Functions to Predict Effects of Crop Water Deficits

A well-advanced research program is aimed at quantitative prediction of relations between principal crops and water. Goals are to: (1) estimate functional relations between crop yield Y and water at all water supply levels; water being defined as seasonal depths of both evapotranspiration ET and irrigation IRR; and (2) optimize water management by maximizing profit or water use efficiency or other objective. Davis, Calif. field and lysimeter studies with corn show: (1) the Y versus ET function is linear, provided unavoidable ET deficits from limited irrigation water coincide with those crop growth stages that influence yield the least; and (2) the Y versus IRR function is convex, reflecting decreasing irrigation efficiency (percentage of IRR utilized ET) as actual crop ET approaches fulfillment of maximum requirements. When IRR equals zero the two functions become one, and ET derives entirely from stored soil water and rainfall.