water movement and loss under frozen soil conditions1
TRANSCRIPT
Water Movement and Loss Under Frozen Soil Conditions1
HAYDEN FERGUSON, PAUL L. BKOWN, AND DAVID D. DiCKEY2
ABSTRACT
Soil water and temperature distributions in a silty clayloam soil were studied in the field under freezing winterconditions. Appreciable upward water movement to afrozen zone occurred in plots in which the unfrozen sub-soil water was held at tensions of less than about 2 atm.and water held at tensions of less than 5 atm. moved to-ward the frozen zone. It appears that the upward watermovement to the frozen zone conributed to overwinterwater losses since soil water losses of 0.36 and 0.50 incheswere measured from those plots in which there was ap-preciable upward water movement. Plots containing 7.9,6.2, 4.2, and 1.4 inches of available water before freezingoccurred conserved —0.36, —0.50, 0.0, and 0.81 inches,respectively, of the 4 inches of precipitation that occurredduring the winter.
WATER MOVEMENT IN; RESPONSE to a temperature gradi-ent and/or freezing zone in unsaturated soil mate-
rial has been documented in numerous laboratory experi-ments and engineering projects; however, few data areavailable indicating the magnitude of water movementthat occurs in agricultural soils because of naturally im-posed temperature gradients. In the northern latitudes acold, often frozen, surface soil overlays a warmed subsoilduring the winter. Such a condition might be expected toresult in an upward movement of water. Willis et al.(1961) detected no upward movement of soil water dur-ing the winter at Mandan, North Dakota; however Edlef-sen and Bodman (1941), Garstka (1944) and Dickey et al.(1964) present field data indicating upward movement ofsoil water to a cold soil zone during the winter.
The source and magnitude of the potential causingwater movement to a freezing zone is subject to some con-troversy. Some movement undoubtedly occurs due to thepotential associated with the temperature gradient alone;however, in some cases where water movement to a frozenzone has been studied, the temperature gradients were toosmall to account for an appreciable vapor pressure gradi-ent (Pchelintsev, 1960). It has been suggested (Tabor,1930; Penner, 1959, and Martin, 1959) that the source ofenergy in moving water to the frozen zone is derived fromthe freezing of supercooled water and that the greaterthe supercooling the greater the moving force. However,Miller et al. (1960) and Globus and Nerpin (1960) at-tribute water movement to a matrix or an osmotic poten-tial that develops within the system because of ice crystals.This hypothesis discounts supercooling, except to the de-gree required to maintain freezing.
The magnitude of the potential gradient formed by thefreezing system should be of major importance in deter-mining the conditions and quantity of water movement tothe frozen zone. Jumikis (1956) used tensiometers to meas-ure the tension developed in a silty soil at freezing andcompared these data with the maximum energy available
"Contribution from Department of Plant and Soil Science,Montana State College, Bozeman and the Soil and Water Con-servation Research Division, ARS, USDA. Approved as Mon-tana Agr. Exp. Sta. Research Paper No. 645. Received Feb.3, 1964. Approved May 19, 1964.
2Associate Professor of Soils. Montana State College; SoilScientist, USDA; and Agricultural Research Technician,USDA, Bozeman, Mont., respectively.
as calculated by the method of Winterkorn (1955). Hemeasured a maximum tension of 33.3 cm. of Hg comparedto a theoretical value of 38.1 cm. of Hg. On the otherhand, Penner (1957) found, by comparing the water con-tent of soils below the frozen zone with a desorption curvefor the soil, that a tension of about 5 bars was developedin a clay soil. He reported that tensions in excess of 7bars have been measured in this manner. Also, Penner(1957, 1958) found that the tensions developed in theunfrozen portion of a soil decreased as particle size in-creased and as density decreased. Thus, depending uponthe pore size and soil density, tensions of considerablemagnitude may be created by freezing a portion of thesystem.
Considering the tensions which can apparently be de-veloped by ice formation, a considerable amount of watermight be expected to move upward to a frozen zone inareas of long, cold winters. This upward movement ofwater may be a contributing factor to overwinter lossesof water which have been measured. Staple et al. (1960)reported overwinter water loss on fallow land rangingfrom 0.06 to 0.66 inch in 7 out of 20 years at Swift Cur-rent, Saskatchewan. The average available water contentof the soil to a depth of 4 feet was 4.51 inches in thefall of the years when water was lost during the winterand 3.63 inches in the fall of the years when no soil waterwas lost. Moreover, each inch of fall-stored water reducedoverwinter conservation of precipitation by about 0.2 inch.In work at Sidney, Montana, overwinter losses of 1.5inches of soil water have been measured in mediurn-textured soils.3 Water losses were measured at all depthsto 5 feet in all profiles that had a water content of greaterthan 15% by weight.
The work reported here is the result of an experimentdesigned to study the influence of different soil water con-tents on the soil temperature-soil water distribution duringthe winter in a silty clay loam soil. Similar results havebeen obtained during three successive winters.
EXPERIMENTAL PROCEDURE
In the fall of 1962, four contiguous plots, 12 feet square,were hoed free of vegetation and three of the plots wereirrigated by impounding various amounts of water in metaldikes. The soil was Amsterdam silty clay loam. Measured bulkdensities of the soil were 1.20, 1.30, and 1.36 in the O- to 6-,6- to 18-, and 18- to 48-inch zones, respectively. Alfalfa hadbeen growing on the plot area and the soil was dry to at least10 feet when the experiment was initiated.
In late December, when the first soil freezing occurred,plots 1, 2, 3, and 4 contained 7.9, 6.2, 4.2, and 1.4 inches,respectively, of available water in the 6- to 72-inch zone andwere wet to approximate depths of 94, 72, 54, and 30 inches.Below these depths the soil was near the permanent wiltingpercentage. A steel neutron access tube was placed in eachplot. Soil water content was measured at approximately weeklyintervals at 6-inch depth increments. The first reading wasmade at 6 inches and no measurements were made of the sur-face soil water content. Thermocouples were placed at 2 feetfrom each access tube at depths of 3, 12, 24, 36, 48, 60, 72,and 84 inches and soil temperatures were read when watercontent measurements were made. Soil water was consideredfrozen at temperatures below 32 °F.; it is recognized that waterin thin films and small pores was not frozen at 32 °F. The exactfreezing temperature is not known since it depends on manyfactors associated with the soil.
3Black, A. L. Personal communication.
700
FERGUSON ET AL.: WATER MOVEMENT AND LOSS IN FROZEN SOIL 701
RESULTS
During the winter of 1962-63 no prolonged periods ofcold weather occurred until late December. The data pre-sented were collected during the period December 20 toApril 26. Soil temperatures in the 4 plots at selected datesare given in Table 1. The soil was frozen in the 24-inchdepth zone for about 2 months. In the spring thawingoccurred from both top and bottom; however, it was morerapid from the top and the upper portion of the profilewas thawed on March 20 while the 24-inch level remainedfrozen.
Soil water data for all four plots are given in Table 2.The soil water contents of Plot 1, for three dates, areshown in Fig. 1. The initial soil-water content, prior tofreezing, is shown by the December 20 data. March 20data show the water content before spring thaw. April 26data show soil-water content about 1 month after springthaw.
Plot 1
Except for the surface 12 inches, the water tension inPlot 1 (wet to 94 inches) on December 20 was between0.7 and 1.2 atm. This was before soil freezing commenced.By January 4 the plot was frozen to 12 inches and a smallamount of water had moved upward from the 30- to 60-inch zone and accumulated in the 6- to 30-inch zone. Ad-ditional water movement occurred during the frozen periodand by March 20 there was a marked buildup of waterin the 12- to 24-inch depth (Fig. 1). All depths below 24inches had lost water. It seems unlikely that the waterloss from the zone below 24 inches was from downwardmovement since this would have increased water contentsin the lower depths. Since all of the water accumulatedabove 30 inches could be accounted for by upward watermovement, it appears safe to assume that none of thewater accumulated above 30 inches by March 20 re-sulted from surface infiltration. Moreover, the surface wasvery wet when freezing occurred and infiltration wouldnot be expected to occur through ice-filled pores. Soiltube sampling during the frozen period showed many icelenses, approximately 1 to 2 mm. thick, in the frozen zone.
Table 1—Soil temperatures within the plots atapproximately 2 p.m. on selected dates.
Plot Soildepth IE/20/12/28/
62 62
Inches1 3
122436486072
2 3122436486072
3 3122436486072
4 3122436486072
32.833.336.137.342.141.542.0
32.033.035.036.938.139.942.0
32.033.134.936.839.040.942.9
32.333.135.037. 139.841.443.0
30.132.034.137.238.840. 141 930.031.034.036.038.035.941.2
30.031.534.135.938.040.042.0
29.932.034.337.639.341.142.4
1/4/ 1/14/63 63
31.131.333.135.937.539.240.7
30.931.233.035.037.139.040.9
31.231.932.935.037.139.041.1
31.130.932.734.837.139.641.1
20.323.832.235.837.239. 340.9
18.622.832.034.436.938.440.7
17.222.131.534.536.839.040.9
13.216.924.532.235.938.740.5
2/5/63
——— D
33.030.831.233.634.736.438.3
32.530.831.131.732.734.836.9
31.630.029.931.433.235.337.7
32.028.627.027.731.034.437.0
Date2/20/63
egrees
31.330.830.932.634.035.637.1
32.931.231.832.634.035.637.7
31.130.431.031.734.535.037.3
30.228.028.028.231.035.437.2
3/13/63
F. ——
30.431.331.532.133.234.836.1
31.431.031.131.833.034.636. 0
31.431. 731.832.634.135.237.7
30.630.731.030.733.034.735.8
3/20/ 4/11/63 63
36.930.230.231.732.133.835.3
42.830.830.832.832.233.43i.O
41.031.131.131.834.734.636.338.731.331.131.032.934.235.4
44.035.032.433.133.934.936.142.034.532.432.833.934.936.1
45.235.032.032.533.234.536.0
42.137.537.136.036.036.036.5
4/15/ 4/26/63 63
53.841.733.833.934.735.937.5
50.240.833.333.034.936.136.9
52.040.933.832.734.735.036.6
51.243.940.537.736.936.83-7.2
54.942.138.136.336.336.337.9
51.942.538.036; 235.936.937.0
54.242.238.237.237. 037. 138.053.044.041.939.840.540.039.2
All of the macro-pores in the zone were completely filledwith ice crystals.
The data show a net loss of 0.50 inch of water from theprofile during the frozen period, December 20 to March20, and a net loss of 0.36 inch for the period December20 to April 26. This decrease in net loss can possibly beattributed to infiltration that occurred after the profilehad completely thawed on March 24. It is possible thatthe neutron meter underestimated the water content inthe 24-inch zone on March 20. McHenry (1963) hasshown that the minimum thickness that a soil zone shouldbe, in order that the probe be influenced only by thatzone, is 10 inches. Since the thickness of the high-waterzone and its exact location are not known, it is probablethat the 6-inch measurement intervals resulted in anunderestimation of the water content of the wet layer.On the other hand, the data (Fig. 1) indicate that upwardwater flow occurred below 72 inches; thus, water lossfrom the total profile may have been greater than indi-cated. Total precipitation between December 20 and April26 was 4.00 inches. This entire 4 inches plus the 0.36inch from the soil was lost from the wet plot duringthe winter.
Plot 2Soil moisture data (Table 2) from Plot 2 (wet to 72
inches) show a large accumulation of water in the 24-inch zone by March 20 and an accompanying decrease inthe water content in the zone between 30 and 66 inches.Water loss from the profile between December 20 andApril 26 amounted to 0.50 inch; nearly all of this lossoccurred after March 20. Water loss from the 36 to 66inch zone accounted for one-half of the profile loss. Aminimum water content of 16.5% developed in the wettedportion of the profile at 36 inches; this corresponds to awater tension of approximately 4 atm.
Plot 3Only a small amount of upward water movement (Table
2) occurred in Plot 3 (wet to 54 inches). The minimum
.19 .21 .23 .25 .27 .29 .31VOLUMETRIC WATER CONTENT
.33
Available water contents by plots were: (1) 7. 9 Inches; (2) 6. 2 Inches; (3) 4. 2 Inches, and(4) 1.4 Inches. Fig. 1—Water distribution in Plot 1 on three selected dates.
702 SOIL SCIENCE SOCIETY PROCEEDINGS 1964
water content that developed in this plot was 15.5% at42 inches, which corresponds to a tension of about 5 atm.Thus it appears that the formation of ice in the profileresulted in a tension of at least this value. Either thetension created by the ice was not much greater than 5atm. or the hydraulic conductivity of the soil at tensionshigher than this is too low to allow appreciable watermovement. Thus, no marked build-up of water occurredat the frozen zone because little water was available attensions below 5 atm. in the zone below 30 inches. Incontrast to the two wetter plots, some infiltration anddownward movement of water occurred by April 11;however, a comparison of water content on December 20and April 26 indicated no total change in the profile.Thus, whereas no net loss occurred from the profile,neither was any of the 4 inches of precipitation conservedduring the overwinter period.
Plot 4
No water movement (Table 2) due to ice formationwas detected in Plot 4 (wet to 30 inches). Some infiltra-tion occurred before March 20 and by April 11 infiltratingwater had penetrated to at least 24 inches. Data in Table1 show that the temperatures above 48 inches were from2 to 5 degrees higher in Plot 4 on April 11 than in thewetter plots. This early warming, together with the dry-ness of the plot, probably accounts for the relativelyhigh infiltration in the spring. Between December 20and April 26, 0.81 inch of the 4.00 inches of precipita-tion was stored in the profile below 6 inches.
Table 2—Water content by soil depths in the plotson selected dates.
Plot Soildepth
Inches1 6
1218243036424854606672
2 61218243036424854606672
3 61218243036424854606672
4 61218243036424854606672
12/20/62
32.128.924.325.326.123.522.522.722.923.922.922.5
31.327.524.526.322.518.920.722.322.120.317.514.1
31.125.724.723.522.519,717.716.915.712.712.512.3
30.324.119.715.713.511.912.311.912.011.911.912.3
1/4/63
33.529.725.726.726.122.722.121.722.723.122.723.132.929.326.524.521.918.119.521.522.520.118.515.9
2/14/63
32.329.325.728.524.121.920.921.122.122.521.321.733.129.323.927.519.717.519.121.321.120.117.516.3
Date2/11/
63- % by volume
32.731.527.532.730.121.920.320.321.522.320.921.133.131.525.332.722,116.918.720.320.118.917.116.5
33.127.325.325.122.317.715.716.316.514.112.112.1
33.124.520.315.513.112.511.912.112.111.912.312.3
3/20/63
29.930.927.733.124.320.119.919.521.120.919.720.530.730.924.932.921.516.718.519.520.118.714.715.9
31.929.125.124.722.716.115.515.715.514.312.511.9
28.127.121.315.714,311.911 511.912.111.711.712.1
4/11/63
29.730.128.530.925.120.920.120.120.521.319.919.929.128.326.330.520.516.518.719.120.118.316.315.926.526.726.128.923.116.715.715.915.714.712.512.5
28.929.125.319.513.911.911.511.912.112.312.112.5
4/26/63
28.328.925.327.725.722.321.321.122.522.320.720.724.726.123.526.721.718.720.320.920.718.717.315.9
24.924.524.123.922.519.117.716.916.114.512.112.3
25.926.723.122.516.911.711.512.312.312. 111.712.1
DISCUSSION
The results clearly indicate that water moves to a frozenzone under field conditions when water in the unfrozenzone is held at low tensions. The amount of movementprobably depends on available soil water, temperaturesof the frozen zone, length of frozen period, and physicalproperties of the soil. Appreciable water movement to thefrozen zone occurred in plots wet to 72 and 94 inches,in which most of the water in the profile was held attensions of less than about 2 atm. No water movementto the frozen zone occurred when the soil-water tensionwas greater than about 5 atm. Apparently tensions of atleast 5 arm. were created as a result of freezing since,in Plot 3, upward water movement resulted in the soildrying to about this tension.
The upward movement of water to a frozen zone con-tributes to overwinter losses of water that have beenmeasured in fallow fields. Soil water was lost only fromthe two wettest plots where a considerable quantity ofwater was moved from the subsoil to the freezing zonenearer the surface and was thereby exposed to the evapo-ration forces operative near the surface. Thawing oc-curred from both top and bottom in the profile but wasmore rapid from the top. This resulted in a frozen zoneat about 24 inches after the top had thawed which tem-porarily prevented downward movement of any accumu-lated water. This perched water was subject to evapora-tion losses. These losses were further accentuated by dailyfreezing and thawing of the soil surface even though frostwas no longer present at lower depths. Even after thawingwas complete, downward redistribution of the accumu-lated water was slow.
Water losses (soil-water plus 4 inches of precipitationduring the measurement period) from Plots 1, 2, 3, and 4were 4.36, 4.50, 4.00, and 3.19 inches, respectively. Thisrepresents a net gain of 0.81 inch on the dry plot and anaverage net loss of 0.43 inch from the two wet plots. Aplot of these gain-loss data against initial available soil-water shows an approximately linear relationship indicat-ing a decrease of 0.22 inch of water stored for each inchof available water. This value is based on a paucity ofdata but is interesting because of the close agreement withthe value of 0.20 inch reported by Staple et al. (1960).
QASHU AND ZINKE: INFLUENCE OF VEGETATION ON SOIL THERMAL REGIME 703