Exploring the Limits to Maize Water Productivity in the Western Corn-Belt.

Unlike the Central and Eastern Corn-Belt where maize is grown almost entirely under rainfed conditions, maize in the Western Corn-Belt is produced under both irrigated and rainfed conditions. Simulation modeling was used to identify and better understand the factors that determine maize water productivity under irrigated and rainfed conditions in the Western Corn Belt.  Yield and water balance were simulated for irrigated and rainfed crops, using 20-year weather records from 18 locations in combination with actual soil, planting date, plant population, and hybrid-maturity data. Regression analysis was performed to evaluate yield variation of irrigated crops in relation to meteorological factors. Boundary functions for water productivity and water-use efficiency were derived and compared against observed data reported in the literature. Yield of irrigated crops depended on the specific radiation / temperature regime during the post-silking phase at each location. Rainfed crops grown in the Western Corn-Belt are frequently subjected to episodes of transient and unavoidable water stress, especially around and after silking. Soil water at sowing ameliorates, but does not eliminate water stress episodes. Boundary function for water productivity had a slope of 28 kg grain ha^-1 mm^-1. This function can be used to benchmark water productivity in farmer’s fields. At high seasonal water supply, yield was weakly correlated with water supply because many crops did not fully utilize seasonally available water due to deep percolation or water left in the ground at maturity. Fitted boundary function for water-use efficiency had a slope (≈ seasonal transpiration-efficiency) of 37 kg grain ha^-1 mm^-1 and an x-intercept around 75 mm (≈ seasonal soil evaporation). Data collected from experiments conducted in a wide range of environments indicated that the boundary function for water-use efficiency is broadly applicable. Estimated maize maximum seasonal transpiration-efficiency is above to that reported for other crops.