Soil Characteristics Influencing the Movement and Balance of Soil Moisture1

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    L. D. BaverUniversity of Missouri

    The soil plays a major role in thedisposition of precipitated water and,consequently, is an important link in thehydrologic cycle. Therefore, the follow-ing discussion will attempt to analyze themovement of soil water in its relation towater conservation and erosion, control asa variable in the hydrologic cycle. Inas-much as there are wide differences insoils as well as several phases in theprocess of the movement of water in soils,it is apparent that 'this variable will bequite complex. It is dependent upon sev-eral variable factors, but probably themost important are the characteristics ofthe soil. Consequently, it will only bethe purpose of this paper to treat soilwater movement as a function of soil char-acteristics.

    It must be emphasized that the soilcharacteristics of the entire profile mustbe considered in analyzing the movement ofwater within the soil. Although, as itwill be shown later, the characteristicsof-the surface layer greatly influence theinitial infiltration of rainfall, theproperties of the entire profile determinethe rate and amount of percolation of ab-sorbed rainfall through the soil column aswell as the amount and movement of waterretained in the soil.

    Soil Factors Affecting the Infiltration ofWater

    Horton (8) defines the infiltra-tion-capacity of a soil as the maximum rateat which rain can be absorbed under a givencondition. Rainfall in intensities belowthe infiltration-capacity is diverted intothe soil where it may contribute to theground water flow, or return to the air asvapor through evaporation and transpira-

    tion, or become a part of the capillarywater content of the soil. Water fallingin intensities in excess of the infiltra-tion-capacity is lost to the soil as sur-face-runoff. From the point of view ofwater conservation and erosion control,therefore, the ideal hydrologic condi-tion within a soil would be one in whichthe infiltration-capacity would equal therainfall intensity, and the field capaci-ty would be high enough to retain suffi-cient water for plant growth. It isobvious, however, that the intensity ofcertain storms is so high as to make theinfiltration of all of the precipitationinto the soil practically impossible. Onthe other hand, it may be possible byvarious mechanical means to temporarilyimpound a part of the precipitation onthe soil surface, thereby permitting thecontinued infiltration of water after theintensity of the storm has decreased be-low the infiltration-capacity of the soil.

    Common observation has shown thatsoils vary considerably in the amount ofwater which is absorbed during rainfall.Experimental observations at the varioussoil conservation experiment stations haveprovided some data to show the magnitudeof these variations. For example, datafrom the Clarinda arid Bethany soil con-servation experiment stations have shownthat on plots cropped to continuous corn,with the rows running up and down theslope, the Marshall silt loam absorbed95.9$ of the annual rainfall and the Shelbyloam 72.31$. In other words, the ratio ofrunoff of the Shelby to the Marshall was.approximately 7. Musgrave (13) measuredthe infiltration-capacity of these soilsand found that the ratio of the Marshallto the Shelby was 7.8 for a one-hour pe-riod. The relationship of the infiltra-tion rate to rainfall intensity for thesetwo soils is shown in figure 1. These

    Contribution from the Department of Soils, Missouri Agricultural Experiment Station JournalSeries No. 486.




    Fig. 1. Cumulative effect of infiltration of waterinto Marshall silt loam and Shelby loam.

    which would impound about 1.25 inches ofwater. The low infiltration-capacity ofthe Shelby makes it impossible to preventrunoff by any practical means of impoundage.

    Musgrave and Free, (14) in furtherstudies with these same soils, observedthat the initial rate of infiltration de-'pended to a great extent upon the physicalcondition of the surface. Cultivating todepths of four and six inches increasedthe total infiltration of water, althoughthis increased infiltration rate was almostcompletely manifested during the firstfifteen to thirty minutes. This is dueto the increased absorption of the rain-fall by the surface layer because of itsgreater porosity. These data clearly il-lustrate .that soil characteristics have apronounced effect upon the infiltration-capacity of the soil. The question arisesas to the nature of the characteristics





    20 40 60 80 %MARSHALL SILT LOAM

    20 40 60 80 %SHELBY LOAM

    Data from U.S.D.A.Tech. Bul.430(l2)Pig. 2. Soil Porosity Relationships

    results indicate that runoff on the Mar-shall could be prevented by any treatment

    that makes possible such wide variationsin soils.


    The movement of soil water througha given volume of soil must take placethrough the soil pore space. (2) The type,rate, and amount of movement will be re-lated to the properties affecting the na-ture of this pore space. These relation-ships are shown in figure 2 for two dif-ferent soils.

    Movement of water through thesepores is brought about by the action ofgravity alone or in combination with capil-lary pull. These forces act against thefriction on the surface of the particlesand the pressure of the soil air. Accord-ing to the dominance of the moving forcethe type of water movement may be dividedinto (1) that which moves in the largernon-capillary pores through the action ofgravity and (2) that which moves throughcapillary and gravity forces from surfaceto surface or in the capillary pores.

    The Downward Movement of Water ByCapillary and Gravitational Forces

    When a drop of water is broughtin contact with a dry soil it wets thesurface of the particles with.a moisturefilm. As the thickness of the film in-creases, the absorbed water fills thewedges between the particles. Capillarywater, therefore, is held about the pointsof contacts of soil particles in spaces ofvarious irregular shapes. Film water isunder the influence of the molecularforces between the soil particles and thewater. Movement from the places withthicker films to places with thinnerfilms is very slow. In other words, thepotential gradient may be high but theconductivity is very low. As the develop-ment of water wedges takes place with in-creasing moisture content, however, andthe capillary column is established in thesmall, irregularly shaped pores, the con-ductivity increases quite rapidly (6); thecapillary potential gradient decreases.At low moisture contents, therefore, thecapillary potential is highly negative andthe conductivity is low; at higher mois-ture contents, the reverse is true. There-fore, capillary movement of water into adry soil is very slow unless part of thesoil is quite wet to give a high potentialgradient. This relationship is discussed

    in the work of Richards (15) to which thereaders are referred for -a thorough dis-cussion of the capillary potential as wellas reference to the studies of other in-vestigators.

    His data show that a coarse-tex-tured soil exhibits a high negative po-tential at a low moisture, content, whilethe fine-textured soils have the same po-tential at a much higher moisture content.This is due to the larger number of con-tacts in the finer-textured soils, there-by reducing the amount of moisture at eachof the contact points with a correspondingdecrease in the radius of curvature of thewater meniscuses in the capillaries. Ifthe soils were placed in tubes in contactwith a free water surface and permitted tocome to equilibrium the sandy soil wouldhave a moisture content of about 4$ at 800centimeters; the clay soil would containabout 2Q%. The curves also indicate atendency for the moisture content to be-come constant at the more negative valuesfor the capillary potential. This sameeffect was observed by Lebedeff (10) instudying the distribution of water incolumns of sand of various heights as afunction of height after they had beensaturated and allowed to drain at the bot-tom. His results are shown in figure 3.








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    f L*beteff 00)

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    4 8 12 16 20 y sand as afunction of the height of the sand column.

    They point out that the moisture contentdecreases with height up to about 40 cm


    after which it remains a constant. Thewater content of this layer is called the"maximum molecular moisture holding capaci-ty" of the sand and the water is held bythe forces of molecular cohesion as in-fluenced by the properties of the surface.This value increases exponentially withdecreasing size of particles and increasingsurface as shown in the insert in figure 3.The interesting part of Lebedeff's studieswas the fact that when a drop of water wasadded to the tube 20 cm high, after it haddrained, a corresponding drop of waterdripped from the tub,e, indicating a hy-drostatic displacement of water. However,considerable time elapsed before the addeddrop caused a dripping from the tubes thatwere longer than 40 cm. When a drop of asolution containing chlorides was added tothe longer tubes and the distribution ofthe chlorides in the column was determinedat the time when the drop of water was dis-placed from the bottom, they were found ata depth of about 40 cm, corresponding tothe end of the zone of the maximum molecu-lar moisture


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