field and laboratory determination of field capacity · voir and its capacity for available...

FIELD AND LABORATORYDETERMINATION OF FIELD CAPACITY

J .M. MurrayMember C.S.A.E.

Agricultural Engineering DepartmentUniversity of Saskatchewan

Saskatoon, Saskatchewan

byC. R. Shanmugham

Madras, India

INTRODUCTION

The efficient application of wateris one of the most important factorsin successful crop production underirrigation. The soil in the root zoneof the plants acts as a moisture reservoir and its capacity for availablemoisture storage is the difference inmoisture content at field capacity, theupper limit, and permanent wiltingpoint, the lower limit. Field capacityis generally understood to be themoisture content of the soil, when,following saturation, the downwardpercolation of water has essentiallyceased. This limit in conjunctionwith wilting point governs the depthof water to apply during irrigationand thereby directly affects the interval between irrigations.

According to Israelsen (5) the essential conditions to be observed indetermining field capacity of soils inthe field are the following:

1. Saturate the soil profile to thedepth under study by adding anexcess of irrigation water.

2. Minimize surface evaporationlosses.

3. Eliminate transpiration losses byworking on non cropped fields.

4. Select plots containing uniformand free draining soil.

5. Observe the time rate of decreasein moisture content.

Gen Ogata and Richards (4) andWilcox (11) have shown that on un-cropped fields protected from evaporation, the rate of soil water loss atany time is proportional to the initialwater content and inversely proportional to the time. Richards (8)working on uncropped fields not protected against evaporation have showna similar relationship. It is assumedthat the same relationship is applicable to fields which are cropped,where an additional moisture lossthrough transpiration occurs. In thiscase, the total soil moisture loss byconsumptive use and drainage is assumed to be equal to the moisturethat would be lost by drainage alonefrom a covered fallow field, whiledraining to field capacity. During the

initial period while drainage is occurring, consumptive use occurs atthe expense of free water that wouldotherwise have drained away. Withthis assumption the problem of fieldcapacity determination on croppedfields reduces to one of determiningwhen rapid drainage has stopped. Nocorrection is required in the time-soilmoisture curve for soil moisture depleted by consumptive use during theinterval.

In view of the time involved andthe inherent problems in determiningthe field capacity in the field, it isdesirable to relate the field results toa more readily measurable soil moisture constant which may be determined by laboratory techniques. Oncesuch a relationship has been established it is possible to estimate fieldcapacity indirectly from the laboratory measurement. Many investigators(2, 6, 7, 9, 10) have developed suchrelationships in other areas usinglaboratory measurements such asmoisture equivalent and moisturecontent retained by a sample at 1/10or 1/3 atmosphere tension. The mainobjections to indirect measurement offield capacity are:

1) The laboratory analyses do notrepresent field conditions inasmuch asthe sample is distributed and oflimited size. 2) Most of the estimatesequate the soil moisture with a givenstress whereas field capacity is recognized as a non equilibrium phenomenon. 3) The field variations in rateof drainage due to differences in subsoil and in rate of moisture absorptionby crops cannot be accounted for inlaboratory determinations. It is suggested that for accurate determinationof field capacity at any one location,it would be necessary to make themeasurements at that location and

that if absolute accuracy is not considered essential, the use of an indirect measurement can be made to

estimate field capacity.

The work described in this paperwas designed to develop a rapidmethod of determining the fieldcapacity of irrigated soils, in the field,under conditions of crop growth. Inaddition, comparisons of moistureholding characteristics determined in

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the laboratory with the field capacityas measured in the field, were madeto establish the relationships betweenthem for the soils under test. The

value of this work is expected to bein its practical utility for formulatingirrigation schedules. Emphasis isplaced on finding an easy and rapidmethod to estimate the field capacityof soils rather than to determine itwith great precision.

MATERIALS AND METHODS

The field studies were conducted

on sprinkler irrigated area of theUniversity Irrigation farm. A soilsurvey report (3) states that "thearea is underlain by a stony glacialtill which may extend in some placesto within 60 inches of the soil sur

face". Soil samples taken during thestudy indicates that glacial till occurswithin the root zone at some profiles.The overlying soil varied in texturefrom sandy loams to clay at differentlocations and depth. A uniform, wellestablished, brome grass-alfalfa foragecrop covered the entire area. Peizome-ter installations showed the water

table to be generally below 10 feet,although one reading of 7 feet belowground surface was noted. All fieldcapacity determinations were made inthe top 4 feet and it is assumed thatfree drainage in this region was notimpeded by the location of the watertable.

Field tensiometers were installed atdepths of 6, 18 and 30 inches, at thestudy sites to indicate the soil moisture tension in each foot of the root

zone at various times following irrigation. The tensiometer readings wererecorded simultaneously with othersoil moisture measurements for several

days following each irrigation.

A Nuclear-Chicago neutron moisture probe was used to follow actualchanges in soil moisture contentthroughout the study. Field moisturecontents were determined for each

foot of depth down to four feet beforeand immediately after irrigation andat progressively increasing intervalsthereafter until all drainage had obviously ceased. Irrigation gauges ofthe type developed by Brooks (1) wereinstalled at the test sites. These gaugeswere essentially a reservoir and an

evaporating device (Black BellaniPlate) calibrated to predict the consumptive use of water by the growingcrop under the prevailing weatherconditions. The daily consumptiveuse of the crop, estimated from theirrigation gauge readings were used inseparating drainage from the total soilmoisture depletion to arrive at fieldcapacity.

Three undisturbed soil core samples were obtained for each foot ofdepth at each site. The moisture retained in the samples, subjected to1/3, 1/5 and 1/10 atmosphere tensionand the moisture equivalent were determined in the laboratory by standard test procedures. The soil coresamples were also used for densitydetermination, needed to convert soilmoisture percentage by volume to soilmoisture percentage by weight.

RESULTS AND DISCUSSION

Field Methods

For each test site, the cumulativesoil moisture loss and cumulative ir

rigation gauge depletion were computed and plotted from measurements taken at frequent intervalsfollowing an excessive application ofirrigation water. Figure 1 is a typicalset of these curves for one test site. As

would be expected the cumulativemoisture depletion curves for eachfoot show a pronounced curvature

during the initial period of wetting,after which time, except for minorvariations they approach a straightline. The 0-48 inch curve is a sum

mation of the data for each foot and

the inconsistency between the plottedpoints and the straight line section ofthe curve is due to cumulative error.The curved portion denotes consumptive use plus rapid drainage. The section which may be approximated by astraight line represents soil moisturedepletion primarily by consumptive

ot

F.C. 3rd. Day - 3.7"

12-24 F.S.C.L.

F.C, 5th. Day - 3.236-48" F.S.C.L.

1.0 2.0 3.0

CUMULATIVE IRRIGATION GAUGE DEFICIT IN INCHES

Figure I. Soil Moisture Depletion vs. IrrigationGauge Deticit.

use. The slope of the straight line forthe 0 to 48 inch depth is approximately 45 degrees indicating that theactual soil moisture depletion is dueonly to consumptive use during thatperiod and that essentially all of theconsumptive use of the crop is takenfrom a 4 foot root zone. The slope ofthe straight line section of the curvesrepresenting individual 1 foot depthsof the root zone probably indicate therelative percentage of the crop's wateruptake which comes from each depth.The point where the curve changes toa straight line represents the timewhen drainage stopped. Field capacityvalues given in figure f were determined by extending the straight lineportion of the curve back to the pointof tangency and determining the soilmoisture content which existed atthat particular time. The time afterirrigation when field capacity wasreached and the field capacity moisture content for all tests are shown in

table 1. The field capacities as calculated above were checked with the soilmoisture level in the field at the timewhen the hydraulic gradient becamezero. Theoretically, at the time whentensiometer readings from differentdepths are the same the soil-watersystem is in equilibrium at fieldcapacity. These two independent determinations of the field capacitycompared well with one another asmay be seen in table 1.

Wilcox (11) has defined an "upperlimit of available moisture" that includes water used consumptivelywhich would otherwise have gone todrainage. He points out that fieldcapacity as normally determined approximates this upper limit and thatthis moisture content occurs at from 1

to 4 days after an irrigation depending on soil texture. The time requiredto reach field capacity, following irrigation in these tests varies between1 and 5 days which is in close agreement with the results of Wilcox (11).

Laboratory Methods

The mean values of four replications, for each of the moisture constants, 1/3, 1/5 and 1/10 atmospheretension and moisture equivalent wereplotted against the correspondingfield capacity values determined fromfield measurements. A linear regression line drawn through the scatter ofpoints in each plot gave the followingempirical relationships.

F.C. = 1.96 + 0.471 x 1/5atmosphere moisture 1

F.C. = 1.72 + 0.436 x 1/10atmosphere moisture 2

F.C. = 2.09 + 0.475 x 1/3atmosphere moisture 3

F.C. = 1.82 -f 0.514 x M.E 4where F.C. = Field Capacity in

inches of water

M.E. =r Moisture Equivalentin inches of water

TABLE 1. FIELD CAPACITY DETERMINED IN THE FIELD.

Depth Station Numbers

(inches) 1 2 3 4 5 6

Texture

0"-12" F.C. from Curves (in)F.C. from Tensiometers (in)

C.L.

3.103.27

L.

3.833.83

L.

3.33

F.S.L.

3.69

Hv.C.

4.00

F.S.L.

3.15

3.15

Texture12"-24" F.C. from Curves (in)

F.C. from Tensiometers (in)

F.S.L.3.103.10

F.S.C.L.3.623.56

F.S.L.3.20

F.S.C.L.3.453.45

C.3.97

F.S.C.L.2.80

24"-36" TextureF.C. from Curves (in)

F.S.C.L.2.53

C.3.98

F.S.C.L.3.06

F.S.C.L.3.69

Si.C.3.50

S.C.2.60

36"-48" TextureF.C. from Curves (in)

HvC.L.3.65

C.4.48

F.S.C.L.3.98

F.S.C.L.3.18

Si.C.4.08

S.C.3.08

Total0"-48" F.C. from Curves 12.42 15.80

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12.80 13.90 16.49 12.22

Figure 2 shows the plot of fieldcapacity versus 1/3 atmosphere, astypical of the method of arriving atthe above formulae. The standarddeviation from the regression werecalculated to be 0.31.1, 0.316, 0.330and 0.324 respectively for the four

/ F.C* 2.09 + 0.475