J. Agronomy & Crop Science 161, 234—237 (1988)© 1988 Paul Parey Scientific Pubhshers, Berlin and HamburgISSN 0931-2250

Agricultural Engineering DepartmentIndian Institute of Technology, Kharagpur, India

Assessing the Evaporation Zone in the Bare Soilfrom the Soil Water Flux and Soil Heat Flux Measurements

A. R. KHAN

Author's address: Dr. A. R. KHAN, Agricultural Engineering Department, Indian Institute of Technology,Kharagpur-721 302, India

With 3 figures

Received November 5, 1987; accepted December 29, 1987

Abstract

A simple approach was undertaken to estimating bare soil evaporation in a soil column in a laboratory. Themeasurement of soil heat flux and soil water flux at various depths provides a practical means of assessing andevaluating the position of the drying front (evaporation zone) in the soil.

IntroductionThe importance of evaporation from soil canhardly be overemphasized when one considersthe vast and regions on the earth. Evaporationof water from bare soils is an important con-sideration in the scheduling of many farmingoperations in both irrigated and dry land ag-riculture. High soil evaporation rates and lowstorage efficiencies can thwart all efforts toincrease crop production.

A large amount of work and study concern-ing this process has been conducted, however,because of the complexity of the soil and thecontinuously changing variables associatedwith atmosphere, much remains to be learned.In some areas, accurate predictions of bare soilevaporation may serve as the basis for decisionto increase the acreage planted with a givencrop. GARDNER and HANKS (1966) have shownthat the measurement of soil heat flux at var-ious depths can give a good indication of thelocation of the zone in which evaporation istaking place. This paper presents an approachto evaluate the evaporation zone in bare soil bythe measurements of soil water flux and soilheat flux.

Materials and Methods

The experiment was conducted in the laborator)' ofAgricultural Engineering Department, Indian Insti-tute of Technology, Kharagpur, India. The plexi-glass cylinder, 60 cm long and of 30 cm internaldiameter was used. The bottom of the cylinder wasclosed with perforated plexiglass (3 mm) sheet. Thesides and bottom were drilled through the cylinder at2.5, 5.0, 8.0, 15.0, 24.0, 30.0, 45.0 and 58.0 cm fromthe top edge of the cylinder for the placement ofthermocouples. The holes were properly sealed fromthe outside after placing thermocouples. Similarly,larger holes of 2.5 cm diameter were also drilledthrough the cylinder at the distance of 15, 30, 45 and58 cm for moisture measurements. Tensiometerswere connected, using 2 mm I.D. semirigid plastictubing, to Hg manometers attached to the cylinderwall. Each tensiometer cup also was connected to anair-tight water reser\'oir that sensed as a port forpurging the tensiometer with deaerated water whennecessary. The tensiometer cups were placed at thecentre of the cylinder. The excavation was thencarefully back filled and compacted to the approxi-mate original bulk density. The moisture contentwas estimated at the above mentioned depths and atdifferent time intervals using tensiometric readings.For this purpose a moisture characteristics cur\'eprepared under separate study was employed.

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Assessing the Evaporation Zone in the Bare Soil 235

Air dry sieved (2 mm) soil was packed in thecylinder to achieve a uniform bulk density of1.25 g cm \ The soil (lateritic sandy clay loam;ultisol) had about 34.4, 22.3, 19.8 and 23.1 per centcoarse sand, fine sand, silt and clay, respectively.After packmg, the cyhnder containing soil wasplaced in a water bowl and wetted from bottom for2—3 days until a free water film appeared on thesurface to avoid crusting. The cylinder was removedfrom water bowl and the excess gravitational waterallowed to drain before it was ready for operation. Itwas covered and allowed to equilibrate until thetemperature within the container was essentiallyconstant throughout the column. The cover was thenremoved and radiant energy was added during exper-imental period by means of continuously glowing250 watt heat lamp placed 50 cm above the soilsurface. At each point in the cylinder the soiltemperature was measured every hour during first 24hours and every three hours on subsequent days.Every cylinder was insulated with the help of 5 cmthick thermocole to obtain one dimensional heatflow. The potential evaporation (mm per day) wasmeasured using an open pan evaporimeter situatedunder similar conditions. Soil water flux was calcu-lated by the method as descibed by FLOCKER et al.(1968) and heat flux were computed using the proce-dure of JACKSON and TAYLOR (1965).

Results

Diurnal Soil Water Flux

Changes in soil water flux at four depths areshown in Figure 1. The soil water flux at thesoil surface increased immediately after theradiation was applied and the evaporation wasallowed to proceed. The highest daily evapora-tion occurred on the first and second day andthen tended to decrease by third day for all thedepths. The flux values at the surface(0—10 cm) are smaller than those at the lowersoil depths. However, the peak flux at alldepth occurred at the same time. As water wasevaporating only from the top few centimetersof soil, the surface dried rapidly. The figurealso shows that the rapid drying of the soilsurface occurred during the first few days. Thiscoincides with the maximum evaporation thattook place during this period. After the thirdday, the water flux at 0—10 cm began to re-duce and reached a constant value indicatingthat the evaporation zone was moving down-ward into the 0—20 cm layer, which, in turnalso started reducing after fourth day and be-come constant from sixth day onward. Similar-

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Fig. 1. Soil moisture flux as a function of time atvarying soil depths

ly, the evaporation zone continued to movedeeper into the soil till the eighth day and theflux assumed a constant value.

Figure 2 presents the cumulative evaporationfrom the entire length of soil column and theopen water pan evaporimeter in relation totime. Comparison of the curves shows thathigher evaporation potential results in greatercumulative water loss. The open water panapparently allowed greater cumulative evap-

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Fig. 2. Cumulative evaporation as a function of timefor soil column and open pan

236 KHAN

oration because of the free water surface whichwas in direct contact of air and radiation of thecontinuously glowing bulb. Whereas, theevaporation rate from the soil column prob-ably depends both on the water and air con-ductance properties of the soil. The loss ofwater is also dependant on the hydraulic con-ductivity of the soil. In column, as the toplayer dried out the capillary continuity wasbroken and as a result water movement couldonly occur in the vapour phase.

Diurnal Soil Heat Flux

Figure 3 presents the relationship between heatflux versus time. It is interesting to observethat at 10 cm layer heat flux increased at firstslowly, then increased rapidly upto 8th day,and then started decreasing gradually till thecompletion of the experiment (15 days). Al-most similar trends were observed in lowerdepths but for 20 cm layer wherein heat fluxcontinued to increase upto 11th day indicatingthat the zone of evaporation was movingdownward with the passage of time.

Discussion

Practical means of reducing the large amountof water loss will come about only as our

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Fig. 3. Heat flux as a function of time <it varying soildepths

knowledge of the evaporation process in-creases. Besides temperature and vapourpressure as controlled by surrounding atmos-phere, evaporation in soil is also a function ofthe water transmission properties of themedium. Soil water potential is an importantfactor affecting evaporation from soil.

Evaporation of soil water after wetting maybe characterized by three stages (LEMON 1956,WILLIS 1960). The first stage is controlled pri-marily by external atmospheric conditions,and lasts as long as the soil profile can supplywater to the surface at a rate sufficient tosatisfy the evaporative potential. For any givensoil, the first stage may last from a few hours toseveral days, depending on the evaporativepotential at the surface. During this stage,most water movement to the surface is in theliquid phase. When the profile can no longersupply water at the rate at which it is beingevaporated, the second stage sets in duringwhich the evaporation rate decreases rapidlyand is governed by the unsaturated conductivi-ty than by the evaporative potential at the soilsurface. Here, evaporation rate reaches a smallbut fairly constant rate. The third stage usuallybeen typified by a low, relatively constantevaporation rate controlled by absorptiveforces acting over molecular distances at thesolid-liquid-interface in the soil (LEMON 1956).Water loss by evaporation in this stage mustdiffuse as vapour through dry soil. The secondand third stages are commonly known collec-tively as the falling rate stage (LEMON 1956,HANKS andGARDNER 1965).

The data of the present experiment also re-veal different stages of evaporation. As thebulb was continuously glowing to supply theconstant heat energy at the soil surface. Tliere-fore, the water flux and heat flux were alwaysupward and downward (unidirectional),respectively during the experimental period.Figure 1 depicits the first stage upto about 24hours when water supply to the surface fromlower depths was not a limiting factor. How-ever, after one day, the soil moisture fluxdecreased considerably and attained a nearconstant value after sixth day at all the fourdepths. This pattern of evaporation seems tohave been controlled by water transmissionproperties of the soil (second stage of drying).A look at Figure 3 would reveal that thermalgradients were directed from surface to lower

I Assessing the Evaporation Zone in the Bare Soil 237

depths in a direction opposite to mass flow. Itis the thermal gradient which is contributingtowards the increased flux in the lower depths.The next flux at any depth is the result ofcombined effects of liquid and vapour flow.This may be the reason which has caused ahigher moisture flux on second day in all thedepths except in 0—10 cm layer.

Soil temperature measurement can also beused for estimating the evaporation zone fromthe bare soil. KIMBALL et al. (1976) evaluatedthe interaction between the movement of heatand water due to the combined effects of tem-perature and moisture gradients.

Zusammenfassung

Bestimmung der Evaporationszone in einemunbewachsenen Boden aus den Messungendes Bodenwasser- und Bodenwarme-Flusses

Es wurde ein einfaches Verfahren zur Bestim-mung der Evaporation unbewachsenen Bodensm einer Bodensaule im Laboratorium entwik-kelt. Die Messung des Bodenwarme-Flussesund des Bodenwasser-Flusses in unterschiedli-chen Tiefen stellt eine praktisch anwendbareund auswertbare Methode dar, um die Lage

der Austrocknungszone im Boden zu be-stimmen.

References

FLOCKER, W. J., M. YAMAGUCHI, and D. R. NIEL-

SEN, 1968: Capillary conductivity and soil-waterdiffusivity values from vertical soil columns.Agron. J. 60, 605—610.

GARDNER, H . R., and R. J. HANKS, 1966: Evalua-tion of the evaporation zone in soil by measure-ment of heat flux. Soil Sci. Soc. Amer. Proc. 30,425—427.

HANKS, R. J., and H. R. GARDNER, 1965: Influenceof different water content relations on evaporationof water from soils. Soil Sci. Soc. Amer. Proc. 29,495—198.

JACKSON. R. D. , and S. A. TAYLOR, 1965: Heattransfer Part. I. In: Methods of Soil Analysis. C.A. BLACK et al. (Editors). Amer. Soc. Agron. Inc.,Wisconsin, 349—360 pp.

KIMBALL, B. A., R. D. JACKSON, R. J. REGINATO,

F. S. NAKAYANA, and S. D. IDSO, 1976: Compari-son of field measured and calculated soil heat flu-xes. Soil Sci. Amer. J. 40, 18—25.

LEMON, E. R., 1956: The potentialities for decreas-ing soil moisture tension on growth of wheat.Canadian J. Soil Sci. 42, 180—189.

WILLIS, W. O. , I960: Evaporation from layered soilsin che presence of a water table. Soil Sci. Soc.Amer. Proc. 24, 239—242.