Estimation of Soil. Moisture Under Corn

Transcription

1 Estimatin f Sil Misture Under Crn by R. H. Shaw Department f Agrnmy AGRICULTURAL AND HOME ECONOMICS EXPERIMENT STATION IOWA STATE UNIVERSITY f Science and Technlgy RESEARCH BULLETIN 520 DECEMBER 1963 AMES, IOWA

2

3 CONTENTS Summary..._._.._._...._ Intrductin _.... _......_.._ Methds _.._ _ Field capacity._......_ Wilting pint._...._ Estimating runff _. 971 Evapratin and evaptranspiratin lss _. 972 Summary f steps required in cmputing sil-misture balance 977 Results and discussin A pril-j une p el'id _._...._ June-August perid... _......_._ _. 978 August-N vember perid..._ _.. _..._. 979 Cnclusins....._ _.._ Literature cited..._......_.._.._ _.._.... _._.980

4 SUMMARY This study was undertaken as an attempt t develp a simplified apprach t a cmplex prblem-the predictin f sil misture under crn which, at times, may have a limited misture supply. Sil misture under crn was estimated fr June, early August and early Nvember by using April, June and August sil-misture survey data as the starting pints. All cmputatins were made n the basis f the amunt f plantavailable water present. T btain this value, bth the field capacity and the wilting pint f a sil need t be knwn. Precipitatin amunts were added t the data fr sil misture supply after deducting a value fr runff. An antecedent precipitatin index, which 'Varied with the seasn, was used t cmpute runff. Fr the April-June perid, evapratin was estimated as 0.1 inch per day fr as lng as any available water was present in the tp 6 inches. After early June, evaptranspiratin was estimated by using pen-pan evapratin data as the measure f the ptential fr evapratin. This was multiplied by a factr t accunt fr crp develpment and, when necessary, by a factr that cnsidered any misture stress present. The prduct f these values gave the actual evaptranspiratin fr each day. The water used in evaptranspiratin was remved frm the shallw depths early in the seasn, with a gradual increase' in the depth f extractin as the seasn prgressed. By Aug. 1, water use was assumed t take place t a depth f 5-feet. Water use was apprtined amng the different depths acti've in water absrptin, with the largest percentage cming frm the shallw depths. Fr the April-June perid, the crrelatin between the bserved and the predicted sil misture was 6. The linear regressin was Y = X where the bserved sil misture was X, and the predicted sil misture, Y. The standard deviatin frm regressin was 0.70 inch. The prcedure underestimated the sil misture in the tp ft by an average f 0.13 inch and verestimated the amunt in the secnd ft by 0.16 inch. The three deepest ft-increments were estimated t be 0.10 inch r less frm the bserved value.. Fr the June-August perid, the crrelatin between the bserved and predicted sil misture was 5. The linear regressin was Y = X. The standard deviatin frm regressin was 0.84 inch. The estimated sil misture fr each ftincrement averaged 0.10 inch r less frm the bserved value. Fr the August-Nvember perid, the crrelatin between the bserved and predicted sil misture was 6. The linear regressin was Y = X. The standard deviatin frm regressin was 0.85 inch. The average difference between the bserved and estimated sil misture in each ft was less than 0.10 inch. 968

5 Estimatin f Sil Misture Under Crn 1 by R. H. Shaw 2 Althugh numerus references can be fund in the literature relating t different methds f predicting evapratin and evaptranspiratin, there has been relatively little research n predicting sil misture under a rw crp such as crn. This study was an attempt t develp a simplified apprach t a cmplex prblem-the predictin f sil misture under an annual rw crp which des nt give a cmplete grund cver, where the sil surface is smetimes dry and where sil misture may at times be limiting. As a further cmplicatin, it was als necessary t estimate runff. In selecting the "best" methd t estimate ptential evaptranspiratin,3 ne is frequently cnfrnted with a paucity f infrmatin relating these methds t accurate water-lss data. Sme f the predictin methds appear t have general applicatin; thers are extremely limited in use. The accuracy f the Blaney-Criddle (1), Thrnthwaite (12) and Penman (9) methds has been checked against Iwa pnd and pen-pan evapratin (15, 16), and the Thrnthwaite and Penman methds als have been checked against evaptranspiratin frm a meadw cver (2). Bth the Thrnthwaite and Blaney-Criddle methds prvide a simplified apprach fr estimating evaptranspiratin, but these methds have a cnsiderably reduced accuracy fr shrt-perid bservatins. Penman's methd requires cnsiderably mre data, sme f which is available at nly a few weather statins, and requires mre cmputatin time than the ther methds. The Penman methd, hwever, was fund t be the mst accurate f the methds tested (2, 9). 1 Prject 1276 f the Iwa Agricultural and Hme Ecnmics Experi. ment Statin, U.S. Weather Bureau cperating. This wrk was par tially supprted under Weather Bureau Cntract Cwb The authr wishes t acknwledge the wrk dne by Mr. Stan Buss, which laid the fundatin fr this study. This cmputatin has nw been prgrammed fr the IBM 7074 by the Iwa State University Cmputatin Center., Prfessr f agricultural climatlgy. Department f Agrnmy. 3 Ptential evaptranspiratin has been defined by the Cmmissin fr Agricultural Meterlgy f the Wrld Meterlgical Organiatin as the amunt f water vapr that evaprates frm the sil-air interface and frm plants when the sil is at field capacity. In sme definitins this is expanded t require an actively grwing crp which cmpletely cvers the glound surface. In develping a technique fr predicting silmisture changes, it was believed that the methd shuld be relatively simple t use but still as accurate as pssible. Results f an earlier study (2) shwed that sil misture culd be predicted as accurately by using Class A evapratin-pan data fr the evapratin ptential as by using Penman's equatin. Evapratin-pan data are recrded at relatively few weather statins, but these data are available fr a perid f years in the climatlgical recrds (13). The evapratin-pan netwrk in Iwa and the immediately surrunding area was dense enugh s that penpan evapratin culd be evaluated fr different parts f the state. Evapratin _ pan data prvide an evaluatin f the meterlgical factrs causing evapratin-that is, the evapratin ptential. Hwever, evapratin pans have a different type f surface than des a crp cver and, at times, represent a wet area surrunded by a large, relatively dry area. The pan data used here were nt adjusted fr surface temperature. This may accunt fr sme f the variatins fund between years.. The study reprted here was cnducted by using the evapratin-pan data as the measure f the evapratin ptential, but the study culd have utilied any f the methds available fr estimating evapratin ptential. The sil-misture data used t check the accuracy f the sil-misture predictin methd were btained frm the Iwa state sil-misture survey, which has usually been cnducted fur times a year since its inauguratin in These surveys prvide sil-misture data fr April, June, August and Nvember and prvide starting and final sil-misture values fr the perids April t June, June t August and August t Nvember. The range f years and lcatins gives a brad sample f Iwa weather cnditins and sil types (fig. 1). The survey sites have little r n slpe but are lcated s that surface water will drain 969

6 PRINCIPAL SOil ASSOCIATION AREAS OF IOWA _ Abupt Bundary ilil Grada.tinal. Bundary -- - Tentative Bundary EXPERIMENTAL FARMS A FARMER'S FIELDS fig. 1. Lcatin,f sil misture survey sampling sites. away frm the area. Except fr very wet perids there is n' standing water n the sites. Lw 'spts were purpsely avided t remve this prblem as much as pssible. On ccasin, since the survey was started, standing surface water has ccurred and water tables have been fund in the 5-ft prfile. In.cnsidering the accuracy f a sil misture predictin methd, it is imprtant t evaluate the sampling errrs invlved in the data. On each date at which sil-misture samples were taken, six brings were made by l-ft increments frm the surface t a depth f 5 feet in a 40 by 40 ft area. The available sil misture was determined frm these samples. Extensive samples taken in a variable glacial till sil in central Iwa (11) were used t estimate the sampling errr in the misture survey. Values f the standard errr f the mean are presented in table 1. These values are believed t represent values near the upper limit f 'Variability that wuld be encuntered n mst sils. Althugh the upper layers have a higher mean sil misture, they are mre unifrm and have the lwest standard errr. On sme f the less sils in Iwa, it is believed that the errr fr each ft is near that fr the tp ft shwn in table 1 and that the errr fr the 5-ft prfile wuld be cnsiderably less -pssibly near 0.5 inch. Table 1. Standard errr f the mean sil misture based n six sam pies per plt in a glacial till sil, Webster silty clay lam, Ames, Iwa, Standard errr f the Depth (feet) mean (inches) , , :' All available data were carefully checked t eliminate bservatins that had bvius errrs r in which factrs ther than thse cnsidered here may have entered int the results. In a few areas, pnding f water ccurred in very wet perids, r undergrund water mvement may have influenced the results. In sme instances, changes in sil misture between different sampling dates appeared t be physically impssible. These errrs are part f the sampling errr, and such sites were mitted when testing the predictin technique. At a few sites, rainfall was recrded a few miles frm the plts. If the Iwa climatlgical data indicated that the ttal rainfall fr any test mnth shwed a nticable gradient in the area f a particular site, this site was mitted. These sites were used when a gradient was nt present, but they usually shwed mre variability between actual and predicted values than did thse sites where rainfall was recrded at the sampling site. Fr each test perid, a sil-misture sample was used as the starting pint. All values were cnverted t inches f plant-available water by subtracting the misture at the wilting pint frm the misture btained at the sampling date. This gave the available water in each ft f the 5-ft prfile. The estimate fr the end f the perid, as btained by the predictin technique, was cmpared with the actual sil misture at the end f the perid t check the accuracy f the technique. The technique varied smewhat between the perids because f the different factrs used in cmputing the water balance. METHODS Several factrs must be cnsidered in estimating sil misture. The field capacity f the sil must be estimated. This is nt the saturated cnditin f the sil, when all air and capillary spaces are filled with water, but is the cnditin existing after a saturated sil is permitted t drain fr a few days until nly the capillary spaces are filled. The wilting pint must als be estimated, since all cmputatins are dne n the plant-available water. The runff must be predicted t estimate the amunt f water that entered the sil. A means fr estimating ptential evaptranspiratin is necessary, and adjustments must be made fr stage f crp develpment, fr dryness f surface sil and fr sil-misture limitatins fr the plants. Field Capacity A field-capacity value must be set t determine hw much water can be held in each ft f the 5-ft prfile. Since there is n satisfactry labratry methd t determine this value,

7 it is best determined in the field. Fr sme sils, experiments were cnducted t determine field capacity. Fr thers, the value used was based n the survey data. Field sampling has shwn that this value will vary with the seasn, prbably because f temperature effects. In this study, field capacity was cnsidered t be the upper limit f the sil misture measured in the field (excluding the April survey) except fr certain special cnditins. Exceptins included the presence f a water table r the lack f time fr precipitatin t perclate thrugh the prfile. Water which perclates int the sil des nt immediately mve thrugh the prfile. Fr this study it was assumed that perclating water takes' 3 days t mve thrugh the prfile and that the water abve field capacity is remved at the rate f ne-third f the amunt each day. Fr example, if the entire prfile was abve field capacity, excess water frm rains within 3 dars was cnsidered t perclate ut f the prfile at the rate f ne-third f the amunt fr each day after the rains. If nly the upper layers were abve field capacity, the excess water resulting frm recent rains perclated t deeper depths at the same rate. In all cases it was assumed that a given ft-increment must have reached field capacity befre any water: mved thrugh it. An adjustment fr recent rains was used nly t accunt fr precipitatin just befre sampling. At ther times, water in excess f field capacity was assumed t have immediately perclated thrugh the prfile r saturated layers. Althugh nt stri.ctly valid, this prcedure simplified the cmputatins. On the April sampling date, misture in the tp ft was frequently higher than the estimated field-capacity value. In this case, this value was set as the revised field capacity fr the April June perid nly, after allwance was made fr recent rains t perclate t deeper depths. This cnditin was usually a result f cld sil temperatures and, in a few cases, frst layers, which may be encuntered in early spring. Sampling shuld be delayed until the sil has been frst free fr several days. When free water was fund in the prfile at the start f any perid, an accurate predictin f sil misture culd nt be made. The disappearance f free water is affected by factrs ther than thse cnsidered here. The nly situatins that shwed any cnsistency were thse when free water was present in June and when n perclatin had ccurred thrugh the tp ft fr at least 4 weeks befre the August sampling. Under these cnditins! the usual fieldcapacity values culd be used In August. In the data presented here, all sites were mitted where free water was present in the prfile n the first sampling date fr the perid. Wilting Pint The wilting pint was set equal t the 15- atmsphere percentage fr all sils. Field sampling f dry sils indicated that this gave a very clse apprximatin f the misture value t which plants culd reduce sil misture. Estimating Runff Runff must be cmputed fr each daily precipitatin amunt. Khler and Linsley (5) have presented a graphic estimatin f runff,?ased n watershed data, in the frm f a hyper-dimensinal diagram shwing the relatinship between runff and several runff-prducing factrs. Using this infrmatin fr the late June perid and strm duratin er, Buss and Shaw (2)?evelped fig. 2, which gives runoff. a;; a. fuij;ctlon f rainfall and the antecedent preclpltation mdex (API). The antecedent precipitatin index was given as where p. is the amunt f precipitatin that ccurred i days prir t the day being cnsidered, and d i is the cn"espnding number f days... Buss (2) develped this methd frm 1954 data. He fund that if the tp 3 feet, r mre, were at field capacity, a crrectin fr excess runff was necessary. An examinatin f several years' data indicated that this ccurred during certain seasns when heavy rainfall amunts were measured. During summer mnths f wet years, use f fig 1 resulted in a cnsistently high silmisture alue when rains ccurred. T avid vercrrectin fr small rains, and t allw fr greater runff frm heavy rains, the precipitatin index was mdified t allw greater runff fr rains f 1 inch r mre. Fr all rains f r r--r--r_-,--.---r--,---, iii 3.0ll t--n u =- I Z 'S"7"-;7"'-1 ::I! u. '" u. l.l bij;...f;7"""7 1 II: PRECIPITATION AMOUNT (INCHES) Fig.1. Predictin f runh frm precipitatin and antecedent precipitatin index (after Buss and Shaw, 2). )( 1&1! 971

8 inch r mre, half f the precipitatin amunt was added t the index fr the day having mre than 1 inch f rainfall. This prcedure gave the revised index API = P 1/d1 + Pdd Pdd, + P/2 (2) where P is the precipitatin amunt fr which runff was being cmputed. The P term was used nly when the precipitatin was 1 inch r greater. This term was er if the precipitatin was less than 1 inch. On subsequent days P/2 was carried in the expressin as Pi. This revisin culd have been accmplished by revising fig. 2, but, since the index is empirical and the revised index is used nly part f the year, the abve mdificatin was cnsidered the simplest t make. Equatin 2 was used t predict runff in the spring when the grund is bare r cver is sparse and in the summer when high-intensity rains are expected t ccur. After Aug. 31, the cmbinatin f a gd crp cver and lw-intensity rains was assumed t result in less runff, and runff was cmputed accrding t equatin 1. Evapratin and Evaptranspiratin Lss The prcedure used t predict the water vapr lss depends upn the time f the seasn and the stage f crp develpment. April thrugh June 6 4 Th:e grund cnditin during the spring perid may vary; The residue frm a previus crp, fr example, may still remain n the surface f unplwed grund, a meadw crp may nt yet be plwed under, the surface may have been plwed r grund recently planted t crn may have a very sparse grund cver. Therefre, all water lss was assumed t take place frm the tp 6 inches f the sil, whether by evapratin frm the sil r by transpiratin thrugh the small plants. Under meadw existing fr a shrt time early in the perid this assumptin is nt entirely crrect. Hwever, because vapr lss at this time is largely frm the tp 6 inches and the cmputatin is much simpler, this assumptin was made. The available water in the tp ft at the start f the perid was assumed t be evenly distributed. If held in an uneven amunt, the extra 0.1 inch was put in the tp 6 inches. Slar radiatin is relatively high during much f this perid (14). The availability f water fr evapratin is believed t be the prime factr that limits water lss. As a first apprximatin, water lss by evapratin and transpiratin was This date cincides with the end f B standard climatlgical week, where week 1 is March assumed t average 0.1 inch per day. Water in the tp 6 inches f the sil was assumed t be lst at the rate f 0.1 inch per day, and meterlgical factrs affecting water lss were ignred. On clear days, there is a large amunt f energy available fr evapratin. As lng as capillary films can supply misture t meet the evaprative demand, evapratin is limited by the energy available. Once these films cannt supply adequate misture, the availability f misture becmes the limiting factr. Lemn (6) stated that this cnditin ccurs as a bare-field sil appraches field capacity. Philip (10) shwed that this ccurred abut 3 days after the sil had been saturated. Penman (8) has demnstrated that, the lwer the evaprative demand, the lwer is the misture cntent at the critical pint. On cludy days, energy may be the limiting factr. The use f 0.1 inch lss per day gave excellent results fr the perid cncerned, and n mdificatin was attempted. This rate, which seems realistic fr Iwa fr this perid, may have t be mdified fr ther areas. June 7 thrugh Sept. 30 As the crn plant grws, cnsiderable change takes place in the grund cver prduced by the crp. June 7 was selected as the date when the predictin technique was changed fr tw reasns: (1) A nticeable change is taking place in the rati f evaptranspiratin t pen-pan evapratin at this time, and (2) it is the start f a week in the standard climatlgical year. Cmmencing June 7, pen-pan evapratin was used as the starting pint fr estimating evaptranspiratin. Data fr Iwa and surrunding states were pltted, and islines f the average daily pan evapratin in inches were drawn fr each week. A typical example is shwn in fig. 3. Althugh daily values might have been mre cn duly Fig. 3. Map shwing islines f average daily pen pan evapr. tin in inches per day, fr week f July 12.18, 1960.

9 .9!i ll:.8 <t > w rt.7 I W Q i= <t ll: ;;:.5 '" <t ll: >-.4 :! w!2 >-. <t a: 3 2 I-' Z <t J "- Ol 15 MAY,./,/ / / V 15 JUNE II v / 15 JULY./ i\ 15 AUG. 9 B 7 1\ 6 \ 15 SEPt 5 ". I 4 Fig. 4. Rati f evaptranspiratin f crn t pen pan evapratin thrughut the grwing seasn (after Denmead and Shaw, 3). On the average, 50 percent f the COrn in Iwa is silked by July 31. cise, an average value f daily evaptranspiratin was cmputed fr each weekly perid t facilitate cmputatin. The weekly average f daily pan evapratin fr each sampling site was read frm the isline map, and this value then was multiplied by a factr t adj ust evaptranspiratin t the prper stage f crp develpment. This factr was btained frm fig. 4. The curve in fig. 4 represents cnditins where surface misture may limit evapratin but misture in the rt ne des nt limit evaptranspiratin (3). The calendar dates used n this figure represent the average date f ccurrence in Iwa f the phenlgical events shwn. Fr very early r late crp develpment, the calendar dates shuld be adjusted t fit the phenlgical dates. The prduct f these tw values (pan evapratin in inches x crp develpment factr in percent) gives the evaptranspiratin in inches when sil misture is nt limiting. Data btained by Denmead and Shaw (4) shw that, under cnditins f high atmspheric demand, transpiratin frm a plant will decrease at a relatively high sil-misture cntent because the plant cannt supply water fast enugh t meet the high demand. At lw atmspheric demand there will be n reductin in water lss until a relatively lw sil-misture cntent is reached. The Denmead and Shaw data were cllected by using lysimeters, which restricted rt develpment, and the data represent nly transpiratin. In the field frm June t August, the rt ne is cntinually advancing, 2 representing a stress cnditin different frm that in a lysimeter, and the water lss in the field is bth by evapratin and transpiratin. Three curves were selected frm Denmead and Shaw's (4) data t represent different stress days. The high-stress curve used represents a transpiratin rate f 0.22 inch per day. Days when pan evapratin was abve 0.30 inch were classed as high-stress days. Days when pan evapratin was between 0.20 inch and 0.30 inch were classed as average, and the curve used represents a transpiratin rate f 0.16 inch per day. Days when pan evapratin was less than 0.20 inch were classed as lw-demand days, and the curve used represents a transpiratin rate f 0.08 inch per day. These three curves are shwn in fig. 5. The percent f available water in the rt ne was used fr entry n the hrintal axis. The relative evaptranspiratin rate was read frm the vertical axis. Data fr Ames fr a 23-year perid shw that these pan-evapratin values have ccurred at the frequencies shwn in table 2. These frequencies wuld vary widely amng different climates. Rting depth was cnsidered t be as fllws: t June 27, 2 feet; t July 4,21;2; t July 11, 3; t July 18, 31;2; t July 25, 4; t Aug. 1, 41;2; and after Aug. 1, 5 ft. Because f the sparse plant cver and the relatively lw rati f evaptranspiratin t pen-pan evapratin, n stress was I- 100 '" 0 a: '" 0..; 80 a: ;:: 60 a: 0: '" II: 40 I '" 20 ;::.. J '" a: '" HIGH - '\ \ \ UlW!AvER... \ '\ [\..., li "'1 g 100, '" '" FIELD 15 ATMOS. CAPACITY PERCENT AVAILABLE SOIL MOISTURE PERCENTAGE Fig. 5. Relative evaptranspiratin rates fr different atmsphric demand rates prir t Aug. 1. ET is actual evaptranspira. tin, ETO is pen pan vapratin (after Denm.ad and Shaw, 4). Table 2. Percent ccurrence f daily pen pan evapratin at Ames, Iwa in high, medium and lw classificatins during June, July and August, Percent f ttal days in each mnth when pan.evapratin ccurred nt specified levels. Level f daily pan eva.pratin June High (> 0.30") Medium ( ") Lw «0.20") July August

10 Table 3. Field capacity and available sil misture n July fr Castana. Iwa. Values given are fr the tp 5 feet f sil. by l ft increments. Field Available misture Depth capacity On July 18 (ft.) (inches) (inches) assumed t ccur befre June 27 as lng as any water was available in the rt ne. The available sil misture was determined as a percentage f field capacity in the rt ne fr each week. T determine the percentage f field capacity fr a given week, the amunt f water in the prfile t the depth t which rts advanced during the week was used. Fr example, fr the perid July 18-25, rts advanced t 4 feet (table 3). The percentage f field capacity fr this week was: = - = 40 percent T simplify the cmputatins, the value cmputed in this manner was used as the percentage f field capacity available fr the entire week. The daily average pen-pan evapratin was 0.32 inch; therefre, the high atmspheric stress curve was used. Frm fig. 5, the relative evaptranspiratin was fund t be 58 percent, since nly 40 percent f field capacity was present. Evaptranspiratin per day, then, is equal t: Pan evapratin (fig. 3) x rati fr crp develpment (fig. 4) x stress factr (fig. 5) = 0.32 x 0.78 x 0.58 = 0.14 inch, instead f the 0.25 inch that wuld have resulted if n stress were present. Rts were assumed t reach a depth f 5 feet by Aug. 1. Since little further penetratin wuld ccur, a different effect f atmspheric stress appeared evident. A different set f curves (fig.!;; 100 "' U 0: IL "',.j BO t II: '" ii 60 0: 0:.. :i :: > 20 ;::: a 0:. IQ. 90 FIELD CAPACITY K r-.- '" 1'\ -r i'- " "" '" '" '" HIGH "- " \ LOW IUM \ "" \ "- \.. '\ ::::::: BD "'."' 15ATMOS. PERCENT AVAILABLE SOIL MOISTURE I GO PERCENTAGE Fig. 6. Relative evaptranspiratin rates fr different atmspheric demand rates after Aug. 1. ET is actual evaptranspiratin, ETO is pen pan evapratin (after Denmead and Shaw. 4) ) was used after July 31. Use f these curves resulted in a greater reductin in evaptranspiratin fr a cmparable stress, cmpared with the perid befre Aug. 1 when the rts were advancing. Water was assumed t be remved frm each ft-increment in the pattern shwn in table 4. When an increment f sil did nt have available water, the water that wuld have been used frm that increment was mved t the ther depths frm which water was being extracted. When an increment f sil ther than the tp ft had n available water, the amunt was prrated amng the ther depths frm which water was being extracted. When the tp ft had n available water, the extractin pattern was shifted ne depth deeper. The amunt nr"'; mally extracted frm the deepest active depth was divided equally amng the ther active depths. Fr example, frm July 12 t 18, the water use wuld nrmally be 60, 15, 15 and 10 percent frm the 1st, 2nd, 3rd and 4th ftincrements, respectively. With n available water in the tp ft, the amunts wuld be 0, 60 plus 1/3 f 10 percent, 15 plus 1/3 f 10 percent and 15 plus 1/3 f 10 percent, r, with runding 0, 63, 18 and 18 percent frm the 1st, 2nd, 3rd and 4th ft-increments, respectively. Anther factr had t be cnsidered fr days when the transpiratin lss was reduced because f misture stress. If recent rains had added water t the sil, higher evapratin wuld be taking place. As lng as this water was present in the tp 6 inches, up t 0.1 inch per day evapratin culd take place. Fr example, n the day when the ptential rate was 0.25 inch, lss wuld be 0.14 inch if n available water were present in the tp 6 inches, but lss wuld be 0.24 inch ( ) if water were available. The amunt f water lst by evapratin culd nt bring the ttal amunt lst abve the ptential rate. Table 4. Water extractin frm the sil prfile at different depths during the grwing seasn. Values fr each date are given as the percentage f evapratin r evaptranspiratin that OCCUrS frm each f the depths listed. Percent f E r ET Dates which cme. frm Depths frm which respective depth. water was extracted t June t 6 inches June 8 t st ft (equally frm each 6 inches) June 15 t st. 2nd ft June 28 t July , 20 1st. 2nd and tp half f 3rd ft July 5 t ,20 1st. 2nd and 3rd ft July 12 t , 10 1st. 2nd. 3rd and tp half f 4th ft July 19 t st. 2nd. 3rd and 4th ft July 26 t Aug , 60, After Aug , st. 2nd. 3rd. 4th and upper half 5th ft lob 1st, 2nd. 3rd and 4th. ft 10, 10, st, 2nd. 3rd. 4th and 5th ft O ht.?"i1. 3rd and 4th ft U.ed nly if first 4 feet all have < 60 percent available misture. b Used if any f first 4 feet have> 50 percent available misture; hpwever. after Aug. 1. the percent available is always cmputed On the ttal available water in the 5 ft prfile.

11 Table 5. Cmputatin sheet fr April t June sil misture, sampled April 9 and June 8, Ames, 1955 (all values are in acre inches}. Available misture by climatlgical weeks Depth (feet) Field capacity Apr. 98 Apr. 11 Apr. 18 Apr. 25 May 2 MaY 9 MaY 16 May 23 May 30 0-% _ O.S % O.S 0.4 O.S 0.7 O.S ) Water Balance Date pb R E Date PRE Date PRE Date PRE Date P R E Date PRE Date PRE Date PRE Dat Ap,. Apr. Apr. Apr. May May May May May " Jun d :1 May IS" O' G (0.6" perc.)f April 9 misture in tp 6 inches was 0.8/2 = 0.4 inch. b P = precipitatin, R = runff and E = evapratin. "N runff n April 13 = net gain f 0.8 inch. Tp 6 inches had 0.3" n 11th, 0.4" n 12th because f gain f 0.1. On 13th evapratin f 0.1" reduces this t 0.3" s it will hld 0.6" mre, bringing it t "; remaining 0.3" added t next deeper layer. d API = 0/1 + 0/2 + 0/ / /9 + /10 + /2 = Frm fig. I, at intersectin f inches f rainfall and API f 0.87, runff = 0.2". Misture in tp 6 inches was 0.3" after evapratin n 23rd, and 0.6" was used t bring it t field capacity. Of the 0.6" which perclated thrugh, the tp secnd 6" t field capacity and 0.4" moves t next layer. 24th t bring secnd ft up t " available water. April 18, misture in tp 6 inches was reduced fr n 18th. t Gain n MaY 9 was greater than capacity f sil; 0 g N evapratin this day because evapratin was su tin, and nne was available befre precipitatin ccurred....::j c:tt

12 (0 :! Table 6. Sample cmputatin sheet fr June t late July sil misture, sampled June 4 and July 31, Nrthwest Iwa Experimental Farm, 1958 <an values in acre Depth (feet) Field capacity June 4 Available misture by climatlgical weeks June 6 June 13 June 20 June 27 July 4 July 11 July 18 Av Ju -.,..._.._..._ ,-l _..._ _ n _ _..._..._ 2.n _ n n n O Wafer aalance Date p R ETb Date PRETe Da.te P R Erd Date PRETe Date P R Ert Date P R ETC Date P R ETh Date June June June June J'une July J'uly July July O.S p = precipitatin, R = runff, ET = evaptranspiratin. bet fr this week Was assumed t be O.ln /day. ET = average daily pan-evapratin x crp COver factr = 0.24 x 0.41 == 0.10." N stress present. d ET fr week = 0.27 x 0.45 = 0.12; 0.12 x 7 = 0.84" which, with runding is O.S." If balance is cmputed fr each day, subtract 0.2" fr first day and 0.1" On last 6 days t allw fr ET f 0.8"; week. Of the water used, 2/3 cmes frm tp ft, equally frm each 6-ineh increment, and 1/3 frm 2nd ft. ET == 0.22 x 0.5 = 0.11; 0.11 x 7 = 0.77 = 0.8. Use frm tp 2 feet, same as in ftnte c. 0.4" perclates t 2nd 6 inches f tp ft frm rains n June and 24. t Rt develpment t July 4 is 2l1. ft. Percent available misture = ( ) 3.5 == = 64 pereent. Divide by 2 t get misture frm 2 t 2% ft. ( ) 5.5 Fr a high-demand perid (pan > 0.30"). adjusted ET 0.19" /day). Hwever. there is water available in tp 6 inch ET back t 0.21"/day. Use is and 20 percent frm Perclatin f 0.6" thrugh 1st 6 inches. r Average-demand perid and n stress, s ET ccurs 1.12; Use 60, 20 and 20 percent frm 1st, 2nd and 3rd f tp 6 inches f 0.6" n 8th and 0.2" n 9th. Perclati 0.2" n bth Sth and 9th. b Lw-demand perid. n stress x 7 days = and 10 percent frm 1st, 2nd, 3rd and 4th ft, r I Average-demand perid. Rt depth t 4 feet by Ju rt ne is 4.0/8..5 = 49 percent. ET is 92 percent f pt since water available in tp 6 inehes, use is per da.y. J Lw-demand perid. Percent availa.hle = 40 percent is 60, 15, 15 and 10 percent frm 1st, 2nd, 3rd and 4th percent available.

13 Oct. 1 and later Transpiratin was assumed t cease Oct. 1- unless terminated earlier by a killing frst r unless delayed plant develpment indicated that transpiratin shuld cntinue later. After Oct. 1, evapratin (r evaptranspiratin when it ccurred) was assumed t be 0.35 f pan evapratin. This value was estimated frm fig. 4. The energy available fr evapratin in Octber and Nvember is lw, and the ptential fr evapratin was cnsidered the best factr fr estimating the lss. Evapratin was assumed t ccur nly frm the tp 6 inches. After Nv. 1, when evapratin-pan data were nt available, evapratin was assumed t be 0.1 inch per week V I'" C 19$7 0 II It!lS... 19!i' X y II 0.4'... O.,4X f IJ 0 96 Summary f Steps Required in Cmputing Sil-Misture Balance 1) Set field-capacity and wilting-pint values fr each increment f sil and determine the plant-avail able water fr each increment. 2) Start with measured sil misture in the prfile n first date f sampling. Fr the first 3 days, cnsider excess water in prfile resulting frm recent rains. 3) Subtract evapratin, evaptranspiratin, r bth, fr each day. 4) Add daily precipitatin after adjusting fr run ff. Sample cmputatin sheets are shwn in table 5 fr the April-June perid and in table 6 fr the June-August perid. The tp part f each table shws the available misture at the end f each week, s that the prcedure fr each week can be checked. RESULTS AND DISCUSSION April.June Perid Cmparisn f bserved and predicted ttal available misture in the prfile The results btained applying this prcedure t sil-misture data fr the perid are shwn in fig. 7. Very gd agreement was btained between the bserved and predicted sil misture as shwn by a,crrelatin fr all years f 6. The crrelatin cefficients fr each year were near 0 r higher and were significant at the 1-percent level-except fr 1954 when, with nly 6 cmparisns, the crrelatin f 0.88 was significant at the 5-percent level. The linear regressin fr all years was Y = X (3) where the bserved sil misture is X and the predicted sil misture, Y. Althugh there was II OBSERVED SOIL MOISTURE -INCHES IN JUNE Fig.7. Cmparisn f bserved and estimated sil misture, inches in June, a clse relatinship between the bserved and predicted value, because f the large sample sie, 4 differs frm 1.00, and 0.49 differs frm 0, between the 5- and 1-percent levels f prbability. The standard deviatin frm regressin was 0.70 inch, which cmpares favrably t the variatin fund in the sil-misture sampling. In use, this regressin equatin wuld be used as a calibratin curve t predict an X value frm a Y value. The sil-misture value, Y, btained by the predictin technique wuld be used t btain a calibrated, r adjusted, X value frm the regressin equatin. Cmparisn f errrs in predictin in each ft-increment In sme situatins, the distributin f the sil misture within the prfile is very imprtant. Althugh the ttal misture in the prfile was generally predicted within 1 inch f the bserved misture, this des nt shw hw well the distributin within the prfile was predicted. The differences between the bserved sil misture and the predicted value fr each ft-increment are summaried in table 7. The tp ft was underestimated an average f 0.13 inch, while the secnd ft was verestimated 0.16 inch. Table 7. Average difference and standard deviatin f difference between predicted and bserved June sli misture and percentage f errrs > 1 inch and < 0.5 inch. Average Depth difference (feet) (inches) : : Std. dev. f diff. (inches) Erl'rs > 1 inch (percent) Errrs f 0.5 inch r less (percent)

14 Fr the three deeper depths, the predicted value averaged 0.10 inch r less frm the bserved value. The standard deviatin f the difference between the bserved and predicted value, was largest in the tp ft, 0.46 inch, and abut 1/3 inch fr the ther depths. There were relatively few errrs greater than 1 inch, with a high percentage f the errrs fr each ft-increment being less than 0.5 inch. Fr the 5-ft prfile, 50 percent f the errrs were 0.5 inch r less and nly 8.6 percent were ver 1 inch. Rating f predicted values An attempt was made t rate the predicted value as t distributin in the prfile and difference in the ttal amunt f misture in the prfile. Althugh arbitrary, such a rating system prvides a quick methd f examining the results. The ratings used are: 1. Each ft-increment within % inch f bserved value. 2. Fur within % inch, ne within % t 1 inch f bserved value. 3. Fur within % inch, ne mre than 1 inch different frm bserved value. 4. Three within % inch, tw within 1 inch f bserved value. 5. Three within % inch, ne within 1 inch, ne mre than 1 inch different frm bserved value. 6. All within 1 inch f bserved value. 7. Others. Each grup was further classified as t the ttal sil misture in the prfile as fllws: a) Ttal within 1 inch f bserved value. b) Ttal 1 inch r mre different frm bserved value. On the basis f these ratings, the accuracy f predicting the distributin within the prfile is shwn in table 8. Table 8. Rating f sil misture predictin fr the spring perid. Numbel' f RntinSl: ccurrences lall. 53 lb a b a...._ b a b... 0 fin._ b..., a... 4 Uh... 7a h... 0 Ratings defined in text. June-August Perid Cmparisn f bserved and predicted ttal available misture in the prfile Percent In 1954, misture samples were taken each mnth. The results are included here fr cmparisn with the results btained fr the June-August perid in ther years. Althugh the 1954 data are 978 fr nly 1 year, the results indicate the cnsistency f the predictin technique fr shrter perids within the lnger perid. The crrelatin between predicted and bserved ttal sil misture fr the June perid was 6. The linear regressin was Y = X (4) where X is the bserved value, and Y the predicted value. Fr the July perid the crrelatin was 8. The linear regressin was Y = X. (5) Only 16 cmparisns were available fr this perid, and 3 did nt test statistically different frm 1.00, nr was 0.28 different frm O. Fr the June-August perid, the crrelatins were cmputed fr each year, except 1954 which was divided int the tw perids already presented. The,crrelatins fr each yea.r were near 0 r higher and were significant at the 1-percent level. Fr all years the crrelatin was 5. The cmparisn between the bserved and predicted values fr is shwn in fig. 8. The linear regressin equatin fr all years was Y = X (6) with 4 being significantly different frm 1.00, and 0.34 significantly different frm 0, between the 5-percent and 1-percent limits f prbability. The standard deviatin frm regressin was 0.84 inch. Cmparisn f errrs in predictin in each ft-increment A summary f the differences between the bserved and predicted il misture fr each ft- 13,055. In8 D ' ;;; 9 II ::::: '" ::! 8 I '" 7 '" ::> Ii; 6 is '"..J is., 4 c '" t; 3 Ci ::! c.. Y r "0.8$ " 3 4 S II OBSERVED SOIL MO'STURE - INCHES IN AUGUST Fig. 8. Cmparisn f bserved and estimated sil misture. inches in August,

15 Table 9. Average difference and standard deviatin f diff.rence between predicted and bserved August sil misture and percentage f errrs > 1 inch and < 0.5 inch. Average Std. Dev. Errrs Errrs f Depth (feet) difference diff. > 1 inch 0.5 inch r le.s (inches) (inches) (p.rcent) (percent) Table 10. Rating f sil misture flredidin fr the summer perid. Number 01 Rating ccurrences 1aa :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 23 2b n :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 8 4b._...._ a :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 3 6h n b Katings defined in text. Percent 4" :{ 6: increment is given in table 9. The tw tp feet were estimated lw, and the three lwer feet estimated high, but all averaged 0.10 inch r less frm the bserved values. The predicted mean sil misture in the 5-ft prfile was very clse t that bserved. Errrs greater than 1 inch were few fr the individual ft increments, and 80 percent f the errrs were less than 0.5 inch in each ft. Fr the 5-ft prfile, apprximately 50 percent f the values were within 0.50 inch. Rating f predicted values Based n the same ratings used fr the spring perid, the accuracy f the predictin technique is shwn in table 10. Sixty percent f the values were in either the 1a r 2a classes, abut 12 percent less than fr the spring perid. August-Nvember Perid Cmparisn f bserved and predicted ttal available misture in the prfile In 1954, samples were taken in early August, early September, late September r early Octber, and early Nvember. Fr the perid f early August t early September the crrelatin between the predicted and bserved sil misture was 6. The linear regressin was y = X. (7) The regressin cefficient 0.85, was statistically different frm 1.00, and 0.65 was different frm at the 5-percent level f significance. 'Fr the September perid, the crrelatin between the predicted and bserved sil misture was 7. The linear regressin was Y = X. (8) ;55. I'I c 12 ItS? Q II '" '" 10 l!; 9 n... 8 I 7 w a: 6 n is '" n 4 t; 3 '" i It5' X.'&0 T03e+a92,)I '-0." II OBSERVED SOIL MOISTURE - INCHES IN NOvEMBER Fig. 9. Cmparisn f bserved and estimated sil misture, inches in Nvember, Fr the late September-Nvember perid the crrelatin between the predicted and bserved sil misture was 8. The linear regressin was Y = X. (9) Fr the years , samples were taken nly in early August and early Nvember. The crrelatin between actual and predicted sil misture was 6. Crrelatins fr individual years were all highly significant. The linear regressin (fig. 9) fr was Y = X. (10) The regressin cefficient, 2, was different frm 1.00 at the 1-percent level f prbability, and 0.38 was different frm 0 at the 5-percent level f prbability. The standard deviatin frm regressin was 0.85 inch. Cmparisn f errrs in predictin in each ft A summary f the differences between the bserved and predicted sil misture fr each ftincrement is given in table 11. All values averaged within 0.10 inch f the bserved value. The standard deviatin f the difference between the bserved and the predicted values ranged frm 0.4 inch t 0.5 inch. Mre than 80 percent f the Tab!e 11. Average difference and standard deviatin f difference between predicted and bserved Nvember sil mistu-e and percentage f errrs > 1 inch and :::; 0.5 inch. AVerage Std. dev. Errrs Errrs f Depth difference diff. > 1 inch 0.5 inch 01' les. (feet) (inches) (inches) (percent) (percent) : 8: :J : 82.8 :{ :

16 values in each ft-increment were 0.5 inch r less frm the bserved value. Rating f predicted values On the basis f the same ratings used previusly, the ratings fr the fall predictin are summaried in table 12. Abut 60 percent were in the 1a r 2a classes, which was lwer than fr the spring perid but abut the same as fr the summer perid. Table 12. Rating af sail maisture predictian fr the fan (lerid. Number f Rating ccurrences 1a& b a b a b a b a b a b... 7s.. h 2 7b... Ratings defined in the text. II Percent CONCLUSIONS Over-all, the methd f predictin develped here is cnsidered very successful. In predicting sil misture under a rw crp such as crn, it must be expected that the methd will require several steps in its cmputatin. This is because the ptential evaptranspiratin values must be mdified t take int accunt,crp cver and misture availability. The results indicated that the prcedure can be used fr a wide range f weather cnditins and may be applicable t areas ther than Iwa if the rting pattern f crn is similar. Runff amunts may vary with the sil type and particularly with the amunt f slpe. The cnstants f the regressin equatins wuld nt necessarily be the same. Further refinements culd be made in the prcedure develped here. Hwever, additinal adjustments further cmplicate the cmputatin, and the effrt expended may nt be wrth the gain in accuracy btained. 1. Blaney, H. F. and W. D. Criddle. Determining water requirements in irrigated areas frm climatlgical and irrigatin data. U. S. Dept. Agr. Sil Cns. Serv TP Buss, S. and R. H. Shaw. Predictin f sil misture under crn. Pm t II. Final reprt U. 8. Weather Bureau Cntract Cwb Department f Agrnmy, Iwa State University, Ames, Iwa. May, Denmead, O. T. and R. H. Shaw. Evaptranspiratin in relatin t the develpment f the crn crp. Agrn. Jur. 51: Denmead, O. T. and R. H. Shaw. Availability f sil water t plants as affected by sil misture cntent and meterlgical cnditins. Agrn. Jur. 45: Khler, M. A. and R. K. Linsley. Predicting runff frm strm rainfall. U. S. Weather Bureau Research Paper N. 34. Washingtn, D. C. Sept Lemn, E. R. The ptentialities fr decreasing sil misture evapratin lss. Sil Sci. Sc. Amer. Prc. 20: Nielsen, D. R. and R. H. Shaw. Estimatin f the 15 atmsphere misture percentage frm hydrmeter data. Sil Sci. 86: Penman, H. L. Labratry experiments n evapratin frm shallw sil. Jur. Agr. Sci. 31: LITERATURE CITED 9. Penman, H. L. Natural evapratin frm pen water, bare sil and grass. Prc. Ry. Sc. Lndn. Series A 193: Philip, J. R. The physical principles f sil water mvement during the irrigatin cycle. 3rd Cng Intern. Cmm. n Irrigatin and Drainage. 8: Shaw, R. H., D. R. Neilsen and J. R. Runkles. Evaluatin f sme sil misture characteristics f Iwa sils. Iwa Agr. and Hme Ecn. Exp. Sta. Res. Bul Thrnthwaite, C. W. An apprach tward a ratinal classificatin f climate. Geg. Rev. 38: U. S. Weather Bureau. Iwa climatlgical data. (Published mnthly and annually.) Waite, P. J. and R. H. Shaw. Slar radiatin and sunshine in Iwa. Iwa State Jur. Sci. 35: Ya, Yi-Ming and R. H. Shaw. A cmparisn f methds f estimating evapratin frm small water surfaces in Iwa. Final Reprt U. S. Weather Bureau Cntract Cwb Dept. f Agrnmy, Iwa State University, Ames, Iwa. May Ya, Yi-Ming and R. H. Shaw. Estimatin f evapratin frm farm pnds. Iwa State Jur. Sci. 32: