an inventory of soil water relationships on woodland, pasture, and cultivated soils1
TRANSCRIPT
Water Relationships Affecting Agronomic PracticesAN INVENTORY OF SOIL WATER RELATIONSHIPS ON WOODLAND, PASTURE, AND
CULTIVATED SOILS1
F. R. DREIBELBIS AND F. A. Posr2
RiCENT studies on soil and water conservationhave emphasized the influence of soil water on
plant growth, and the role played by plants and soilon the disposal of precipitation. The importance ofthe relationship of soil water to various forms of pre-cipitation disposal, such as runoff, percolation, andevaporation-transpiration, also has been stressed, butfew data including all these measurements have ap-peared largely because of the difficulty involved inobtaining contemporary measurements of all of thesefactors.
The hydrologic installations 011 the North Appa-lachian experimental watershed at Coshocton, Ohio,afford a good opportunity for studying soil waterconditions and their relationships to the precipitationdisposal factors mentioned above. The present reportcomprises a study of soil water conditions under dif-ferent land use practices and their relationship toprecipitation, runoff, percolation, evaporation-trans-piration, and storage in the soil.
Attention recently has been directed ( i ) 3 to theexcellent work in soil and water conservation re-search by Wollny (14) who, 50 years ago, pioneeredinvestigations in the field of hydrology in its rela-tion to soil physics. With measurements of intercep-tion, infiltration, runoff, soil moisture, and percola-tion he attempted to portray the role played by soiland by plants in the disposition of precipitation.
A joint report by the Forest Service and the SoilConservation Service (12) has summarized the morerecent studies on the influence of vegetation andwatershed treatments on various phases of hydrolo-gic activity. Of the factors affecting precipitation dis-posal, vegetal cover was recognized as the principalone that can be controlled by man. It was aptly stated£12) that "vegetation is man's strongest ally inbringing about maximum utilization instead of dis-sipation of precipitation." A number of investigatorshave pointed out that there is no accretion to the
water table as long as the water consumed by vegeta-tion exceeds the infiltration.
Transpiration has long been considered as theprimary cause of water removal from soils. Experi-mental evidence seems to indicate that a plant willtranspire the maximum amount of water when thesoil has sufficient moisture, but that an increaseabove the optimum amount will produce no furthereffect. However, when the water content has beenreduced to the wilting coefficient, or below, and theplant is badly wilted, the rate of transpiration maybe only 3 to 5% of that prevailing under optimummoisture conditions (9). Although various methodshave been used for measuring transpiration, no satis-factory method yet has appeared for measuringthe amount of transpiration under field conditions.Defects in any of the methods used could be attribu-ted either to the partial disruption of the natural lifefunctions of the plant or to artificial climatic and soilconditions.
Collison and Mensching (3) found that on a Dun-kirk silty clay loam cropped with maize, small grains,and hay, about 33% of the annual precipitation(32.52 inches) was lost through evaporation-transpi-ration and that 18% was lost through transpiration.The "water requirement" of corn during the growingseason was found by Burr (2) to vary in the sameorder as the precipitation. He also found the totalyield of dry matter both for spring wheat and cornwas almost directly proportional to the amount ofwater used during any growing season, and variedconsiderably from year to year. In some years twiceas much water was required to produce a pound ofdry matter as in certain other years.
Previous investigations reported from this stationinclude studies on soil moisture relationships (4),and a summary of hydrologic data embracing meas-urements of precipitation, runoff, percolation, andsoil moisture (n).
Contribution from the Hydrologic Division, Office of Research, Soil Conservation Service, Coshocton, Ohio.2 Assistant Soil Technologist and Junior Agronomist, respectively. The authors gratefully acknowledge their obligation to Mr.
H. S. Riesbol, formerly Hydraulic Engineer, Soil Conservation Service, Coshocton, Ohio, for releasing the data on runoff andprecipitation and for reviewing the manuscript; to Mr. W. H. Pomerene, Assistant Agricultural Engineer in charge of lysimeteroperations, Soil Conservation Service, Coshocton, Ohio, for the data on percolation; and to Dr. R. E. Yoder, Chief, Departmentof Agronomy, Ohio Agricultural Experiment Station, Wooster, Ohio, for valuable suggestions given.
8Numbers in parenthesis refer to "Literature Cited", p. 473.
462
DREIBELBIS AND POST'. SOIL WATER RELATIONSHIPS 463
EXPERIMENTAL PROCEDURE
The soil water relationships studied are classified intowater of accretion, accretion being used in the broader senseand representing any addition of moisture to the watershed,of storage, and of depletion, and all are expressed on a com-mon basis, namely, in terms of inches of water. The datawere obtained from four experimental watersheds, each ap-proximately 2 acres in area, one in woodland (No. 131),one in pasture (No. 102), and two in rotated crops (No. 109on Muskingum silt loam and No. 123 on Keene silt loam).( See Table 2.) The watersheds are all located within a radiusof approximately a quarter mile. The woodland consists of astand of second-growth mixed hardwoods and has been pro-tected from grazing. Unimproved practices were used onboth the pasture and the cultivated watersheds. On the formerthe vegetation consisted largely of poverty oatgrass, Canadabluegrass, and weeds, a stand commonly found on low-fertili-ty acid soils in this region. A more detailed description of thewatersheds was given in previous publications (4, n).
Precipitation constituted mostly, but not all,- of the waterof accretion and was measured by recording and standardrain gages located at strategic points in the experimentalarea. A central meteorological station equipped with varioustypes of rain gages, anemometers, and other weather instru-ments gave additional information pertaining to meteorologi-cal conditions. Unaccounted accretion was determined by anequation described later. Both forms of accretion were to-taled for periods beginning and ending whenever soil mois-ture was determined.
Storage of soil water was expressed in terms of activesoil water, which was previously described (4) as the differ-ence between the total soil water and the minimum amountof water held in the soil under field conditions at the driestperiod of the year. This active soil water is similar to theterm "available" water referred to by some investigators.The latter term has been avoided by various individualslargely because of the abuse this term has received inscientific literature. Total soil water was determined bysampling the soil by means of a soil auger or a King tubesampler at approximately weekly intervals to a 4O-inchdepth during a 12-month period beginning August i, 1939,and the moisture determined gravimetrically. This time cov-ered the entire period of the wheat crop grown on the cul-tivated watersheds. Four to 10 locations were sampled induplicate at each watershed. Conversion to inches of waterwas made from volume weight data previously obtained (5).
Depletion of soil water consisted of runoff, of percola-tion, and of net depletion (largely evaporation-transpiration).These depletion values were totaled for periods beginningand ending with soil moisture sampling periods.
Runoff was measured with water-level recorders in con-nection with precalibrated flumes. These instruments meas-ure quantities and rates of runoff from the drainage areasabove as it leaves the watershed. Dikes are used to directthe water around the borders of the watersheds to preventsurface inflow of water from outside sources into the experi-mental areas.
Percolation was determined from monolith lysimeters,each 0.002 acre in area and designed primarily to evaluateprecipitation disposal factors, including percolation. A batteryof three lysimeters was located adjacent to each of the
watersheds studied, except the forested area. LysimetersYIOI, YiO2, and Yi03 were adjacent to watershed Nos.102, 109, and 123, respectively. These lysimeters embodynumerous features recognized as essential for a study oftrue natural percolation and other hydrologic factors. Theprofile depth of the soil is 8 feet, the soil extends to a depthof approximately 3 feet, disintegrated rock extends 2 or 3feet below this, and the lowest 2 or 3 feet consist largely ofundecomposed parent rock. All the layers are left in theirnatural structure, and the rock rests directly on the perco-late collector pans that form the bottom of the lysimeters.Water that passes through the monolith is called percolation.Plant roots seldom, if ever, penetrate below a 4O-inch depthin this region. Steel strips serve to prevent any flowof water likely to occur between the soil block and the sidewalls of the lysimeter. Detailed descriptions of these lysi-meters were previously given (7).
The equation, Precipitation = Runoff + Percolation -f-Evaporation-transpiration ± Storage, is essentially correctfor periods of time, which may or may not include precipi-tation, that begin and end during periods of no precipitation.In the present problem the length of period varied from 6to 33 days, being usually about 7 days in length except duringthe winter when the sampling was more infrequent because ofthe difficulty involved in making the measurements. Net de-pletion was determined from the equation, D = P — R — G —{Si—Si), when the resultant quantity is positive and whereD = net depletion, largely evaporation-transpiration to theatmosphere; P = precipitation, including rainfall, snowfall,sleet, and hail, but not fog or dew; R = runoff as measuredthrough a flume at the outlet of a small watershed, the ex-perimental watersheds referred to in this work ranging from1.37 acres to 2.21 acres in area; G = water of percolationwhich may be defined as gravitational water that percolatesthrough an 8-foot profile as measured from the lysimeterspreviously referred to (7) ; Si = active soil water determinedfrom samples obtained at the beginning of the period ob-served ; S2 = active soil water determined from samples ob-tained at the end of the period observed.
When the resultant quantity obtained in the equation aboveis negative, unaccounted accretion results. As an aid in theinterpretation of results, watershed condition records, con-sisting of detailed observation of agronomic and soil surfaceconditions, were utilized. These observations were madeevery 2 weeks throughout the year, except in winter whena daily record of frost penetration was taken under woodland,pasture, meadow, and wheat whenever the soil was frozen.
RESULTS
Summarized data are plotted for the woodland,pasture, and two cultivated watersheds in Figs, i, 2,3, and 4, respectively, the data in Fig. 3 representinga cultivated area on Muskingum silt loam and thosein Fig. 4 on the Keene silt loam. For accretion anddepletion the data are plotted showing total accumula-tion for each of the periods extending from one soilmoisture sampling period to the next as well as aver-age daily rates. The rate is shown by the slope of theline, while total accumulation is represented by the
464 SOIL SCIENCE SOCIETY PROCEEDINGS 1941
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FIG. i.— An inventory of the soil water relationships on Muskingum loam (woodland) from August 8, 1939,' to August 7, 1940
FIG. 2.—An inventory of the soil water relationships,on Muskingum silt loam (pasture) from August 8, 1939, to August 7, 194°'
DREIBELBIS AND POST: SOIL WATER RELATIONSHIPS 465
AUG. SEPT. OCT. NOV. DEC. JAN. FEB. MAR APR. 22 MAY JUNE JULY AUG- I 8 15 22 29 6 13 19 17 26 7 1521 29 6 1319 3 K> 12 4 19 28 4 H j 1 8 21 29 5 12 T9 26 9 ISZ33Q 7
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FIG. 3. — An inventory of the soil water relationships on Muskingum silt loam (cultivated) from August 8, 1939, toFEB.
12AUG. SEPT.
I 8 6 22 29 6 B 19OCT. NOV. DEC. JAN.
17 26 7 6 21 29 6 13 19 3 10MAR. APR. MAY , JUNE
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August 7, 1940.JULY AUG.
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iFIG. 4.—An inventory of the soil water relationships on Keene silt loam (cultivated) from August 8, 1939, to August 7, 1940.
TABL
E i.—
An -i
nven
tory
of p
reci
pita
tion
disp
osal
on
wood
land
, pas
ture
, and
cul
tivat
ed s
oils
from
Aug
ust 1
939
to A
ugus
t 19
40. E
xpre
ssed
in
term
s of
inch
es.
Dat
e
Aug
. i, 1
939
Aug
. 8, 1
939
Aug
. 15,
193
9A
ug. 2
2, 1
939
Aug
. 29,
193
9Se
pt. 6
, 193
9Se
pt. 1
3, 1
939
Sept
. 19,
193
9O
ct. 1
7, 19
39O
ct. 2
6, 1
939
Nov
. 7, 1
939
Nov
. 15,
193
9N
ov. 2
1, 1
939
Nov
. 29,
193
9D
ec. 6
, 193
9D
ec. 1
3, 1
939
Dec
. 19,
1939
Jan.
3, 1
940
Jan.
10,
194
0Fe
b. 1
2, 1
940
Mar
. 4, 1
940
Mar
. 19,
194
0M
ar. 2
8, 1
940
Apr
. 4, 1
940
Apr
. n,
1940
Apr
. 22,
194
0M
ay i
, 194
0M
ay 8
, 194
0M
ay 2
1, 1
940
May
29,
194
0Ju
ne 5
, 194
0Ju
ne 1
2, 19
40Ju
ne 1
9, 1
940
June
26,
194
0Ju
ly 9
, 194
0Ju
ly 1
6, 1
940
July
23,
194
0Ju
ly 3
0, 1
940
Aug
. 7, 1
940
Tota
l
Prec
i-pi
ta-
tion
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0-39
0.40
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0.19
0.24
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0.98 2.66 1-54
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640.
06O
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0.14
0.09 2.6
32.4
30.5
60.7
20.9
91-
952.
640.
130.
90 0-57
2.13
0.98
2.14 1.69
1.24
2.23
0.71
0-93
2.07
0.28
35-9
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0.31
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7.60
8.51 7.56
7.39
7.07 7.83
6.35 6.99
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6.71
5.78
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6.94
6.76
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8.36
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7.02
6.60
5-95
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4-37 5-4
15.5
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DREIBELBIS AND POST! SOIL WATER RELATIONSHIPS 467
vertical line. Active soil water is plotted as storagefor the particular days in which the soil was sampledfor this purpose. Summarized data for all the water-sheds appear in Table i.
ACCRETIONThe source of soil water is largely from precipita-
tion which varies in amounts and intensities fromyear to year. The total precipitation for the 12-monthperiod studied was 35.91 inches. The 30-year aver-age for this locality is approximately 40 inches. Al-though the total precipitation for the period studiedwas 4 inches below the annual average, the distribu-tion and intensity was such that the soil conditionsduring the growing season in 1940 reflected an un-usually wet period. Precipitation for the calendaryear 1940 was approximately 7 inches above normal.The distribution of the rainfall for the periods studiedmay be observed in Figs, i, 2, 3, or 4 and also inTable i. Except for a few storms in October andApril, the rainfall distribution and intensity, in gen-eral, favored high infiltration and low runoff. At-mospheric humidity is generally rather high in thisregion.
Unaccounted accretion. — Unaccounted accretionmay be due to a number of sources that will be dis-cussed briefly here. It is probable that part of thisaccretion may be the result of lateral movement ofsoil water into the column being sampled, whichmovement may be favored by the slope and directionof the contours and particularly by the presence ofimpervious layers frequently found in this area belowthe 4O-inch depth. By this method the accretion tothe watershed actually may be greater than is attribu-ted to it by rain gage measurements.
Experimental error in the sampling operations alsomay have contributed to unaccounted accretion. Thedetermination of soil water at any given time is theresult of sampling. Should the samples representsoils wetter than the average condition of the water-shed, there is likely to be unaccounted accretion. Ifdrier than the average, the error will tend to increasethe net depletion.
Another source of unaccounted accretion may be
the interception of precipitation by vegetation. Thisis particularly true iri the woodland where intercep-tion of fog drip and of drifting snow is likely to beappreciable. Such accretion varies considerably withother factors, such as wind velocity, humidity, densi-ty, and height of vegetation, and is rather difficultto measure.
Another source of unaccounted accretion is thatresulting from condensation. Lebedeff (8) has re-ported an annual accretion of 72 mm (2.84 inches)of water due to condensation. In a 3-year study thenumber of days in which condensation occurredvaried from 126 to 179 days per year. These meas-urements, however, were made in a semi-arid regionin Russia.
To obtain some idea of this accretion, condensa-tion was measured by two Fergusson recording raingages4 for a 66-day period from September 22 toNovember 26, 1940, in which 13 days showed a dailyaccretion varying from 0.005 to 0.04 inch, giving atotal of 0.209 mcn f°r the period.
Although water movement by capillarity also maybe a possible source of unaccounted accretion, it is be-lieved this amount is negligible in Muskingum soils.
The fact that percolation measurements have beenmade from a profile of 8 feet rather than of 40 inchesalso may constitute a possible source of unaccountedaccretion. For the entire annual period this wouldmake no appreciable difference, but for shorter pe-riods when the percolation is rather high, the unac-counted accretion may be affected to some extentfrom this source. Previous studies (7) have indicatedthe soil to be very shallow in these lysimeters andthat the percolation passes through true soil approxi-mately 40 inches in depth, while in the lower part ofthe profile percolation proceeds through disinte-grated rock (shale or sandstone) and the undecom-posed bed rock. Furthermore, it is not establisheddefinitely that lysimeter percolation is truly repre-sentative of natural percolation. However, these per-colation values are assumed to be the most accuratethat can be obtained.
The soil water content for all watersheds waslower at the end of the experiment than at the be-
4The Fergusson rain gages were used in the following manner to measure condensation: A K-inch mesh hardware cloth wasplaced over the collector bucket of each on which was placed a pan containing in situ surface soil. The diameter of this pan was 7inches and the depth 3 inches. On one gage the pan contained in situ soil only, while the other contained in situ soil on which grasssod had been established. The gages were placed in a pit, so that the soil in the pans was level with the surrounding area. The pitwas covered with a tarpaulin, exposing only the surface of the pans containing the soil, in order to maintain the units at a tem-perature comparable to the surrounding soil. Any increase in weight was recorded on a daily chart, each space representing an ac-cretion of 0.02 inch of water. Whenever precipitation occurred no good records were obtainable, and such periods were not in-cluded in the total measurements, although it is probable that condensation took place during these periods of intermittent rain-fall. Thus the total amount of condensation reported represents a value less than the true value. The soil with the vegetation gaveslightly higher values for a similar period. This work is still in progress.
468 SOIL SCIENCE SOCIETY PROCEEDINGS 1941
ginning. This decrease is shown in Table 2 and re-veals the additional amounts of water disposed fromthese areas expressed both in inches of water and inpercentage of total accretion.
Probably all sources of unaccounted accretionmentioned above at some time contributed to the soilwater on each of the watersheds studied. In thespring when both litter and soils are saturated in thewoodland, water of condensation may readily bepassed on to the soil. Litter from deciduous trees hasbeen found to intercept as much as 0.2 inch of waterduring a 24-hour period of wetting. On the culti-vated areas during the winter, the structure of thefrozen soil very likely had some influence. The struc-ture of the Keene silt loam surface soil (when frozen)often resembled a concrete structure, while that ofthe Muskingum silt loam more nearly resembles ahoneycomb structure, thus providing a better meansof absorbing moisture. During the summer the great-er size and number of wheat stems on the Muskin-gum soil may have resulted in higher interceptionstorage by plants from dew and fog. In general, thewatershed condition records have been very usefulin the interpretation of these data, explaining certainphenomena for which otherwise it would be difficultto account.
STORAGE
Active soil water.—The active soil water is a more
useful value than total soil water, as it represents theamount of water that may be accumulated or releasedby the soil under field conditions at any time duringthe year. Active soil water is considered here as waterstored in the soil. In the driest part of the year, whenthe minimum amount of water is present in the soil,the active soil water is represented as zero. Thistime will vary from year to year, but commonly oc-curs in September or October which is near the closeof the high transpiration period and when the precipi-tation is often the lowest. The peak of soil moistureusually occurs in late winter or early spring. Themaximum values obtained were 9.39, 8.36, 7.09, and8.03 inches for the woodland, pasture, and two cul-tivated areas (Muskingum, Keene), respectively.
In general, the active soil water is low as a resultof high evaporation-transpiration, low precipitation,or both, but seldom, if ever, due to percolation alone.The active soil water in pasture amounted to morethan 7 inches in January before appreciable percola-tion occurred. After percolation started it continued,even though the active soil water was lower than 7inches. Seemingly, once the column of water startsto percolate, it continues, even though the soil watercontent may be much lower than when starting.
During the growing season there was a corres-ponding decrease in active soil water with increasein plant growth. This was particularly evident when
TABLE 2.—A summary of accretion and disposal of water for the year August 8,1939, to August 7,1940, expressed in inches andin percentage of total accretion.
Landuse
Woodland(Watershed
131,2.21 acres)
Pasture(Watershed
102,1. 25 acres)
Cultivated(Watershed
109,i. 69 acres)
Cultivated(Watershed
123,1.37 acres)
Soil,type
Muskin-gum loam
Muskin-gum siltloam
Muskin-gum siltloam
Keenesilt loam
Accretion
Precipita-tion
Ins.
35-91
35-91
35-91
35-91
%
77-2
85-7
79-4
84.9
Unac-countedaccretion
Ins.
8.08
3-51
7.16
4.98
%
17.4
8-4
15.8
u.8
Soil waterdecrease*
Ins.
2-53
2-47
2.15
1-39
%
5-4
5-9
4.8
3-3
Total
Ins.
46.52
41.89
45-22
42.28
%
IOO
100
IOO
IOO
Disposal
Runoff
Ins.
O.I I
0.60
6.09
6.35
%.
O.2
i-4
13.5
15-0
Percola-tion
Ins.
13-01}
12.52
5-49
2.12
%
28.0
29-9
12. 1
5-o
Net deple-tion f
Ins.
33-40-
28.77
33.64
33-81
%
71.8
68.7
74-4
80.0
Total
Ins.
46.52
41.89
45-22
42.28
%
IOO
IOO
IOO
IOO
*Difference in soil water between the beginning and end of the experiment,flnto the atmosphere, largely evaporation-transpiration.JEstimated from data on pastured lysimeters.
DREIBELBIS AND POST: SOIL WATER RELATIONSHIPS 469
the plants were at the flowering stage. On the culti-vated areas the active soil water was more nearly afunction of crop growth than on the pastured water-shed. High percolation and runoff account for mostof the reduction of soil water outside of the growingseason, while during plant growth soil water deple-tion to a great extent is the result of evaporation-transpiration.
When the active soil water storage was within 2inches of the maximum, only 52% of the volume ofrunoff occurred on the Muskingum silt loam andonly 46% on the Keene silt loam, both areas in culti-vation. The remaining runoff occurred at timeswhen the active soil water was relatively low. Thisseems to indicate that the reservoir capacity of a soilis sufficiently great to hold the runoff for most pe-riods of the year, particularly during the summermonths when rainfall intensities are usually the high-est. If conservation practices can be adopted to pre-vent this water from running off, the reservoir ca-pacity of the soils will be able to retain the water formost of the storms normally occurring in the sum-mer months, when both soil and water loss oftenis the greatest in this region. Any additional accre-tion to the soil water during the summer months, asthe result of improved practices, is likely to be dis-posed of through evaporation-transpiration ratherthan through percolation.
The average maximum daily increase of soil waterwas highest in the woodland where it amounted to0.52 inch November 15 to 21. On the pasture itamounted to 0.37 inch October 17 to 26, while on thecultivated areas it amounted to 0.34 inch on the Mus-kingum silt loam June 5 to 12 and 0.44 inch on theKeene silt loam October 17 to 26.
The average maximum daily decrease of soil water(July 30 to August 7) also was greatest for the wood-land, where it amounted to 0.50 inch. On the pastureit was 0.25 inch and on the cultivated areas 0.23 inchand 0.32 inch on the Muskingum silt loam and Keenesilt loam, respectively. This-maximum daily decreaseoccurred in the same period for all the watershedsstudied.
DISPOSALRunoff.—The volume of water disposed through
runoff is low for the woodland and pasture water-sheds in comparison to water 'lost by other means.On the wooded watershed the only runoff recordedwas 0.04 and 0.07 inch in March and July, respective-ly. In the pastured area the occurrence of runoff wasmore frequent and in larger amounts, the highest
total quantity recorded for any one period being 0.27inch. The cultivated watersheds were sown to wheatin the fall of 1939 and went from wheat to clovermeadow in the summer of 1940. The highest runoffon the cultivated Muskingum silt loam occurred inwinter between January 10 and February 12 when2.23 inches'of runoff were recorded. The total vol-ume of runoff was 0.26 inch higher on the Keenesilt loam cultivated area than on the Muskingum siltloam. The larger volume of runoff from the Keenesilt loam for various periods, as compared to theMuskingum silt loam area, probably was due to thehigher transmission capacity of the latter soil, thusfacilitating percolation, while in the former, due to thehigher soil moisture content and low transmissioncapacity, a greater volume of runoff resulted. Thisevidence is confirmed by the comparative percolationvalues for these respective periods. Since the amountof litter was the highest in the woodland (lowest run-off) and lowest on the cultivated areas (highest run-off), it is clear that runoff is affected by litter.
A potential source of runoff not. mentioned aboveis the seepage water coming to the surface of a water-shed and contributing to the runoff from that area,as has been pointed out by Riesbol (10) who citeda specific case where seepage from a small watershedcontributed to the runoff from a larger watershed onboth of which precipitation and runoff records wereutilized. It is probable that one phase of the unac-counted accretion described in this paper constitutesa transitional stage of soil moisture increase passinginto seepage water and eventually into runoff. Thiscondition is apt to result where impervious layersoutcrop on slopes. It is believed that on the water-sheds reported in this study, little, if any, runoff canbe attributed directly to seepage water. The runoffper unit area on fields farther down the valley mayeasily be greater where seepage water constitutes apartial source of the total runoff from that area.
Percolation.—The percolation of water throughsoil is influenced by soil characteristics, including itsmoisture content, as well as by precipitation and tem-perature. The water removed from the soil by trans-piration will reduce the soil moisture content andtend to retard percolation. Thus, soil type, weather,and land use practices influence percolation. Perco-lation thus may be considered essentially as waterthat infiltrates into the soil and is not utilized byplants nor lost by evaporation in that immediate area.
As will be noted in Figs, i, 2, 3, and 4, most of thepercolation occurs during the winter and earlyspring. The greatest amount of percolation occurred
470 SOIL SCIENCE SOCIETY PROCEEDINGS 1941
from April n to April 22, when 2.17 'inches wereobtained from the pastured watershed, 2.36 inchesfrom the cultivated watershed (Muskingum series),and 1.49 inches for the same land use on the Keenesilt loam. In comparing percolation on these twocultivated watersheds, the Muskingum silt loamshows more frequent and greater amounts of perco-lation than the Keene silt loam. This is the resultlargely of soil conditions, as the latter soil has a layerof heavy silty clay at a 24-inch depth, thus impedingpercolation considerably, while the Muskingum soilis quite permeable throughout the profile.
Mechanical analysis data (4) have revealed thatthe soils on the pastured watershed are lighter in tex-ture than on the cultivated area, hence the greaterpercolation on the former probably is due to soilconditions. It will be noticed that percolation occurson the pastured area even when the active soil water
>is low. Once water gets below the root zone in thissoil it continues to percolate rather than become avail-able to plants in that immediate area. On the Mus-kingum silt loam (pasture) percolation occurredalmost throughout the entire year, except in latesummer and fall. The parent material apparentlyinfluenced percolation considerably, although theprevailing practice that resulted in a cover consist-ing mostly of poverty oatgrass and weeds also hadsome influence. The parent material of the Muskin-gum silt loam on the pastured area consists of acoarse-grained sandstone, while that on the culti-vated area, also on Muskingum silt loam, consistslargely of shale. Percolation proceeds more readilythrough the sandstone material than through theshale.
As lysimeters are not available on the woodedareas, percolation was estimated from data obtainedfrom lysimeters adjacent to the pastured watershed.The soils on both areas belong to the Muskingumseries, the woodland having a loam texture and thepastured watershed a silt loam. Both are of sand-stone origin and the woodland soils are somewhatmore immature, thus favoring a somewhat greaterpercolation. The total -percolation in the woodlandwas estimated to be equal to the percolation fromthe lysimeters in the pasture plus any additionalamount of water lost through runoff from the latterarea. It is believed the estimate for the woodland isnot far from the actual value, although these datashould be interpreted with caution because they arebased on estimates. It is probable that during thewinter and spring percolation is actually higher inthe woodland than the amount estimated, and dur-
ing the summer when transpiration is high, it isprobably lower.
It should be recalled that the soil moisture datain this paper apply to a 4O-inch depth, while percola-tion measurements were made on an 8-foot profile.With soils of the Muskingum series this probablyentails no serious error, as water percolates quitereadily from these soils and little capillary rise ofwater is likely to occur from the lower depths up-ward into the 4O-inch zone. In the woodland the rootpenetration is deeper and water is utilized to lowerdepths. On the Keene silt loam percolation for theperiods studied is usually lower and the total annualamount is appreciably lower. It is probable that waterfrom the heavy clay layer below the 4O-inch depthmoves upward into the 40-inch zone as indicated bylower amounts of percolate collected from this soiltype.
On the lighter soils, particularly the coarser tex-tured Muskingum soils, it is probable that some ofthe gravitational water is utilized by plants duringthe growing season when rains are often rather fre-quent. Further study on this work is desirable. Thefunction of capillarity in these soils seems to be tohold the water in place until it is utilized by the plantroots.
Net depletion.—Net depletion to the atmosphereaccounts for the major loss or disposal of precipita-tion and is highest on all the areas during the grow-ing season. It is particularly high on the cultivatedareas at the time of flowering and at the soft doughstage of wheat. On the pasture the vegetal cover isdormant during the fall period and little water dis-posal occurs at this time through transpiration.
The maximum daily value for net depletion for allwatersheds was found for the period from July 30to August 7 when more than 4 inches of water weredisposed by this means in the woodland and from2.06 inches to 2.85 inches in- the other watersheds.This probably was the result of both high tempera-tures with low humidities and of a high amount ofactive soil water stored during the previous period.Vegetal observations showed the rate of growth to behighest during this period. Records showed thatgrowth of pasture plants and wheat varied in muchthe same order as the net depletion values obtainedfor these respective periods. During the late fall andwinter net depletion values were low when transpira-tion was at a minimum. The average daily rate of netdepletion on the four watersheds was computed forthe various seasons and appears in Table 3. In thelate summer this rate in the woodland exceeds that
DREIBELBIS AND POST: SOIL WATER RELATIONSHIPS 471
TABLE 3.—Average daily rate of net depletion (largely evapora-tion-transpiration) on woodland, pastured, and cultivated
•watersheds in inches.
Season
Late summer . . . .Fa l l . . . . . . . . . . . .Winter. . . . . . . . . .
Late spring andsummer. . . . . . .
Wood-land*
0-1550.037
0.170
Pasturef
0.1340.0250.097
0.144
Wheatf
0.0980-035
0.168
Wheatt
0.113
0.0230.118
0.176*Muskingum loam.tMuskingum silt loam.jKeene silt loam.
in the other areas. In early spring the rate is slightlyhigher in the wheat. In late spring and summer, thehighest daily rates were found for all the areasstudied.
DISCUSSIONA summary of both the accretion and disposal of
water for the year studied appears in Table 2. Pre-cipitation, as measured by rain gage catches, variedfrom 77.2% (woodland) to 85.7% (pasture) of thetotal amount of water disposed on the watershedsstudied. The total unaccounted accretion for theyear was highest in the woodland where it totaled8.08 inches. The least occurred in the pasture whereit amounted to 3.51 inches. The source of unac-counted accretion may be due to the following:
1. Subsurface flow of water into the soil massfrom which the samples were taken.
2. Interception of precipitation, such as fog dripand snowfall, by vegetal cover which is espec-ially appreciable in woodland areas.
3. Condensation.4. Capillarity. It is probable the amount of un-
accounted accretion from this source is verylow, particularly in Muskingum soils.
5. Experimental error in the sampling operations.All watersheds had less soil water at the end of
the experiment than at the beginning, the amountvarying from 1.39 inches on the Keene silt loam to2.53 inches on the Muskingum loam (woodland).
The potential storage capacity of the soil for ad-ditional water is a factor that bears directly uponthe immediate possibility of the occurrence of floods.The upper 40 inches of soil was found to vary over7 inches on a Muskingum silt loam cultivated soiland over 9 inches on a Muskingum loam woodlandsoil during the 12-month period studied. Soil waterstorage also is important for crop production as wellas for runoff control. Field crops rarely have accessthroughout the growing season to a supply of soil
moisture sufficient for optimum growth in thisregion.
In accounting for the disposal of water the moststriking factor seems to be the large amount of waterlost through net depletion, largely evaporation-trans-piration, and the small amount through'runoff. Thewoodland disposed only 0.2% of the water throughrunoff, the pasture 1.4%, and the cultivated areas13.5% and 15.0% for Muskingum and Keene siltloams, respectively. The amount disposed throughevaporation-transpiration varied from 68.7% (pas-ture) to 80.0% (cultivated Keene silt loam). It isalso interesting to observe that for the growing sea-son the cultivated areas (wheat) showed a highernet depletion than either woodland or pasture. Thiscontrast between the wheat and the woodland maybe due partly to the higher evaporation from the soilin wheat than that in the woodland, even though thetranspiration may be higher in the latter. Practicallythe same amount of water was disposed in producinga 25.7 bushel per acre yield on the Muskingum siltloam as in producing a 17.9 bushel per acre yield on
, the Keene silt loam.In a study of the effect of environmental factors on
the transpiration of tomato plants, Foster and Tat-man (6) found a highly significant effect of soilmoisture on the total amount of water transpired, onthe water requirement, and on the fruit yield. Wid-stoe and Merrill (13) have found that the amount ofwater utilized by the crops varied with the quantitiesof irrigation water applied. For wheat the amountof water lost by evaporation-transpiration was 18.74acre inches when 5 inches of irrigation water wereapplied, while 63.74 inches were lost when 50 incheswere applied. For corn the amount lost ranged from13.04 inches to 60.54 inches, depending upon theamount of irrigation water applied.
The amount of water disposed through percola-tion varied from 5.0% on the Keene silt loam cul-tivated soil to 29.9% on the Muskingum silt loampastured area. Percolation values are significant,especially in connection with the maintenance of thewater table which is important for industrial as wellas for agricultural purposes.
In a study of the relationship of land use and soiltype to soil water storage and to the disposal factorsof runoff, percolation, and evaporation-transpiration,many complex biological and physical factors areinvolved. Precipitation, soil water storage, and evapo-ration-transpiration all influence runoff and if onefactor is affected, some one or some combination ofother factors will be influenced. Both precipitation
472 SOIL SCIENCE SOCIETY PROCEEDINGS 1941
and vegetal cover exerted a great influence on theactive soil water and on the disposal factors of runoff,percolation, and evaporation-transpiration. Soil typealso has had an appreciable effect on the storage ofsoil water and on percolation, while a lesser influenceupon runoff and evaporation-transpiration has beenobserved.
Meteorological records show that the amounts andintensities of precipitation, the temperature, sunshineduration, wind velocity, and atmospheric humidityvary daily and from year to year. The soil moisturecontent, as well as percolation and evaporation-transpiration, are effected to a great extent by thesemeteorological conditions, and may likewise be ex-pected to vary daily from year to year. The datapresented in this paper apply only to the particularyear studied; however, they probably represent con-ditions close to the average.
SUMMARYThis investigation comprises a study of soil mois-
ture conditions under different land use practicesand their relationship to precipitation, runoff, perco-lation, evaporation-transpiration, and storage ofwater in the soil.
The moisture content was determined on soils toa 4O-inch depth on four experimental watersheds inwoodland, pasture, and two cultivated areas, eachof the latter being located on a different soil type.The moisture was determined 39 times during a 12-month period, beginning in August 1939 and endingin August 1940.
Total amounts of precipitation, runoff, and perco-lation also were determined and tabulated on awatershed basis for periods beginning and endingwith successive soil moisture sampling dates.
Net depletion, largely evaporation-transpiration,and unaccounted accretion were calculated from anequation using soil moisture data from the beginningand end of these periods, together with data on pre-cipitation, runoff, and percolation. The factors ofaccretion and depletion were plotted, showing totalamounts and'average daily rates for periods extend-ing from one soil moisture sampling period to thenext.
The data indicate that precipitation constituted77.2, 85.7, 79.4, and 84.9% of the total accretion forthe woodland, pasture, cultivated Muskingum siltloam, and cultivated Keene silt loam, respectively.Each watershed had approximately 2 inches lesswater at the end of the experiment than at the begin-ning. Unaccounted accretion constituted the remain-
der of the total accretion. The possible sources ofthis unaccounted accretion were discussed. In thewoodland and pasture, these values were 8.08 inchesand 3.50 inches, respectively. For the cultivated areasthey amounted to 7.16 inches on the Muskingum siltloam and 4.98 inches on the Keene silt loam.
Soil moisture was expressed as active soil water,this amount representing water above the minimumcontent found for the annual period and represent-ing an amount that could be accumulated or releasedunder field conditions. The amount of active soilwater varied considerably during the year, beinghighest in late winter and early spring and lowest inlate summer or early fall. The maximum amount ofactive soil water found during the year was 9.39,8.36, 7.09, and 8.03 inches for the woodland, pasturecultivated Muskingum silt loam, and cultivatedKeene silt loam, respectively.
The relatively low soil moisture content fromJune to September, inclusive, when rainfall inten-sities often are the highest in this region, reveal apotential reservoir capacity of these soils sufficientlygreat to store most of the precipitation if land usepractices can be modified so as to induce sufficientinfiltration and prevent runoff.
Active soil water is important both in connectionwith crop production, as well as in the control ofrunoff.
Net depletion values constituted by far the great-est percentage of water disposal for all the areasstudied. The percentage of the total accretion dis-posed by this method was 71.8 in the woodland, 68.7in the pasture, and 74.4 and 80.0 for the Muskingumand Keene silt loam cultivated areas, respectively.The average daily rate of net depletion for the variousseasons was computed for all the watersheds studied.
The percentage of the water disposed through per-colation ranged from 5.0 on the Keene silt loam (cul-tivated) to 29.9 on the Muskingum silt loam (pas-ture). Percolation for the woodland was estimatedfrom data obtained from lysimeters in pasture andthe data from woodland are, therefore, possibly sub-ject to error. Percolation values are important inconnection with the maintenance of the water table.
Runoff constituted only 0.2% of the disposal inwoodland, 1.4% in the pasture, while the cultivatedareas lost 13.5% and 15.0% for Muskingum andKeene silt loams, respectively.
The active soil water as well as the water disposedthrough runoff, percolation, and evaporation-trans-piration has a direct bearing upon the occurrence offloods and a knowledge of these values should prove
DREIBELBIS AND POST: SOIL WATER RELATIONSHIPS 473
useful in flood control studies, especially of the re-gion represented in this study.
Both precipitation and vegetal cover affect soilwater storage and the disposal factors of runoff, per-colation, and evaporation-transpiration. The effectof soil type on active soil water and on percolationwas apparent, while a lesser influence of soiltype onrunoff and evaporation-transpiration was observed.
Considering the difficulties involved in obtainingevaporation-transpiration values, it is believed thedata presented on net depletion afford a reasonablyaccurate measure of evaporation-transpiration underfield conditions. The inventory of soil water and itsrelationships, as revealed by the data presented,should prove useful in hydrologic studies, especiallywhere an evaluation of the effects of land use onwater conservation are sought.