A Benchmark to Diagnose and Improve Water Productivity in Maize Cropping Systems.

Useful benchmarks are those based on understanding of biophysical processes that determine crop productivity in response to environment x management interactions. The challenge is translating these complex processes into decision-support that is useful to farmers and policy-makers. The present study defined a benchmark for maize water productivity (WP) based on the relationship between simulated grain yield and water supply (n = 859). Yields were simulated for irrigated and rainfed conditions in 18 locations in Western Corn-Belt using 20-year weather records and site-specific soil properties and management practices. Three parameters were defined for the WP benchmark: x-intercept (~ soil evaporation), slope (~ water-use efficiency), and asymptote (~ yield-potential). The WP benchmark was validated against observations collected from crops grown under near-optimal conditions in a wide range of environments, water regimes, and irrigation systems (n = 123) and used to identify management practices to improve WP in irrigated commercial fields in central Nebraska (n = 777 fields). The defined WP benchmark had x-intercept, slope, and asymptote equal to 100 mm, 19.3 kg ha^-1 mm^-1, and 14.4 Mg ha^-1. When validated against field data, most of the observations were distributed around the WP benchmark, except for five rainfed crops with severe water deficit during silking-pollen shed window. On-farm WP analysis indicate that farmer’s fields are, on average, 20% below the WP benchmark and a substantial number of fields (55%) had water supplies in excess of water requirements to achieve yield-potential. Major opportunities for increasing on-farm WP include turning surface to sprinkler systems, adoption of conservation tillage in fields under soybean-maize rotation, and better irrigation schemes, which together could reduce district-level water requirements by 32% without yield reduction. The WP benchmark defined in this study can be used in other maize-producing regions after calibration of benchmark parameters to site-specific evaporative demand and yield potential.