Illegal water diversions and lax rule-enforcement are common on irrigation canals. We present a mathematical programming model of a watercourse, calibrated to a canal in Maharashtra on which farmers voted to cooperate to control water theft. The model solution computes the crop choices and profits of individually optimizing farmers who differ in their location. It reveals the spatial distribution of gains and losses from cooperation. It illuminates why voluntary bargaining will rarely achieve an efficient water allocation. It also shows that landless laborers might well be against local cooperation, if the expropriated water nurtures labor-intensive crops.