This paper uses mechanism design theory to (i) propose a mechanism to price irrigation water when farmers are heterogeneous in their production technologies (adverse selection) and their individual water uses are unobserved (moral hazard) and (ii) discusses briefly when such a mechanism might be economically unreasonable. Unmetered irrigation water is often priced by imposing per-acre fees on cultivated acreage or by charging per-unit fees on observable inputs or outputs. The offered pricing procedure is based on the observed output and achieves a first-best outcome when implementation is free of transaction costs.