Flow and water quality observations are necessary in virtually every water quality model simulation. Observed chemical constituent and recorded flow values are important for hydraulic and water quality simulations as the justification for parameter value assignments. Model applications
in EPA's BASINS (Better Assessment Science Integrating Point and Nonpoint Sources) are designed to simulate these processes at a watershed scale. The main caveat for using these models is to, as a minimum, have a meaningful set of hydrologic and water quality data closest to the point
at which the water exits the watershed. The models in BASINS provide a means for calibrating hydrologic simulations by comparing model output with observed conditions. However, consistent and frequent water quality observations at the desired location within a watershed are not always available.
Ideally, the calibration should encompass several calendar years to represent a range of conditions. Gaps and inconsistencies in the temporal records make it difficult for even experienced modelers to calibrate water quality for a given area. While acquiring additional observations to support
the model calibration typically necessitates retrieving data from external sources such as EPA's STORET (Storage and Retrieval of U.S. Waters Parametric Data), this study is based upon the premise that BASINS' users will rely only upon the data supplied by EPA on the distributed
compact discs or website downloads. To test the availability and adequacy of the BASINS data sets, we randomly selected a set of hydrologic units throughout the U.S. and evaluated each watershed on the basis of data availability using general guidelines for developing a Total Maximum Daily
Loads (TMDL). This paper discusses the results of these random samples and their efficacy in modeling water quality simulations at a watershed scale and for TMDL calculations as an indication of the potential utility in BASINS
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