Iraq is experiencing water-resource shortages which are expected to become more severe in the future. It is believed that rainwater harvesting will be one of the solutions to overcome this problem. In this paper, rainwater harvest modeling techniques were applied to the Sinjar area of northwest Iraq for agricultural purposes. A watershed modeling system (WMS) and linear programming (LP) optimization techniques were applied to maximize the irrigated area, which could be supplied by each selected reservoir for the period 1990-2009. This technique proved to be efficient for solving large-scale water supply problems with multiple parameters and constraints, including the required input data for the model. Two scenarios of operation were considered for each main basin. In the first, each reservoir was operated as a separate unit while, in the second, all reservoirs within the basin were operated as one system. Both scenarios gave encouraging results but Scenario 2 provided better results.