Fluctuations in water availability, either in the form of precipitation or stored water in surface and groundwater bodies, will affect agricultural productivity and farmers income. Climate science tells us that much of these fluctuations will be in the form of shifts in the timing and intensity of precipitation. Understanding these relationships and the accurate estimation of their economic effects may, therefore, help in the designing of effective agricultural public policies to mitigate drought and climate change impacts on agriculture. In this context, this paper introduces a novel hydro-economic model in which the timing of rainfall and supplementary irrigated water supplies affect the productivity of a partially irrigated agricultural system. The specification of the production function and water availability is designed to reflect shifts in monthly precipitation totals and to show how the opportunity cost of supplementary irrigation supply varies with changes in the timing of precipitation. Results show that shifts in monthly precipitation parterns have indeed significant impacts on agricutltural income and that the coarser the temporal resolution that the modeler chooses, the lower is her ability to precisely measure them. Acknowledgement :