Effect of Slope on Water Balance Under Center-Pivot Irrigation1

A. Y. HANNA, P. W. HARLAN, AND D. T. Lewis2

ABSTRACTThe effects of slope on soil water storage and other water balance

components under center-pivot irrigation were studied in Gage County,Nebraska. Soil water, surface runoff, precipitation, and irrigation weremeasured for soils of low infiltration on slopes of 2, 4, and 8% undercorn (Zea mays L.). Evapotranspiration was calculated. Soil water con-tent (0- to 137-cm depth) during the growing season was higher in soilson 8 % slopes than in soils on slopes of 2 and 4 %. Runoff from all slopeswas not associated with sprinkler irrigation in the crop production sys-tem studied. As slope increased, surface runoff caused by rain increased,and internal drainage beyond the 137-cm depth decreased at approxi-mately the same rate. The overall water depletion during the growingseason was not significantly affected by slope.

Additional Index Words: infiltration rate, water depletion, water flux.

Hanna, A. Y., P. W. Harlan, and D. T. Lewis. 1983. Effect of slopeon water balance under center-pivot irrigations. Soil Sci. Soc. Am. J.47:760-764.

UNIFORM WATER DISTRIBUTION by sprinkler irri-gation is necessary for more efficient use of irri-

gation water. To attain maximum water use efficiency,water should be applied as the crop needs it and in quan-tities governed by the capacity of the soil to retain it forcrop use. However, application rates from center-pivotirrigation systems sometimes exceed soil infiltration rates(5), especially toward the outer end of the pivot. Re-sultant runoff moves from high areas to nearby low areascausing uneven water distributions across the field. Theuneven water distribution affects water balance compo-nents of the soils and may decrease irrigation efficiency.The amount of water available for infiltration and cropuse could, therefore, be reduced.

Infiltration and redistribution of water within a soilprofile are important aspects of water conservation. Mea-suring the net amount of water moving through the soilprofile is necessary if a favorable control of both waterand solutes is to be achieved in relation to crop produc-tion (6). Soils in the study area have been classified asunsuitable for irrigation (10) on the basis of their erosionpotential as estimated from data on their slope, texture,water-holding capacity, depth, infiltration rate, etc. It isimportant that the adequacy of these estimates be eval-uated for soils under field conditions. The objective ofthis study was to evaluate the effect of slope on waterbalance components of some representative soils duringthe growing season under center-pivot irrigation.

MATERIALS AND METHODSThis study was conducted in the SE 1/4, Section 30, T5N,

R7E, Gage county, Nebraska. Three soil mapping units thatdiffered in slope were selected from within the irrigated areaas follows:

Site 1: Crete silty clay loam (fine, montmorillonitic, mesicPachic Argiustoll), 2% slope.

Site 2: Wymore silty clay loam (fine, montmorillonitic, mesicAquic Argiudoll), 4% slope.

Site 3: Wymore silty clay loam, 8% slope.Sites selected for study were located 8.1 m from wheel track

7, one-fifth of the way between towers 7 and 8 of an eight-tower center-pivot system. The center-pivot in the study areahad a 381.5-m length with operating pressure at the pivot of5.4 to 6.2 kg/cm2. All sites were on a south-facing slope. Thearea was chisel-plowed in November after the corn (Zea maysL.) was harvested. Corn stalks were left on the soil surface. Atthis time, 160 kg/ha of N were applied. In early April, the soilswere described and sampled just before a single disking andcorn planting on the contour on 18 April. The field was culti-vated on 7 June.

Hydraulic properties of the soils to a depth of 137 cm weredetermined on a nonwheel-track row at each site. Six mercurymanometer tensiometers were placed in a row at the 5-, 15-,30-, 50-, 91-, and 122-cm depth at each site. A soil water de-sorption curve for each horizon was obtained from clod samples.Soil water content was determined using the gravimetric tech-nique from 0 to 1.07 m and tensiometers from 107 and 137 cm.

Precipitation and irrigation water were measured by a re-cording rain gauge and by 3.78-L containers with funnels 10.2cm in diameter placed in triplicate above the corn plants (250cm in height) at each site. Runoff plots (11 m2) were con-structed at the three sites. The plot area was surrounded witha metal border. Runoff water from plots was collected in a pitlined with polyethylene sheets to prevent infiltration. Runoffwas measured using graduated buckets and then was convertedto centimeters of runoff based on the plot area.

Soil particle-size distribution was determined by the pipettemethod (3, 4). Soil bulk density was determined by the clodmethod, modified by the National Soil Survey Laboratory. Or-ganic C was determined by the procedure proposed by Peechet al. (7). Water content at —15 bar matric potential was de-termined using pressure membrane apparatus (8). A pressureplate was used to determine water content at —1/3 bar matricpotential from natural soil clods.

RESULTS AND DISCUSSIONProfile descriptions and particle-size distribution of each

soil site are similar (Tables 1 and 2). The sand contentranges from 1.4 to 5.4% at all sites. Most of this is veryfine sand. Soils at all sites have low infiltration rates andslow permeability because of the high silt and clay con-tent in all horizons and firm B horizons. Bulk density andpercent water by volume for each horizon suggest thatthe soils are similar in their water-holding characteristics(Table 3). Organic C content differences are small andprobably do not significantly affect water-holding capac-ities (Table 2) among the sites.

Field DataTotal amount of water from the surface to the 137-cm

depth at the first sampling date (18 June) was 517, 515,and 536 mm for sites 1, 2, and 3, respectively. Thus, soilson the three slopes contained about the same amount ofwater as a starting point. Soil water content decreasedafter 4 July and reached its lowest amount during thesecond half of July and the first week of August (Fig.1). The amount of water depletion from the soils up tothis time was 88, 52, and 55 mm for sites 1, 2 and 3,respectively, based on the 18 June starting date. About

1 Contribution from the Nebraska Agric. Exp. Stn., Journal Seriesno. 6900, Lincoln. Received 7 June 1982. Approved 15 Mar. 1983.2 Graduate Student, Assistant Professor, and Professor, AgronomyDep., Univ. of Nebraska-Lincoln, Lincoln, NE 68583.

760

HANNA ET AL.: EFFECT OF SLOPE ON WATER BALANCE UNDER CENTER-PIVOT IRRIGATION 761

Table 1—Morphological descriptions of the sites.

Horizon

MottlesT

Depth,cm

Moistcolor Texture

SizeAbundance contrast Color Structure! Cons.t

Concre-tions f Bndyf

APBlB2H

B22tB3

Cl

APBlB21t

B22t

B3

Cl

C2

APBl

B21t

B22t

B23t

B3

C

0-13 10YR3/213-20 10YR2/220-51 10YR2/2

51-69 10YR3/269-91 2.5Y4/2

91-152 10YR5/2

0-18 10YR3/218-23 10YR3/223-38 10YR3/2

38-66 10YR4/2

66-81 10YR4/3

81-117 10YR5/3

117-152 10YR5/4

0-13 10YR3/213-20 10YR3/2

20-28 10YR3/2

28-51 10YR4/3

51-69 10YR4/3

69-89 10YR4/3

89-152 10YR4/3

siclsicsic

sicsicl

sicl

siclsicsic

sicl

sil

siclsic

sicl

sicl

sil

Site 1, Crete, 2% slope

If

IP

10YR5/4

10YR5/6

Site 2, Wymore, 4% slope

ccmc

Id

2pIP2pIP

10YR5/1

10YR5/110YR5/610YR5/14YR5/8

Site 3. Wymore, 8% slope

3fIf

IdId

2pIP

10YR5/210YR5/6

10YR5/25YR4/5

10YR5/25YR4/6

IfgrImsbk2mpr2mpr

to2msbk2cprParting to2msbk

IfgrImsbk2mpr

to2vfsbk2mprParting to2fabk2mprParting to2msbk

IfgrImabk

andIcabk2mpr

to2vfabk2mprParting to2fsbkImprParting to2fsbkImprParting toImsbk

mfrmfimfrmfi

mfr

mfr

mfrmfimfr

mfr

mfr

mfr

mfr

mfrmfi

mfr

mfr

mfr

mfi

mfr

Fe, MnCa-carb

Fe,MnCa-carb

Ca-carb

Fe, MnCa-carb

Fe,MnCa-carb

Fe, Mn,Ca-carb

ascscscs

ascscs

ascs

T Abbreviations and terms from Soi7 Survey Manual (Soil Survey Staff, 1951).

35 mm more water were lost from soil on 2% slope thansoils on 4 and 8% slopes. This suggests that there wasless water available for crop use at this site. Soil watercontent began increasing from the second week in Augustto the first week in September as a result of high pre-cipitation and lower crop demand during this period. Thetotal amount of water in the soil (151 cm) on 8 Septem-ber, the last sampling date, was 513, 512, and 517 mmfor sites 1, 2, and 3, respectively. Net water loss fromthe soils (0-137 cm) during the growing season was 4,3, and 18 mm; and average water content for the growingseason was 461, 487, and 504 mm for the respective sites.Total water content (0- to 137-cm depth) and that atvarious depths were the highest in soil on the 8% slopeand least in soils on the 2% slope (Fig. 1). In other words,the soil on a 2% slope appeared to be somewhat moredry than those on 4 and 8% slopes during most of thegrowing season. From observations, no differences in plantgrowth existed and most populations among the three

sites were nearly the same. In addition, similar corn yieldswere obtained, namely, 11.6, 11.1, and 11.8 t/ha for sites1, 2, and 3, respectively. Therefore, the differences in soilwater contents among the sites did not appear to be re-lated to differences in crop vigor or density. If differencesin water were real, they may have been due to the land-scape position. The 2% slope was at a higher position onthe landscape than the 4 and 8% slopes. The 2% slopewas at the lower part of a summit (divide); the 4% slopewas at the upper part of the backslope, and 8% slope wason the mid-backslope. Soils on backslope positions ap-peared to maintain higher water levels than upper posi-tion on the slope. Although slope would determine therate of water movement on the soil surface, points loweron the landscape would have greater infiltration oppor-tunity because they receive water as runoff from higherelevations. In addition, there is a migration of water inthe subsurface horizons from topslope to lower positions(2).

762 SOIL SCI. SOC. AM. J., VOL. 47, 1983

Table 2 — Particle-size distribution and organic C for the soilsat three sites.

Soilhorizon

APBlB2HB22tB3Cl

APBlB21tB22tB3ClC2

APBlB21tB22tB23tB3C

Depth

cm

0-1313-2020-5151-6969-9191-152

0-1818-2323-3838-6666-8181-117

117-152

0-1313-2020-3838-5151-6969-8989-152

Sand2000-50 n

Sitel5.25.42.52.84.23.8

Site 24.33.22.62.42.32.31.8

Site 32.82.62.31.72.23.23.3

Silt50-2^

———— 9!

64.366.446.349.960.264.9

58.858.654.558.967.069.772.3

57.555.257.262.065.467.172.2

Clay<2,,

30.528.251.237.335.630.3

36.938.242.938.730.728.025.9

39.742.240.536.332.429.724.5

OrganicC

1.601.420.930.580.270.15

1.451.280.890.550.280.180.13

1.421.200.670.440.290.200.14

Table 3—Bulk density, percent water content at several levels ofsuction, and available water.

These results show that there were no differences intotal water content in the soil between the three slopesat the time when enough water was supplied by eitherirrigation or rainfall. During the dry periods (waterstress), however, soil on 2% slope (0-137 cm) containedabout 35 mm less water than soils on 4 and 8% slopes.

Hydrologic BalanceThe water balance of a plant root zone is

where P is the precipitation and irrigation, E is the eva-potranspiration, F is the drainage at the bottom of theroot zone, AW is the change in stored water within theroot zone, and G is the surface runoff (1).

Amounts of rainfall and irrigation for the 85-d studyperiod are given in Table 4. The period between 25 Juneand 24 July was the driest one. Most crop water needsduring this time were supplied by irrigation. Irrigationwater was applied six times from 18 June to 21 July. Anaverage of 20 mm of water were applied during each

E

H 600-1 1 1H 560

8 520-]

440-

400

—— SITE I""""" SITE 2—— SITE 3

100-

50-

, RAINFALL1 IRRIGATION

pi lif. filpnn[iii,.^liT -v.l10 20 30 10 20 30 9 19 29 8 18

JUNE JULY AUG. SEPT.Fig. 1—Soil water content in toe 0- to 137-cm depth of three sites from

18 June to 8 September.

Soil water content

Hori-zon

APBlBitB22tB3Cl

APBlB21tB22tB3ClC2

APBlB21tB22tB23tB3C

Depth

cm

0-1313-2020-5151-6969-9191-152

0-1818-2323-3838-6666-8181-117

117-152

0-1313-2020-3838-5151-6969-8989-152

Bulkdensity

g/cm"

1.381.401.391.471.451.50

1.351.381.401.451.481.391.34

1.291.331.461.481.421.391.37

-0.1 -0.33bar bar

Sitel43.543.847.344.145.841.6

Site 243.645.545.644.542.545.248.2

SiteS46.142.042.841.443.544.946.6

-1bar

% by volume -

38.139.343.840.140.639.6

40.840.441.740.939.742.443.6

42.840.040.439.139.841.743.8

30.537.435.934.731.4

35.636.134.432.733.432.4

33.136.833.633.131.329.5

-15bar

18.619.027.226.927.826.6

22.024.026.926.425.222.021.3

23.024.626.726.324.122.019.6

Availablewater

cm'lcm*

0.190.200.170.130.130.13

0.190.160.150.150.150.200.22

0.200.150.140.130.160.200.24

irrigation at an intensity of 20 mm/h. After 21 July,precipitation supplied the crop water needs. The totalamount of water applied by precipitation and irrigationwas 610 mm. From this, 120 mm were supplied by ir-rigation and 490 mm by rainfall. Similar evapotranspir-ation rates were assumed at all sites, since all slope gra-dients had the same aspect and all conditions includingcrop, rainfall, field operations, and irrigation were thesame. Daily evapotranspiration rates were calculated us-ing mean daily temperature at Beatrice, Nebr. (9). Totalamount of evapotranspiration from 10 June to 8 Septem-ber was 494 mm.

Surface runoff occurred nine times on some or all sitesand was caused only by rain. Total runoff from 18 Juneto 8 September was 48, 80, and 108 mm for sites 1, 2,and 3, respectively. Total runoff appeared to increase asthe slope changed from 2 to 4%. Further increase to 8%caused more runoff, but the increase was not as great asthat from 2 to 4% slopes (Fig. 2).

Soil water depletion represents the total amount of sur-face runoff, evapotranspiration, and internal drainage be-yond the 137-cm depth for a specific period of time. Thetotal amount of water removed from the soil calculatedfrom daily water depletion was 600, 590, and 642 mm

120-

£ 10O-

O 80'a: 60-UJ

§ 40-20-

RUNOFF

SLOPEFig. 2—Amount of runoff and drainage for different slopes.

HANNA ET AL.: EFFECT OF SLOPE ON WATER BALANCE UNDER CENTER-PIVOT IRRIGATION 763

Table 4—Amount of rainfall and center-pivot irrigationat the study site.

Date

16 June19 June21 June24 June2 July6 July7 July

11 July13 July17 July21 July24 July28 July1 August4 August5 August8 August

11 August14 August15 August19 August22 August25 August26 August28 August31 August1 September3 September4 September

TotalTotal rainfall

Rainfall Irrigation————————— mm —————————

28

29t12f

421

21833

7101040t22t25f86t2186

19T53

80t19T

490mm+ total irrigation

20

1922

172220

Total 120 mm= 610 mm

t Rainfall resulting in runoff on one or more slopes.

for sites 1, 2, and 3, respectively. The total amount ofrainfall plus irrigation for the period was 610 mm. Storedsoil water for the period decreased 4, 3, and 19 mm forsites 1, 2, and 3, respectively.

The amount of water that moved below 137 cm (in-ternal drainage) at each site was calculated from thewater balance equation. Total amounts of runoff, eva-potranspiration, and internal drainage calculated fromsoil water depletion were 15 and 24 mm less at sites 1and 2, respectively, and 12 mm more at site 3 than thatcomputed from the water balance equation (Table 5).This means that relying entirely on water content mea-surements to calculate the water depletion rate under-estimated the amount of depletion by 2 to 5% on slopesof 2 and 4% and overestimated it by 2.5% on the 8%slope. The average error was ± 3%.

Relative amounts of total runoff, total evapotranspir-ation, and total internal drainage beyond the 137-cmdepth are shown in Fig. 3. Differences of bar heights fromthe total precipitation and irrigation represent decreaseof soil water storage at the site. As the slope increased,surface runoff increased and internal drainage decreased.

800-

600-EEa, 400-Ultt^ 200-

n.

RAINFALL PLUSSITE 1 SITE 2 SITE 3 IRRIGATIONn

i

1

ffi

11m

FH

JS

|11

D EVAPOTRANSPIRATIONS RAINFALLH RUNOFF

S IRRIGATIONDRAINAGE BELOW137cm

Fig. 3—Measured total runoff, evapotranspiration, calculated drainage,and rainfall plus irrigation from 16 June to 8 September.

For the 85-d study period, total runoff accounted for7.9, 13.1, and 17.6%, and total drainage accounted for12.0, 6.6, and 4.7% of the total amount of irrigation andrainfall for 2, 4, and 8% slopes, respectively. Internaldrainage losses result from leaching when water in excessof that required to wet the rooting zone was supplied byrainfall. The losses after a heavy rain continue until ex-traction by plants and evaporation from the soil surfacecreate an upward hydraulic gradient throughout the rootzone. Slope increases the internal movement of waterdownslope. However, the high clay content of the B21thorizon at site 1 (2% slope) restircted water movementfrom lower horizons to the upper ones, especially at theearly stage of plant growth when most of the plant rootswere above this layer. This caused water to move below137 cm after heavy rains, thus increasing the amount ofinternal drainage losses at this site.

CONCLUSIONS1. Surface runoff caused by rain appeared to increase

as slope increased, while total internal drainage beyondthe 137-cm depth seemed to vary inversely with slope.

2. The total amount of water depletion did not seemto be affected by slope, since slope increased runoff andreduced internal drainage at the same rate.

3. Soil water content (0- to 137-cm depth) was higherin the soil on the 8% slope than in soils on 2 and 4%slopes during most of the growing season.

4. Calculation of water depletion from the soil profileusing water content measurements only gave an error of± 3% from measured water depletion using the waterbalance equation.

REFERENCES1. Black, T.A., W.R. Gardner, and C.B. Tanner. 1970. Water storage

and drainage under a row crop on a sandy soil. Agron. J. 62:48-51.

Table 5—Measured runoff, evapotranspiration, and calculated drainage from water balance equation compared with that determinedfrom soil water depletion from 16 June to 8 September.

SitelSite 2SiteS

Rainfallplus

irrigation(A)

610610610

Runoff(B)

4880

108

Evapotrans-piration

(C)

494494494

Drainage(D)

734028

Differencein soil

moisture(E)

-4-3

-19

Water losses

Waterbalance

(B+C + D)

615614630

Waterdepletion

gravimetric

600590642

764 SOIL SCI. SOC. AM. J., VOL. 47, 1983