Incorporating and Evaluating Environmental Instream Flows in a Priority Order Based Surface Water Allocation Model

As the remaining water resources in river basins around the world are appropriated for human uses, it is critical to protect environmental instream flows in order to preserve aquatic and riparian species and ecosystems. It is widely recognized that an adequate environmental flow regime consists of a range of flow conditions. In Texas, a statewide planning process was established in 2007 for determining environmental flow recommendations for the state?s river basins. The environmental flow recommendations, which consist of subsistence flows, base flows, and high flow pulse events, are determined for each basin by a team of scientists and committee of stakeholders. The recommendations are considered by the Texas Commission on Environmental Quality (TCEQ) in developing environmental flow standards which are incorporated into the state?s prior-appropriation water rights permitting system. The environmental flow standards for the Colorado River basin and Trinity River basin are incorporated in daily time-step versions of the authorized use scenario water availability models using existing and recently added features of the Water Rights Analysis Package (WRAP). Various metrics are developed by this research to characterize the degree to which the environmental flow standards are attained, given their junior position in the priority sequence. The techniques used to model instream flows in the Colorado and Trinity river basins contribute to the body of knowledge for modeling flow standards in other basins. Metrics describing the degree to which environmental flow standards are attained will assist scientists and decision-makers in the evaluation and revision of the standards.