The effects of transient temperatures and pressures on soil water hysteresis were
studied under controlled laboratory conditions. Variations in ambient pressure and
temperature caused small changes in soil water hysteresis. These changes suggest that
at least two mechanisms may be present, neither of which is easily rationalized by the
classical hourglass-shaped pore liquid jump model.

The variation between duplicate samples of a silty clay, a silt loam, and a loamy sand
suggested that normal temperature and pressure changes in the field will not cause
changes in soil water hysteresis that are larger than those due to natural soil
heterogeneity. Consequently, soil water hysteresis curves measured in the laboratory on
representative samples may be used in computer models of field situations

Cary, J.W.

The effects of transient temperatures and pressures on soil water hysteresis were studied under controlled laboratory conditions. Variations in ambient pressure and temperature caused small changes in soil water hysteresis. These changes suggest that at least two mechanisms may be present, neither of which is easily rationalized by the classical hourglass-shaped pore liquid jump model. The variation between duplicate samples of a silty clay, a silt loam, and a loamy sand suggested that normal temperature and pressure changes in the field will not cause changes in soil water hysteresis that are larger than those due to natural soil heterogeneity. Consequently, soil water hysteresis curves measured in the laboratory on representative samples may be used in computer models of field situations. Scientists constructing comprehensive com-puter models of soil water flow are increasingly concerned about soil water hysteresis. A com-mon model for the hysteretic mechanism is a

J. W. Cary

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NOTE SOIL WATER HYSTERESIS: TEMPERATURE AND PRESSURE EFFECTS

Northwest Irrigation & Soils Research Laboratory Publications

Soil Water Hysteresis: Temperature and Pressure Effects

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Son, Saxrice	 Vol. 120, No. 4Copyright Q 1915 by The Williams & Wilkins Co.	 Printed in U.S.A.NOTESOIL WATER HYSTERESIS: TEMPERATURE AND PRESSURE EFFECTSJ. W. CARYWestern Region, Agricultural Research Service, U.S. Department of Agriculture, Snake River ConservationResearch Center, Route 1, Box 186, Kimberly, Idaho 83341Received for publication June 17, 1974ABSTRACTThe effects of transient temperatures and pressures on soil water hysteresis werestudied under controlled laboratory conditions. Variations in ambient pressure andtemperature caused small changes in soil water hysteresis. These changes suggest thatat least two mechanisms may be present, neither of which is easily rationalized by theclassical hourglass-shaped pore liquid jump model.The variation between duplicate samples of a silty clay, a silt loam, and a loamy sandsuggested that normal temperature and pressure changes in the field will not causechanges in soil water hysteresis that are larger than those due to natural soilheterogeneity. Consequently, soil water hysteresis curves measured in the laboratory onrepresentative samples may be used in computer models of field situations.Scientists constructing comprehensive com-puter models of soil water flow are increasinglyconcerned about soil water hysteresis. A com-mon model for the hysteretic mechanism is anhourglass-shaped pore, which may empty or fillwith water in discrete jumps when triggered bypressure changes across the air-water interfacesof smaller interconnecting pores. However, notall experimental data can be explained by thisconcept. For example, the hourglass-shapedpore model suggests that the volume of en-trapped air would be less during drainage andgreater during wetting, but the reverse occursmore often at low water tensions (Cary 1967).The hourglass-pore concept also suggests thatthe hydraulic conductivity will not be a uniquefunction of water content, because the resist-ance to liquid flow in unsaturated soil increasesrapidly as the thickness of the water filmsdecreases. For any given unsaturated soil watercontent, the resistance to liquid flow should beleast when all the films have a maximumthickness as compared to a nonuniform distri-bution where some interconnecting films re-main thin at the expense of filling large hour-glass pores. However, as shown by Vachand andThong (1971), and others, hydraulic conductiv-ity is very nearly a single-valued function ofwater content, but metric potential is not.In spite of these conceptual problems, func-tions are being developed that describe soilwater hysteresis reasonably well and are amena-ble to modeling with digital computers (Mua-lem 1973). These functions are based on labora-tory observations of soil water hysteresis underconstant temperature conditions. When theflow models are applied to practical problems,one must know whether fluctuations of temper-ature and air pressure in the field significantlyaffect soil water hysteresis. The work reportedhere concerns this question.METHODSSix cylinders, each containing 300 cm C of soil(Fig. 1), were filled with duplicate air-driedsamples of three soils—a silty clay, B(bulk density 1.35 glcm 3), a loamy sand aridsurface soil (bulk density 1.4), and a silt loamarid surface soil (bulk density 1.3). The cylin-ders were submerged in a water bath so that thetemperature could be held constant or varied asdesired. The soils' liquid and gas phase pres-sures were controlled independently to developthe desired water tensions.All treatments were subjected to water pres-sure cycles of 5 to 550 to 5 cm water tension withfive steps for desorption and five for adsorption.308WONT BethLevelBrassCylinderFrilled GansCylinderBuretIndividuallyCentroled AirPressuresSealSOIL WATER HYSTERESIS	309As shown in Table 1, hysteresis was measured atconstant temperatures of 5°, 15°, and 25°C.Measurements were also made as the tempera-ture cycled through continuous 24-hr periods of25° to 15° to 25°C, and, in a more extreme test.as the temperature was continuously cycledfrom 25° to 5° to 25°C during an 8-hr period inF1G, 1. Cross section diagram of the experimentalapparatus.which the 20°C temperature change occurredover 3-hr intervals. All the water contents weremeasured by observing the level in the buretswhen the soil temperature was 25°C. Soil gasphase treatments included one test at 200 cm ofwater below ambient, and a second where thegas phase varied from 800 to 250 to 800 cm ofwater below ambient as the liquid pressureremained constant at 805 cm of 1-1 10 below am-bient. Three to five days were allowed for equili-brium between each increment of pressurechange. The complete experiment extendedover 21 months.RESULTSTypical hysteresis curves for the three soilsare shown in Fig. 2. All the data were plottedsimilarly, and the areas enclosed by individualloops were used as a criterion for whetherhysteresis had been influenced by the treat-ments (Table 1). Significant differences werecalculated individually for each soil texturefrom analysis of variance of the duplicate sam-ples making up the seven treatments.Some of the means for the silty clay weredifferent at the 5 percent level. Based ontreatment 7 as the laboratory standard, thecycling temperatures did affect hysteresis in thesilty clay. Peak temperature gradients duringthe 8-hr cycle exceeded 0.5°C/cm and caused acalculated net vapor flux of about 0.04 mm thatreversed direction every 4 hr. This reversal,TABLE IAverage areas enclosed by soil water hysteresis loops as affected by changes in temperatures and ambient airtemperature. Units of the area are (1% water content change) x (10 cm H tO tension). The treatments are listedin the order in which they were appliedNo.Treatments AreasAir pressure(cm 14,0 belowatmospheric)Water pressure(cm I-1,0 belowatmospheric)Temperature °C LoamysandSiltloamSiltyclay1 0 5-550-5 25-15-25 108 100 5024-hr period2 0 5-550-5 5 105' 92 573 0 5-550-5 15 76 91 434 0 5-550-5 25-5-25 104 99 448-hr period5 200 205-750-205 25 100 101 48°6 800-250-800 805 25 126 124 377 0 5-550-5 25 100 86 34LSD, 95% confidence level NS NS 10" Based on a single hysteresis loop; missing data point used in statistical analysis.100 200 300 400 500 800Sall Water Tension. cm H 2O310	J. W. CARYFIG. 2. Volume fraction of water withdrawn fromthree soils as a function of soil water tension understandard conditions of atmospheric pressure and25°C.coupled with any air-water interface instabilitycaused by the cycling temperatures and evapo-ration-condensation sequences, did not drivethe system far enough toward interior scanningcurves to reduce hysteresis. Rather, the hystere-sis increased and appeared to be influencedmore by low temperatures than by cyclingtemperature per se (treatment 2, Table 1).Some of the data in Table 1 may be timedependent. Even though the samples had gonethrough several adsorption-desorption cyclesbefore treatment 1, they tended to hold morewater each time they returned to the 5 cm ofwater tension starting point during the first 10months of the experiment. This amounted to a 1to 3 percent by volume increase in water contentand may have been accompanied by a changetoward decreasing hysteresis. For example, theareas of the loops shown in Fig. 2 were 5 to 8percent greater than the areas created by treat-ment 7. Data shown in Fig. 2 were obtainedbetween treatments 1 and 2, but under the sameconditions as those for treatment 7.Although statistical analysis for the silt loamand loamy sand did not show any significantdifference at the 95 percent confidence level,this is not conclusive evidence that no realdifferences occurred. Treatments 6 and 7tended to differ for both soils. Hysteresis ap-pears to have been increased by reduced soil gaspressures. If significant amounts of entrappedair associated with the hourglass-pore modelwere involved, one might have predicted anopposite effect because the density of air beingentrapped during desorption would be lowerthan that in the normal system under atmos-pheric pressure (Peck 1969; Cary 1967).The three soils responded differently to thetemperature and pressure variables. The 5°Cconstant temperature increased the hysteresisin the silty clay, but not in the silt loam orloamy sand. There was no difference betweentreatments 6 and 7 for the silty clay. buttreatment 6 tended to increase hysteresis in theother soils. Although the trends for the loamysand and silt loam may not be real, the data dosuggest that soil water hysteresis may involve atleast two different mechanisms: One is sensitiveto temperature and is associated more with claysize particles; the other is sensitive to airpressure and is associated with larger soil parti-cles and pores. Wilkinson and Klute (1962)noted that the temperature coefficient for soilwater tension was about twice as great as onemight calculate from the properties of purewater, and the coefficient increased as the soilparticle size decreased. In the study reportedhere, holding the soil water tension constant at95 cm while lowering the temperature from 35°to 5°C resulted in soil water inflows of 0.01volume fraction for the silty clay, 0.0085 for thesilt loam, and only 0.0065 for the loamy sand.These inflows also indicate the greater tempera-ture sensitivity of the clay-water system. Whilemechanisms sensitive to air pressure may beinvolved with air-water interfaces and particlerearrangement, mechanisms sensitive to tem-perature are more likely associated with clay-water interactions, and/or involve the amor-phous silica that coats most soil particles (Jonesand Uehara 1973).The data in Table 1 indicate that normalfluctuations of temperature and air pressurewill not cause large changes in soil water hys-teresis as compared to changes resulting fromthe soil heterogeneity inevitably encountered inthe field. Exceptions may occur in the surface 2to 3 cm when temperature variations are ex-treme, or for fine-textured soils with low tem-peratures. For most practical problems of com-puter simulation, normal temperature and airpressure changes will not significantly affect therequirements for programming hysteresis in thewet soil water range. Rose (1971) reached aSOIL WATER HYSTERESIS	311similar conclusion for hysteresis in the drier soilranges.REFERENCESCary, J. W. 1967. Experimental measurements ofsoil-moisture hysteresis and entrapped air. SoilSci. 104: 174-180.Jones, R. C. and G. Uehara. 1973. Amorphous coat-ings on mineral surfaces. Soil Sci. Soc. Am. Proc.37: 792-798.Moslem, Yechezkel. 1973. Modified approach to cap-illary hysteresis based on a similarity hypothesis.Water Resources Res. 9: 1324-1331.Peck, A. J. 1969. Entrapment, stability and persist-ence of air bubbles in soil water. Aust. J. Soil Res.7: 79-90.Rose, D. A. 1971. Water movement in dry soils. H. Ananalysis of hysteresis. J. Soil Sci. 22: 490-507.Vachand, Georges and Jean-Louis Thong. 1971. Hys-teresis during infiltration and redistribution in asoil column at different initial water contents.Water Resources Res. 7: 111-127.Wilkinson, G. E. and A. Klute. 1962. The temperatureeffect on the equilibrium energy status of waterheld by porous media. Soil Sci. Sac. Am. Proc.26: 326-329.

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Soil Water Hysteresis: Temperature and Pressure Effects

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