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.