In attempting to relate water supply to crop yield, both soil water and time have to be considered. An integrated soil water deficit, which can be estimated from climatologlcal and soil data alone, is defined. Data for 19 years from a commercial coconut estate of 800 acres were used to derive a regression equation relating the yield in a particular crop year with the integrated soil water deficit over the 29 months preceding the start of the crop year. The correlation coefficient between yield and the integrated deficits is - 0.81, significant at the 1% level. Yield is more closely related to the integrated deficit than to rainfall.