Income differences attributed to differential access to water in irrigation systems are common. Prior studies of farm‐level water use in developing areas have typically been limited to using number of irrigations as a proxy for water use. We develop a volumetric measure in Sri Lanka's Kirindi Oya Irrigation System through recent farmer recall and use it in production function estimation and welfare analysis. Findings indicate substantial differences in water use by farms across seasons and across subareas of the irrigation district. Alternative plans for allocating additional water among seasons and subareas to elevate net rice revenues are examined. The recommended plan predicts higher net returns from rice and greater equality in the distribution of household income than would be generated if the additional water were allocated based on the current allocation criteria. Concurrent application of fertilizer at recommended levels would further increase net revenues and reduce income inequality.