Irrigation Water Management MANAGEMENT 4.6. Irrigation Water Management Water management is an important element of irrigated crop production. Efficient irrigation systems and water
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P R O D U C T I O N M A N A G E M E N T
4.6. Irrigation Water Management
Water management is an important element of irrigatedcrop production. Efficient irrigation systems and watermanagement practices can help maintain farm profitabilityin an era of limited, higher-cost water supplies. Efficientwater management may also reduce the impact of irrigatedproduction on offsite water quantity and quality. However,measures to increase water-use efficiency may not besufficient to achieve environmental goals in the absence ofother adjustments within the irrigated sector. As is oftenthe case, technology is not the whole solution anywhere,but part of the solution almost everywhere.
Why Manage Irrigation Water?. . . . . . . . . . . . . . . . . . . . . 225
Use of Improved Irrigation Technology and Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226
Irrigation Technology and Environmental Benefits . . . . 233
Factors Affecting Technology Adoption . . . . . . . . . . . . . . 236
Policies and Programs Promoting Improved Irrigation Water Management. . . . . . . . . . . . . . . . . . . . . . 237
The U.S. Department of Agriculture identifiesimprovements in water management as one of theprimary agricultural policy objectives for the 1990s(USDA, 1994). Irrigation water management (IWM)involves the managed allocation of water and relatedinputs in irrigated crop production, such thateconomic returns are enhanced relative to availablewater. Conservation and allocation of limited watersupplies is central to irrigation management decisions,whether at the field, farm, irrigation-district, orriver-basin level.
Why Manage Irrigation Water?
Irrigation water is managed to conserve watersupplies, to reduce water-quality impacts, and toimprove producer net returns.
Water Conservation. Water savings throughimproved management of irrigation supplies areconsidered essential to meeting future water needs.Irrigation is the most significant use of water,
accounting for over 95 percent of freshwaterwithdrawals consumed in several Western States androughly 80 percent nationwide (see chapter 2.1, WaterUse and Pricing). However, expanding waterdemands for municipal, industrial, recreational, andenvironmental purposes increasingly compete foravailable water supplies. Since opportunities forlarge-scale water-supply development are limited,additional water demands must be met largelythrough conservation and reallocation of existingirrigation supplies (Moore, 1991; Schaible and others,1991; Vaux, 1986; Howe, 1985).
Water Quality. Improved water management can alsohelp minimize offsite water-quality impacts ofirrigated production. Irrigated agriculture affectswater quality in several ways, including higherchemical-use rates associated with irrigated cropproduction, increased field salinity and erosion due toapplied water, accelerated pollutant transport withdrainage flows, degradation due to increased deep
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percolation to saline formations, and greater instreampollutant concentrations due to reduced flows.Strategies to improve the Nations water quality mustaddress the effect of irrigation on surface and groundwater bodies (National Research Council, 1996).
Farm Returns. Finally, improvements in IWM canhelp maintain the long-term viability of the irrigatedagricultural sector. Irrigated cropland is important tothe U.S. farm economy, accounting for about 40percent of total crop sales with just 15 percent of theNations harvested cropland in 1992 (USDC, 1994).Water savings at the farm level can help offset theeffect of rising water costs and restricted watersupplies on producer income. Improved watermanagement may also reduce expenditures for energy,chemicals, and labor inputs, while enhancing revenuesthrough higher crop yields and improved crop quality.
Use of Improved Irrigation Technology andManagement
How producers respond to higher water costs andlimited water supplies is important to policymakers.Producers may reduce water use per acre by applyingless than full crop-consumptive requirements (deficitirrigation), shifting to alternative crops or varieties ofthe same crop that use less water, or adopting moreefficient irrigation technologies. In some cases,producers may convert from irrigated to drylandfarming or retire land from production. Manyirrigators have responded to water scarcity throughthe use of improved irrigation technologiesoften incombination with other water-conservingstrategiesand irrigators will likely look totechnology as one of several means of conservingwater in the future.
Various management practices and irrigationtechnologies are available to enhance efficiency ofapplied water in irrigated agriculture (see box,"Irrigation Water-Use Efficiency"). Irrigationimprovements often involve upgrades in physicalapplication systems, with improved field applicationefficiencies and higher yield potentials. Improvedwater management practices, such as irrigationscheduling and water-flow measurement, may also berequired to achieve maximum potentials of thephysical system. In addition, management ofdrainage flows may be an important concern in manyirrigated areas (table 4.6.1). In some cases, theeffectiveness of improved irrigation practices may beenhanced when implemented in combination withother farming practices such as conservation tillageand nutrient management.
Irrigation Application Systems
Irrigation application systems may be grouped undertwo broad system types: gravity flow and pressurizedsystems. (For an explanation of irrigation systemsdiscussed here, see boxes, "Gravity (Pressurized)Irrigation Systems and Practices," pp. 229-230.)
Gravity-Flow Systems. Many irrigation systems relyon gravity to distribute water across the field. Landtreatmentssuch as soil borders and furrowsareused to control lateral water movement and channelwater flow down the field. Water is conveyed to thefield by means of open ditches, above-ground pipe(including gated pipe), or underground pipe, andreleased along the upper end of the field throughsiphon tubes, ditch gates, or pipe valves. Fields are
Irrigation Water-Use Efficiency
Water-use efficiency measures are commonly used tocharacterize the water-conserving potential ofirrigation systems. Alternative efficiency measuresreflect various stages of water use and levels ofspatial aggregation. Irrigation efficiency, broadlydefined at the field level, is the ratio of the averagedepth of irrigation water beneficially used(consumptive use plus leaching requirement) to theaverage depth applied, expressed as a percentage.Application efficiency is the ratio of the averagedepth of irrigation water stored in the root zone forcrop consumptive use to the average depth applied,expressed as a percentage. Crop-water consumptionincludes stored water used by the plant fortranspiration and tissue building, plus incidentalevaporation from plant and field surfaces. Leachingrequirement, which accounts for the major differencebetween irrigation efficiency and applicationefficiency, is the quantity of water required to flushsoil salts below the plant root zone. Field-levellosses include surface runoff at the end of the field,deep percolation below the crop-root zone (not usedfor leaching), and excess evaporation from soil andwater surfaces. Conveyance efficiency is the ratio oftotal water delivered to the total water diverted orpumped into an open channel or pipeline, expressedas a percentage. Conveyance efficiency may becomputed at the farm, project, or basin level.Conveyance losses include evaporation, ditchseepage, operational spills, and water lost to noncropvegetative consumption. Project efficiency iscalculated based on onfarm irrigation efficiency andboth on- and off-farm conveyance efficiency, and isadjusted for drainage reuse within the service area.Project efficiency may not consider all runoff anddeep percolation a loss since some of the water maybe available for reuse within the project.
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generally rectangular with water runs typicallyranging from one-eighth to one-half mile in length.Gravity systems are best suited to medium- andfine-textured soils with higher moisture-holdingcapacities; field slope should be minimal and fairlyuniform to permit controlled water advance.
Although total acreage in gravity systems hasdeclined by 20 percent since 1979, gravity-flowsystems still account for over half of irrigated acreage
nationwide (table 4.6.2). Gravity-flow systems areused in all irrigated areas, and are particularlypredominant in the Southwest (California, Nevada,Arizona, New Mexico), Central Rockies (Wyoming,Colorado, Utah), Southern Plains (Texas, Oklahoma),and Delta (Arkansas, Louisiana, Mississippi) regions.The predominance of gravity systems in arid regionsof the West reflects early project development onbroad, flat alluvial plains; high crop water-consumption requirements; and increased soil salt-
Table 4.6.1Irrigation technology and water management: conventional methods and improved practices
System and aspect Conventional technology or management practice
Improved technology or management practice
Onfarm conveyance Open earthen ditches. Concrete or other ditch linings; above-groundpipe; below-ground pipe.
Gravity application systems:Release of water Dirt or canvass checks with siphon tubes. Ditch portals or gates; gated pipe; gated pipe
with surge flow or cablegation.Field runoff Water allowed to move off field. Applications controlled to avoid runoff;
tailwater return systems.Furrow management Full furrow wetting; furrow bottoms
uneven.Alternate furrow wetting; furrow bottomssmooth and consistent.
Field gradient Natural field slope, often substantial; uneven field surface.
Land leveled to reduce and smooth fieldsurface gradient.
Length of irrigation run Length of field, often 1/2 mile or more. Shorter runs, 1/4 mile or less.
Pressurized application systems:Pressure requirements High pressure, typically above 60 psi. Reduced pressure requirements,
often 10-30 psi.Water distribution Large water dispersal pattern. More narrow water dispersal through
sprinkler droptubes, improved emitterspacing, and low-flow systems.
Automation Handmove systems; manually operatedsystems.
Self-propelled systems; computer control ofwater applications.
Versatility Limited to specific crops; used only to apply irrigation water.
Multiple crops; various usesirrigation,chemigation, manure application, frostprotection, crop cooling.
Water management:Assessing crop needs Judgment estimates. Soil moisture monitoring; plant tissue
monitoring; weather-based computations.Timing of applied water Fixed calendar schedule. Water applied as needed by crop; managed
for profit (not yield); managed for improvedeffectiveness of rainfall.
Measurement of water Not metered. Measured using canal flumes, weirs, andmeters; external and inpipe flow meters.
Drainage Runoff to surface-water system orevaporation ponds; percolation to aquifers.
Applications managed to limit drainage;reuse through tailwater pumpback; dual-usesystems with subirrigation.
Source: USDA, ERS.
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leaching requirements. Furrow application systemscomprise nearly 60 percent of all gravity-flowsystems; border/basin and uncontrolled-floodapplication systems account for the remaining acreage(table 4.6.3).
Water losses are comparatively high under traditionalgravity-flow systems due to percolation losses belowthe crop-root zone and water runoff at the end of thefield. Field application efficiencies typically rangefrom 40 to 65 percent, although improved systemswith proper management may achieve efficiencies ofup to 85 percent (Negri and Hanchar, 1989).
Various land treatment and management measureshave been developed to reduce water losses undergravity-flow systems (table 4.6.1). Measures includeimproved onfarm water-conveyance systems,precision field leveling, shortened water runs,alternate furrow irrigation, surge flow andcablegation, and tailwater reuse.
Improved water-conveyance systems are an importantpotential source of farm-level water savings. Systemupgrades include ditchlining, ditch reorganization, andpipeline installation. According to the 1994 Farm andRanch Irrigation Survey (FRIS), traditional open-ditchsystems remain the principal means of onfarm waterconveyance for gravity-flow systems, with almost 60percent of gravity-acreage served (USDC, 1996).Above-ground pipelinesincluding gatedpipeaccounted for a third of gravity-flow acreage
served, with underground lines serving the remainingacreage.
Improvements in traditional gravity technology canincrease the uniformity of applied water, whilereducing percolation losses and minimizing waterrunoff. Gated-pipe systems are concentrated in theNorthern and Southern Plains and Delta regions.Surge-flow and cablegation systemsdesigned tocontrol water deliveries from gated pipeare used on5 percent of gravity-flow acreage, predominantly in
Table 4.6.3Irrigation application systems, bytype, 1994
System Acres Share of allsystems
All systems 46.4 100
Gravity flow systems 25.1 54Row/furrow application 14.2 31
Open ditches 5.0 11Above-ground pipe 7.4 16Underground pipe 1.8 4
Border/basin application 7.5 16Open ditches 5.1 11Above-ground pipe .9 2Underground pipe 1.5 3
Uncontrolled flooding application
Open ditches 2.3 5Above-ground pipe .0 0Underground pipe .0 0
Sprinkler systems 21.5 46 Center pivot 14.8 32
High pressure 3.2 7Medium pressure 5.9 13Low pressure 5.7 12
Mechanical move 3.7 8Linear and wheel-move 3.0 7All other .6 1
Hand move 1.9 4 Solid set & permanent 1.0 2
Low-flow irrigation (drip/trickle) 1.8 4
Subirrigation .4 1
Note: Percents may not sum to totals due to multiple systems onsome irrigated acres and rounding.Source: USDA, ERS, based on USDC, 1996.
Table 4.6.2Changes in irrigation systemacreage, 1979-94
System 1979 1994 Change 1979-94
Million acres Percent
All systems 50.1 46.4 -7Gravity-flow systems 31.2 25.1 -20Sprinkler systems 18.4 21.5 17
Center pivot 8.6 14.8 72 Mechanical move 5.1 3.7 -27 Hand move 3.7 1.9 -48 Solid set and
permanent1.0 1.0 2
Low-flow irrigation (drip/trickle)
.3 1.8 445
Subirrigation .2 .4 49
Source: USDA, ERS, based on USDC, 1982 and 1996.
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Gravity Irrigation Systems and Practices
Open-ditch conveyance systems have been the traditional means to supplying gravity irrigation systems. Open ditches maybe earthen, although improved systems are typically lined with concrete or other less permeable materials to reduceseepage loss. Water is delivered to gravity-flow fields through siphon tubes, portals, or ditch gates.
Furrow systems, the dominant gravity application system, are distinguished by small, shallow channels used to guide waterdownslope across the field. Furrows are generally straight, although they may be curved to follow the land contour onsteeply sloping...