Matching cropping systems to available water supply is a major element in the complex process of choosing a suitable cropping system. This manual, using examples from studies conducted in Pakistan, describes an integrative mathematical model to assist in this process. The first step in evaluating cropping systems relative to water supply is to determine the climatic potential for crop water use, or "potential evapotranspiration" (Etp). Etp may be defined as the water use of some standard reference crop growing as if water were not limiting. (The Penman and the Modified Jensen-Haise methods of calculating Etp are used.) The next step is to estimate actual evapotranspiration that will occur in the field; that is, the crop evapotranspiration that would normally occur plus the excess transpiration that would occur from crop and soil surfaces that may temporarily be abnormally wet from irrigation or rainfall. A computer program has been developed to calculate consumptive use for eight common crops in Pakistan -- citrus with a cover crop, citrus without cover, cotton, berseem (Egyptian clover), maize, rice, sugarcane, and wheat. The input required for these calculations is a file of weather data containing the necessary information for calculating Etp. Next, calculations of crop consumptive use (for both single crops and crop mixes) are integrated with the irrigation supply requirements by use of a simulation model. The model is programmed in the HPL language for use on the Hewlett Packard model 9825A desktop calculator. In a less technical final section, use of the linear programming (LP) technique for crop selection and/or water pricing is explained. Possible strategies include combining crops whose peak irrigation demands are complementary, selectively stressing some crops in order to stretch a fixed supply of water, and substituting more or less water-intensive crops for one another. It is shown that the LP technique complements the simulation model, with the latter providing essential data for LP optimization. The manual includes a list of references (13 entries, 1948-79) and two appendices (methods for calculation of important model parameters and a program guide, documentation and listings).