Submitted to: Natural Resources Research Update (NRRU)
Publication Type: Research Technical Update
Publication Acceptance Date: July 31, 2009
Publication Date: July 31, 2009
Repository URL: http://ars.usda.gov/Research/docs.htm?docid=15371
Citation: Ahuja, L.R. 2009. Using Computer Models to Explore Alternative Scenarios for Managing Limited Irrigation Water. Natural Resources Research Update (NRRU). Update #244313. Technical Abstract: Crop water stress due to low precipitation and high temperatures are the main limiting factors for agricultural production in the Great Plains. Corn is grown under either rainfed or irrigated regimes. Irrigation can improve corn profitability in this region, but over-irrigation accelerates depletion of ground water, which may result in limits to agricultural water use. Agronomic practices which increase water use efficiency (WUE) need to be developed to maximize returns from a combination of limited irrigation and optimum utilization of the precipitation received. Such water management strategies should be based on knowledge and planning that considers the underlying natural climate variability. This can only be achieved through long-term field studies with cropping systems under limited irrigation conditions for specific climates and soil regimes. These studies can be very expensive and do not account for the future changes in management practices or climate. Agricultural system simulation models of the soil-water-crop-atmosphere-management system provide an alternative approach. Agricultural systems models are potential state-of-the-art tools, that help us integrate and synthesize knowledge gained in field studies. These models can be set up to evaluate the changes in crop growth and development in response to soil, water, and nutrient management alternatives. Scientists with the Agricultural Systems Research Unit of the USDA-ARS used the CERES-maize model for developing water production functions for corn. The scientists used field studies conducted at the Central Great Plains Research Station, USDA-ARS, in Akron, Colorado. The model accurately simulated the irrigated corn grown at this location. The ARS scientists further used the model to simulate irrigation scenarios using 94 years (1912-2005) of weather data recorded at Akron. Fixed amounts of irrigation (4 to 40 inches), were split between the vegetative (V) and reproductive (R) stages in the following manner: 20:80; 40:60; and 50:50 (V:R) 18 weeks after planting. Within the V or R stage, irrigation amount was equally split between weeks and applied at weekly intervals. Simulations showed declining grain yield with irrigation amounts greater than 16 inches. This research showed: ' The 20:80 split irrigation between V and R stages is preferred over other split treatments. In this model simulation study, nitrogen (N) fertilizer was 225 lbs/ac. Irrigation water was applied in increments of 4” from 0 to 40 inches. Three splits in irrigation water between vegetative and reproductive growth stages (V:R) were tested with the model: 20:80, 40:60, and 50:50. The text below describes the effect of increasing N fertilizer and irrigation water on N uptake by the plant, N leached below the root zone, and residual N in the soil. For the 20:80 split: ' N-uptake by the plant increased significantly with increasing irrigation up to 16 inches of applied water. Above 16 inches of irrigation water the uptake of N increased only slightly, peaking at 24 inches then N-uptake began to decline. ' N-leaching was essentially unchanged between 4 inches of applied irrigation water and 24 inches then N leaching began to increase. This increase in leaching matched the decrease in N uptake, indicating that while the plant had stopped taking up additional N the additional irrigation water began removing N from the root zone. ' Root zone residual-N decreased significantly between 4 inches and 16 inches of applied irrigation water (in inverse proportion to N-uptake) but began to level off above 16 inches of applied irrigation water. For the 40:60 split: ' N-uptake peaked at the 24 inch level of irrigation and then declined steadily beyond this point. ' N-leaching was similar to leaching to the 20:80 split with leaching increasing above 28 inches applied irrigation water but the increase in leaching above 28 inches was greater for the 40:60 split ' Root zone residual-N followed an almost identical pattern for the 40:60 split as the 20:80 split with a steady decline followed by a leveling off above 16 inches of applied irrigation water. For the 50:50 split: ' N-uptake peaked at the 20 inch level of irrigation and then declined steadily beyond this point. ' N-leaching was similar to leaching for the 20:80 and 40:60 splits but with leaching increasing above 20 inches applied irrigation water and overall was higher than the other two splits ' Root zone residual-N followed an almost identical pattern for the 40:60 split and the 20:80 split with a steady decline followed by a leveling off above 16 inches of applied irrigation water.