Irrigation Quantity and Uniformity and Nitrogen Application Effects on Crop Yieldand Nitrogen Leaching

X. P. Pang, J. Letey,* and L. Wu

ABSTRACTThe combined effects of irrigation and N management on crop yield

and NOf leaching have not been extensively investigated. The objectiveof this study was to quantify the relationships between irrigationmanagement (including uniformity) and N management on corn (Zeamays L.) yield and NOf leaching. Yield and N leaching were simulatedusing the CERES-Maize (version 2.10) model for various combinationsof irrigation amounts and uniformity and N amount and timing ofsplit N applications for semiarid conditions typical of Tulare Countyin California. Simulated grain yield increased, reached a plateau, andthen decreased with increase in applied water under uniform irrigation.The amount of applied water above which yield decreased was higherfor the higher N application rate and the later simulated split Napplication. The simulated amounts of N leached were consistent withthe yield results. The higher water applications that lead to reducedyields were associated with higher N leaching for a given N applicationamount. The effects of irrigation were simulated assuming Chris-tensen's Uniformity Coefficient (CUC) of 100, 90, and 75. The resultswere only slightly affected by CUC = 90 compared with 100. A CUCof 75 caused a reduction in yield and increase in N leaching comparedwith uniform irrigation. The lowest CUC required a higher N applica-tion to achieve the same yield as uniform irrigation. Under nonuniformirrigation, it is impossible to manage either water or N application ina manner to achieve high yields without considerable NO.f leaching.High yield and low NOf leaching are compatible goals and can beachieved by appropriate irrigation and fertilizer management forirrigation systems that have a CUC of 90 or greater.

WHEREAS THE ORIGINAL GOAL of agricultural ptO-ducers was to maximize productivity, presently

this goal is moderated by the need to protect environmen-tal quality. Farmers have new constraints on their man-agement decisions. The scientific community is chal-lenged to provide quantitative technical information toguide the farmer in making management decisions thatoptimize the dual goal of high crop yield and low environ-mental degradation.

Elevated NOf concentrations in water, particularlygroundwater, are prevalent and fertilizer use is one con-tributing factor. Application of nitrogenous compoundsto land is a source of N that may eventually reachnontarget ground or surface waters. Nitrogen that re-mains in the crop root zone is not a pollutant unless itis transported by water flow to ground or surface water.Thus, both N and water management are influentialfactors in the quest for producing high yields with mini-mal water quality reduction. Nitrogen and water arecodependent management factors that cannot be com-pletely evaluated as independent factors, as has beenpredominant in past research.

Department of Soil and Environmental Science, Univ. of California,Riverside, CA 92521. Research supported by California Fertilizer Re-search and Education Program. Received 18 Dec. 1995. *Correspondingauthor (Letey @ ucrac 1. ucr. edu).

Published in Soil Sci. Soc. Am. J. 61:257-261 (1997).

Irrigation uniformity is a critical factor affecting irriga-tion management. The effects of irrigation uniformityon crop yield have been documented (Heerman et al.,1990; Seginer, 1978; Warrick and Gardner, 1983). Leteyet al. (1984) evaluated the effects of uniformity of infil-tration rates on optimal levels of water application. Intheir study, crop water production functions were devel-oped between applied water, yield, and drainage fordifferent levels of irrigation uniformity. Hunsaker et al.(1991) conducted field studies to evaluate the effects ofvariations in soil water content and field surface onuniformity of infiltrated water for a level basin irrigationsystem. Warrick and Yates (1987) evaluated crop yieldas a function of amount and uniformity of irrigation.

None of the previous studies investigated the combinedeffects of irrigation uniformity and N applications oncrop yield and NOf leaching. Hereafter, NOf leachingwill be quantified by the amount of NOf-N leachingand will be referred to as N leaching. Large amountsof nutrients such as NOf could be leached from the rootzone, causing economic loss and contamination ofgroundwater, with excessive water, which probably oc-curs in parts of the field with low irrigation uniformity.The cost associated with increased fertilizer need or adecrease in yield due to N leaching must be includedin economic analyses (Letey et al., 1984). The costsassociated with water quality reduction must also beconsidered.

Experimentally quantifying the combined effects ofirrigation and N management on yield and N leachingis expensive. Computer simulation models that integratethe effects of soil, climate, and management on cropgrowth and availability and movement of N in the rootzone provide a less expensive means of obtaining approxi-mate relationships. One such model is the Crop Environ-ment Resource Synthesis (CERES)-Maize (version 2.10)model (Jones and Kiniry, 1986). The CERES-Maizemodel simulates corn growth and has the capability ofsimulating the impact of various irrigation and N fertilizermanagement scenarios on crop yield and N leaching. Inthis study, the CERES-Maize model was used to simulatecrop yield and N leaching for different irrigation and Napplication levels under different uniformities using soiland weather data of a given location.

Although models can synthesize information quicklyand inexpensively, the value of the model is contingenton the degree to which the model accurately reflectsthe natural processes. The CERES-Maize model wasevaluated by Pang et al. (1997) using experimental dataon irrigation and fertilizer management trials on cornconducted at Davis, CA, during 1975 to 1977 (Tanji etal., 1979). The model simulated the measured yield andtotal N uptake under Davis, CA, conditions where corngrowth relies primarily on irrigation. It was concludedthat the CERES-Maize model may be applied with confi-

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258 SOIL SCI. SOC. AM. J., VOL. 61, JANUARY-FEBRUARY 1997

N. ka/ha Date180 5/10180 6/01180 6/20240 5/10240 6/01240 6/20

120 160 200 240

Total irrigation, cmFig. 1. Simulated corn grain yield as a function of seasonal irrigation

amounts for N applications of 180 and 240 kg/ha. One-half of theN applications was applied at planting (15 April) and the secondhalf was applied on 10 May, 1 June, or 20 June.

dence to study the effects of N and irrigation managementon corn yield and N uptake under irrigated semiaridconditions. Measurements of NOa" leaching were notmade so no comparision between measured and simu-lated NOf leaching could be made. However, since themodel predicted N uptake and yield quite well, one wouldexpect the simulated NOf leaching to be reasonable.

The goal of this study was to quantify the relationships

200

N. ka/ha Date180 5/10180 6/01180 6/20240 5/10240 6/01240 6/20

80 120 160 200 240

Total irrigation, cmFig. 2. Simulated N leaching as a function of seasonal irrigation

amounts for N applications of 180 and 240 kg/ha. One-half of theN applications was applied at planting (15 April) and the secondhalf was applied on 10 May, 1 June, or 20 June.

between irrigation management (including uniformity)and N management on corn yield and NOf leaching.

PROCEDURESThe sequence of simulations was as follows. Yield and N

leaching were computed as functions of applied water foruniform irrigation and different quantities and timing of Napplication. These data were then used to compute yield andN leaching as functions of applied water for three levels ofirrigation uniformity and two N application rates. The relation-ships between yield and applied water were simulated for threelevels of irrigation uniformity with the nonlimiting N conditionsimposed on the model. The optimal irrigation amount wasselected for each irrigation uniformity from these simulatedresults. Finally, the yield and N leaching were computed asfunctions of N application for the optimal water applicationsfor each irrigation uniformity.

The simulations were conducted using conditions typical ofTulare County in California. The procedures to determine soiland crop genetic input data of the model were given by Panget al. (1997). The soil data were based on the soil survey ofTulare County, CA (Soil Conservation, Service, 1982) andthe weather data were retrieved from the California IrrigationManagement Information System (CIMIS).

Split N applications of 180 and 240 kg/ha were used in thesimulations. One-half of the N was applied at planting (15April) and the second half was applied on 10 May, 1 June,or 20 June. Fourteen equal irrigation applications were usedin this study. The timing of irrigation was determined by theCERES-Maize model such that irrigation was applied when thewater amount in the soil profile reached 50% of the extractablewater. Irrigation intervals based on the model simulation wereabout 1 wk. This irrigation schedule was consistent with theschedule suggested by the Cooperative Extension advisor(1995, personal communication). The seasonal simulated irri-gation amounts ranged from O to 280 cm and were assumedto be applied uniformly in the first set of computations.

The detailed procedure for calculation of irrigation unifor-mity effects on yield was given by Letey et al. (1984). Thedepth of applied water (AW) at a given point in the field isgiven by AW = fc(AW), where b is a parameter whosedistribution across the field must be known and AW is averagewater application for the entire field. The values of b wereassumed to be normally distributed across the field (any distri-bution could be analyzed if it was known). For computationalconvenience, b was approximated by a discrete distribution inwhich b takes a finite number (W) of distinct values b\, b2,... bfi. The fractional area of the field in which b = bn isequal to the probability that b = bN. The yield and amountof NOf leached out of the root zone was computed for eachfractional area of the field based on the amount of infiltratedwater in that area and then integrated for the whole field.

Irrigation uniformity was characterized by the CUC, anindex commonly used by irrigation professionals. The CUCwas calculated by CUC = 100[1 - (Lx/Mn)], where x is theabsolute value of the deviation from the mean, M, of theapplied water observation, and n equals the number of observa-tions. The normal distribution of AW across the field thatprovided the CUC values equal to 100 (uniform), 90, and 75were used in the simulations.

The effects of nonuniform irrigation on corn yield and Nleaching for a range of water applications with the model setat the nonlimiting N mode were computed. Based on theserelationships, the optimal irrigation amount for each of thethree irrigation uniformities was identified. The model was

PANG ET AL.: IRRIGATION AND NITROGEN APPLICATION EFFECTS 259

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N = 180 5/10

UNIFORM- CUC 90- CUC 75

50 100 1500

Irrigation, cm50 100 150

Fig. 3. Relationship between simulated corn grain yield and seasonal irrigation amounts for various levels of irrigation uniformity: (A) Napplication of 180 kg N/ha with the second applicition on 10 May; (B) N application of 240 kg N/ha with the second application on 10 May.

then run under the optimal irrigation amount for a givenirrigation uniformity for a range of N applications.

RESULTS AND DISCUSSIONThe simulated crop yields are illustrated as a function

of seasonal irrigation for different N treatments underuniform irrigation in Fig. 1. Grain yield increased sharplywith an increase in irrigation in the range of O to 75 cmfor all N treatments. Grain yield remained at maximumvalue in the range of irrigation from 75 to 100 cm forN application of 180 kg/ha and from 75 to 140 cm forN application of 240 kg/ha. Grain yields decreased withan increase in irrigation beyond these ranges. The higherthe N application rate and the later the second N applica-

tion was applied, the longer the yield stayed at themaximum and the less the yield decreased.

The simulated N leaching as a function of seasonalirrigation amount for different N treatments under uni-form irrigation is shown in Fig. 2. There was no Nleached by irrigation of <75 cm. Seasonal N leachingincreased with an increase in irrigation, and there werelarge differences in N leaching among the N treatments.The higher and the earlier the N application, the morethe N leaching.

The results depicted in Fig. 1 and 2 are consistentwith each other. Application of excess water causedleaching, which induced a N deficiency and resulted inyield reduction. Delaying the time for the second Napplication reduced the opportunity for leaching and

1500 50 100 150Irrigation, cm

Fig. 4. Relationship between simulated N leaching and seasonal irrigation amounts for various levels of irrigation uniformity: (A) N applicationof 180 kg N/ha with the second application on 10 May; (B) N application of 240 kg N/ha with the second application on 10 May.

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SOIL SCI. SOC. AM. J., VOL. 61, JANUARY-FEBRUARY 1997

120

UNIFORMCUC 90CUC 75

50 100 150 200 250

Irrigation, cmFig. 5. Relationship between simulated corn grain yield and seasonal

irrigation under N nonlimiting condition for various levels of irriga-tion uniformity.

n 100 -

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UNIFORM (lr=80cm)CUC 90 (lr=90cm)CUC 75 (lr=120cm)

100 200 300 400

N-application, kg N/haFig. 7. Simulated N leaching as a function of N application rates for

various levels of irrigation uniformity.

resulted in a higher yield for a given water and Napplication. The consistency between simulated yieldand N leaching provides evidence that the simulated Nleaching, though not previously evaluated by measureddata, is realistic. The higher water application amountswere unreasonable for uniform irrigation, but might oc-cur on some sections of the field under nonuniformirrigation. The data presented in Fig. 1 and 2 providethe basic information required for computations undernonuniform irrigation.

Examples of simulated yields as a function of seasonal

14

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PO) 6•OSBO 4H5P UNIFORM (lr=80cm)

CUC 90 (lr=90cm)CUC 75 (lr=120cm)

100 200 300 400

N-application, kg N/haFig. 6. Simulated corn grain yield as a function of N application rates

for various levels of irrigation uniformity.

irrigation for N applications of 180 and 240 kg/ha underthree values of CUC are shown in Fig. 3. A CUC of90 had relatively little effect on yield, compared withuniform irrigation, but the yields were consistently re-duced at a CUC of 75. The reduction in yield startedat 40 cm irrigation and was maximum at irrigation of80 cm. At this point the relative reduction in yielddecreased with increase in irrigation, particularly for the180 kg/ha N treatment. The maximum yield reductionfor the CUC = 75 was about 10% for both N applicationrates.

The predicted N leaching for the conditions depictedin Fig. 3 are shown in Fig. 4. Similar to yield, therewere larger effects of nonuniform irrigation on N leachingat CUC = 75 than for CUC = 90. Irrigation nonunifor-mity increased N leaching except at very high N amounts.The maximum increase in N leaching due to irrigationnonuniformity was about 10 to 15 kg/ha.

The simulated grain yields vs. seasonal irrigation undernonlimiting N for three levels of irrigation uniformitiesare presented in Fig. 5. Optimal seasonal irrigationamounts were selected at 80, 90, and 120 cm for CUCof 100, 95, and 75, respectively. Irrigation at seasonalevapotranspiration (ET) value of 80 cm produced maxi-mum yield for the uniform irrigation, but a 9% yieldreduction compared with CUC of 100 for the least uni-form irrigation.

Relationships of simulated grain yields as a functionof N application rate for three levels of irrigation unifor-mity are shown in Fig. 6. The above selected optimalirrigation amounts were used for each level of uniformity.The yields were reduced by the lowest irrigation unifor-mity for a given N application rate. Thus, more Nmust be applied for a given yield under low irrigationuniformity. The simulated N leaching as a function ofN application rate for three irrigation uniformities are

PANG ET AL.: IRRIGATION AND NITROGEN APPLICATION EFFECTS 261

presented in Fig. 7. Nitrogen leaching was extremelylow when N application was less than 240 kg/ha foruniform irrigation and only slightly higher when CUCequaled 90. The value of 240 kg/ha represented the totalcrop N uptake. When N application was greater than240 kg/ha, N leaching increased dramatically for allthree uniformity levels. The N leaching at a CUC of 75was much higher than that at CUC = 100 or 90. Thepoor irrigation uniformity resulted in very high N leach-ing, even when N application was less than the totalpotential crop N uptake. Nitrogen was leached out ofthe root zone under poor irrigation uniformity becausesome areas of the field received large amounts of water.The decrease in yield observed in Fig. 6 for the lowestirrigation uniformity is a result of the amount of Nleached out of the root zone. Note that the approximate40 kg/ha N leached under a CUC of 75 (Fig. 7) is theamount of additional N that would be applied to bringthe nonuniform irrigation yields equal to the uniformirrigation yields (Fig. 6).

Farmers are commonly reported to apply more N thanUniversity recommended rates. One consideration is thatfarmers are wasteful or that they apply excess N asinsurance against deficiency. Another explanation canbe made based on our results. The University experimentsare usually conducted on small plots with relatively uni-form irrigation. Farmer fields are much larger and irriga-tion is frequently less uniform. Under these conditions,the farmer would have to apply more N to achievethe same average yield as achieved in the Universityexperimental plots.

CONCLUSIONSThe amount and uniformity of irrigation and the

amount and timing of N application are all interacting

factors that affect crop yield and NOs leaching. Of thesefactors, irrigation uniformity is the most critical oneto improve. Without uniform irrigation, it is impossibleto manage either water or N application in a manner toachieve high yields without considerable NOf leaching.High yield and low NOf leaching are compatible goalsand can be achieved by appropriate irrigation and fertil-izer management for irrigation systems that have a CUCof 90 or greater.