Texas recently established a surface water rights permit system, similar to those of other western states. A simulation study of the Brazos River Basin identified issues and concerns that illustrate the practical complexities of administering and modeling a water allocation system. Key considerations involve sharing of limited supplies by numerous water users during drought, reservoir storage priorities, multiple reservoir system operations, water quality constraints, return flows, hydrologic data compilation, and reliability assessment. The issues affecting evaluation of water availability within the Texas water rights system are representative of other states as well. Includes 5 references, tables, figure.