A CRITICAL ASSESSMENT OF WATER QUALITY AND WATERSHED MODELS FOR THE TEXAS TOTAL MAXIMUM DAILY LOAD PROCESS

A project is underway for the state of Texas focused on establishing methodologies to determine Total Maximum Daily Loads (TMDLs) for all impaired waterbodies within the state. The project is prosecuted in three phases: establishment of a state-wide geographic information system (GIS) database, delineation of the watershed areas and hydrological characteristics for each of the state's classified waterbodies, and evaluation of current modeling techniques in order to establish optimum modeling practices for Texas. This paper summarizes the third phase of this project: the evaluation of current modeling tools for use in the Texas TMDL process.The State of Texas exhibits a wide range of hydroclimatologies, from the humid, water-rich regions of the east, bordering Louisiana and Arkansas, to the arid deserts of the west. There is also a variety of land-surface and topographic regions, in which land uses encompassing rangeland, row-crop agriculture, and urban development. The coastline of Texas includes seven major estuarine embayments that receive runoff and inflow from the interior and support major commercial and recreational fisheries. The variability of rainfall in the state has made reservoirs the keystone of water management, and there are numerous reservoirs sprinkled across the Texas landscape. Clearly, no single water-quality model will be capable of depicting such a range of watercourse types and hydrology. This project undertook an assessment of watershed models, as well as watercourse models, to determine their appropriateness for application to TMDL analyses in the Texas environment. Among the models reviewed are EPA's Water Quality Analysis Simulation Program (WASP5), QUAL2E, Hydrologic Simulation Program-Fortran (HSPF), and Natural Resources Conservation Service's Soil and Water Assessment Tool (SWAT).A multi-tiered screening analysis was employed to evaluate over 50 existing water quality and watershed models incorporating three successive exclusionary levels of review. The first level was concerned with general model capabilities, computer requirements, public availability, and recent applications of the model. The second level focused on model processes and solution techniques and the final level concentrated on the model's applicability in Texas, including its potential for integration into a GIS framework. This paper addresses the general development of the screening criteria, with particular attention to the first-level screening. Brief summaries of a few model reviews are presented along with an overview of the final recommended models. In addition, because the three screening levels considered the possibility of the model's integration into a GIS-based system, this paper discusses the feasibility of model interfaces in the GIS environment. The paper presents the characteristics necessary to incorporate a model into a GIS- based user interface, along with advantages and disadvantages of the interface integration. Based on the information studied, the review recommended a “short list” of 13 models for consideration for TMDL projects in the state.