Norman L. Klocke

NITRATE LEACHING IN IRRIGATED CORN AND SOYBEAN IN A SEMI-ARID CLIMATE

Nitrate-nitrogen leached from the root zone of land in intensive corn production is a major groundwater contaminant in some of the intensively irrigated regions of the western Cornbelt, including central and western Nebraska. To obtain a clearer understanding of the amount and timing of nitrate leaching losses from irrigated crops, 14 monolithic percolation lysimeters were installed in 1989-1990 in sprinkler irrigated plots at the University of Nebraska’s West Central Research and Extension Center near North Platte, Nebraska. The lysimeters were used to provide a direct measure of leachate depth from continuous corn and a corn-soybean rotation. Both cropping systems were sprinkler irrigated and used current best management practices (BMPs) in the region for water and nitrogen management. Leachate was collected from 1990 through 1998 and analyzed for nitrate-N concentration. Results for the period 1993- 1998 are reported here. In the semi-arid climate of West-Central Nebraska, the interaction of rainfall patterns with the period of active uptake of water by crops played a major role in defining leaching patterns. Careful irrigation scheduling did not eliminate leaching during the growing season. There was no significant difference in drainage depth between continuous corn and the corn-soybean rotation. The average drainage depth among the lysimeters was 218 mm yr–1. This was more than expected, and in part resulted from above normal precipitation during several years of the study. No water quality benefit was found for the corn-soybean rotation as compared to continuous corn. Nitrate-N concentration in the leachate from continuous corn averaged 24 mg L–1, while that from the corn-soybean rotation averaged 42 mg L–1. Total yearly nitrate leaching loss averaged 52 kg ha–1 for continuous corn and 91 kg ha–1 for the rotation. This represents the equivalent of 27% and 105% of the amount of N fertilizer applied over the six years of study. In calculating N fertilizer needs for corn in Nebraska, the recommended legume N credit of 50 kg ha –1 for a preceding crop of soybean may be too low under irrigated production.

EVAPORATION FROM SOIL INFLUENCED BY CROP SHADING, CROP RESIDUE, AND WETTING REGIME

ABSTRACT Soil evaporation (E) was measured with mini-lysimeters 225 mm deep and 150 mm in diameter over the growing season in 1986 and 1987. Mini-lysimeters were placed in com field plots under dryland, limited irrigation and full irrigation water regimes, with or without canopy shading and with or without straw mulch. Soil water content in the mini-lysimeters was periodically matched to that in the surrounding field. Mean daily E was calculated and the effects of canopy shading and straw mulch were analyzed. No significant differences in E between mini-lysimeters at different row positions under the corn canopy were observed. Shading by the corn canopy significantly reduced E 0.3 to 0.5 mm d-i under dryland, 0.6 to 0.7 mm d-i under limited irrigation, and 1.2 mm d-i under full irrigation. The presence of a straw mulch significantly reduced E 0 to 0.1 mm d-l under dryland, 0.5 mm d-i under limited irrigation, and 0.9 to 1.1 mm d-i under full irrigation. The crop canopy played a more important role in reducing soil E than straw mulch under dryland. Under limited or full irrigation the crop canopy and straw mulch contributed equally to E reduction. Combined reduction of mean daily E by the crop canopy and straw mulch was approximately 0.5, 1.0, and 2.0 mm d-l for dryland, limited irrigation, and full irrigation wetting frequencies, respectively. KEYWORDS. Soil evaporation, Lysimeters.

Soil water evaporation and crop residues.

Crop residues have value when left in the field and also when removed from the field and sold as a commodity. Reducing soil water evaporation (E) is one of the benefits of leaving crop residues in place. E was measured beneath a corn canopy at the soil suface with nearly full coverage by corn stover or standing wheat stubble. E was also measured from a soil surface that was partially covered with corn stover without crop shading. E was measured with mini-lysimeters that were 300 mm in diameter and 140 mm deep. Surface coverage and amount of dry matter of crop residues influenced E. E was reduced nearly 50% compared with bare soil E when corn stover and wheat stubble nearly covered the surface under a corn canopy during the growing season. Partial surface coverage, from 25% to 75%, with corn stover caused small reductions in E compared with bare soil when there was no crop canopy. Full surface coverage reduced energy limited E 50% to 65% compared with E from bare soil with no shading. No-till management, using crop residues to significantly reduce E, required soil surfaces to be nearly covered. Economic benefits of crop residues for E suppression during the growing season can be as much as

Field Scale Limited Irrigation Scenarios for Water Policy Strategies

365 ha-1.

CROP ROTATIONS WITH FULL AND LIMITED IRRIGATION AND DRYLAND MANAGEMENT

Approaches to reducing irrigation inputs to crops have been studied for the past 50 to 60 years in research settings. Fewer efforts have been made to document limited irrigation responses over a number of seasons on commercial fields. This study compared farm-based irrigation management (FARM) with best management practices (BMP), late initiation of irrigation (LATE), and a restricted allocation (ALLOC). These irrigation management strategies each occupied 1/8 of a center pivot system in southwest Nebraska in continuous corn production, on four cooperating farms, which were replicated at the same sites for 3 to 6 years. Irrigation variables were achieved by irrigating or not irrigating, or by speeding up or slowing down the center pivot. When the grain yields and irrigation amounts were normalized each year using the FARM treatment as the basis, on average for three of four locations, the BMP treatment yielded equal to the FARM treatment, the LATE treatment yielded 93% of the FARM treatment and the ALLOC yielded 84% of the FARM treatment. At the same time, it took 76% and 57% of the water for the LATE and ALLOC treatments, respectively, to achieve these yields. The adjusted gross returns (yield . price – irrigation treatment costs) of the irrigation treatments were analyzed for each location. When the gross returns were normalized using the FARM treatment as the basis, FARM and BMP returns were equal across combinations of high and low input commodity prices and pumping costs. The LATE treatment gross return was 95% of FARM return. The gross return for the ALLOC treatment was 85% to 91% of the FARM treatment. The higher the water costs, the lower the difference between the highest and lowest returning water treatments. Relationships between evapotranspiration and grain yield were developed for two sites over the limited range of water applications of the projects. Regressions indicated more variability between the commercial field data and research plot environments. Much of this difference may have been due to yearly replication in this study rather than plot-to-plot replication in the research center study. Yield and irrigation data were normalized on the basis of the FARM treatment. Normalized yield – irrigation results over years and locations for three of the four locations showed declining yields as irrigation decreased. The same regression was used to normalize the locations with soil textures from fine sand to sandy loam, which suggested that the three locations behaved similarly with respect to the management treatments.

Design, Installation, and Performance of Percolation Lysimeters for Water Quality Sampling

ABSTRACT Irrigated cropping systems need to maximize the economic value of both rainfall and irrigation water, especially in areas of declining groundwater. This study compared water management systems in a winter wheat (Triticum aestivum, L.)-corn {Zea mays, L.)-soybean (Glycine max, L.) (W-C-S) and continuous corn (CC) rotation in west central Nebraska for dryland, limited irrigation (150 mm/yr), and full irrigation. Crop yield, evapotranspiration, and soil water storage were determined from field studies conducted at North Platte, Nebraska, on a Cozad silt loam (Fluventic HaplustoU) soil. Dryland com used 21.5% more evapotranspiration (ET) in the W-C-S rotation compare to CC. ET for the limited and full irrigation com was 4.6% and 4.9% more for the W-C-S rotation compared to the CC and was statistically significant at the P > 0.08 level. Water use efficiency, defined by the slope of the linear relationship between grain yield and ET (3Y 3ET-^), was the same for com in the W-C-S and CC rotations. Com grain yield response to irrigation and ET was more than the yield response of winter wheat and soybean. The W-C-S rotation increased com grain yields in two out of three years at this location for dryland management and increased the seasonal ET of corn compared to continuous corn. Full irrigation management did not consistently increase winter wheat and soybean grain yields above the limited irrigation treatments. Soil water storage for the full irrigation management was greatly reduced compared to dryland and limited irrigation management for both rotations.

Corn Yield Response to Deficit Irrigation

Lysimeters are the primary research tool for measuring percolation and water quality. Monolithic percolation lysimeters were evaluated for measuring the quantity and quality of leachate from the root zone of irrigated crops. Six percolation lysimeters were installed in a continuous corn (Zea Mays, L.) cropping system near North Platte, Nebraska, during the fall of 1988 and spring 1989. The lysimeters were 0.9 m in diameter and 2.4 m deep. They were filled with undisturbed soil using a hydraulic pull-down method. Porous stainless steel extractors were installed vertically upward into the lysimeter bottoms, and leachate was extracted from the unsaturated soil. Leachate volume, volumetric soil water content, soil temperature, and soil bulk density were measured. Isolation of a large soil monolith in the lysimeter did not significantly affect plant growth, soil bulk density, or temperature. Soil water content near the lysimeter bottoms was greater than in the surrounding field after a wet spring in 1991. However, extractors removed most excess water and adequately matched unsaturated drainage in the field. Though leachate varied among lysimeters, they behaved similarly over time. Results will help relate irrigation management and scheduling strategies to potential leaching of soil water and associated chemicals into groundwater.

Water Allocation Model for Limited Irrigation

Because dwindling water supplies are limiting crop production, a field study was conducted during 2005-2009 in southwest Kansas to determine the yield response of corn to irrigation and evapotranspiration (ETc), and to document plant growth parameters and soil water use. Corn was grown in a five-year rotation of corn-corn-wheat-grain sorghum-sunflower. Results from the corn after sunflower and corn after corn are presented here. Six irrigation treatments were produced by applying 25 mm of irrigation every 5 to 17 days. Year-to-year grain yields averaged over irrigation and crop sequence appeared to be correlated with leaf area index, which possibly reflected the severity of hail events that occurred in four of the five years of the study. However, dry matter accumulation per plant did not vary across irrigation treatments. Surface residue coverage from the previous years crop was 38% for sunflower and 61% for corn. ETc and productivity, also known as water use efficiency (WUE), decreased significantly as irrigation decreased. The deficit irrigation treatments used more of the previous non-growing season precipitation than the fully irrigated treatment due to greater soil water storage capacity in the drier soil profile. Furthermore, these treatments extracted more soil water during the growing season as irrigation decreased. Linear models of ETc predicted grain and dry matter yields with R2 values of 0.67 and 0.59, respectively. The relationship of relative grain yield and ETc was also linear and more pronounced, with an R2 value of 0.82. In contrast, the relationship of relative yield and irrigation followed a curvilinear model. During the five-year study, variability in yields increased as irrigation decreased, illustrating a greater income risk with less irrigation. Yield response to irrigation, especially over multiple years, is essential information to build economic studies of cropping alternatives, deficit irrigation management, and income risk. These relationships need to be developed regionally to characterize the effects of environmental factors, especially precipitation.

Equations for Drainage Component of the Field Water Balance

For irrigation managers with limited water resources, irrigation management decisions begin well before the irrigation season. Irrigation managers with limited water supplies from restricted well capacities or water allocations need to anticipate crop selections, plan for crop rotations, and project water deliveries to each crop. A water allocation model [Crop Water Allocator (CWA) available at www.oznet.ksu.edu/mil] has been built to evaluate growing-season water allocations among two to six crops over five possible divisions of land area. Users input crop pricing, production costs, irrigation costs, and maximum crop yields. The program iterates all possible combinations of the water allocation by 10% increments over all possible crop combinations and a chosen land division. Net economic return to land, labor, and irrigation equipment is calculated for each crop mix/water allocation/land division combination. Net returns are ranked, and several of the highest are presented to the user for evaluation. The influence of one variable input on another, such as water allocation, commodity prices, crop yields, annual rainfall, irrigation system efficiency, and irrigation operating costs on net return can be evaluated through multiple executions of the model.

Long-Term Response of Corn to Limited Irrigation and Crop Rotations

Accurate estimates of the drainage component of the field water balance are needed to achieve improved management of drainage in irrigated crop production systems and obtain improved estimates of evapotranspiration (ET) from soil water measurements. Estimating drainage for numerous soil and field conditions necessitates the use of simple, yet accurate, drainage equations containing easily measured parameters. The Wilcox drainage model is a relatively simple mathematical equation with a high degree of accuracy and applicability to field conditions. Our objectives were to develop Wilcox-type drainage rate equations for three coarse-textured soils of the west-central Great Plains and assemble previously determined, but fragmented, Wilcox-type drainage equations and supporting information for three medium-textured soils of the region. Drainage plots for collection of data for development of Wilcox-type drainage equations were established on two coarse-textured soil profiles in 2008 near Garden City, Kansas. Total water content of the soil profiles was measured over time during ~48-day drainage events. Total water was plotted against drainage time on log-log scales, and the linear regression equation relating the two variables was determined. These linear equations of profile water (log10) vs. drainage time (log10) were used to develop Wilcox-type drainage equations in which drainage rate (dW/dT in mm/day) is expressed as a function of soil profile water content (in mm). Drainage rate equations in this article can be used to estimate the drainage component of the field water balance for improved irrigation water management and more accurate estimates of ET from soil water measurements.