Rate of Flow of Capillary Moisture - AgEcon...

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MICROCOPY RESOLUTION TEST CHART MICROCOPY RESOLUTION TEST CHART NATIONAL BUREAU OF STANDARDS1963-A NATIONAL BUREAU OF S1ANDARDSmiddot1963-A

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q ~~~-~-~~~~~ Technical Bulletin No 5i9 ~ October 1937

UNITED STATES DEPARTMENT OF AGRICuLTURE

WASHINGTON D C

RATE OF FLOW OF CAPILLARY MOISTUREil2

By M R LEWlG

lrrigation Cllgilecr Divis-ion of hrigation Bnreau oj AgricuLLu1ll Engineering and Soils DeTIOrt11le-nt Oregon Agricultural Experiment Station

CONTENTS

Pnge _Page Introouction _bullbullbullbullbullbullbull__ bullbullbullbullbullbull bullbullbullbullbullbullbullbullbull _bullbull 1 Resultsbullbullbullbullbullbullbullbull ___bullbullbullbullbullbullbullbullbullbullbullbullbullbullbullbullbullbullbull__ bull 16 Otber studlesl Rate of water movement 16 Experimental procedure_ bullbull__ bullbullbullbullbullbullbull_bullbullbullbullbullbullbull_ 8 Effect ot soil type on rate or flow _ 19

SlIIDplng uppnr tus 0 Effect of grn-ity on rale or flowbullbullbullbullbullbullbullbullbull_ 20 I a Moisturecontent gradient for different rute$

Evaporation cbamber 10 or flow___bullbullbullbull___bull __ _bullbull _bullbullbullbull 21 Steady f1owbullbullbullbullbull__ __ bullbull __bull__bullbull 11 Conversion or units_bullbullbullbull_ 22

Supplementary experiments bullbullbullbullbullbullbull __bullbullbullbullbull 13 Discussion _bullbullbull_bullbullbullbull __ bullbullbull__ _bullbull__ bull_ 22 Movement as vapor bull __ _bullbullbull__ 13 Summary nocl conclusions_ 25 Ellcct ot Intermittent additlon ____ bullbull_bullbullbullbull 15 Llteruture clted_ bullbullbull_ __ bullbullbullbullbullbull_bullbullbullbullbullbullbullbullbullbullbull l7

INTRODUCTION

The rate of flow of moistWc through unsatWated soils is a problem that has attracted It grent denI of interest nmong soil investigators It is of direct importance ill fnnll operations for many reasons and is of great technicn interest because of its indirect bearing on other problems

Perhaps the question thnt has been most genemlly considered is the loss by direct evnpomtion from the surface of the soil and the effect of cultivlLtion in preventing that loss This problem has been _ attacked by both field and laboratory methods and somewhut conshytr~dictory results have been obtained

A second important problem isthnt of supply of soil moistlUe to plant roots The relative importance of the moyament of moisture through the soil to the absorbing loots and the movement of roots tlrough tbe soil to the moistlUe supply has been dabuted at conshysiderable length in technicnlliteratureat is generally recognized that w~tar is held in the soil abov~ the ulmer surface of the zone of saturntlOu (the water tnble) by capIllary force The soil zone so moistened is known as the capillary fringe Some of the earlier investigators considered the movement of moisture

I Received tor publication Jan 28 1937 J A report ot Investigations carried on under u cooperntlv~agream~nt between the Bureau of Agriculturnl

Engineering U S Department ot Agrlculture_ and tbe Oregon Agricultural Experiment Station rhe laboratory assistants nnd computers were employed by the Works Progress Administration of Oregon

1511160middot-lj~middot1

2 TECHNICAL BULLETIN 5i9 U S DEPT Ot AGRICULTURE

from the zone of saturation into or through the capillary fringe to the root zone to be of very great importance nnd thought that it might take place through distances of many feet More recently its importance has been denied and some workers have held thllt the roots must always go to moisture rather than that moisture may move to the roots Oonclusive evidence as to the real importance of the movement of capillary moisture has not been available

In irrign tion practice cnpillary movement is important in at least three ways In furrow flnd corrugation irrigation the lateral movement of moisture through the space between the furrows is essential to the moistening of the root 70ne In many cases the raLe of this moveshyment is the governing factor determining the length of time dwing which wnter must be applied The downward movement of moisture lUl(ter the combined influences of gravity and capillary forces is also of primary importance The possibility of producing maximum crops under irrigntion without permitting the loss of watcr by deep pershycolation is bound up with this question of the rate of flow of cl1piUary moisture The third phase of nnportance in inigation farming is the upward movement of moisture carrying (lissolved salts This moveshyment acCOtmts for the accumulation of alkali at the surface of irrigated lands

If as seems entirely possible the mte of movement of moisture through the soil to the roots is at times the limiting factor in plant growth it is probable that a knowledge of the ability of soils to transmit water will furnish another criterion of the agricultural value of different soils

1be foregoing may be considered the more immediately practical phases of the problem There nre in addition some fundamental questions that need further study These are particularly concerned with studies of the capillary potential and the capillary conductivity of different soils

In the range between the field capacity lLnd the wilting point methods heretofore used have not yielded satisfactory data as to the value of the capillary potential The vnpor-prcssure method is useful with moisture contents below the wilting point Above that point modemte increases in moisture content result in extremely small decreases in yapor pressure This fnct raises considerable doubt regarding the shape and position of the moisture-content versus capillnry-potential curYes as they may be determined by the vaporshypressure method in the moister soils The porous cup method is useful with very moist soils but it has been found incnpable of obtaining values with soils much (lrier thnn the field cllpacity It is obvious thnt the intermediate zone (between the wilting point and the field capacity) is of primary importance in studies of plant-soilshywater relations

The capillary potential is an expression for the force with which moisture is held at a given time and at a particular point in the soil by capillrry action As used in this bulletin it mny be defined as the work performed by the capillary force in moving a unit mass of water from a free water surface to the point in question Since no values are given it is not necessary to fix the units or determine whether the potential is positive or negative The capillary-potential gradient is the rate of change from point to point of the capillary potential

The capillary conductivity of the soil is a measure of its ability to transmit water when in an unsaturated condition The capillaryshy

3 UA1E OF liLOWOli CAPILLAHY llOISTUltE

conductivity factor as used in this bulletin may be defined as the mtio of the rate of flow of moisture in an unslttulI1ted soil to the capillaryshypotential gradient Here again no vulnes are given and the units need not be specified

Certain theoretical studies of the nbility of soil to transmit moisture in capilhuy form huye been mnde Ilndldew vrunes for the conductivity factor hnve been secured

Heretofore most studies of cllpillitry movement have been oased on the movement of moisture from a free water supply or a moist soil into nir-dry soil Obviollsly plants do not grow in air-dry soil nnd the movement of moistlUc into such soil is of compnrutively little importance in actual farm operntions A smaller number of studies havebeenmnde of the movement into soil containing some moisture but seldom have soils moist enough to support rnpidly growing crops been used loreovel most of the experiments concerned the maxishymum distance through which moisture would advance into the dry soil and the IIIte nt which thnt advance would take place In most instances no Ilttempt has been made to evaluate the Ilctual quuntity of water flowing through soil of any pnrticulnr moisture content

The ability of a soil to tmnsmit wnter to the absorbing roots may be compared to the ability of nn aquifer to yield water to a well In either case the total quantity of water held may be very great as may be alBo the availoble capacity in the one case and the specific yield in the other but unless the ability of the soil or the aquifer to trnnsmit water to the point of use is reasonably high neither the plant roots nor the well will be satisfactorily supplied

In this investigation the actual rate of flow of moisture under the influence of capillary forces acting in soil columns of different and varying moisture contents (chiefly between the wilting point and the field capocity) has b~en lleastUed This range is obviously of paramount importance m agncultme but has been generally neglected in other studies Measurements have been made on several soil types and on soils taken from different depths The effpct of gravitation on capillaTY flow hns been mensured and will be used in determining the capIllary potential and transmission factor

OTHER STUDIES

The importl1nee of n knowledge of the l11te of the enpillory How WIlS

lecognizecl early in the modern period of soilreselUch in the United States Briggs lind Lnpham (9)3 say

Further in order to distinguish in the field between the effect of extensive root systems and capillary action it is neeessary to know the maximum limit of capillary action in a given moist soil together ith the rate at which water can be supplied under ghen conditions

They carried on SOllle experiments with 1 fine sand in which 109 cc of water per sqUlre centiIm~ter per day was lifted 85 cm from a free water surface nnd evaporated at the surface The sand transshynutted 015 cc pel square centimeter pel day through a column 165 em high but did not moYe any water through a height of 180cm

Livingston and Koketsu (32) were also impressed with the imshyportance of the water-supplying power of the soil They devised the soil-point method of measuring this vnlue The fnct that they defined the length of tinle during which the soil point shouldmiddot be kept in contact

bull Italic numbers in parentheses refer to Literature Cited 1 XI

4 TECHNICAL BCLLETIN 579 U S DEPT Ol~ AGHlCUUrURE

with the moist soil indicates that they recognized the dynamic character of the problem

Recently Magness (37) has reiterated the importance of this problem He says

We need much more evidence OIL the rate of capillary movemeIit of moisture ill soils of different textures It seems probable that the Iate at which water is supplied to roots by the soil depends ill considerable part upon the rate of this capillary movement Thus a 11eavy soil will apparently supply water morc slowly but for a longer period while a light soil may supply water much more rapidly with the available supply being rather quickly used up by the trees

The distance through which wnter may be expected to move by capillary action has been the subject of study by many investigators King (2930) devoted several pages to the subject (30) He reported I1nextreme case where quartz sand raised 4409 surface inches of water through a distance of 675 inches in 24 hours There was some loss from tt depth of 10 feet and Tates of rise from free water of 1 or 2 pounds of watbr per square foot per day through 1 to 4 feet of a fine sand and a clay loam From shallow depths the rise wns at a greater rate through the sllnd but almost the same through both soils from a depth of 4 feet No dltta were reported as to the moisture content of the soil King concluded that capillary rise through a few feet is of importance but not sufficient to meet the needs of crops

1eGee (34) concluded that the ground water at an average depth of about 30 feet in the plains section of Kansas supplies 5 to 10 inches annunlly of water to crops He believed that this water rises to the root ~lone by capillary action In another publication (33) he conshycluded that moisture will move under capillnrity with sufficient freedom to affect the growth of crops from a depth of 3 to 4 feet

More recently thought along this line nas gone GO the other extreme and mnny investigators have concluded that capillary movement is of little importance in crop growth Shull (45 46) holds the view that wilting of plants at a rather definite moisture content was due to the slowness of movement of moisture from soil particle to soil particle and agnin (46) points out that the whole relation of the root and soil to the soil water is a dynamic one On the other hand he describes a case where lt buckwheat root had grown through the soil and had apparently rerooveCl aU the availftble moisture from a cylinder of the same diameter as the root with its root hairs the inference being that the root moved to the moisture rather than the moisture to the root

McLuughlin (35) Shaw (43) and OOlllad pnd Veihmeyer (11) all conclude that the movement of moisture by capillarity is middotof little if any importance in supplying plttllt roots The last say It appears Very probable that capillarity cannot be counted on to move moisture appreciable distnnces from moist soil into soil that has been dried by root extraction

The importance of the movement of capillary moisture through short dilStances within the root zone is not definitely established Some investigators believe that the rate of movement is so small as to be of no practical importance while others hold that observed plant responses to differences in soil moisture may be explained only on the gIgtund of capillary movement of soil moistu~e

Veihmeyer (52 53) Hendrickson and Veihmeyer 24 25) Veihshymeyer and Hendrickson (54 55 56) Beckett Blaney and Taylor (6) and [aylor Blaney and McLaughlin (47) are of the opinion that

5 RArrE OF 1LOW OlCAPILLARY MOISTUHE

plants do not suffer for moisture until the soil in some portiont of the

root zone is reduced to about the wilting point They believe that when the moisture content in the soil adjoining the roots is reduced to the permanent wilting percentage and soil with a higher moisture content is available the roots will grow into the moister soil

On the other hand Aldrich and Work (12) Aldrich Work and Lewis (3) Lewis Work and Aldlich (31) and Work and Lewis (57) are of the opinion that the moistme must move through the soil to the roots and that it is this condition which causes plants to slow up in their growth when the average moisture content in all parts of the root zone is well above the wilting point This condition has been noted by FUlT and Degman (14) Furl and Magness (15) and Bartholshyomew (5) Shantz (42 p 711) says Plants as a nue n~quiIe mOle water during drought periods than during periods of abundant supply and may be greatly damaged by extreme conditions even if the soil is still well supplied with water

As pointed out by Vasqu87 (51) Magness (37) Aldrich and Work (1) and Work and Lewis (57) the explunation of the suffering of plants often noticed when uVRiluble moisture is still shown by ordinary soil sampling lies in the slow movement of capillary water through the soil

Shantz (42) pointed out the fact that there is little definite inform ashytion as to the movement of soil moisture in the field under conditions where the subsoil is permanently moist This is an important obsershyvation and muy serve to point the way to a reconciliation of some of the contradictory opinions on the importance of capillary movement of water

Hanis and Turpin (23) carried on muny e~periments showing the extent of movement of moisture from a moist soil into it dry soil ~1any others have calJied on experiments with the lI1te and extent of movement of moisture from free water into a dry soil

Alway and McDole (4) and KI11Taker (28) studied the flow of moisshyture from a moist soil into drier soils of different moisture contents The former used soils with moisture contents between 05 and 15 times the hygroscopic coefficient The moister soils were thus approxshyimately nt the wilting point Thp latter used soils slightly more moist but little if any above the wilting point Except under lI1ther extreme conditions such as occur in dry farming irrigation with very Jimited water supplies or during serious droughts field soils below the surface few inches are seldom dried to the wilting point and almost never below thnt point These experiments do not cover the zone of moisture contents which is most usunlly met with in the field und which is of most importance

Shaw and Smith (44) cnrried on an experin1ent with soils which hod been satmated and allowed to drain Tubes 4 6 8 and 10 feet long were used and moistme was allowed to rise from a free water surface and to evaporate nt the top of the tubes Vith Yolo loam evapshyoration wns 360 192 100 and 016 surface inches monthly from 4- 6 8- and 10-foottubes respectively in 321 to 324 days From Yolo srmdy lonm 184062 -13 and -08 surface inches respectively were lost monthly in 87 to 96 days from tubes of the lengths mentioned The minus vnlues are due to the fact that the experiment was started before all of the excess water had drained out of the tubes At the end of the ePeriment the moisture content of the soil at different

TECHNICAL BrLLEIlN i370 U S DBPT OIl AGIllCUlrUfm6 distances from the ends wus determined The resulting curves are not dissimilar to those securecl in the experiments reported herein These results and those which Briggs and JJupllilll1 (9) and ICing (29 30) quoted ure in accord with field experience and numerous experiments in showing thut considemble ([uantities of water may be raised by capillarity tlllough moist soils from a free water surface

The question of the quantity of water that moves tlUough the soil in the vapor phase is tLlso raised by some investigators Scofield and Wright (41) in reporting field studies of moisture conditions dmw the conclusion that moisture wus lost from certain of their fLuId plots by eTaporation well down in the soil und diIJusion of the vapor through the soil and into tbe atmosphere at the surface They go 011 to say that evidence both in the field and in tbe laboratorYI indicates that movement takes pluce quite as much by nlbernate vaporization and condensation ns by the capillary movement of liquid water Other workers had been led to the concl usion that movement of water in vapor form is of considerable importance

Buckingham (10) studied this question and Cleme to the conclusion that loss of moisture by vaporizution below the surface and difrusion tluough moist soil is very small He found rates of loss of 14 surface inches peryenr into still air nnd 43 surface inches per Tear into a 3-mile-per-llOur breeze tllrongh 2 inches of conrse dry sand Through 1 inch of Ionm he found It loss of 271 surface ll1ches per yeur and through fine siLudy loam 252 inches per yenr Through 12 inches of sandy loam the loss was less than 02 snrface inch per year All of these tests were from a saturated atmosphere below tIle colman of soil fLncl into n current of air from a fnn lhc soils aU appear to have helclless moisture thf~n tIle wilting point at the time these mtes were determined

Bouyoucos (8) gives data showing that the movement of water vapor from a warm moist soil into a cold dry soil is very small even with temperature diflelences of 20deg and 40deg O

A number of investigators have applied tIle general laws of hydroshydynamics tothe soil-moisture problem Buckingham (10) defined the capillary potential and carried on e~-periments to determine the relashytion between the capillary potential and the moisture content of the soil He set up tubes filled with different soils which he allowed to stand fo periods rnnging from 2 to 10H months with the tops proshytected Jrom evnpomtion and the bottoms in free water On the asshysumption that static equilibrium had been reached he then detershymined the moisture content at different heights above the water tnble and thus arrived at tIle relation between the two quantities His experimental data indicated a wide variation in different soils

Gardner and others of the Utl1h Agriculturel Experiment Station also have studied this problem from the point of view of the physicist In this work they have (1617181920212638484950) always stressed the dynamic nature of the problem In the early work Gardner (18) defined a transmission constant as a single-valued funcshytion for each soil This transmission constant has the physical dimen-

M~L sionsof ----rr- Later Gardner (19) Gardner and Widtsoe (21)

Gardner Isrnelsen Edlefsen and Olyde (20) Israelsen (26 27)and Richards (38) have stressed the use of the capillary potential in probshylems of capillary movement A great dealof study has been given to

L

7 RATE OF FLOW OF C~PILLAnY ~lOISTUl~

the relation between the moisture content of the soil and the capillary potential

E A Fisher (12) R A Fishel (13) fLnel Haines (22) have made applications of ml1thematicI11 and physict11 methods to capillary flow problems

Thomas (48 49) Immd that the vapor pressure lowering at the wilting point with three dliferent soils ranged from 02 to 046 mm of mercury He found that the vapor pressure was approximately proportional to the leciproclll of the moisture content and states that the vapor pressure lowering was about 002 rom at the moisture equivshyalent but that it cannot be accurately determined

Israelsen (26) concluded from a theoretical study that the moisture content in H deep uniform soil at equilibrium between capillarity and OlfIrity would demease from the water table upwarcl He gave the following equ[Ltiol1 for the ]~elntion between thl moisture content und capillary potential

(P -a) C1I+b)=O

where pi equals the moistule content 11 equals the cnpillary potential and a b and 0 are constants

Richards (38) described the porous-plate method of measuring the capillary potential and gave curves showing the relation between the moisture content and the capillary potential He pointed out that this method cannot be used for capillary potentials in excess of about OIle 1tmosphere If0 (39 40) carried the work further and determined the lelation of both the capillary potential and the capillaryconducshytiYity to the moisture content He also developed formulas for the apphcation of these factors to specific problems of capillary flow His measuremen ts of the capillary potential were made by the porousshyplnte method and therefore are limited to values of less than one atmosphere

Bodman and Edlefsen (7) discussed the soil-moisture system and pointed out that measurements of the capillary potential above the wilting polli~ and below about the field capacity have not been yery successful

Harris and Turpin (23) lknd 1cLaughlin (35 36) found that water moved downward from a moist soil into a dry soil and from a supply of free water into a dry soil more ntpidly than it moved upward under the same conditions but didllOt attempt to interpret their data in terms of the gravitational potential

The studies heretofore made of capillary movement of soil moisture may be somewhat arbitrarily divided into two groups In the first group the approach has been through laboratory and field studies of soil-moisture movement middotwith the principal emphasis on the physical results obtained These results have been used to explain other field observations In the second group the experiments have aimed to apply the fundamental laws of physics to soil-moisture movement The studies reviewed have not furnished satisfactory data on the movement of soil moisture in the range between the wilting point and the field capacity They have however shown the need for such data and an evolution of ideas leading toward a clearer conception of the relations between soil and water The experiments herein deshyscribed were undertaken for the purpose of adding to the accumulashytion of datu obtained by other investigators

8 TECHNICAL BDLLE1lN 579 U S DEPT OF AGHlCULTURE

EXPERIMENTAL PROCEDURE

In the main the attempt has been to use soils with their natural field structure In a few instances soils have been used that ]Hve been broken up sifted and mixed in the laboratory It is hoped that a conductivity factor fmd the relation between the moisture content and the cfLpillary potential may be found fOi difterent soils For the second part of the study difterent rates middotof flow vertically both up and down and different lengths of soil columns have been used with samples of a single soil type taken at the same depth and as near together as possible in the field or samples uniformly prepared in the laboratory

For the first objective fl few replicntions only were used often with only one rate of flow and in one direction only The number of replications used in the seel1nd type of study ranged from 5 to 20 In most instances it was found that four 01 five replications gave reasonably consistent results

This bulletin presents the data obtained up to the present on the first phase of the problem Much of the data herein reported will be analyzed in a study of the more fundamental aspects of the problem

The experience of other investigators has shown that long periods of time are necessary if approximately static equilibrium is to be secured with long soil columns For this reason short columns 2 4 and 6 inches in length were used The differences in field soil make it necessary to replicate the tests and it wns desired to make tests on a large number of soil types Moreover part of the incentive for undertaking this experiment was the peculiar fluctuations in soil moisture found in the soil at depths of several feet in certain orchards in western Oregon The cost of securing large numbers of undisturbed field samples of large diameter at depths of several feet was prohibitive Moreover the equipment available precluded the possibility of using much space for samples For these rensons soil columns of small diameter were used

It appears possible to secure a state of steady flow much more quickly than to secure static equilibrium in soil columns of moderate length and moisture content These experiments were conducted on the basis of steady rates of flow

By working with different rates of flow both upward nnd downward the gravitntional potential can be used to evaluate the capillary potential Where flow takes place in an upward direction the two potential gradients are in opposition whereas with downward flow they work together This fact gives a basis for using the known gravitationnl potential in determining the value of the unknown capillary potential

From the datn it is possible to plot curves between the mte of flow of moisture and the moisture-content gradient (i eth~ mte of change with distance of the moisture content) at different moisture contents By extrapolatin~ these cmves to zero flow conditions at static equilishybrium can be estlllillted Theoretically the curves for upward flow and for downward flow should coincide at zero flow

The experiment consisted essentinlly in setting upa series of soil columns exposed at one end to a current of nil at constunt temperature and humidity and adding water atdifferent predetennined rates at the other end of the columns In the earliest trial it was felt necessary to add the water in smnll quantities nt very short intervals It was

9 HA1li) Oli FLO OF CmiddotUlLLAUY MOISTUHIJ

SOon discoveredhowever that wl1ter could be added at intervals of 12 bours without creating too great irregularity in the results The columns were left in the eVlpolation chamber until the rate of loss by evaporation from the exposed ends became approximately constant and equal to the mte ot addition This rate of loss ordinarily averaged over the final 24-holll period has been used as representing the rate of flow thro~h the tubes

In the earher trials water was added from the burette directly on the surface of the soil at the closed end of the soil column It was found however that this tended to puddle the soil and thereafter small pieces of cotton felt were placed between the soil and the stopper These served to hold the moisture in contact with the soil column and also to protect the soil from the puddling action of the dropping water In the early trials as noted above 2- 4- and 6-inch soil columns were used Howeyer even with the heavier soils the 2-inch columns were of little use and the later trials were made with 4- and 6-ioch sam pIes only

In certain cases where the rute of addition of water was great enough to cause a portion of the soil in tbe tubes to become saturated the replacement of the rubber stoppers after adding water developed pressure in the tubes This pressure forced water thlough the soil column in some instances DUling the latter part of the experiment small holes were bored in the side of the tube opposite the felt pnds and closed by rubber bands so arranged us to prevent the building up of pressure in the tubes

After approximate dynumic equilibrium had been attained the soil was removed from the tubes in small trl1nsycrse sections I1nd the moisture content determined Approximately huH of the tubes were plnced so that the movement of moisture wns vertically upward and in the remainder downward In most instances duplicate sets were used for upward and donward flow In some cases where datu were required for some plLrticulnr soil depth or soil type a single set of tubes with flow either upwnTd or downward was used

SAMPLING APPARATUS

The use of the King soil tube is practically stundnrcl in soil-moisture ork by the Division of Irrigl1tion and this method of sllmpling llfls been used in the present pl)eriment A special point for the King tube was prepared by ndcling stellite to the cutting edge of fL standard point and grinding out as shown in figure 1 Samples of transpl1rent tubes made of u transparent plastic with all inside diameter of 0840 imlt were obtained and in the figure are shown the washer and rivet through the body of the steel tube fOl~ holding the smnll sample tubes ill the King tube Unfortunately when it came time to order n supply of the tubes the manufacturers were out of the size required and white celluloid tubing was substituted It is believed tbnt transparent tubing would hl1ve made it possible to detect breaks in the soil columns I1nd might have permitted the sorting out of samples which gtlYC erratic results

Considerable difficulty was encountered in securing samples in the field when the soil was very wet This was purticularly true with the Medford clay adobe and the vVillamette sil t loam When the moisture content was slightly below the field capacity no difficulty was encountered The small difference in inside diameter between the

10 TECHl~lCAL BULLETIN 570 US DBP1 01~ AGlUCULTUlm

point and the celluloid tube is necessary in order tbat the sample m1y slip inside the tube without too much compacting With wet Ilnd sticky soils it was found helpful to thoroughly wet the inside of the tubes just before taking a sample Even when this was done it was sometimes necessary to mllke severn attempts before the sllmple in

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the tube proved to be approximately ns long as the distance the tube had been driven After the samples were secured one end of the tube was covered with fl piece of cheesecloth and the other end closed with It rubber stoppel

gyAOUA1ION CHAM BER

Figure 2 shows the evnpomtioll chamber used in the experiment A plywood box 16 by 20 by 39 inches vIlS fitted with galvanized-iron transitions on both ends At one end the box WitS connected to

Gulnnlzed Iron w========r~~__~__~__~__ __=5_r~~m~ff~~_=__

6 - __ - 1----751 =--=-R-~ ___

PLAN (TOP RENOPELJ)

LONGTUL1wAL SECl70N TRAIYSPERSE SEC17tH JIGllUE 2-gllporutiO( chamber in whih lubos were exposed to secUIo sleady flow

another gnlvltnized-iron box used us fin air-eonditioning chamber while Itt the other end a fan was installed in the opening to dmw air through the chamber Inside the main box were rllcks arranged to hold the sample tubes in a vertical position The racks which were 13 inches long were supported by mellns middotof solid boards at each end These boards forced the ail to pass through n 4-inch space between the two rucks The upward-flow tubes were held in the lower of the

--

11 RATlJ Ol~ FLOW 01 (APILLAHY MOlfiTUHE

two boxlike Tucks with their cloth-covered ends Hush with the top of the rack Similarly the cloth-covered ends of the downwardmiddotflow tubes were flush with the bottom of the upper rucks Thus all the tubes were exposed to the current of air in the 4-il1chrestricted space Air entered and left the box through a series of holes at euch end It was expected that the air curren ts would thus be made uniform Extra space in the chamber was utilized for a thermohygrogrnph and thermostat

The current of air drawn through the chamber was hea ted in n second box by lamp banks or Nichrome resistance wire The apparatus was set up in the bnsement where the fluctuations in temperature ordinarily were not very greut There were 11Owever steam pipes in the room and the steain in these pipes was cut ofi over the week ends Occasionally the reshysulting drop in temperature was greater than the henting elements in use at the time could overcome In a few instunces also the COllshytact on the thermostat stuck and the temperature in the evapoJashytion chamber run high Some cases were noticed when the rate of loss from all tubes would be somewhat low or high depending on whether the temperature was below or above the normal In the Uain however it was imposshysible to detect nny correlation beshytween these accidental variations in temperature and the rates of loss In fact it was not unusual for part of the ttl bes to show marked inc reuses in mtes of loss at the same time others showed fl constant or evelllower rate

STEAD) FLOW

In order to determine when the fiow had reached an approximately steady state the tubes were weighed before mId ufter enclt

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f---~ I I I I I i i gto z

OCJOOOOOO or 0 C1l lt0 ltt CJ

(ILj 6w) SSOll0 3JV~ lJGultE 3-Hute(lrlossof wnterfroUl O-incb caresal

beach sund to wll[(h wnter wus added It different rutes

addition of wllter In some cuses all the tubes of 11 group were weighed while in other cases only one or two of fl group were weighed i1s indices of the rate of loss After the index tubes hud reached flll npproxinmtely

--

12 rECHNlCAL- BULLECIN57D u S DBPT Oil AGl1ICUUl~Um~

steady state of flow all of the tubes of the group were weigh(ld before and nftel ndding water at intervals during the last 24 hours before breaking down the soil columns As was to be eA1)ccted

t-f-~ I-f-~

II-f-~ ~ I-I-~ ~ ~ t-I-~ ~~ l~) I lll I-I-~ ~ i 1-I-I-~ ~ ~4 1shy- I-~ ~ ~ - I-~~ - IS t-~

I I

I

-f-J - ~

1

f--~-~

--~ ~ ~

~~ ~-Il ~

~+-~-~ ~ ~ lt)i

lj t ~Vi V1

V 1

I 1

l-

V _-xV

o (J

x

t I

Ishy-

x rlt) i

I)

1

r I 6- shyi

t i ~

1

e

~ o ~

Q)

t)

-- shy0000000 v (J 0 III lD V tJ

- (J4 6w) 8801 10 3111~ FIOUllE 4-RateoCIossowllterromO-inchcorllS g~IJ~lm~ JI~er~~t~utt~s~middotlJiCh wuter wus

moist soil samples lost water rapidly at first The rate of loss dropped off rapidly as the portion of the soil colshyumn near the open end dried out Thereafter if the rate at which water was added was high the rate of loss increased sometimes temposhyrarily becoming considerably higher than jihe rate of addition In planshyning the experiment it was feared that wave motion might be set up in the water flowing through the tubes It was thought that the soil at the open end of the tubes might dry out more rapidly than the moisture could move up and that as a result the dry zone would gradually extend back toward the closed end of the tube until soil moist to the field cashypacity or above wns encountered Then the moisture migh t move forshyward into the dry soil until the open end was reached and the drying-out process would then stnrt over again

Onreiul consideration led to the conclusion however that a condishytion of dynamic equilibrium would be reached with the soil at the open end of the tubes lust moist enough tt) cause evaporation to take place as rapidly IS moisture was supplied The moisture-content grudient ~ack of the open end would then be sufshyficient to move the moisture at the rate of supply and the moisture conshytent at the closed end would depend on the rate of supply and the length of the tube This conclusion has been confirmed by the experinlental work Figure 3 shows the rate of loss of water from several tubes filled with a rather coarse beach sand Each curve shows the average of four tubes ThB curve for the 6shyinch tubes with the highest rate of

flow seems to indicate 1 tendency toward the wave motion which was feared In some other cases with the beach sand a similar tendency was observed However in m08t cases with the sand and in pmcshyticallyevery case with undisturbed soils the conditions illustrated by figure 4 were found

The data for groups of tubes filled with Willllmette silt loam are shown in figure 4 The portion of ench curye for the last few hours

13 RAJE OF FJOW OIr CAPIILAltY lIOJRlU1U)

which is separated from the rest of the curve represents average vaitles for all five tubes in each group instead of the values for the index tubes alone It will be noted thllt in ellch instance the lnte of loss approached the rate of addishytion and became nearly constant at a value close to that at which water was added This judicntes that the rate of flow hlld become approximately steady before the tubes were broken down Thc curves of figure 4 are typical of the curves obtained in these exshyperiments SUPPLEMENTARY EXPERIMENTS

MOVEMENT AS VAPon

In order to determine whether an important part of the llloeshyment through the tubes was in the vapor phase a special group of tubes was set up In these tubes a break in the soil column was provided by placing two pieces of 30-mesh sereen about 1 mm apart at a predetermined point in the column Breaks were placed at 1 cm from the open end in one group of eight tubes at 2 cm in a second group at 4 cm in a third at 7 cm in a fourth and n control group was provided without any break Each group of eight was divided into two subgroups of fCllr One set of subgroups was supplied with water at the rate of 11 mg per hour and the other set at the rate of 44 mgper hour All were set with the flow upward because it was feared that the soil on the wet side of the break might become satmatAd und allow free water to drip across

The rates of loss from the set receiving 11 mg per hom l11e shown in figur3 5 lhedata as shown are the avernges for the four tubes of each subgroup The frrst weighing was made nbout 12 homs after the tubes were set up and the effect of the breaks was alrel1dy apparent

11

-~ ) c-~~~~ I

-~ 1-~1Jjr- I

-l-l-~~~j ~ gt

-~~Q)~~~ _ l~~~

Io~~ -~~S~IS -~~~~~~ _s~~~~~

~sqsqsqsqs I

-~ I If gt -Cf)-~ I II

to

-i-Cl bull ~ bull I l- ~ o

It) LL o

I

I I

I 7 I

I

I o i rltl

m COJ

i bull to 1 COJ

1 ~~ l-

CD ~

~ COJ 00middot -- r

p

a ct

o 0 0 0 000 oE lt3 COJ Q CX) CD v COJ

(J46w) SS0110 3111~

FIGURE 5-Rllte of Joss of water from 6-lnch cores of Wlllamette silt loam hwng breaks tit dfferent distances from tho ond when water WIIS added at tho rate of 11 mg per hour

The tubes without a break showed the greatest rate of loss the tube with 7 cm of soil between the break and the open end lost the second

1

14 rEOHNICAL BULLETlN 579 U S DBPT OF AGIUCUurlJRE

highest mte and so on in regular order The difference between the groups having breaks 1 and 2 cm from the open end is very small

As the experiment progressed the curves representing the rate of loss from the broken soil columns crossed each other in regular

_ sequence Before the second weighshying the rate of loss from the tubes

it with the break 2 em from the open 2 end became less than that from the

r-~r-- - ta tubes with the break 1 cm from the r-~ r- ~ ~ mJ end By the end of the third dayI-~ ~ ~ 3 3

31i~ the curve of the 4-cm tub e had r-~ 1 m

I bullbull crossed both the 2-cm and the I-cm r-~ ~ ~ ~ ~ ~ ~ ~ J~~Q) curves These curves indicate that1- 1ampr-I( ~~sect~ It rather definite quantity approxishy

I-~ ~ mately 16 mg per hour of moisturet-~ ~ ~ ~ ~ ~ I

S ~~~~~ diffused across the break and throughII- ~H~~ 1 em of soil Smaller quantitiesI-~ S CQ CQ CQ CQ ~ were able to pass tluough the longer~oQrvx I If)

I-~I I I I c soil cohunns An exph1nation of the

bull A Ir--lt2bull x x ltt fact that the movement of moisture

~ through 7 cm of soil appeared to be I I

rlt) 5 almost as rapid as through 4 cm apshy

I~ t- peared when the columns were broken IK ~ ~ down as wiU be shown h1ter

I I tigt During the first 145 hours the difshy I ~ fcrent groups in the set leceiving 44 I I [ E mg per hour acted exactly as did eX those receiving 11 mg per hour asuJ

I J ~ may be seen in figure 6 Howeverrlt)

at the end of that time (AplI) theo rlt)

~ iii subgroup bloken at 7 cm followed a 11 ~ day or so later by the 4-cm group

q m

suddenly began to lose water at an I increasing rnte The explanation apshyI gJ peared to be that water or moist soil

l 1 was being forced thlough the screens r-shy~ by the pressure set up when the rubshy-r- V ber stoppers were seated in the tubesV middotId Ppon breaking down the tubes this

explnnntion was found to be correcto o o o o o o o m ltr The average moisture contents of

(Jy fiw) SSOl ~o 3lll the soil at different distances from the open end for each subgroup of the

IOUitE fl-Hnte o[ loss of wilter from 6middotinch cores o[ Willllmette silt loam huving breaks lit ll-mg set are shown in figure 7 The (litTerent distnnces [rom the cud when wnter WIIS added at tho rale of H 109 per hour sharp changes in moisture content at

the breaks in the soil colmnns are ouvious II the moistUle were moving through the soil in the vapor phase there should be no breaks in the curves since the movement of vapor through the open spnce between the screens would be fully as rapid and should be more rapid than tluollgh the soil The moisshyture content just below the screens in every instance was well above the field capacity while just above the screen it was much lower in every case In the columns broken nt 1 and 2 CIll the moisture content just above the screens waswell below the wilting point

RArE m FLOWOl CAPILLAltY MOISrUJE 15 It is interesting to note that the portions of all of the curves either

to the left or to the right of the break are approximately parallel to each other and to the lower and upper portions respectively of the curve for the unbroken column These data seem to iudcate thnt moisture distilled across the brenk in the column and built up a moisture content or capillary gradient above thot point It appears to have required n difference of lUoisture content of 12 to 17 percent to force 85 to 12 mg per hour of wnter in the vltpor phnse ncross nn open space of nbout 1 30 mm These rates are equivalent to 24 and -- 28 -- -- -- _- -shy34 mgper square cen- ~ 26 00- -- -shytimeter per hour or l--0UJ

~ i069 and 097 inch in ~ 24 -- -- -i--- --bull- bullx -- --shydepth (surfnce inches) ~ 22

xshy

per month If this is ~10-Cl

true the movement ~ 20 -- through the soil pores ~ 18 P

~o1 o~breo-ks shymust be negligible and I 6 ~ the conclusion seerns ~ iK il7 cOitl5f- shy~~brokel7 Ico~justified thnt the movl- ~ 14

VlUeut of moisture ob- - 7shy~ 12

r V IsenTed in these experi- z ments wns predomi- ~ 10 nntelyifnotexclusive- 5 8 41 V Iy in liquid form u V

1 The observntions on ~ 6

~rvmiddotHolsltlre c0l7tell1 Yo disT~dce Ithe soil columns 1e- 4 1

ceiving water nt the U) tgtltI~rate of 44mg per hour ~ 2 were not satisfnctory o because of the forcing o 2 4 6 8 10 12

DISTANCE FROM OPEN END (cm)of wet soil through the screens The subshy lGlRI 7-scmge moisture content throughout hngth of five roups

of four amiddotinch cores of iIIUIIICLlO silt 103m 1I1-iug 1-1I1111 brenks atgroup blOken nt 1 cm ditTercnt dlstallces from the open end Wllter WIS ndded to nIl tubes did not show [my of at u rnte of upproxilllntely 11 Illl( pcr huUI It IVU los froUl the unmiddot

1)roken core~nt un nvcmge rute of 101 Ill per hour and nt rates ofthis eff e c t an d its 120 102 $5 and 00 mg per hour frOIll cons lltwirtg breaks I 2 middot1

und 7 (n respectively from the open eil(l~moisture-coutent-v shydistnnce ollnc is ery similar to the corresponding CUITC 011 figmc 7 However the datil of figure 6 indicnte thut so lOllg as wMer was comshypelled to cross the breaksin the yopolphnse the1110 Cll1ent cOlTesponded (losely with that shown by the set of tubes receiviug 11 mg Pl hour

ElFECl OJ INlEUJlUllENT ADUITIOK

The method adopted ill this experiment required that wllter be added in batches mther tbiLIl continuously At first it was thought tlmt satisfactory Tesults could be secured only if the additions were small and mtlde fit very frequent inteJYnlsmiddotmiddot Since this procedure added greatly to the clif1ieultyof callying on the work n specia] test WilS made to determine tbe efrect of nddinp n eompulHtlyely large yolume of wnter nt one time A set of 20 tubes contnining 1(wbclp clay loam was used All these slllllpIes wen tahll within tl fe feet of the same point and ilt the same llcpth Yater WtlS added fit 12-hour intervnls until the 1ute of loss ns indicated by two indCx tubes becnme stolldy and np]1lodmntely equnl io the lflte of i(ldition At

16 TECHNICAL BULLETLJ 5iO U SbullDEPTOF AGHIOUUrURE

the beginning and end of the last 12-hour periodfin tubes were weighed and the rate of loss was found to be about equal to the rate of addition

When the tubes were llndy J2 drops approxiruately 400 mg of water was ndded to every tube at as nearly the same time as possible The first tube was broken down immediately j the next three at 5shyminute intervals the next tIuee at 10-minute interyals jand the others at increasing time intervals the last tube being broken down 8 hours and 45 minutes niter the Wilter wns added It was expected that the added water could be traced through the series of tubes as a sort of wave However analysis of the dltta) both by individunl tubes and by groups of foUl iailed to show any legulnr progression of It wave of high moisture content through the tubes In fad it was impossible even in the tubes broken down almost immediately after ndding the wlIter to detect the position oithe slug of wate] In breaking down the tubes the section of soil next the open end wns lcmomiddoted first and that at th( end where water was added lnst It took about 10 minutes to complete one tube so even in the first tube broken down the wnter hnd several minutes to distribute itself tluough 11 portion of the soil

These results together with the fnet that the mte of loss nppears to be only slightly nffected by differences of several hoUls in the length of time between ndditions of water appenr to indicate that adding the wnter in batches at intervnls of 12 homs or less gives practicnlly the sume rosult as would be secured by continuous nddishytion This is It point that needs more study however

RESULTS

RATE OF WATER MOVEMENT

Jh( primnry pmpose of the experiment was to determine the disshytal~e tluough which water in nppreciahle qtll1ntities can be moved by capillnry forces in the runge of moistme content between the field eapacity and the permanent wilting percentage It appears possible that eltentually fl single-valued conductivity factor mny be determined for any given soil thnt cnn be used to determhh1 the late of movement fOT fLny given set of conditions of temperllture moisture content

direction of flow etc Until such a factor and its relation to other eonclitions hilS been found we must be content with experimental determinations of the rate of movement under a variety of conditions Thp results are shown in table 1 The distanc(gt ghmiddoten in column 9 is the disttLOce through which the moisture content fell from the upper to the lower limits shown in columns 6 and 7 wlrile steady capillnry flow wus taking plnce at the late shown in column 8 For example the data of no 7-fL are from the experiment shown in thecmve for the unhroken column in figure 7 TIle moistme content at a point 112 em from the open end after 11 stnte of steady flow had become estnblished was 17 percent of the dry weight while at a point 29 em from the open end the moisture content wns 9 percent the permanent wilting percentage for this soil The rate of flow was 29 mg per square eentimeter per hom This rate of pound10- therefore took place through a distance of 83 em upward under the influence of a drop in moisture content from 17 percent to 9 percent

TAUJ] l-middotmiddotJ)islancc throlIlh which dijJerent qllunlitlcs oj water moiled itl l(lri(lIl~ s(iilllJ1(~ ultder Ilw injlllCllc(l oj 1It(ii~ule-coltclll dijJcrttlccs opprrxi1llatcly blwccn the willillJ7 point 07(1 the fitlet c(l7lOcitll

~~~ontent _-- ---~-~--__________ )i~tIIlL~) Yor

I hetwcon bullbull age

I lommiddot Rematks

llI1plQ Wilting Field (It pcrirncnts fiov (ontenl 110 pornmiddot ]Joint parity I __ limits llro

Nol SOIl t)pc II~llth orl I I I lill1its of exmiddot I HUleof 1II0isturemiddot1 Dlrcd~~ M I rllbe~

~ Lower I V PIler

~ ~

11Ichpound Prrcw~ Prrcwl Ipercent Percenl glcmlhr em fYumtltr o 1~

i 00 middot1lmiddotiJlCh tuhes 0111) nil of moisturemiddot o13 10 H 22 284 102 p 1 n contenlcurvcsllboel1eld capacity

lob I JllllO Mild 83 do __ J2 824 I-II ~ a 10 150 l-c I 3 O 74 8U bull dobullbullbull J2 852 ~

-niiT io 1 27 In 6 25 O I 03 __ do 810 t-lt2~n 12-h II G-l~ 106 27 13 I 29 bull ___ do 510jg middotnch lIllIl -lnch slimpIes from (HI)IH2 10n 24 65 95 DowlI bullbull 11 810 ~ 2~1 810 inch dCgt1h tl-Inchsflmplcs frout2middotd I (Iwhlllis 10Imbullbullbullbullbull j IH2 106 27 ]34 U4 _- do 10 2-e H28 1106 ~ ~i 10 tl 27 74 42 Fp 6 838 H2 inc I depth Largo po~tion or o

12 0 ~7 151 16 lt10 I 83c lI1(listllrcLOIltent curc oboyo field ~ 2-( 100 27 73 48 Down 6 amp18 cnpllcity o106 27 H U 53 lt10 bullbull i 83S

3~0 ii-j1 I iJS t 31 Kg 32 18 27 Up 15 82-1 0

tf I 3~h HS 12 95 2 5 Dowl1_~ Jj 824 Irge lIumbels (If tubes u501 in Illi

4-18 H g 31 93 42 Up_ 15 835 IIUempt to sutooth out curves nndI-c Ij~Wbl~rg d~middot kltU 835 j secure data on clTect or grnity ~ 3-d H8 11 80 54 Down H 30 11 ]8-24 148 33 86 15 ____ lt10 lQ 821 4~1 16-22 188 317 188 32 3 ~ 68 Up J2 817 lj 1~b 18 S 30 3 II 0middot1 lJOWIL J2 81 I HI

188 3U 5S 70 tpbull __ )0 7ll 7 EnCh group lrCltldell rlur 2middot IIc1l 4-c middotIll 1)own_ 7U~ i (ollr 4-lnoh and eighlOmiddotineh somples4-d ISS 337 01 Hi ~

4-(1 middoti2ir - J88 2U 6r JU Up 8 2middotiucllllnd 4-inch slIIples only o -I-f lcgtcr dn) 1lobo m28-0 _bullbullbull _ ISS 27 56 1 2 ])01111_ 4 H

34-30 188 28 50 14 do Ht~ 14-18 _ 18 8 31 7 5 9 middot15 IL I ~~ d024-28 188 337 56 4-1 Down bull middot14-1 lj188 320 50 na do_~ middot1 mo -I-k 1S-2~ 188 317 188 337 52 middot10 I _10 a m3 =J 4-1

J-j 32-36 _ bullbull bull

]88 270 50 53l lOp __ 8 60 140 154 114 1)OWII __ - bull na5-0 ~~ I~g ~tl middot10 140 96 lJfgt do ~75-b AIf 6middotinch Sllmples 1Il ixed 111 labOramiddot40 lJO 45 100 _10 _ ns5-e (ory Mnch of moisture-content60 144 11 rp _ nsbull LOBltlysnnd (Hermistonl ~ ~_--~~= 140 U

n4 ]5-d 1 curves obove fieid capacity ] middottO 1middot10 75 30 do TIIHl (in _do __ bull 1 ~96-( bull _bull--1 40 14n 55 bull

1heso values IIrc only npproxinullc I Theso nlnes wern determined by C S Schusler on soil SlIutpies token nl lIoiuts vcr) ciostl to UtI phlLCS where the samples 10 ill Ih oxperiments wero taken --J

~

TABLE 1-1)i~lante thrOlloh which differenl qllanlil1cs of water mOiled in tarillll~ ~oil 11I11ell 111lder Ihe injllcnce of moi8tnrc-content differenes i)approxim(ltely belleen the willing 110inl alfllhr field C(1)(lcitll-ContinllccI

__-- ~-- Moisturo contentcontent l-3

lJeo A-er No DeJ)h of Limit ageBun typo Limits o ex- Rnleosample perin Direction o I ruhltl5 temshy amp1perilllentsWilting Field ClI- lIow flo Remnrks

pernshy zpoint parity tnro Q

I shy____L~wer~IIt~pper t---- t~~-shymiddotmiddotmiddot----1lncha Pe-rctn Percell Ipercent Percenl mucmlhr

f

OR tC6middotu 2middot1-28 jXurnlJa l oti-h nO 2-10 90 190 50 3 do____ f 51 ~624-28 00 170 tfl-tl 03 5 DOWI1 5 au30-10 ItO 2middot0 636-d 2 lIL_ ii ~816-10 ~ 00 210 08lI-e 2 r~on ~ I ~2 l-3110 240G- 03 o Cp 0 n2110 2middot10Willllmeliesill loom 68 5 Don bull_ 5 f m2 Zll=fi l0 2middot10 113 4 lmiddotp_ 5 ms ~90 240 03 s6-1 5 Down -15100 180lI-J 39 9 Up 5 ~8 SD90 160 39Il-k ~ I~own ~ I ~5110 220 tL20- - lJl_ - - - I n4 ~ 90 19 t) 61 o Down~___ il J -q

O-n 00 200 87 8 Up _ 5 I rn ti-m II

~o00 ~JO S5i-n -jllolll((i ~ silt 1lt1111 _ I Don 5 I ~6 job

1 90 170 211 3 [p 4 m2 tjmiddot1 I 0 1 98-n It 8 flo middot1 s tl6-10 n-I Hfifl

24-28 1 I$O~ lOOO lB-b 5 flo 1 ~7 R-c 8 tlo 1 ~716-10 16bull 12 28271 - -Sod 18-62 t 1507 I do 4 ~7 o2587S-e middot1 do 4OO-(H t 1511-1 2414R- a do -I 71i S Il72-76 159middot1 241-1 gS-u 11 DOWll 76 Sti-IO lbullJ) 2400 shykit 5 do 412-10 1170 2l37 o2 do lij [lCOritOn silly (-tty Ion ilL 24-28 ISS 1006smiddot) - do middot1 758 ~ J2-18 II 7 2917 ~SmiddotI I lIL 10 770 S-I 84 do ton bullbull 0 Iil I)

)(-hI 2501 8S i - 110 10U7 o 9 1 2tl0 107)11 SI tlo 10 ~ 8-0 n shy 140 t middot0 58 1Donn7 5 - oIOS 110 j 14 I8-Tl Il j ~81 -I 0_ ~lq 108 do bull iI1117 210 II S do t=l

t Ih(lse t)lUtS afro t1ert~rlt1ill~tl hy ( ~ fifhuslrr fJll ioii ~nIllJ)tlS Illktl1 nL l)uinlS cry (middotIose 10 ttwl)hUllR llllre liltl snrlipltls IlCtl in tht$o t~~perirurnts Wt~rr taken

ItA]$ OF 1~LOW OF CAPILLARY llOI5TUltE 19 rhe other data in the table may be interpreted in the same way

Where possible the field capacity and the permanent -ilting pershycentage have been used as the upper and lower limits respectively because this is the ranampe middotof moisture content middotof primary importance in plant growth and it IS believed that the data ale more trustworthy within this rnnge

EFFECT Ot SOIL TYPE ON RATE OF FLOW

The data of table 1 are intended primarily to show the rate of flow of capillary moisture in difleient soils So many fnctors nre involved however thatitis very difficult to determine directly from the table 34 f-+-+--+--+--+--J-l--+--J--+-+--+-l the effect of individual 32 J--+--+--l--i--l-7-+---t--t-+--+--t--t- shyfactors on the rate of - V flow Figures 8 and 9 ~ 30 V clenrly indicnte the 3 28 J--+--+--l---+-l--t---I--l--t--t-t-+--l

effect of soil type The ~ 26 ---l--+--l-6-+-l--t---I--l--t--t-t--t--l curves on these figures ~ I( nre sinulnr to those of ~ 24 I ~~_ figure 7 in thnt they 0 ~-i--+--+--I--_-fs--=t---+---22 1------t-++---c1i- showtheaverngemois- lZ L~ ture content of the soil ~ 20 I-+--t-J~V--l-+--l---V+--+-II-+-+-I at different distances 181---I--t--1I--+---f-rIs-i--l--+--1r-+--t----i ~1from the open end in - I G I---I-HI-I---I-7=-t----I--+-J--J--f--t-I--l groups of tubes under r t (0 _ - - shysimil ar conditions ~ 14 u ~-

Til e s e euryes s11 0 0 ~2 f-+l-+-ltyen-Ir--+--r--j-i--f---+--II--+--t----i

that for any given lUtc 3 10 IV 1 x

of flow of water the W 11 l x

moiatule-content gra- ~ J-I)(-i-li~r-+--t---t-i--l--+---r-+--t----i8 f-lu-l-l client is steepest at the 6 6 pf1--rRATE Or now SOIL TYPE I- shy

i~taibi~~~~escl~~~ ~ 4 f1~-~~~~ -z~ r shy2 I-+-I-O--Opound2171fc1~r ml(7mete 57rL17t7P7[shy

steep as the moistmc 0 x--XpoundS1lrCd1~r L17t)71yStrl1tT content increases2 3 4 5 6 7 8 9 10 II IZ 13There is a decided di1- 0 DISTANCe FROM OPEN END (em)ferencein the moistureshybull I t 1IGtlI1 8--MoisLuru contont lhroughout len~th of ~roups of cores of COIl ten II grac len Ul1ny diffcrent soil typcs with 11lIllrodllllltely thesllmoratosof now upwllrdgivenlnoistult con te]) t ngnillst grnit~middot

for the difleren t soil types lc]Jlcsen ted by the ClLlyeS in these two figures Mechanical analyses of these soils are not avttihtble tlnd the soils

aTe clulSsified as mnpped in the soil-survey reports On this basis there are two cases in these two groups where fL soil of a finer texture tlllllsmits water more leadily thiUl one of ooarser texture The Villamette silt loam shows a lower moisture content and flatter moisture-content gradient than does the Chehalis loam with practi shycally the same rnte 01 flow rhe Callton silty cln~y loam also shows a lower moisture content and flattor gntdient than the Newberg clay loam with the same rate of flow Itmay be but probnbly is not significant that both tho Chehalis and N ewherg soils ure Tecent riYer bottom soils

POlhnps the most interesting point brought out by this comparison of the late of movemen t tluough difielent soil types is the very sroa11 middotdiffeTences in the rate of capillmy flow under similal conditions as compared to the vcry grent differences in late of flow tluough satushy

20 TECHNICAT BULLETIN Gi9 U S DEPT OF AGRICUUlURID

rated soils of the same classes Figure 10 shows that with pll1ctically the same moisture-cont~nt gll1dient between 10 and 22 percent the flow-through Meyer clay adobe Chehalis loam and Cnllton silty day loam was 35 61 and 88 mg per squnre centimeter per hourrespectively

Jfor two of the soils the clay adobe and the loam data nre available on the rate of infiltration of wutel applied at the surface During the

third consecutive hour 36 during which wuter 34 stood on the surfnce

~ 10- the clny adobe abshy 3 2 sorbed 04 inch of washy~ 30 ter while the loam () I

~

absorbed four times as V much or 16 inches

gt- 26 ~10reover the rate ofa o 24 I inftltrution into the-- k-0- r- clay adobe was rapidshyo 22 ly decreasing and in ~ 20

VI ~ fact became negligible u V within a few hours ~ IB I - 0 1 whereas the move-

- 16 f - ment in the loam apshy ~~II V penred to have reachedr5 14 shy ~ a constant rate ----~I

)lt --- The lI1tio middotof flowz o

12 shyu 10 VI - under saturated conshy I ditions was then 4 to 1

0 1-0 0 l B (and eventually much IJ (J) 6 ~ degRATE OF FLOIr SOIL TYPE_I-

more) in the Chehnlis o ~ ilt-86 m~cl119l1r New-oefY ClOYLoom

loam as compared to 4

o--laquoU mgcm9l1r CtTrltol1SilfyCltTy LotTm- the Meyer clay adobe 2 ~X- --85 mgcl11Yl1r H7lamette 0517 LOlm _ while under nonslitushy0--r96rgj11191r I Lfol11 SQftT rlited conditions the1o lI1tio for the same soils o I 2 3 4 5 6 1 B 9 10 II 12 wns less than 2 to 1

DISTANCE FROM OPEN END (cm) Specmcwt)ter-conduc_ lWl]IIE 9-middot0Isturo contcnt throughout lcngth of groups of coros of tivity mensurements

difTerent soil typOS with Ipproximlltcly tho slime rilles (If flow downmiddot LS (lefinecl by Israelsenward with grllllty lt l

(26) are not lmlilable for these soDs It will be noted on this figllle that the rate of flow of the Chehnlis loam is Jess thnn that of the Carlton silty clay loam a condition similm to thut noted in figures 8nnd 9

EFFECT 0)0 GRAVITY ON RAlE OJ joLOW

Pairs of curves are shown in figure 11 illustll1ting the difference in the moisture-content gradient for flow upward as compured with flow downward With each of the three soils illustll1ted it required 2 pershycent more difference in the moisture content between the two ends of a soil column 12 em long to force the water to move upward against the gravitational potential gradient and the flictional resistance than it did to force water downward with gll1vity but agninst the same frictionru resistance at the same rate It is hoped that use can be made of this effect of gravity on the rate of fiow to evaluate the capilshylary potential and a conductivity factor ill terms of ordinary units of

21 HA~~E OJ ELOW 011 UAllLLAHY lOl$TUHE

mass fome and distance As has already been stated these dlta make it possible to plot curves showirlg the relation between the moisture-content gradient and the rate of flow both when the cnpilshylarypotential gradient and the gravitational potential gradient nre opposite and when they work in conjunction

MOISTURE-CONTENT GRADIENT FOR DIFFERENT RATES OF FLOW

The curves of figures 12 and 13 show thediffercnces in the moistureshycontent gradients at given moistme contents required to force different quantities of wILter through the soil eitber upward or downward The slope of the cm-ves at any given moisture content represents the rate of change or gn1dient ~4 ------ shy

of the moisture content at the given moisture 32

V [ shy

content and f 0 l the 30 [lshyconditions represented X 28by the CU1ve Thus C1 17

if a tangent be drnwll w 26 1--shy~ 3

~V shyto one of the curves at 1 a given moistmmiddote COll- ~ 24

tent the difference be- ~ 22 tween the m 01 stu]o 0 ~ conteuts indicnted by ~ 20 V~ the intersections of tIlls ~ 18

tangent with uny two ~ 16 il rt vertical1inesreprescnt- ~ Jing a distance apart of I- 14 7 V 1 centimeterwillbethe ~ 12

1Itte of change ox grn- ~ 10 1Idient of moistllJe con- 8 tent in percent pel w 8 A centimeter g 7

Figme 12 shows thnt n 6 SOIL TYPiat i moisture content 0 4 [-~~c~ Nf7er- CA7jAdobe of18percentagrudicut ~ ~o---ltgt88 mgcm2)r curtofl $17ty C1Y Lim of 32 percent per cen- 2 ~161 fgcm2)r I C1ell17f8 Lf171111timeter is required to o I 2 3 4 5 6 7 8 9 10 II 12oforce water upward itt DISTANCE FROM OPEN END (em)tbe rate of 107 milli-

F1GLIU 1ll-MoisturQ content throughout length of groups of cores of grams per square ce11- (iitTerent ~oil types ~hnling differences in rutes of finw (upwardtimetel l)er hOLir 2 8 Ilgllil)sl grJlit)J wit h IJriproxillllllel~ uniform lIIoisturemiddotconlent

~rndlOlIls percent per cen tmlcter for 88 millignLIDs per squnre centinleter per hoUl 17 percent per censhytinleter for 63 milligrums per sqlHLre centimeter per bour and only 06 percent per centimeter to force water upward tit the rate of 47 millishygrnms per square centimeter pel hoUt Figure 13 shows at tbe same moisture content tL gradient of 18 percent per centimeter will force water downwnrd at the ratc of 91 milligrams pel squnre centimeter per hour 14 percent per centimeter for 72 milligrams pel square centishymeter pel honral1dO6 percent percentimetel will force water downshyward the mte of 54 milligralDs per square centimeter per bour The 6shyinch tubes useil were not long enough to build up it moisture content of 18 percent with a flow of 40 milligrams per square centimeter per hour

From another point of view it mny be noted that a moisture-content gradient of2 percent per centimeter will force 40 milligrams per

22 rlECHNlOAL BULl~I~TlN 570 P S DEPT 011 AGJtlCUVrulU~

square centimeter per hour downward through CmltoH silty clay loam at a moisture content of5 percent 54 miHigrams per squarecenti shymeter per hour at 12 percent 72 milligrams per square centimeter per hour at 16 percent and91milligrums per squltle centimeter per hour at a moiampture content of 17 percent

CONVERSION OF UNITS

The equipment used in these experiments gave dotit in terms of milligrams per hoUl througha soil column 084 inch in diameter In table 1 and the moisture-content versus distance curves these data have been converted into terms of milligrams per square centimeter per hour In considering the field npplicotion of the data it may be

remembered that 1 milligrUll per square

32 cPlltimetel per hour is ~ ~~ - flpplOximntelyequivashy

lent to 000945 inch ill~V depth of water (someshy1~t11o~ shy311 J --f- - - times called surface

gt- V inches) per dov or 1 ~ jfl1 zjh(-~ 24 x I ~N inch in 106 days

I C IG 1jC~~ 22 131d DISCUSSIONJ~ ~~ Pr (nZftmiddot~ 20 II ~lt (fIg C These data serye to I

1 8middot~ ~ 18 give some idea of the J V

rate at which water II 16 - I may move through theIIv ~II - 14

x soil undertheinfiuence z ~ of difiercuces ill moisshyvr~~ 12 ZVz tUJe content Table 2

8 10 shows the number of ~~ days req uired for 1inch

of water to move fromFLOIY sOIL TYPE soil ot the field capacshy~ ==== own ~ Clellults Loum itythrough different

~ wn INeyer Cluy Aoo6e distances either up or2 shy down to soil at the o ~wn Jmtmrte$iiLlt7ltf wilting point in several

o 2 3 4 5 6 7 8 9 10 II 12 soil types NIovement DISTANCE FRON OPEN END (em) latcmlly mny reasonshy

IGUltt ll-Moisture content throughout length of 1111irs of groU(l$ of u bly be ossunled to be cores of differont sulls hell lIow anhe snnw rutc is tllkill~ 111llC upmiddot in tcrrnediH te betweenward or downward

the Tates in these direcshytions It will he observed that in the case of the Meyer clny ndobe the flowupwUd appears to be more Teldy than the flow downward in that only a slightly longer tinle was req uired to move nOll inch of wnter 28 inches upward than to moye the same quantity 19 inches dowllshyward This case is almost unjque in these experiments l1nd probnbly serves to show the difrerences in soil samples mther thnn to prove that the Meyer clny adobe hlw some stmnge power to couuteract the gravitatIOnal field The results moo hl1rder to explnin in thnt the lower pmmiddott of each of thecorresponding curves represents the data from 16 tubes and the upper pllrt from 8 tubes

23 HATE OJ FLOW OI(AlJLIAIty MOISTUH1~

TABlpound 2-Dtsallce through which 1 inch oj water will move in a giter inio through soils oj d(fJcreltt types from Z01US at field capacity to zones at lhe uviliing 1)0111

M lt+- _ bull -shy ~~

J)recDlrecmiddot I Dis Soil InJltJ ~Ihlle

Dismiddot Iion of Soil typo rime tnne ~ lion of tunlc flow r ~ Itcil ]Jals Inchbull1 31- 1Jp 14 1 2 Up

IOIlJllY 811ndbullbullbullbullbullbullbullbullbullbullbullbull 8 11 Do 11 4 5 Down

rhlllnll5 IOal 3 ( Up10 l7 DOW11 21 WiIlallloU() silt I01l1ll 12 3 o DownS 25 Do 11 17 Up 12 3 o UpIcwbl~rg (IllY Inlllu 12 21 Down (nrlton ~my cillY l(lltl 12 35 Down 18 28 tp~f ~y~r clay odobebullbullbullbullbullbullbull _ 17 111 Down

~ ~---- -

Ii it be assumed that the lIIoisturccontcnt aL the ground surface is at the wilting point find thnt a Jew inches below it is at the field eapacity the loss by evaporation might beshy 30

eome appTeciable ~For instan(e with the 1-

V ~ r-Willamette silt loam if ~ the soil 36 inches be- ~ 24

V -shylow the surface were ~ 2~ _ -shymaintained at the field ~I y Cl

tapaoity an in ell of ~ 20 xl water might be lost I- 18

eyery 21 days If the ~ I 1 soil 3 3 inches below ~ I 6 I the surface were main- ~ 14 7 bull tained h nl f w u y be- - II j

u d

j

tween the wilting point ~ 12 0

and the field capacity ~ 10 71 ~ it would take 37 days 8 I for 1 inch of water to 0 Il 1ltbe lost4 The (ondi- ~ RATE OF FLOIY tions of theexperi- ~ 4

Ipmiddot x-x fa Hlffcm7l7r 1shylllent do not aford di- Vl 88 mgcm7hr0----0

)(___x 2 63 m~ltr I- shyIect measurements of ~

0---0 1 bullIlnyencm~nrthe losses through 0 greater distanees but 0 23 4 5 6 1 8 9 10 II 12

from the aboye data DISTANCE FROM OPEN END (em) it lnuy be inferred tilat FlO U1m l2-1-Toisturc ~ontent throughout length of groups of eore~ ofit would take abou t 3 CurltoD silty clay loam wh~tlllpwrd IInw Is t1King plneont different

rntes~

months for 1 inch to be lost from Willamette silt loum if the soil a foot below the surface were maintained at the field capacity Where crops arc growing they will of course reduce the moisture tontent well below thnt point within a week or two after ltlin or irrigation These data then conshyfirm the general opinion that wl1CJe field crops are growing and the nter table is deep losses by evnporation of moistmc brought to the surface by cnpillary action are not ~leat

There appeal to be no data IWl11lable as to the length of roots or root hairs that are effective in absorbing moisture in the soil at any one time Schuster 5 has found thnt a soil-tube sample 075 inch in

bull Fronl no lmiddotn in table I lind line 15 onp 22 2l IIlIl ]lcr sounre centimeter ]ler hour equals 0027 inch per ltlny

(nPublished dntn

24 ~ECfNLCAL BULLETIN G7lJ 1 K DJ~Pl OF AGHlCULTURI~

diameter by 12 inches long which contains 80 to 100 cm tf fine roots represents a good root distribution If all of this root length absorbs water it indicates that on the average there is about 1 CIn of abshysorbing root pel cubic centimeter of soil (Root hairs up peal to be absent or nearly so on both walnut und peal roots in this ureu and if present are so short thnt in effect they onJy increuse the diameter of the absorbing root n millimeter or less Their influence therefore is not considered in the present dismIssion)

If it be nSRumed that only 10 percent of 1he length of fine roots is water absorbing that the 100t zone is 4 feet deep and that the diameter of the water-absorbing Toots is 1 mm the aren of watershy

absorbing root surface28 per squnre centimeter ~ 26 of ground surfnce will C)

w 24 ~ be 37 squnre centishy3 ~~ meters On these asshygt- 22 a V i sumptions and the furshy

~ o 20 ther assumption thu t II- v the root distribution is o 18 ~ -- -shyt shy - uniform themuximum Z w 16 j~ --- distance wuter would u shy~ 14

~ JlUve to move through n - the soil to the roots

- 12 I - shy-- would beabout45 cmt- - - OIl thefurtherussumpshy~ 10 - shyi- ~ i~ j-o tion that the water reshyZ o 8 l quuement of a mature u

6 0-

tree is 2 acre-inches Jw 0

-I-I--shya x__x RATE OF FLO)f per nere pel week 01 gt I shy

4 ~ l- ~I mgcmYr l- 003 cubic centimeter (I) e 72 mlcm2r0--0 per sq uare centimeter ----)( $4- mgcm2r I-- shyo 2 0---100 I 10 mgcm~Ir of ground surface per

1--1~ 0 hour the flow through o I 2 3 4 5 6 7 8 9 10 II 12 thesoiludjacenttothe

DISTANCE FROM OPEN END (cm) absorbing roots would JoWtlItE 13--jloisturc content throughout length of grouJls of (Ires need to be itt the rate dlr~~~~O~li~~Y cla IOUlIl whell dowllwlIrd tlOIlis takillg pillcllat of 0008 cub i c centishy

meters 0 I 8 m g pel squilre eentimeter pel hOlil TIll dutu of table 1 indicate thutwith most of the soils tested flow u t this Tate through distances of n few centimeters will take piaeo under the inJiuenee of soil-moisture differshyenCes l~oughly between the field eupndty flud the wilting point

The flow of moistme townrd n root is somewhat annlogous to the flow of water toward fl well After a rnill 01 an irrigation the soil mass maybe assumed to be wet to the field capacity Immedintely the roots begin to nbs tract wuter At first the water comes from the soil adjacent to the root but very soon u moisture-content or capilluryshypotential grndient is set up nnd fiow townrd the root through the soil tnkes place As the loss of water continues the sphere of influence about each rooiexpands ltnd in time interferes with the sphere of influencenbuut adjoining roots The flow toward any root crosses concentric cylindrical smfaces nud therefore the maximum rnte in volume per unit area per unit of time occurs ndjacent to the root Assuming a root diameter of 1 mmthe mte of flow 1 em from the root would be only one-twentieth as great as that at the root Moreshy

25 RATE OI IltLOW Or~ OAPILLARY MOISTUHFJ

oyer part of the supply is always being drawn from the soil near the root and does not need to be forced toward the root from the outer region of the sphere of influence Taking these factors into account together with the foregoing assumptions it seems that the moisture content midway between roots might be well below the field capacity before the rate of flow through the soil toward the roots would become the limiting factor of tree ~IOwth Whether the assumption that 10 percent of the fine roots IS water absorbing is even approximatclr true is unknown so far as the writer can learn

Admittedly this discussion is based on a number of assumptions Pild is somewhat speculative However it is believed that it throws some light on plant-soil-moisture relations It is hoped that as the investishygation progresses it will be possible to work out this phase more definitely

SUMMARY AND CONCLUSIONS

The experiments reported herein were designed to furnish data on the rate at which capillary water will move through various soil types under the influence of gradients in the moisture content

The IDlportance of these data for the solution of many field and orchard problems is discussed

Most of the recent pertinent literature in English on the subject is briefly reviewed

The study was inuuguruted with the intent of securing both expershyimental data on the mte of flow in many soil types and fundamental information on (1) the relation between the moisture content of the soil and the capiUaIY potential (2) the relation between the capillaryshypotential gradient and the rate of fiow of capillary moisture (lIld (3) the specific conductivity of the soil for moisture

Only the first part of the study is reported here but enough comshyparisons are shown to give some idea of tbe principal factors wbich affect tbe rate of flow

For several reasons short small-diameter soil columns of undisturbed structure were used A steady state of flow can be secured much more quickly in short than in long tubes Several duplications of small samples were considered more trustworthy than one or two larger samples Detailed data for different depths in the soil profile were desired The ease of securing samples at considerable depth with the King soil tube was an important considerution

The experiment consisted of setting up series of soil columns exposed to the evapornting influence of fill nil current at one end and adding water at predeterniined rates a t the other end Vhen a state of steady flow was secured the soil columns were broken down and moisture conshytent of each short trnnsyerse section was determined

That approXIDlUtely steady flow had been attained was determined by weighing the tubes before and after adding watereach time and plotting up the rate of loss Vhen the rate of loss became and COllshytinued approximately uniform and equal to the rate of addition it was evident that dynamic equilibrium had beellleached

It was found that a very small portion of the water could be moving through the moist soil in the vapol phase Only 8 to 12 mg pel llOUl of water crossed a screened space of about 1 mm in these tubes If no more than that quantity of water will cross a practically open space it seems impossible that any appreciable quantity would pass through the pores of the soil

26 1ECHNIC~l BULLETIN mo U S DEP]~ OF AGHICUUlUltE

Ideally the water should be added to the soiL column in this experishyment continuously rather than intermittently A series of tests indishycate that the addition of comparatively large quan~ities of water at12-hour intervals did not seriously distort the dataThe distances through which water flowed at different rates underthe influence of certain differences in moisture content at the two endsof the soil columns in a number of soil types and at different depthsare tabulatedBecause of the large nurnbel of factors involved it is difficult to makedirect comparisons from the tttbulated values and a number of comshyparisons are presented glllphicallyCurves showing the moisture content throughout the length of soilcolumns of diffeJent soil types when moisture was fiowing throughthem at the snme rate are shown These curves show that in generalthe finer textured soils wiIlllot tmnsmit capillary water as readily aswill coalser textured soils They show that for steady flow in anyparticular case the moisture-content gradient is alwnys steeper as themoisture content becomes lessThe difference in the ability of different fine-textured 10ils to transshymit cnpillary moisture as compared with coarse-textured soils is muchless than the corresponding differellee in the ability of similar soils totransmit moisture under saturated conditionsThe eflect of the gravitational field on the moisture-content gradientreguired to force capillary water through the soil is shown graphicallyVlth each of three soils represented it required about 2 percent morediflerence in moisture content at the ends of 12-cm columns to drhewater upward thun downwardFor one soil moisture-eontent-versus-distance curves are shown forfour different 1n tes of flow both upward and downward These curveshow graphically the different moisture-content grndients required atvarious moisture contents to force different quantities of capilll1rywater through the soilThese data show thnt beiiween npproximntely the field capacityand the wilting point the several soil types will transmit 1 inch ofwater through 1 to 4 inches in 8 to 20 daysFrom these dnta it is obvious that with differences in moisturecontent between tIle field capacity and the wilting point water insufficient quantities to support crops eould be raised a feT inchesfrom a moist subsoil but only a few inches Examination of thecuryes shows that most of the movement takes plnce at moisturecontents well above the wilting point Losses by evaporation at the soil surface of water moved upwardhy capillary action nre not greatThe portion of the soil mass midway between absorbing roots mightbe well below the field capacity but still weH above the wilting pointbefore the movement of moisture through the soil would become thelimiting factor in tree or fruit growthAt moisture contents below the wilting point the moisture-contentgradient is extremely steep for the rates of flow used in this experimentThe different opinions noted earlier in the bulletin on the imporshytance of capillary flow are reconciled At high relative moisture conshytents and with low rates of flow wnter may move through considerabledistances At low relative moisture contents the distance throughwhich moisture will move is very small

27 HATH OF FLOW 01 CAP1LLAltY MO[STUHI~

LITERATURE CITED

(1) ALDRICH Vr W and WORK R A 1933 PJlELIMINAItY REPORT OF PEArt fREE ltERPONSS 10 VAHlATlONIl IN

AVAILABLE SOIL MOlSTUJlB IN crAY ADOllE SOIJJ Amer SOC Hort Sci Proc (1932) 29 181-187 illus

(2) --- and WORK R A1935 EVAPORATINO POWloJR OF lU] Alit AND TOl-lWOl~ HAIIO IN nElLAlIOli

TO ItATE OF Il~Al FRU ENLARGElIlENl Amer Soc Hort Sri

Proc (1934) 32 115-123 (3) --- VOJlK R A and LEWIS M R

1935 lEAltJlOOT CONCJNTItAfION IN UEJATlOl 10 SO]I-MOlSTUlUJ lJXshyTUACTJON IN HllAYY CLA SOIL Jour Agr Research 50 975-987

(4) ALWAY F V and McPoLJ O R 1917 ILELAIION OIlt MOYBMENT OJlt WATEIt IN A SOlLTO 118 HYGUOSCOPICIT(

AND UliTIAL MOlSTNl~SS lour Agr Research 10 391-428 illus

(5) BAUTHOLOMEW E T 1926 INTlmNAL DJWLINE O LBMONS Ill WATEU DEFIClI IN LEMON

IltHUITS CAtSElgt BY EXCESSIVB LEAF BYAlOllArlON Amer Jour

Bot 13 102-117 illus (6) BECKETT S H BLANEY H F and TAYJOIt C A

1930 IltUlOATION WATER ltEQUIHEJIIBNT STUDUJS O CITHUS AND AYOCADO IInlES IN SAN DIEGO COUNTY CALTFOHNIA 11)20 AND )1l27 Calif Agr Expi Sta Bull 489 51 pp illus

(7) BODMAN O H and EDIIWSIJN N E 1934 THE SOIJ-JIIOISTUJtJ~ SYSTEM Soil Sei 38 425-444 iIlus

(8) BouyoucOS ObullJ1915 EF]oBCI 0]0 TEMP1~UArUUB ON SOME OF lHE Mosr lMIOHTANI PHYSICAl

1IWCBSSES IN SOILS Mich Agr Expt Sta Tech Bun 22 63 pp illus

(9) BRrGGs L J and JlAJHAJII M R 1902 CAllLLARY SfUDlJoJS AND llWHAIlON OF CLAY IHOM SOlI SOIUlIONS

U S Dept Agr Bur Soils Bull 19 40 pp iIlus

(10) BUCKINGHAM E1907 BTUDHS Of THJo] MOYEMENr m SOli 1I0ISIUItE U S Dept Agr

Bnr Soils Bull 38 61 pp illtls (11) CONRAD1 P and VmlHMFJYElt F J

1929 IWOI Dl~VELOlMENT AND SOIL MOlSTUUE Hilgllrdiu 4 (113]-134 illUs

(12) FlSIIEU E A1923 BO~nJ JoACIOHS AIIEClING IIIE EYAPOHAIlON 010 WAIEU FIWII SOli

JonI Agr Sci England1 ~3 [1211-143 illus (I3) lISHEH R A

1926 ON J11E CAJllLAllY FOltCES IN AN IDEAL SOIL COlntECTION at JoOHMU)AE GIVEN BY W B HAINES Jour Agr Sci [England]

16 [402J-505 (14) FURlt J R anti DEGMAN E S

1932 ItErAlION OF lolOISTUltB SUIILY TO STOMAlT JlEJAV10It O~ ~rnE APPLE Amer Soc Hort Sci Proe (1931) 28 547-551 mus

(15) --- and MAGNESS J H H13l PUEVIJlITNAUY UEIOUI ON UELAIION 01 SOIL MOlSTUHE TO SJOIlAlAL

ACTIVITY AND joUUII GlWWTH 01 AllVES Amer Soc Hort Sci Proc (1930) 27 212-218 illus

(16) GAUDNIltJR W1919 THE JOVEMEN 01 IIOISTUUB IN SOIL DY CAIlLLAHll)middot Suil Sci

7 313-317 illus (17)

1919 CAlILLAJU JllOISTUllE-liOJDIN( CAIAClIY Soil Sci 7 319-324 illlls

(18) 1920 A CAllLJJAUY IltANSMISSION CONSTANl AND METHODS 01 DEIEIshy

MINING 1 EXPEItlMENTALLY SoilScL 10 103-126 illus (19)

1920 THE CAIIILARY 10lENTlAL AND ITS UELAJION IO SOIL-MOISTURE CONSTANTS Soil Sci 10 357-359 illus

(20) ---ISllAEtSEN O W EDLEFSEN N E and CLYDEfI S 1922 THE CAIIVLAUY )OlENTIAL FUNCTION AND 118 J1ELTlON TO lnIUGAshy

TION lUACTIC~ 1l1ys Rc (2) 20 196-197

~l

28 TECHNICAL BULLETIN m9 U S DlDPT OF AGRICUVrURE

(21) GARDNER W and WIDTSOE J A

1921 THE MOVEMENT OF SOIL MOISTURE Soil Sci 11 215-232 illus F

(22) HAINES B W

1927 STUDIES IN THE PHYSICAL PROPERTIES OF SOILS IV A FURTHER CONshyTRIBUTION TO THE THEORY OF CAPrLLARY PHENOMENA IN SOIL Jour Agr Sci [England] 17 [264]-290 illus

(23) HARRIS F S and TURPIN H W

1917 MOVEMENT AND DISTRIBUTION OF MOISTURE IN THE SOIL JOl~ Agr Research 10 113-155 illus

(24) HENDRICKSON A H and VElHMEYER F J

1929 IRRIGArION EXPERIMENTS WITH PEACHES IN CALIFORNIA Calif Agr Expt Sta Bull 479 1-56 illus

(25) --- and VEIHMEYER F J

1934 IRRIGATION EXPERIMENTS WITH PRUNES Calif Agr Expt Sta Bull 573 44 pp illus

(26) ISRAELSEN O W

1927 THE APPLICAlION OF HYDRODYNAMICS TO IRRIGATION AND DRAINAGE PROBLEMS Hilgardia 2 [479]-528 mus

(27) 1932 IRRIGATION PRINCIPLES AND PRACTICES 422 pp mus New York

and London (28) KARRAKER P E

1920 THE EFFECT m THE INITIAL MOISTURE IN A SQL ON MOISTURE MOVEMENT Soil Sci 10 143-152 mus

(29) KING FH 1889 SOIL PHYSICS Wis Agr Expt Sta Ann Rept 6 189-206 illus

(30) 1904 TEXT BOOK OF THE PHYSICS OF AGRICULTURE Ed 3 604 pp

mus Madison Wis (31~ LEWIS M R WORK R A and ALDRICH W W

1935 INFLUENCE OF DIFFERENT QUANTITIES OF MOISTURE IN A HEAVY SOIL ON RATE OF GROWTH OF PEARS Plant Physiol 10 309-323(32) LIVINGSTON B E and KOKETSU R

1920 THE WATER-SUPPLYING POWER OF THE SOIL AS RELATED TO THE WILTING OF PLANTS Soil Sci 9 469-485

(33) MCGEE W J

1913 FlElD ltECORDO RELATING TO SURson WATER U S Dept Agr Bur Soils Bull 93 40 pp

(34) 1913 WELLS AND SUBSLIL WATER U S Dept Agr Bur Soils Bull 92

185 pp(35) McLAUGHrIN W W

1920 CAPILLARY MOVEMENT OF SOIL MOISTURE U S Dept Agr Bull 835 70 pp illus

(36) 1924 iHE CAPILLARY DISTRIBUTION O MOISTURE IN SOIL COLUMNS OF

SMALL CHOSS SECTION U S Dept Agr Bull 1221 23 pp illus(37) MAGNESS J R 1935 STATUS OF ORCHAHD SOIL MOISTURE RESEARCH Amer Soc Hort

Sci Proc (1934) 32 651-661 (38) RICHARDS L A

1928 THE USEFULNESS OF CAPILLARY POTENlIAL TO SOII-MOISTURE AND PIANT INVESTIGATORS Tour Agr Research 37 719-742 illus

(39) 1931 CAPILIAHY CONDUCTION OF LIQUIDS THHOUGH POROUS MEDIUMS

Physics 1 318-333 illus (40)

1936 CAPlILAHY CONDUCTIVITY DATA FOR THREE SOILS Jour Amer Soc Agron 28 297-300 illus

(41) SCOFIEIJD C S and WmGHT C C 1928 THE WATER UELATIONS OF YAKIMA VAIIEY SOILS Jour Agr

Research 37 65-85 (42) SHANTZ H L

1925 SOli MOISTURE IN RELATION TO TIm (mOWTH OF CROP ]LANTS Jour Amer Soc Agron 17 705-711

I

29 HAJEJ OJ JLOW OI UAllLLAHY MOl~TUHE

(43) SHAW C F 1927 THE NORMAL MOlSTUltE CAPACITY OF SOILS Soil Sci 23 303-317

illus (44) --- and SMITH A

1927 MAXIMUM HEIGH OF CAPILLARY RiSE STAltTING WITH SOIL AT CAPIIJLARY SATUUATION Hilgardia 2 399-409 iIlus

(45) SHULL C A 1916 MEASUItEMENT O TilE SUItACE FORCES IN SOILS lioi Gaz

62 1-31 illus (46)

1930 ADSOltPTlON O WATER ]lY PLANTS AND rIlE FonOES INVOlVED Jour Amer Soc Agrou 22 459-471

(47) TAYLOH C A BLANEY H F and McLAUGHLIN W V 1934 TilE WlITING-HANGE IN CEIlTAIN SOILS AND 1IIE UIJlIlATE WILTHWshy

POINT Amer Gcophys Union Ann Meeting 15 436-444 illu8 (48) THOMAS MD

1921 AQUEOUS VAPOR IHESSUllE mSOILS Soil Sci 11 409-434 iIlus (49)

1924 AQUBOUS VAPOll PltESSUIlE OF SOILS 11 81UDlIS IX DltY SOliS Soil Sci 17 1-18 ill liS

(50) --- and-HARUls K 1926 THE MOISTunE EQUIVALENT OtSOILS Soil Sci 21 411-424 illus

(51) VASQUEZ N F 1933 EL AGUA DEL SUELO EN RELAlt16N CON LA PLANTA Min Fomento

Dir Agl y Ganadera [Peru] Cire 20 21-30 (52) VELHMEYEll F J

1927 SOUl FACTOIlS A~FECTING THE lRltIGATION REQUIREMENTS Ot DEshyOIDUOUS OROHAUDS Hilgardia 2 [125]-284 illw

(53) 1932 THE DISIOSAL OF MOISTURE UOM THE AERATED POUTlON Ot SOIlS

Amer Geopbys Union Trans Ann Meeting 13 330-331 (54) --- and HENDRICKSON A H

1927 SOlL-lIIOlSTUnE CONDITIONS IN RELATION TO PIANT GllOWTIl Plant Physiol 2 71-82 illus

(55) --- allel HENDlllCKSON A H 1930 ESSENTIALS O~ lRUlGATION AND CULIVAllON 01middot OHCHAHDS Calif

Agr Ext Serv Cire 50 24 pp (Revised 1936) (56) --- and HENDRICKSON A H

1934 SOME PLANT AND SOIL-MOISTURE UELAIlONS Amer Soil Survey Assoc Bull 15 76-80 ilus

(57) WORK R A aud LEWIS M R 1936 THE UELA7ION OF sou JllOlSTUltE 10 PEA It IHEE WILTING IN A ilEA Y

CLAY SOIL Jour Amer Soc Agroll 28 124-134 illus

ORGANIZATION OF THE UNITED STATES DEPARTMENT OF AGRICULTURE WHEN THIS ]UBUCATION WAS LAST JRINTEI)

Seeretary of Agriculture l ~lIder Seeretary_____ _

Asnstant Secretar1l Direcor of Exlell~i(ln Work Director of Finance Director of Informalion_ Director of Personnel _ Director of Re~earch __ Solicitor_________ _ _ bullbull _ bull Agricultuml Adjllstllumt fltllli1ti~trali()1t Bureau of AgriclIUural Economics Bureau of AgricIIUural ElIgineering Bureau of Animal Jlldll~tll1 Bureall of Biological liurIJe1l_ _ _ _ _ Bureau of Chemistr1l (nd Soil bull 0 0 ___

C01llIlodily Exchange A dlltinitmtion _ __ _ Bureau of Dairy Jndll~try__ _ o __ ___ _

Bureau of Entolllology and Pllud (JlIIL1(ntinc_ Ojficc of Experhllen Btatioll~ _ _ Farm Sectlnty Administration __ ltood and Drllg Admini~t1Cllion _ _ __bull Fore~t Sennce_____ _____ bull _ _ ___ _bull ___ BU1ca1lof Home Economics 00 __ _______ _

Librarll____________ _--------BlIrelLU of Plant Indu~trll__ ___ _ __ _ BUrellll of Public Roads _ _ __ __ bull ____ _ Soil Conservation Service bull bull _ ____ _____ _ Weather Bureau_____ bullbull _______________ _

IfENHY A ~LLACB M L WILSON

HAHltY L BHOWN

C Yo AIlBCH1middotON

W ATuM M S EISENIIOWlm

W T 8TOCKIHItClEH

JAMES T bullJAHDINl~

MASTIN G VHIT~]

H It TOLL~w Alhmnistrator A G Br_ACK Clrief S H McCHOHY Chief JOHN R MOIlLr]It Chief IHA N GAlIltHLSON Chief HENHY C INlGflT Chief J W T Duv] Chief O R REg) Chief LEt A SlltONG Chief rAMIS T bull TAH)IN~l Chief r W ALEXANDFH Administrator ALTEH C CAMPBELL Chief FEltDINAN) A Sncox Chief Loursg STANLEY Chief CLAHIIH1L 11 BAHNETT Librarian Flm)gHICK D RIClfgY Chief THOMAS H MACDONALD Chief H H BrNNEfT Chief IIS R CnEGG Chicf

This bulletin is II contribution from

Bureau of Agricultural Engineering _________ S H MCCnOHY Chief Dilrision of Irrigation W W McLAUGHLIN Chiefe ___________

30

U S GOVERNMENT PRlIITtWC OfFIC[ 1937

-------------- shy -~--FI)r ~lJC by the Sup(rintllldlmt of DOCtJlII(lltR Wn~hillltoll D C - - Price1i ceots

6 N-

u5~-

q ~~~-~-~~~~~ Technical Bulletin No 5i9 ~ October 1937

UNITED STATES DEPARTMENT OF AGRICuLTURE

WASHINGTON D C

RATE OF FLOW OF CAPILLARY MOISTUREil2

By M R LEWlG

lrrigation Cllgilecr Divis-ion of hrigation Bnreau oj AgricuLLu1ll Engineering and Soils DeTIOrt11le-nt Oregon Agricultural Experiment Station

CONTENTS

Pnge _Page Introouction _bullbullbullbullbullbullbull__ bullbullbullbullbullbull bullbullbullbullbullbullbullbullbull _bullbull 1 Resultsbullbullbullbullbullbullbullbull ___bullbullbullbullbullbullbullbullbullbullbullbullbullbullbullbullbullbullbull__ bull 16 Otber studlesl Rate of water movement 16 Experimental procedure_ bullbull__ bullbullbullbullbullbullbull_bullbullbullbullbullbullbull_ 8 Effect ot soil type on rate or flow _ 19

SlIIDplng uppnr tus 0 Effect of grn-ity on rale or flowbullbullbullbullbullbullbullbullbull_ 20 I a Moisturecontent gradient for different rute$

Evaporation cbamber 10 or flow___bullbullbullbull___bull __ _bullbull _bullbullbullbull 21 Steady f1owbullbullbullbullbull__ __ bullbull __bull__bullbull 11 Conversion or units_bullbullbullbull_ 22

Supplementary experiments bullbullbullbullbullbullbull __bullbullbullbullbull 13 Discussion _bullbullbull_bullbullbullbull __ bullbullbull__ _bullbull__ bull_ 22 Movement as vapor bull __ _bullbullbull__ 13 Summary nocl conclusions_ 25 Ellcct ot Intermittent additlon ____ bullbull_bullbullbullbull 15 Llteruture clted_ bullbullbull_ __ bullbullbullbullbullbull_bullbullbullbullbullbullbullbullbullbullbull l7

INTRODUCTION

The rate of flow of moistWc through unsatWated soils is a problem that has attracted It grent denI of interest nmong soil investigators It is of direct importance ill fnnll operations for many reasons and is of great technicn interest because of its indirect bearing on other problems

Perhaps the question thnt has been most genemlly considered is the loss by direct evnpomtion from the surface of the soil and the effect of cultivlLtion in preventing that loss This problem has been _ attacked by both field and laboratory methods and somewhut conshytr~dictory results have been obtained

A second important problem isthnt of supply of soil moistlUe to plant roots The relative importance of the moyament of moisture through the soil to the absorbing loots and the movement of roots tlrough tbe soil to the moistlUe supply has been dabuted at conshysiderable length in technicnlliteratureat is generally recognized that w~tar is held in the soil abov~ the ulmer surface of the zone of saturntlOu (the water tnble) by capIllary force The soil zone so moistened is known as the capillary fringe Some of the earlier investigators considered the movement of moisture

I Received tor publication Jan 28 1937 J A report ot Investigations carried on under u cooperntlv~agream~nt between the Bureau of Agriculturnl

Engineering U S Department ot Agrlculture_ and tbe Oregon Agricultural Experiment Station rhe laboratory assistants nnd computers were employed by the Works Progress Administration of Oregon

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7 313-317 illus (17)







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I

I o i rltl

m COJ

i bull to 1 COJ

1 ~~ l-

CD ~

~ COJ 00middot -- r

p

a ct


(J46w) SS0110 3111~



1




















q m










xshy



















RESULTS







I lommiddot Rematks



~ Lower I V PIler

~ ~






3~0 ii-j1 I iJS t 31 Kg 32 18 27 Up 15 82-1 0










~






f


O-n 00 200 87 8 Up _ 5 I rn ti-m II

























~





















V [ shy













CONVERSION OF UNITS























~ ~---- -







u d

j







rntes~




(nPublished dntn









6 0-
























LITERATURE CITED





























7 313-317 illus (17)







~l




(22) HAINES B W








(26) ISRAELSEN O W










(33) MCGEE W J















I






62 1-31 illus (46)

























HAHltY L BHOWN



W T 8TOCKIHItClEH


MASTIN G VHIT~]




30



1




















q m










xshy



















RESULTS







I lommiddot Rematks



~ Lower I V PIler

~ ~






3~0 ii-j1 I iJS t 31 Kg 32 18 27 Up 15 82-1 0










~






f


O-n 00 200 87 8 Up _ 5 I rn ti-m II

























~





















V [ shy













CONVERSION OF UNITS























~ ~---- -







u d

j







rntes~




(nPublished dntn









6 0-
























LITERATURE CITED





























7 313-317 illus (17)







~l




(22) HAINES B W








(26) ISRAELSEN O W










(33) MCGEE W J















I






62 1-31 illus (46)

























HAHltY L BHOWN



W T 8TOCKIHItClEH


MASTIN G VHIT~]




30






xshy



















RESULTS







I lommiddot Rematks



~ Lower I V PIler

~ ~






3~0 ii-j1 I iJS t 31 Kg 32 18 27 Up 15 82-1 0










~






f


O-n 00 200 87 8 Up _ 5 I rn ti-m II

























~





















V [ shy













CONVERSION OF UNITS























~ ~---- -







u d

j







rntes~




(nPublished dntn









6 0-
























LITERATURE CITED





























7 313-317 illus (17)







~l




(22) HAINES B W








(26) ISRAELSEN O W










(33) MCGEE W J















I






62 1-31 illus (46)

























HAHltY L BHOWN



W T 8TOCKIHItClEH


MASTIN G VHIT~]




30







RESULTS







I lommiddot Rematks



~ Lower I V PIler

~ ~






3~0 ii-j1 I iJS t 31 Kg 32 18 27 Up 15 82-1 0










~






f


O-n 00 200 87 8 Up _ 5 I rn ti-m II

























~





















V [ shy













CONVERSION OF UNITS























~ ~---- -







u d

j







rntes~




(nPublished dntn









6 0-
























LITERATURE CITED





























7 313-317 illus (17)







~l




(22) HAINES B W








(26) ISRAELSEN O W










(33) MCGEE W J















I






62 1-31 illus (46)

























HAHltY L BHOWN



W T 8TOCKIHItClEH


MASTIN G VHIT~]




30






I lommiddot Rematks



~ Lower I V PIler

~ ~






3~0 ii-j1 I iJS t 31 Kg 32 18 27 Up 15 82-1 0










~






f


O-n 00 200 87 8 Up _ 5 I rn ti-m II

























~





















V [ shy













CONVERSION OF UNITS























~ ~---- -







u d

j







rntes~




(nPublished dntn









6 0-
























LITERATURE CITED





























7 313-317 illus (17)







~l




(22) HAINES B W








(26) ISRAELSEN O W










(33) MCGEE W J















I






62 1-31 illus (46)

























HAHltY L BHOWN



W T 8TOCKIHItClEH


MASTIN G VHIT~]




30








f


O-n 00 200 87 8 Up _ 5 I rn ti-m II

























~





















V [ shy













CONVERSION OF UNITS























~ ~---- -







u d

j







rntes~




(nPublished dntn









6 0-
























LITERATURE CITED





























7 313-317 illus (17)







~l




(22) HAINES B W








(26) ISRAELSEN O W










(33) MCGEE W J















I






62 1-31 illus (46)

























HAHltY L BHOWN



W T 8TOCKIHItClEH


MASTIN G VHIT~]




30






















~





















V [ shy













CONVERSION OF UNITS























~ ~---- -







u d

j







rntes~




(nPublished dntn









6 0-
























LITERATURE CITED





























7 313-317 illus (17)







~l




(22) HAINES B W








(26) ISRAELSEN O W










(33) MCGEE W J















I






62 1-31 illus (46)

























HAHltY L BHOWN



W T 8TOCKIHItClEH


MASTIN G VHIT~]




30








V [ shy













CONVERSION OF UNITS























~ ~---- -







u d

j







rntes~




(nPublished dntn









6 0-
























LITERATURE CITED





























7 313-317 illus (17)







~l




(22) HAINES B W








(26) ISRAELSEN O W










(33) MCGEE W J















I






62 1-31 illus (46)

























HAHltY L BHOWN



W T 8TOCKIHItClEH


MASTIN G VHIT~]




30





CONVERSION OF UNITS























~ ~---- -







u d

j







rntes~




(nPublished dntn









6 0-
























LITERATURE CITED





























7 313-317 illus (17)







~l




(22) HAINES B W








(26) ISRAELSEN O W










(33) MCGEE W J















I






62 1-31 illus (46)

























HAHltY L BHOWN



W T 8TOCKIHItClEH


MASTIN G VHIT~]




30










~ ~---- -







u d

j







rntes~




(nPublished dntn









6 0-
























LITERATURE CITED





























7 313-317 illus (17)







~l




(22) HAINES B W








(26) ISRAELSEN O W










(33) MCGEE W J















I






62 1-31 illus (46)

























HAHltY L BHOWN



W T 8TOCKIHItClEH


MASTIN G VHIT~]




30











6 0-
























LITERATURE CITED





























7 313-317 illus (17)







~l




(22) HAINES B W








(26) ISRAELSEN O W










(33) MCGEE W J















I






62 1-31 illus (46)

























HAHltY L BHOWN



W T 8TOCKIHItClEH


MASTIN G VHIT~]




30



















LITERATURE CITED





























7 313-317 illus (17)







~l




(22) HAINES B W








(26) ISRAELSEN O W










(33) MCGEE W J















I






62 1-31 illus (46)

























HAHltY L BHOWN



W T 8TOCKIHItClEH


MASTIN G VHIT~]




30



~l




(22) HAINES B W








(26) ISRAELSEN O W










(33) MCGEE W J















I






62 1-31 illus (46)

























HAHltY L BHOWN



W T 8TOCKIHItClEH


MASTIN G VHIT~]




30








62 1-31 illus (46)

























HAHltY L BHOWN



W T 8TOCKIHItClEH


MASTIN G VHIT~]




30










HAHltY L BHOWN



W T 8TOCKIHItClEH


MASTIN G VHIT~]




30



Rate of Flow of Capillary Moisture - AgEcon...

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Transcript of Rate of Flow of Capillary Moisture - AgEcon...