Soil Characteristics Influencing the Movement and Balance of Soil Moisture1

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<ul><li><p>SOIL CHARACTERISTICS INFLUENCING THE MOVEMENTAND BALANCE OF SOIL MOISTURE1</p><p>L. D. BaverUniversity of Missouri</p><p>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.</p><p>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.</p><p>Soil Factors Affecting the Infiltration ofWater</p><p>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-</p><p>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.</p><p>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</p><p>Contribution from the Department of Soils, Missouri Agricultural Experiment Station JournalSeries No. 486.</p><p>431</p></li><li><p>432 SOIL SCIENCE SOCIETY PROCEEDINGS 1936</p><p>SURFACE INCHES</p><p>Fig. 1. Cumulative effect of infiltration of waterinto Marshall silt loam and Shelby loam.</p><p>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.</p><p>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</p><p>SOILVOLUME</p><p>;TOTAL ?OROSITY[NON-CAR CAP. DEPTH</p><p>SOILVOLUME</p><p>{TOTAL POROSITY;Noi-CAP. CAP.</p><p>20 40 60 80 %MARSHALL SILT LOAM</p><p>20 40 60 80 %SHELBY LOAM</p><p>Data from U.S.D.A.Tech. Bul.430(l2)Pig. 2. Soil Porosity Relationships</p><p>results indicate that runoff on the Mar-shall could be prevented by any treatment</p><p>that makes possible such wide variationsin soils.</p></li><li><p>SOIL CHARACTERISTICS INFLUENCING THE MOVEMENT AND BALANCE OF SOIL MOISTURE 433</p><p>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.</p><p>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.</p><p>The Downward Movement of Water ByCapillary and Gravitational Forces</p><p>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</p><p>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.</p><p>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.</p><p>HEIGHTCM.</p><p>100</p><p>80</p><p>60</p><p>40</p><p>20</p><p>V^&gt;*!</p><p>data fron</p><p>UJoe</p><p>W&gt;30</p><p>SUl</p><p>Date ol</p><p>Ii</p><p>RADi from $1 *,,...</p><p>ilOOcm\,ube\</p><p>Vl</p><p>f L*beteff 00)</p><p>" .</p><p>30 ZOOUS MICRONS</p><p>~" --L^\</p><p>4 8 12 16 20 y sand as afunction of the height of the sand column.</p><p>They point out that the moisture contentdecreases with height up to about 40 cm</p></li><li><p>434 SOIL SCIENCE SOCIETY PROCEEDINGS 1936</p><p>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 holding capactiy. Thus, theadded water penetrated this part of thesand layer solely by the force of gravityand not by the forces of hydrostatic pres-sure which would cause a constant dis-turbance of the equilibrium in the lowermoist layer of sand.</p><p>Alway and McDole (l) found that theamount of water held by the soil was re-lated to the surface properties of thesoil as determined by its hydroscopicity.Their data indicate that surface soils losewater by downward movement until the mois-ture content is reduced' to about "two tothree times the hygroscopicity coefficient..The water is retained in the upper layersfor use by plant roots. It is interestingto note that this value will probably cor-respond quite closely to that of the maxi-mum molecular moisture holding capacitysuggested by Lebedeff.</p><p>The rate and amount of downwardmovement of water through capillary forcesis determined by the amount and fineness ofthe capillary pores. The smaller thepores the lower will be the capillary con-ductivity and the more negative will bethe capillary potential. The amount andsize of the pores is largely determined bytexture and degree of compaction, althoughgranulation effects may reduce the capil-lary pore space content in favor of the</p><p>non-capillary porosity.This type of water movement is slow</p><p>in comparison with that in the largerpores and is not as significant in affect-ing the infiltration rate as the water mov-ing under a gravitational potential.Nevertheless, the capillary pores are ofextreme hydrologic importance in that theyare responsible for the water retained inthe soil after the excess mobile water hasdrained away.</p><p>Downward Movement of YJater by GravitationalForces</p><p>There are several important factorsthat determine the amount and rate of in-filtration of water into and through thesoil by the force of gravity. Perhapsthe most important of these is the. non-capillary porosity of the various layersin the soil profile. The amount of waterpercolating through a soil is determined bythe permeability of the least pervioushorizon. Various experimental observationshave shown that permeability is ratherclosely related to the non-capillarityporosity. The porosity relationships inthe Marshall silt loam arid Shelby profiles,as shown in figure 2, will illustratethis point. These figures were made fromdata on volume weights and capillary satura-tion reported by Middleton, Slater, andByers (12). There are"several significantfacts in this chart that should be em-phasized. In the first place, it.is seenthat the Marshall silt loam has a uniformcapillary and non-capillary porositythroughout the entire profile. This is nottrue of the Shelby loam since the maximumporosity occurs in the upper twelve inches.The Shelby loam has a higher soil volumethan the Marshall, which is indicative ofits greater density. The most impermeablelayer in the Shelby occurs at about 22 to26 inches where the non-capillary porosityis only about 5$ of the total volume. Theminimum non-capillary porosity of theMarshall is about 5$. This makes theratio of the non-capillary porosity of theMarshall to that of the Shelby equal toabout 5. Musgrave found the infiltrationcapacity of the Marshall to be about 7times that of the Shelby.</p><p>The Marshall soil has about 50% ofits total porosity made up of capillary</p></li><li><p>SOIL CHARACTERISTICS INFLUENCING THE MOVEMENT AND BALANCE OF SOIL MOISTURE 435</p><p>pores and the other half of non-capillary.This permits rapid infiltration and alsoprovides a sufficient retaining power forwater. A sandy soil would have a muchgreater non-capillary porosity but thecapillary moisture-holding capacity of thesoil would be inadequate to provide waterto growing plants over any considerableperiod. The Shelby, on the other hand,only has a total porosity of about 25$ inits subsoil, 89$ of which are capillarypores. Its water capacity is too high forits air capacity to insure adequate per-colation downward.</p><p>The density of the Shelby subsoillayers is evidenced by a soil volume of65$. The field volume weight of theselayers ranged from 1.72 to 1.85; the sur-face layers varied from 1.3 to 1.43. Onthe other hand, the volume weight of the &gt;Marshall ranged from 1.06 to 1.16. Bodman(5) has shown that the permeability ofsilt loams and clay loams is very low atdensities exceeding 1.5. In light ofthese data it is easy to visualize thecause of the low infiltration capacity ofthe Shelby loam.</p><p>The non-capillary porosity of a soil may be the result of a coarse tex-ture, a. loose and granular structure,cracks and fissures produced by soilshrinkage, or biologic channels such asold root channels or worm holes. The re-lationship of texture and granulation tosoil porosity has been fairly well estab-lished. The reader is referred to the in-vestigations of the writer for a discus-sion of the factors affecting granulationand soil porosity (2, 3, 4). Volumechanges are of great importance in produc-ing cracks in soils containing largeamounts of clay. In many instances cracksdevelop to depths of four and five feet.Water enters these fissures very rapidlyand the infiltration rate is high untilswelling seals the openings. Old rootchannels and worm holes also serve asdrainage ways through impervious soils.It is extremely doubtful if there is anyappreciable gravitational movement of waterdownward through heavy clay layers otherthan through cracks, fissures or b...</p></li></ul>

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