A procedure has been developed to increase irrigation efficiency by estimating probabilistic irrigation requirements for lowland rice cultivation. The procedure uses a water balance equation with random components to maintain minimum desired water levels in paddy fields at the end of each irrigation period. The procedure estimates weekly pan evaporation and rainfall at different probability levels, which are then used to estimate weekly irrigation requirements through the water balance relationship. The method has been applied to the Kalawewa irrigation scheme in Sri Lanka. Different statistical transformations were used to normalize observed weekly rainfall and evaporation data and to describe their variations. The power transformation was best able to transform the weekly data to normality. Comparison of the use of the model and current system practices shows that a significant amount of water can be saved even when the system is operated at high probability levels.
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