The Water Rights Analysis Package (WRAP) is a generalized river basin simulation model for evaluating water availability within the framework of a prior appropriation water rights system, such as the permit system recently implemented in Texas. WRAP is used to evaluate water supply reliabilities for permitted water users and unappropriated streamflows available for additional permit applicants. The historic hydrologic period-of-record is simulated using a monthly computational time step. For each monthly interval, a network flow programming algorithm allocates streamflow and reservoir storage resources in accordance with water rights. The model is generalized for application to river/reservoir systems of essentially any configuration. WRAP and its application to the Brazos River Basin illustrate the use of network flow programming in modeling the allocation of a limited water resource to numerous users within a complex institutional setting.