- A water availability modeling system was implemented during 1997-2004 by the Texas Commission on Environmental Quality and its contractors, in collaboration with the water management community, under a mandate provided by comprehensive water management legislation enacted by the Texas legislature in 1997. The availability and reliability of water resources are assessed based on simulating river/reservoir system management and water allocation practices using sets of historical naturalized monthly streamflow sequences to represent basin hydrology. The prior appropriation water rights permit system and other institutional mechanisms for allocating streamflow and reservoir storage resources among numerous water users are considered in detail in evaluating basinwide impacts of water management decisions. The generalized modeling system and lessons learned in its implementation in Texas are applicable to river basin management throughout the world. Journal of Water Resources Planning and Management ASCE.