evapotranspiration 692 mm (27.24") 564 mm (22.20") 548 mm (21.57") Precipitation 243 mm (9.57") 285 mm (11.21") 289 mm (11.38") Groundwater Use 449 mm (17.68") 279 mm (10.98") 259 mm (10.20")

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correspond to AZMET value1). This is an annual amount, however, that would include December through April evapotranspiration, which precludes a direct comparison with mesquite and sacaton evapotranspiration. Subtracting full annual precipitation of 232 mm (2003 value taken from Scott et al., 2008a) from total annual alfalfa ET gives 1563 mm (61.54") of alfalfa groundwater use. If one assumes that no mesquite transpiration occurs from December through April, which Scott et al. (2006b) document, then average annual alfalfa groundwater use would be 3.37 times that of mesquite woodland (18.27") for this period, or conversely, groundwater use by mesquite woodland would be 29.7% that of alfalfa. This assumes that the water which alfalfa receives perfectly balances its evapotranspiration needs, which never occurs. The amount of irrigation water applied to alfalfa can be up to 40% higher if flood irrigation is used (assuming a 70% efficiency). Thus this comparison is conservative and likely to underestimate the actual ratio. The total evapotranspiration for all sites includes that of understory vegetation, or the plants growing beneath, around, and between individual trees. This evapotranspiration constitutes approximately 10% of the total (Scott et al., 2006b). Groundwater use is calculated on the basis of millimeters or inches per unit of ground area per year by a particular vegetation. Neither mesquite nor all crops completely cover the ground surface. Groundwater usage calculated per unit of foliage area per year for mesquite would be approximately one-third greater, or 627 mm (24.7"), assuming a linear relationship. This would be the value for a mesquite woodland having 100% canopy coverage. Note that this value is still less than half that of alfalfa groundwater use. One can use this value to determine the approximate groundwater usage by areas of mesquite having different percentages of canopy cover. Data Set 2 This data set includes complete annual evapotranspiration measurements for 2003–2005, adding the months of December and January through April. While adding data from these months increases total evapotranspiration, it has little effect on actual groundwater use because of a lack of transpiration during this time. Any additional winter precipitation evaporates back to the atmosphere, which results in nearly the same groundwater use. Again, these values were computed by forcing closure of the energy balance. They are conservatively high and represent maximum potential mesquite and sacaton water use. These values give an average annual mesquite woodland ET of 40.5% that of alfalfa ET. Alfalfa groundwater use would be nearly the same as for Data Set 1, 1568 mm (62.22"). Thus average annual alfalfa groundwater use for this period would be 3.20 times that of mesquite woodland (19.28"), or conversely, average annual groundwater use by mesquite woodland would be 31.25% that of alfalfa. The groundwater use per unit of mesquite foliage area would be 662 mm/year (26.1"), which approximates that of mesquite woodland canopy covering 100% of the ground surface.

1 The difference in the reference evapotranspiration, ETo, calculated with the Penman-Monteith and AZMET methods at established Arizona meteorological stations varies from 3% to 17% (Brown, 2005). This conversion needs to be done by fully calculating the AZMET value for the San Pedro sites. Instead, it is based upon the ratio at the Safford station and will not be accurate.

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Average alfalfa groundwater use for this period would be 4.1 times that of mesquite shrubland and 4.27 times that of sacaton grassland, or conversely, average annual groundwater use for mesquite shrubland and sacaton would be 24.3% and 23.4% that of alfalfa, respectively. Again, such comparisons assume a perfect use of irrigated water by alfalfa for evapotranspiration, which is never achieved. The relatively small differences in evapotranspiration for the woodland, shrubland and grassland communities are especially notable. Intuitively one would assume that mesquite would use far more water than sacaton grass. Data Set 3 Data set 3 gives evapotranspiration values for the San Pedro mesquite and sacaton sites for two additional years but without adjusting measured heat values to achieve a full energy balance. This reduces ET values for mesquite woodland by 6.6% and for mesquite shrubland and sacaton grassland by 16.8% and 25.8%, respectively. These values should provide lower bounds on actual water use. In this case, mesquite woodland evapotranspiration is 38.6% that of alfalfa Average annual alfalfa groundwater use for this period would be 3.5 times that of mesquite woodland, or conversely, average annual groundwater use by mesquite woodland would be 28.6% that of alfalfa. Groundwater use per unit of foliage area per year would be 607 mm (23.89"). Average annual alfalfa groundwater use would be 5.6 times that of mesquite shrubland and 6.05 times that of sacaton grassland, or conversely, mesquite shrubland and sacaton grassland groundwater use would be 17.8% and 16.5% that of alfalfa, respectively. These values are considered to represent the minimum potential riparian vegetation water use. Closure of the energy balance was not forced in this case because the study focused on quantifying net ecosystem production (NEP), ecosystem respiration (Reco), and gross ecosystem production (GEP), which assess carbon fluxes. Researchers are uncertain whether these ET values or those resulting from forced closure are closest to actual water use. Actual values should lie somewhere between the two (Russell Scott, personal communication, 2014). Other Studies Other studies that systematically address mesquite evapotranspiration in riparian areas are scarce. One additional significant study that attempts to do so is that done in 2010 for the Mojave River of Southern California (Neale et al., 2011). The Mojave River flows northward from the San Bernardino Mountains northeast of Los Angeles and empties into the Mojave Desert, passing through the community of Apple Valley near the mountain front and Barstow to the east in the desert (Figure 8). This study was undertaken by the Bureau of Reclamation in cooperation with the Mojave Water Agency and the University of Utah to assess saltcedar (tamarisk) evapotrans-piration as part of an ongoing program to increase water availability through saltcedar removal. This study also determined yearly evapotranspiration for riparian mesquite in the floodplain. Analysis of ET was based on calibration of aerial multispectral and thermal infrared imagery with ground-based measurements of crop reference ET. This procedure contrasts with that used with San Pedro mesquite, which was based upon eddy covariance measurements acquired with ground-based instruments. Spectral readings were transformed to evapotranspiration values for different classes of vegetation using either the SERBAL or Two-Source model. Both modeling packages produced similar results, and the Two-Source model was chosen for all analyses.

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Multispectral and thermal infrared data from both 2007 and 2010 were analyzed and compared for four contiguous areas (Figure 10; Table 5). The Alto groundwater management area is closest to the San Bernardino Mountains, and subsequent management areas are progressively farther away to the north and east. Groundwater levels have been increasing in the Alto and Alto Transition subareas and declining in the Centro and Baja subareas. The lower evapotranspiration values in the Baja management area reflect this increasing distance from the mountain front and a falling water table. Total yearly ET for Mojave River mesquite is significantly less than that measured for San Pedro River mesquite (Scott et al. 2008a), averaging 461 mm vs. 727 mm and 661 mm (18.14" vs. 28.6" and 26.0"), respectively, for woodland and shrubland. However, this area also has significantly less precipitation per year, 110 mm or 4.33". If one assumes that all precipitation isincluded in these ET values as is done for the San Pedro sites, then subtracting it would give an annual mesquite groundwater consumption of 350 mm, or 13.82". This is very similar to that of Scott’s San Pedro mesquite shrubland (381 mm or 15.0") and about 75% that of his mesquite woodland (490 mm or 19.3") (Scott et al., 2008a). This compares favorably and reinforces the conclusion that alfalfa groundwater use is at a minimum 3 to 4 times that of riparian mesquite.

Figure 8. Location of the Mojave River north of the San Bernardino Mountains, southern California, and the four groundwater management areas analyzed for tree and plant evapotranspiration. Modified from Neale et al. (2011).

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Table 5. Mesquite, cottonwood–willow, and saltcedar riparian evapotranspiration along the Mojave River, Southern California, averaged for 2007 and 2010.

Subarea Mesquite Cottonwood–Willow Saltcedar Alto 511 mm

(20.11") 932 mm (36.69")

747 mm (29.41")

Alto Transition 402 mm (15.83")

835 mm (32.87")

814 mm (32.05")

Centro 524 mm (20.63")

813 mm (32.01")

782 mm (30.79")

Baja 408 mm (16.06")

743 mm (29.25")

714 mm (28.11")

Averages 461 mm (18.15")

831 mm (32.72")

763mm (30.01")

Regarding other vegetation, evapotranspiration for cottonwood in the Alto subarea (932 mm or 36.69") is nearly the same as that for cottonwood at the Charleston San Pedro site (966 mm or 38.0"). The Alto subarea has perennial water, and thus its water regime is similar to the Charleston site. Downstream along the Mojave River, water flow is increasingly intermittent, and groundwater is deeper. This is reflected in lower cottonwood ET rates, as occurs along the San Pedro River. Average saltcedar ET values (763 mm or 30.01") are very similar to those obtained for saltcedar stands along the Rio Grande River that reservoirs do not periodically flood (750 mm or 29.5") (Dahm, 2002). Accurately determining water usage for all vegetation requires assessing specific site characteristics and acquiring data over a given interval of time. Comparing Mesquite Woodland Evapotranspiration Throughout the Year

As mentioned above, of particular interest is how mesquite water use compares with that of a standard reference crop such as alfalfa. Figure 9 is modified from Scott et al. (2008b) and shows total evapotranspiration over the year from the three San Pedro communities (or sites) compared with the reference crop evapotranspiration, ETo, calculated for those sites. As noted earlier, alfalfa ET is ~95% of the reference crop ETo. This diagram thus gives an approximate comparison of mesquite and alfalfa water use for the year. On average, Charleston mesquite woodland (the heavy black line) reaches its maximum water usage around August 20 at 78.5% that of reference ETo. Overall, total annual riparian mesquite evapotranspiration is approximately 40% that of alfalfa, as noted above.

This relationship is further demonstrated in Figure 10 taken from Nagler et al. (2013). This figure continuously compares vegetation ET vs. reference ETo for five years, 2003–2007. Reference ETo is again approximately equal to alfalfa ET. Nagler et al. (2013) note that maximum ET values for these three vegetation types were about 60% of the maximum reference value and were only 42% of total ETo on an annual basis. This again confirms that total alfalfa ET for the year would be about 2.4 times mesquite woodland ET.

Reference evapotranspiration (ETo) and alfalfa evapotranspiration peak in early June in the hottest and driest part of the year and then decline during the monsoon season when the humidity rises and average temperatures decline (Figure 1; Erie et al., 1982). Mesquite, however, does not use nearly as much water during this time. Mesquite evapotranspiration peaks later in August while monsoon moisture is greatest, limiting groundwater use. During the late summer, alfalfa

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Figure 9. Comparison of annual evapotranspiration for mesquite and sacaton with the annual standard crop reference evapotranspiration, ETo, which approximately equals that of alfalfa. This graph represents the approxi-mate ratio of water use of these three communities to alfalfa water use throughout the year. Modified from Scott et al. (2008b). and mesquite evapotranspiration approach one another (Figure 10), a relationship that Don Decker of the Natural Resources Conservation Service noted in the 1990s (Sayre, 2004). This set off some farmers to aggressively removing mesquite bosque thinking that they would save vast quantities of water. To understand the full relationship between mesquite and alfalfa (or crop) water use, however, one must consider the full year’s use and the mixture of water sources (groundwater vs. precipitation). Viewed in this context, mesquite woodland uses much less water than alfalfa. When precipitation is subtracted, mesquite woodland uses one-third or less the groundwater that alfalfa does. Reasons for Lower Mesquite Water Usage As noted in this review, woodland mesquite uses less water annually than most crops. Several factors account for this: 1. Mesquite dormancy. Mesquite is dormant for approximately six months of the year (Figure 11), which is perhaps the most important reason for less water use. The mesquite growing season is approximately bounded by the date of the latest frost in the spring and the earliest frost in autumn. In general, mesquite does not begin to leaf until early May and is not fully leafed and transpiring until early June. Mesquite water usage begins to steeply decline in early to mid- September and essentially ends by early November (Scott et al., 2006b). Thus mesquite is effectively dormant for approximately six months of the year. Forage crops such as alfalfa and bermuda or rye are actively grown for nine months or more, which increases their water usage.

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Figure 10. Figure 2b from Nagler et al. (2013) showing the evapotranspiration for the three San Pedro River communities (ETa) vs. the reference evapotranspiration (ETo, red line) for 2003–2007. Alfalfa ET would be ~95% of ETo; thus the red line approximates alfalfa ET. CM = Charleston mesquite, LSM = Lewis Springs mesquite, and LSS = Lewis Springs Sacaton.

Figure 11. Evapotranspiration of the three communities through the year for 2003. The mesquite growing period is confined to the days between the last and first freezes of the year and is somewhat less than 6 months. Mesquite is dormant for the other 6 months, a major reason for its lesser water usage. Modified from Scott (n.d.); derived from Scott et al. (2006b).

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2. Hydraulic redistribution and storage of precipitation. During the summer monsoon season from July through September, mesquite roots pump surface precipitation downward at night for storage in deeper soil layers (Hultine et al., 2004; Scott et al., 2008a; Figure 12). The tree then uses this stored water later during drier times, reducing groundwater consumption. Downward water transfer in a riparian mesquite woodland can reach 7 liters/night (~2 gallons) per tree. This transfer is closely tied to precipitation events. Lateral roots near the surface gather the water and move it toward the trunk, and then the tap root moves the water downward. Summer nocturnal reverse flow in this manner can reach 25%-50% of daily upward transpiration flow (Hultine et al., 2004).

Figure 12. A. Downward transfer (hydraulic redistribution) of precipitation via the mesquite tap root at night during the summer monsoon season. Negative values indicate water flow away or downward from the base of the tree, while positive values denote water transfer toward the base of the tree. Lateral roots gather the water, and the tap root moves it to deeper soil layers. Downward transfer is closely tied to precipitation events, noted in B. This transfer can reach 7 liters/night (~2 gallons/night). Originally published in Hultine et al. (2004) and later referenced in Scott et al. (2006b), the source of this figure. This is from the Charleston mesquite woodland site on the San Pedro River. Mesquite also collects and pumps precipitation downward for storage during the winter dormancy period (Hultine et al., 2004; Scott et al., 2008a; Figure 13). This downward transfer is nearly continuous through the end of April, as long as precipitation has occurred. During the start of the growing season, essentially all water used is from this winter storage via the tap root. For mesquite trees in upland grasslands, the amount of water stored in this way can equal the yearly amount of water used. Desert upland mesquite typically takes up and transpires about one-third of the total yearly precipitation (Ansley et al., 2005; Scott et al., 2008a). While this

water storage

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Figure 13. Hydraulic redistribution (downward transfer) of winter precipitation by mesquite during the dormant winter and early spring months. As in the previous diagram, negative values indicate water flow away, or downward, from the base of the tree, while positive values denote water transfer toward the base of the tree. Downward transfer and storage approaches 12 liters (3 gallons) per day. Water transfer is predominantly downward through mid to late April. Figure is from Scott (n.d.), derived from Scott et al. (2008a), and is from mesquite savanna in the Santa Rita Experimental Range south of Tucson, Arizona. water storage does not reduce overall evapotranspiration for the year, it does diminish groundwater use in riparian areas by utilizing surface water more efficiently. 3. Limited canopy ground coverage. The canopy in a mesquite woodland covers the full ground surface only in special situations where the depth to groundwater is optimum for growth. The majority of the time canopy coverage is significantly less than 100%. This is in contrast to 100% coverage by forage crops such as alfalfa and bermuda grass. In Scott’s studies, dense mesquite woodland canopy covered 74% of the ground surface, and sparser mesquite shrubland canopy covered 55%. The initial mesquite shrubland canopy that he studied at Lewis Springs covered only 32% of the ground surface (Scott et al., 2006b). This reduced foliage results in less water usage per unit area by mesquite than crops. 4. Arid-adapted leaf structure and physiology. While mesquite does transpire at significantly higher rates than other desert vegetation, its leaves and physiology are adapted to an arid environment, and the plant naturally adjusts to a wide range of moisture conditions. Crops such as alfalfa and pastsure grass are not so adapted. Mesquite thus needs less water physically than forage crops to prosper. During hot and dry weather or when ground water is less accessible, mesquite adjusts the stomatal openings of its leaves to reduce moisture loss, folds its leaves to reduce leaf surface area, and prunes itself of excess leaves if necessary. Its leaves are also covered with a waxy coating, and leaf area is smaller than more broadleafed plants (Nelle, 2014).

water storage

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Mesquite can survive under very low soil water conditions by reducing leaf area, increasing leaf cuticle thickness, and almost completely ceasing to grow. It also uses leaf orientation, wax accumulation, and reduction in canopy development to avoid drought (Wan and Sosebee, 1991). Because of these adaptations, mesquite naturally conserves water when necessary and can endure low water availability.

DISCUSSION

Riparian and upland mesquite acquire and use water differently and require different management approaches. Recent work by Scott (2014) shows that evapotranspiration by upland mesquite in Arizona does not differ from associated grassland and that removing it will not increase water yields for offsite use. Thus removing mesquite that does not depend upon groundwater is more a matter of preference for different vegetation than increasing water availability. Riparian mesquite on the other hand does access groundwater, and water use may be of concern. Without measuring mesquite evapotranspiration at a specific location over the growing season, it is difficult to determine whether newly established mesquite is accessing groundwater, as mesquite in both upland (solely precipitation dependent) and riparian (precipitation and groundwater dependent) environments can be similar in size, at least in the first several years of growth. If mesquite is accessing groundwater, ET values will be high prior to the commencement of monsoon precipitation. Whether or not mesquite will eventually access groundwater on abandoned agricultural fields or other areas depends on depth to groundwater. Optimal riparian mesquite development occurs where groundwater is 5–6 m (16.4–19.7 ft) or less below the surface (Stromberg et al., 1992; Stromberg et al., 1993). Data show that as depth to groundwater increases from 6 to 15 m (20 to 50 ft), trees are increasingly shorter, have smaller crowns, and depend more on precipitation (Stromberg et al., 1992; Stromberg, 1993; Snyder and Williams, 2000). Trees established at shallow groundwater levels become increasingly stressed as groundwater levels fall. When groundwater depths reaches 15 m (50 ft) or greater, these trees either perish or die back severely, becoming dependent solely on precipitation (Stromberg, 1993). Thus the maximum depth to groundwater that new mesquite might access must be less than 15 m (~50 ft). No one has determined what the maximum water depth that a new tree can effectively access to support its growth, only that the depth will be less than 15 m (50 ft). When a mesquite seed germinates, the resulting tree must depend entirely on precipitation until it reaches a certain age and size. Some abandoned agricultural fields may now be too far above the water table for this mesquite to access groundwater, and any reestablished mesquite will thus remain smaller, be more dispersed and depend entirely upon precipitation. Fields underlain by shallower groundwater may facilitate mesquite groundwater use, allowing a true woodland to eventually develop. Determining whether mesquite is capable of accessing groundwater beneath these areas and confirming whether it eventually does will be important to long-term management plans if water use is of concern.

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Addressing this situation is a matter of cultural values. In some circumstances we may want to encourage other vegetation such as grass to revegetate disturbed areas or to replace mesquite. Conversely, riparian mesquite per se is renown for its high biological value, and we may wish to encourage its growth in certain areas for conservation purposes. Whether mesquite is removed from the landscape or discouraged from recolonizing an area, some form of vegetation must replace it. Removing it merely for the sake of removing it can increase erosion and reduce plant productivity and associated habitat, which may be difficult to reestablish. As good stewards of the land we must also choose what to replace it with. If water use is of concern when revegetating an area, comparing and quantifying how much water different types of vegetation use will be important. Given the cost, time and effort required to establish alternative vegetation and the fact that it uses water also, allowing mesquite to naturally establish itself may at times be a more feasible both economically and environmentally. Mesquite’s potential for higher water use may be acceptable given these tradeoffs, and this may be the most practical way of protecting the land’s surface.

CONCLUSIONS While researchers in the past have attempted to measure water use by both upland and riparian mesquite, this review focuses solely on water use by riparian mesquite because of its greater hydrologic impact. All direct field measurements of riparian mesquite water use have been made either along the Gila or San Pedro Rivers in Arizona. Upland mesquite depends entirely on precipitation, which limits its growth and water use, whereas riparian mesquite accesses and uses groundwater in addition to precipitation if the water table is sufficiently shallow. Evapotranspir-ation by riparian mesquite may be up to three times as great as upland shrubland mesquite, whereas transpiration or actual water use by riparian mesquite may be up to seven times as great. Past measurements of riparian mesquite water use have been very sparse and difficult to obtain. The primary difficulty for researchers has been devising a sound and reliable method to measure it. Early methods to measure evapotranspiration of native vegetation used three strategies: (1) measuring water used by plants grown in tanks (lysimeters), (2) measuring the water balance in the aquifer over the period of plant water use, and (3) measuring daily fluctuations in water levels in wells (transpiration well method) within vegetation stands. Only the second and third methods has been used with riparian mesquite and then only in two instances. The difficulty in devising an accurate computational model for these methods introduces large uncertainties into the results and overestimates water use. In the 1990s and 2000s these methods gave way to instrumented measurements of environmental factors within tree stands, first by the Bowen-energy ratio method and subsequently by the eddy covariance method. Only two attempts have been made to use the Bowen-energy ratio method with riparian mesquite. Both gave results that were questionable because of either instrumentation unreliability (Unland et al., 1998) or because equipment was not located within a representative mesquite stand (Scott et al., 2000). The eddy covariance method provided the measurements summarized in this review and is considered more reliable. In addition to these methods, researchers have also attempted to quantify riparian vegetation water use by remote sensing methods. Spectral measurements of vegetation must be calibrated

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with ground measurements of water use for the same vegetation, usually by the eddy covariance method. This method results in significant uncertainty in the results and has been used with mesquite in just a single instance for the Mohave River in southern California. Of all the methods used, the eddy covariance method is the most reliable. Scott and colleagues have acquired up to nine years of water-use data for riparian mesquite using this method within the San Pedro Riparian National Conservation Area, and these data are the primary source of the conclusions presented herein. This method in itself results in uncertain results because researchers do not know whether forcing closure of the energy-balance equation produces more or less accurate values. Forcing closure maximizes water-use and has been more generally used for formal presentations of water use because it produces conservatively high values. Measurements of crop water use fall into two categories, (1) actual evapotranspiration (consumptive use) by the crop, and (2) the amount of water applied to achieve the necessary evapotranspiration for good yields. Applied water can be up to 50% or more greater than actual consumptive use and is an important consideration when comparing water use with natural vegetation. In contrast to crops, mesquite uses only the water that is naturally available and adjusts its growth to it. Crop evapotranspiration is typically carefully determined by measuring water use by plants grown in tanks (lysimeters) or under carefully monitored and controlled field conditions. The goal in applying water and measuring its use in these procedures is to determine the amount required to maximize yield without overwatering. Such exact applications of water are much more difficult to achieve under actual growing conditions, and farmers either under- or overwater their crops to some degree. A primary criticism of the tank or lysimeter method is that it overestimates evapotranspiration somewhat. In some instances, agricultural scientists are now using the eddy covariance method to determine crop evapotranspiration. The primary comparison between mesquite water use and crop water use in this report is for evapotranspiration or consumptive water use. By this comparison, mature mesquite woodland uses approximately 40% of the water used by a reference crop such as alfalfa. A more appropriate comparison, arguably, is the amount of groundwater or external water that mesquite and crops use. When this is considered, mesquite water use declines by approximately the amount of precipitation a woodland receives, whereas crop water in general increases because of irrigation inefficiencies. In terms of groundwater use and irrigation efficiency, mature riparian mesquite uses 25%-30% of the water that a crop such as alfalfa does. No crop grown in southern Arizona uses less water than a mature mesquite woodland. Even lower-evapotranspiration crops such as wheat and barley use 50% or more water by this measure. While the evapotranspiration of vegetable crops is generally less than that of riparian mesquite, when applied or groundwater usage is considered, all vegetable crops use significantly more water. In addition, mesquite water use diminishes with diminishing canopy coverage of the ground and is not a constant, set value. Thus this review shows that riparian mesquite does not use vastly more water than cultivated crops as some have come to believe. To the contrary, in essentially all cases mesquite uses less

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even when fully developed into a mature woodland. The most comparable crop comparison is with grains, whose evapotranspiration is similar or slightly less. Removing mesquite from riparian areas and replacing it with irrigated crops or pasture grass will thus not reduce water loss or improve stream-flow conditions. In fact, doing so may exacerbate stream-flow conditions given the greater water requirements of most crops, especially alfalfa or pasture grass. Much of the irrigated agricultural land along riparian areas in arid regions is used to grow forage for livestock. While mesquite colonization of abandoned fields used for this purpose may gradually result in increased groundwater use if the water table is sufficiently shallow, that use will not exceed prior use unless fields were kept fallow for up to two-thirds or more of the time. Forage crops such as bermuda and rye grass or alfalfa would have to be grown less than once every two to three years to use less groundwater. Even if mesquite became fully reestablished as a mature woodland, its water use would be less than most agricultural use. Acknowledgments. I thank Russell Scott of the U.S. Department of Agriculture’s Agricultural Research Service in Tucson, Arizona for providing data and references on mesquite water use as well as comments on this review. Michael Ottman and Paul Brown of the University of Arizona’s Cooperative Extension Service provided background on alfalfa and pasture grass water use, respectively. Doug Hunsaker of the Agricultural Research Service in Phoenix, Arizona, provided water use data for cotton and wheat. I also thank Phillip Hedrick of Arizona State University’s School of Life Sciences and Scott Wilbor, a recent graduate from the University of Arizona’s School of Natural Resources and the Environment, for their reviews and comments. About the author: Norm “Mick” Meader is Chair of the Conservation Committee of the Cascabel Conservation Association and a Board Member of the Lower San Pedro Watershed Alliance. He has a B.A. and M.S. in geology and is a retired staff member of the Department of Geosciences, University of Arizona, Tucson. You may reach him at 3443 E. Lee Street, Tucson, AZ 85716, phone 520-323-0092, email nmeader@cox.net.

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Ottman, M., 2010. Suboptimal irrigation strategies for alfalfa in the Lower Colorado Region, 2009. 2010 Forage & Grain Report, College of Agriculture and Life Sciences, University of Arizona, 8 pp. Available from http://arizona.openrepository.com/arizona/handle/10150/203769. Accessed July 27, 2014. Putnam, D., and M. Ottman, 2002. Emerging issues for alfalfa in California and Arizona. Available from http://alfalfa.ucdavis.edu/+symposium/proceedings/2002/02-013.pdf. Accessed June 15, 2016. Rogers’ Heaven Sent Ranch, n.d. Certified organic alfalfa (website), http://rogershsr.com/ certified-organic-alfalfa. Accessed June 15, 2014. Salinity Management, 2007. Salinity Management Guide, Estimate the water needs of landscape plants (website), http://www.salinitymanagement.org/Salinity%20Management%20Guide/ew/ ew_2.html. Accessed July 1, 2014. Skaggs, R., Samani Z., Bawazir A.S., and Bleiweiss M., 2008. Yield response to water in irrigated New Mexico pecan production: Measurements & policy implications. In: Gooch, R.S., and Anderson, S.S., (Eds.), Urbanization of irrigated land and water transfers, Proceedings of the U.S. Committee on Irrigation and Drainage Water Management Conference, Scottsdale, Arizona, May 28-31, 2008, pp. 369-480. Available from http://digitool.library.colostate .edu/R/?func=dbin-jump-full&object_id=117070. Accessed September 9, 2014. Snyder, R. L., and Bali, K. M., 2008. Irrigation scheduling of alfalfa using evapotranspiration, In: Proceedings, 2008 California Alfalfa & Forage Symposium and Western Seed Conference, San Diego, Calif, December, 2-4, 2008. Available from http://www/alfalfa.ucdavis.edu/+symposium/ proceedings/2008/08-95.pdf. Accessed June 16, 2014. Teegerstrom, T., and Clark, L., 1999. Arizona Field Crop Budgets, 1999–2000. University of Arizona Cooperative Extension Bulletins #AZ1114–AZ1122. Values for all counties and crops are available at https://ag.arizona.edu/arec/pubs/fieldcropbudgets.html in separate documents. Accessed November 26, 2014. UA Cooperative Extension, n.d. Alfalfa FAQs, Arizona forage & grain crops information (website), University of Arizona, http://cals.arizona.edu/forageandgrain/alfalfa-faqs. Accessed June 14, 2014. UA Cooperative Extension, 2001. Alfalfa report, Yuma County, Arizona, December 31, 2001(website), University of Arizona, https://cals.arizona.edu/crops/counties/yuma/ alfalfareports/2001/alfalfarpt123101.html. Accessed June 15, 2014. Watson, J., and Sheedy, M, 1992. Crop water use estimates, 3 pp. Available at http://connection.ebscohost.com/c/articles/17040181/estimating-cotton-evapotranspiration-crop-coefficients-multispectral-vegetation-index. Accessed June 30, 2014.

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MESQUITE AND RIPARIAN REFERENCES ADWR (Arizona Department of Water Resources), 1991. Hydrographic Survey Report for the San Pedro River Watershed. General Assessment, Vol. 1. Arizona Department of Water Resources. Filed with the Court, November 20, 1991, 604 pp. Ansley, R.J., 2005. How much of a water thief is mesquite? The Cattleman 92(1), 20-32. June 2005 issue. Dahm, C.N., Cleverly, J.R., Allred Coonrod, J.E., Thibault, J.R., McDonnell, D.E., and Gilroy, D.J., 2002. Evapotranspiration at the land/water interface in a semi-arid drainage basin. Freshwater Biology 47, 831-843. Gatewood, J.S., Robinson, T.W., Colby, B.R., Hem, J.D., and Halpenny, L.C., 1950. Use of water by bottom-land vegetation in the lower Safford Valley, Arizona. United States Geological Survey Water Supply Paper 1103, 210 pp. Available from http://www.pubs.usgs.gov/wsp/1103/report.pdf. Accessed June 19, 2014. Gazal, R.M., Scott, R.O., Goodrich, D.C., and Williams, D.G., 2006. Controls on transpiration in a semiarid riparian cottonwood forest. Agricultural Forest Meteorology 137, 56–67. Goodrich, D.C., Scott, R., Qi, J. et al., 2000. Seasonal estimates of riparian evapotranspiration using remote and in situ measurements. Agricultural Forest Meteorology 105, 281–309. Johnson, A.I., 1967. Specific Yield – Compilation of Specific Yields for Various Materials, Hydrologic Properties of Earth Materials, Geological Survey Water-Supply Paper 1662-D, 74 pp. Leenhouts, J.M., 2006. Hydrologic requirements of and evapotranspiration by riparian vegetation along the San Pedro River, Arizona, U.S. Geological Survey Fact Sheet 2006-3027, 3 pp. Available from http://www.pubs.usgs.gov/fs/2006/3027/. Accessed June 18, 2014. Leenhouts, J.M., Stromberg, J.C., Scott, R.L. (Eds.), 2006. Hydrologic Requirements of and Consumptive Ground-Water Use by Riparian Vegetation Along the San Pedro River, Arizona, U.S. Geological Survey, Scientific Investigations Report 2005-5163, pp. 107–152. Available from http://www.pubs.usgs.gov/sir/2005/5163. Accessed June 18, 2014. Nagler, P.L., Glenn, E.P., Nguyen, U., Scott, R.L., and Doody, T., 2013. Estimating riparian and agricultural actual evapotranspiration by reference evapotranspiration and MODIS enhanced vegetation index. Remote Sensing 5, 3849-3871. doi:10.3390/rs5083849. Neale, C.M.U., Taghvaeian, S., Geli, H., et al., 2011. Evapotranspiration water use analysis of saltcedar and other vegetation in the Mojave River floodplain, 2007 and 2010, Mojave Water

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Agency Water Supply Management Study Phase 1 Report. Prepared by the Department of Civil and Environmental Engineering, Utah State University, and the Bureau of Reclamation, 99 pp. Available from http://www.usbr.gov/lc/socal/reports/MWAStudy/Phase1.pdf. Accessed July 3, 2014. Nelle, S., 2014. Misunderstood mesquite. San Angelo Standard Times, Thursday, April 24, 2014. Website: http://www.gosanangelo.com/news/2014/apr/24/misunderstood-mesquite/. Accessed July 9, 2014. Qashu, H.K., 1966. Estimation of the elements of the water balance of an ephemeral stream channel with riparian vegetation. Ph.D. Dissertation, University of Arizona, 141 pp. Qashu, H.K., and Evans, D.D., 1967. Water disposition in a stream channel with riparian vegetation. Soil Science Society of America Proceedings 31(2), 263-269. Sayre, N.F., 2011. A history of land use and natural resources in the middle San Pedro River Valley, Arizona. Journal of the Southwest 53(1), 87-137. Spring 2011. doi: 10.1353/ jsw.2011.0002 Scott, R.L., Shuttleworth, W.J., and Goodrich, D.C., 1998. Water use of two dominant riparian vegetation communities in southeastern Arizona. Summary of poster presented at the American Meteorological Society, Special Symposium on Hydrology, Phoenix, Arizona, 11-16 January 1998, Session 1: Integrated Observations of Semi-Arid Land-Surface-Atmosphere Interactions, HYDRO FA P2.7. Available at http://www.tucson.ars.ag.gov/salsa/archive/publications/ams_preprints/ scott.html. Accessed September 26, 2014. Scott, R.L., n.d. (no date). Riparian water use (PowerPoint presentation). Presented to the Community Watershed Alliance, Benson, Arizona. Available from http://cwatershedalliance.com/ TAC_pdf/Scott_RiparianWaterUse.pdf. Accessed June 18, 2014. Scott, R.L., Shuttleworth, W.J., Goodrich, D.C., and Maddock III, T., 2000. The water use of two dominant vegetation communities in a semiarid riparian ecosystem. Agricultural and Forest Meteorology 105, 241–256. Scott, R.L., Watts, C., Garatuza, J., Edwards, E., Goodrich, D., Williams, D., and Shuttleworth, W.J., 2003. The understory and overstory partitioning of energy and water fluxes in an open canopy, semiarid woodland. Agricultural and Forest Meteorology 114, 127–139. Scott, R.L., Edwards, E.A., Shuttleworth, W.J., Huxman, T.E., Watts, C., and Goodrich, D.C., 2004a. Interannual and seasonal variation in fluxes of water and carbon dioxide from a riparian woodland ecosystem. Agricultural and Forest Meteorology 122, 65–84. Scott R.L., Goodrich, D.C., Levick, L, McGuire, R., Cable ,W.L., Williams, D.G., Gazal, R., Yepez, E., Ellsworth, P.Z., and Huxman, T., 2004b. San Pedro Riparian National Conservation Area (SPRNCA) water needs study: A research effort funded by the Upper San Pedro

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Partnership, 79 pp. Available from http://www.denix.osd.mil/nr/upload/UPPER-SAN-PEDRO-1.PDF. Accessed June 18, 2014. Scott, R.L., Huxman, T.E., Williams, D.G., Goodrich, D.C., 2006a. Ecohydrological impacts of woody plant encroachment: Seasonal patterns of water and carbon dioxide exchange within a semiarid riparian environment. Global Change Biology 12, 311–324. Scott, R.L., Williams, D.G., Goodrich, D.C., Cable, W.L., Levick, L.R., McGuire, R., Gazal, R.M., Yepez, E.A., Ellsworth, P., Huxman, T.E., 2006b. Determining the riparian groundwater use within the San Pedro Riparian National Conservation Area and the Sierra Vista Subwatershed, Arizona (Chapter D). In: Leenhouts, J.M., Stromberg, J.C., Scott, R.L. (Eds.), Hydrologic Requirements of and Consumptive Ground-Water Use by Riparian Vegetation Along the San Pedro River, Arizona, U.S. Geological Survey, Scientific Investigations Report 2005-5163, pp. 107–152. Available from http://www.pubs.usgs.gov/sir/2005/5163. Accessed June 18, 2014. Scott, R.L., Cable, W.L., and Hultine, K.R., 2008a. The ecohydrologic significance of hydraulic redistribution in a semiarid savanna. Water Resources Research 44, 12 pp. W02440, doi:10.1029/2007WR006149. Scott, R.L., Cable, W.L., Huxman, T.E., Nagler, P.L., Hernandez, M., and Goodrich, D.C., 2008b. Multiyear riparian evapotranspiration and groundwater use for a semiarid watershed. Journal of Arid Environments 72, 1232-1246. Scott, R.L., 2014. Hydrologic aspects of mesquite encroachment. Presentation to Science on the Sonita Plain Symposium 2014, Appleton-Whittell Research Ranch, Elgin, Arizona, June 8, 2014. Available at https://www.youtube.com/watch?v=jubTGn0vAzQ. Accessed July 10, 2014. Scott, R.L., Huxman, T.E., Barron-Gafford, G.A., Jenerette, G.D., Young, J.M., and Hamerlynck, E.P., 2014. When vegetation change alters ecosystem water availability. Global Change Biology 20, 2198–2210. Stromberg, J.C., Tress, J.A., Wilkins, and S.D., Clark, S.D., 1992. Response of velvet mesquite to ground water decline. Journal of Arid Environments 23, 45–58. Stromberg, J.C., 1993. Riparian mesquite forests: A review of their ecology, threats, and recovery potential. Journal of the Arizona-Nevada Academy of Science 27(1), 111-124. Stromberg, J.C., Wilkins, S.D., and Tress, J.A., 1993. Vegetation hydrology models: Implications for management of Prosopis velutina (velvet mesquite) riparian ecosystems. Ecological Applications 3, 307–314. Tromble, J.M., 1972. Use of water by a riparian mesquite community. In: Proceedings of the National Symposium on Watersheds in Transition. American Water Resources Association and Colorado State University, pp. 267–270.

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Unland, H.E., Arain, A.M., Harlow, C., Houser, P.R., Garatuza-Payan, J., Scott, P., Sen, O.L. and Shuttleworth, W.J., 1998. Evaporation from a riparian system in a semi-arid environment. Hydrological Processes 12, 527–542. Wan, C., and Sosebee, R., 1991. Water Relations and transpiration of honey mesquite on 2 sites in west Texas. Journal of Range Management 44(2), 156-160, March 1991. Available at https://journals.uair.arizona.edu/index.php/jrm/article/download/8578/8190. Accessed July 9, 2014. White, W.N., 1932. A method of estimating ground-water supplies based on discharge by plants and evaporation from soil–Results of investigation in Escalante, Valley, Utah. U. S. Geological Survey Water-Supply Paper 659-A, 105 pp. Williams, D.G., Scott, R.L., Huxman, T.E., Goodrich, D.C., Lin, G., 2006. Sensitivity of riparian ecosystems to moisture pulses in semiarid environments. Hydrological Processes 20, 3191–3205. Williams, D.G., and Scott R.L., 2009. Vegetation-hydrology interactions, Dynamics of riparian plant water use. In: Stromberg, J.C. and Tellman, B. (Eds.), Ecology and Conservation of the San Pedro River, pp. 37-56, University of Arizona Press, Tucson.

APPENDIX A

Table 1A. Mesquite evapotranspiration and groundwater use calculated from all studies.

Study Method % Canopy Coverage ET GWA GW100% Notes

Gatewood (1950) Transpiration well

47.4% 22.54" (573 mm)

15.46” (393 mm)

32.52" (826 mm)

Determined for woodland with >60% canopy coverage; extrapolated to entire woodland

Gatewood (1950) adjusted

Transpiration well

47.4% 19.58" (479 mm)

12.50" (318 mm)

26.3" (668 mm)

Using 1944 data only; partial 1943 data were discordant

Qashu (1966) Water balance 100% 51.57" (1310 mm)

Precipitation not removed;

Qashu (1966) Water balance – 9.4 mm/ day June

2 month’s data only; total transpiration 328 mm (12.92”)

Qashu (1966) Transpiration well

– 8.8 mm/ day June

2 month’s data only; total transpiration 312 mm (12.30”)

Tromble (1972) Transpiration well

80% – 10.7 mm/ day June

13.4 mm/ day June

2 months data only; total transpiration 291 mm (11.44")

ADWR (1991) Adapted from Gatewood (1950)

80% 34.81" (884 mm)

23.81" (65 mm)

29.76" Precipitation of 11" removed to calculate GWA and GW100%

ADWR (1991) Adapted from Gatewood (1950)

40% 24.12" (613 mm)

13.12" (333 mm)

32.80" Precipitation of 11" removed to calculate GWA and GW100%

ADWR (1991) adjusted

Adapted from Gatewood (1950)

80% 29.28" (744 mm)

18.28" (464 mm)

22.85 Gatewood (1950) full-year 1944 data used as basis

ADWR (1991) adjusted

Adapted from Gatewood (1950)

40% 20.29" (516 mm)

9.29” (236 mm)

23.23 Gatewood (1950) full-year 1944 data used as basis

Unland et al. (1998) Bowen energy ratio

80% 33.39" (848 mm)

19.01" (483 mm)

23.76" (604 mm)

14.38" (365 mm) of Nogales 1995 precipitation subtracted to obtain groundwater use

Scott et al. (2000) Bowen energy ratio

32% 14.76" (375 mm)

5.04" (128 mm)

15.75" (400 mm)

GW100% increases to 19.33” if May-November data used

Scott et al. (2008b) Eddy covariance 74% 28.6" (727 mm)

19.3" (490 mm)

26.1" ( mm)

Charleston mesquite woodland site

Scott et al. (2008b) Eddy covariance 55% 26.0" (660 mm)

15.0" (381 mm)

27.3" (693 mm)

Lewis Springs mesquite shrubland site

ET = evapotranspiration for given canopy coverage; GWA = groundwater use for given canopy coverage; GW100% = groundwater use for canopy coverage extrapolated to 100% ground cover.

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APPENDIX B

Detailed Review of Qashu (1966), Qashu and Evans (1967) and Tromble (1972) Qashu (1966) attempted to measure the yearly transpiration of a small, confined mesquite woodland (< 2 acres) developed on channel alluvium in the Walnut Gulch Experimental Watershed of southeastern Arizona. He did so with two approaches: (1) a water-balance method, and (2) measuring diurnal water table changes and applying the method of White (1932) modified by Troxell (1936). Qashu and Evans (1967) subsequently published this study. The study area was somewhat unusual in that it focused on the channel of Walnut Gulch where it cut into granodiorite bedrock, with alluvium averaging just 3–4 m thick and the channel varying from 70–100 m wide (Figure A1). The underlying granodiorite acted as a seal that limited downward percolation of water, allowing the alluvium to fill with water during the monsoon season and then slowly drain through subsurface flow, bedrock seepage, and transpiration during the fall, winter and spring. Because of this limited sediment thickness and continual water loss, the water-bearing thickness before commencement of the monsoon season could be less than 1 m thick. Mesquite woodland also developed directly on bedrock next to the channel, which may have affected the results.

Figure A1. Google Earth view of the Qashu and Tromble site on Walnut Gulch showing the mesquite woodland studied and its relationship to bedrock. Qashu divided the year into four distinct periods to which he adapted a general water-balance equation that he modified to calculate water use for each period. The equation included surface

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water inflow and outflow, subsurface water inflow and outflow, soil moisture changes, precipitation, evaporation and transpiration. The four periods of the year were defined by distinct changes in Arizona’s precipitation and plant growth regime: (1) January to April, (2) April to July, (3) July to October, and (4) October to January. Qashu and Evans (1967) gave a total yearly evapotranspiration for the mesquite woodland of 131 cm (4.3 ft or 51.57"). This value, however, does not match the summed evapotranspiration for the four periods given separately, which is 161 cm (63.4"). These values are unrealistically high for the reasons discussed below. As an alternative method of determining transpiration, Qashu used Troxell’s (1936) approach of measuring diurnal changes in water-well levels to determine transpiration from mid-April to mid-June (Figure A2). The transpiration values he obtained compared favorably with those calculated using the water-balance method for the same time period. The total transpiration (evaporation was not included) for this period by these two methods was 31.24 cm (12.30”) and 32.82 cm (12.92”), respectively (my determination).

Figure A2. Diurnal water-level changes for 1965 in a well associated with the mesquite woodland. These changes were used to calculate evapotranspiration for mesquite during this time. Because the summer monsoon prevented determinations of all components of the water-balance equation, Qashu did not measure diurnal water table fluctuations during the summer but

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extrapolated the transpiration values for June to July, August and September. He did this by multiplying the June transpiration value by solar radiation ratios or potential evapotranspiration ratios for these other months computed with respect to June. Qashu attempted to determine evapotranspiration values for October to December using just the water-balance method, and he calculated no values from January 1 to mid-April, assuming that no evapotranspiration occurred. A plot of his April to June calculations paradoxically shows mesquite transpiration peaking in early June and then declining precipitously to early growing season levels prior to the onset of the summer monsoon in July (Figure A3). This decline reflects decreased water availability in the aquifer and diminishing recharge capacity. Water levels dropped 2.7 m (8.86 ft) from October 1, 1964 through the beginning of the monsoon season in July 1965, with an approximate 80-cm (31.5") drop from the beginning of the growing season in April through July (Figure A4).

Figure A3. Transpiration values calculated for the mesquite woodland from diurnal changes in water well levels. The sharp drop in transpiration values after early June reflects exhaustion of groundwater in the aquifer. In addition, the transpiration values he includes in his summary table for this period do not match a plot of all values that he calculated. He gives an average transpiration value of 8.8 mm/day for the first two weeks of June using the Troxell method, which compares with a maximum plotted

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Figure A4. Water level fluctuations for 1964-1965 in the water well associated with the Walnut Gulch mesquite site. The sharp rise in water levels in July reflects recharge associated with the beginning of the monsoon season. value of 8.0 mm/day for approximately June 10. Calculated transpiration values drop sharply on either side of this date. His maximum transpiration value using the unmodified White (1932) method is 5.9 mm/day, which occurs on approximately the same date. In addition, his water-balance calculations show mesquite in December using 35%–40% of the water it uses in June when mesquite evapotranspiration should be essentially 0. Such high values indicate that his determinations of the other components of water outflow are too low, or his calculated subsurface water inflow is too high. In addition, the transpiration values he includes in his summary table for this period do not match a plot of all values that he calculated. He gives an average transpiration value of 8.8 mm/day for the first two weeks of June using the Troxell method, which compares with a maximum plotted value of 8.0 mm/day for approximately June 10. Calculated transpiration values drop sharply on either side of this date (Figure A3). His maximum transpiration value using the unmodified White (1932) method is 5.9 mm/day, which occurs on approximately the same date. In addition, his water-balance calculations show mesquite in December using 35%–40% of the water it uses in June when mesquite evapotranspiration should be essentially 0. Such high values indicate that his determinations of the other components of water outflow are too low, or his calculated subsurface water inflow is too high.

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Tromble (1972) expanded this year-long study with two more years of diurnal water table measurements for 1966 and 1967. From these he calculated transpiration values for April through June using the Troxell method but did not attempt to extrapolate them to other periods of the year. Tromble states that the woodland had a canopy coverage of 80% and was growing above a perched water table, which was at a depth of ~3–4 m (10–13 ft) during the period of measurement. Granodiorite bedrock was as shallow as 1.5 m (5 ft), with pockets of deeper sediment within it. The water-bearing alluvial layer was less than 2 m (6.5 ft) thick in places, with the underlying bedrock acting as a barrier to deeper percolation. Tromble first measured the rate of groundwater decline through subsurface seepage into bedrock during the winter. He then subtracted this seepage value from diurnal groundwater fluctuations to determine transpiration, an adjustment that Qashu did not make. Rather than calculating average transpiration values for a given time interval as Qashu did, he calculated values for specific dates, comparing them to Qashu’s average value for the time interval in which his dates fell. He calculated a maximum average transpiration value of 10.7 mm/day for June 17 of 1966 and 1967. The total transpiration calculated from his measurements for the April 1 to June 20 interval was 29.06 cm (11.44") (my determination). This value is similar to what Qashu had calculated by both diurnal changes in water levels and the water-balance method for this period. Potential Sources of Error The primary source of error in using diurnal water table changes to calculate transpiration stems from overestimating the daily recharge rate as mesquite draws down the water table. This can result from overestimating the effective sediment porosity (or specific yield) and permeability (or hydraulic conductivity) of the aquifer. Such overestimations increase the calculated rate of water inflow to the area beneath the woodland where transpiration is occurring, forcing an increase in the transpiration rate to balance the water held in the aquifer. These overcalculations would then be transferred to interpolations of transpiration for July through September. For water-balance calculations of transpiration, either the assumed subsurface outflow is too low, or the calculated subsurface water inflow is too high. Either condition forces an increase in the evapotranspiration rate in order to maintain the water balance of the system. One can determine subsurface outflow or seepage only by determining changes in the volume of stored water in the aquifer when no water is being lost through evaporation or transpiration. In Qashu’s study, this could be done only between January and mid-April, which he did by measuring the gradual decline in the water table with time across the study area. Qashu calculated subsurface water inflow based upon the hydraulic gradient across the area and the hydraulic conductivity of the aquifer. These determine how quickly water can recharge an area. Hydraulic conductivity is a function of permeability, which is in part a function of porosity. Again, overdetermining these will increase the calculated inflow of water into the system, forcing an increase in evapotranspiration rates if subsurface outflow is held constant. Calculations of excessive transpiration rates from both diurnal water-level changes and the water-balance method thus result from the same factor: overdetermining effective porosity (specific yield) and/or permeability.

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Qashu’s high transpiration values indicate that how he measured these supporting variables produced erroneously high values for them. Specific yield, for example, is time dependent and increases with the length of time that the material is allowed to drain (Figure A5). The specific yield of alluvium measured over the course of a day – the timescale of transpiration fluctuations – will be a fraction of the specific yield measured over a period of weeks or months, which is how Qashu effectively measured it. In addition, Qashu determined permeability by measuring the time for water to flow radially toward an open, evacuated borehole and restore the water level to that of the surrounding water table. This method may produce a higher value than if he could measure the time for water to flow laterally across a broad area of confined alluvium to restore the water table after a transpiration drawdown.

Figure A5. Variation in measurements of specific yield with time for Nebraska alluvium. Qashu is measuring specific yield in Walnut Gulch alluvium in the winter by letting the water table equilibrate over several weeks or months. Thus the specific yield he is measuring would fall to the right of this graph. The specific yield over the time of daily mesquite transpiration, however, would fall to the left side of this graph. Qashu’s determination of specific yield would thus be approximately twice what it would be over the daily transpiration cycle, which would double transpiration values. (From Johnson, 1967.) An additional factor to consider is that in some parts of the study area riparian mesquite is growing directly on exposed granodiorite bedrock and must be obtaining its water from water-filled fractures. Mesquite could also be doing this in areas of shallow sediment thickness that may not have an alluvial water table beneath them. The groundwater aquifer would thus be a combination of bedrock and local sand and gravel, which would greatly reduce the aquifer’s effective porosity and result in a disproportional drawdown of water in the alluvium when transpiration occurred.

Additional Comments on Qashu Methods For this study Qashu measured specific yield indirectly with a neutron probe, which is used to measure changes in soil water content immediately above the water table in the capillary fringe as the water table dropped. He then subtracted that from the water content he determined below the water table.

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He first calibrated the counts from the neutron probe with gravimetric measurements of the water contents of alluvial samples. He then calculated how much water the capillary fringe could hold and used the ratio of that water volume to alluvium volume to calculate specific yield. Specific yield itself is time dependent and increases with the time allowed for drainage. The specific yield of alluvium measured over the course of a day will be a fraction of the specific yield measured over a period of weeks or months, which is effectively what Qashu did. To determine permeability Qashu drilled a hole into the alluvium past the water table and then inserted a pipe into the hole so that it extended below the water table but not to the bottom of the hole. He then evacuated the hole and measured the time required for water to fill the pipe to the level of the water table in the surrounding soil. He used these time measurements and several distance measurements to calculate permeability and thus hydraulic conductivity. The high values for permeability that he apparently obtained would indicate that the rate at which water flowed into the hole to equalize water levels in the pipe were greater than the rate at which water would flow through confined alluvium.