tems, and second, develop tools allowing decisionmakers to make use of that understanding. The overall goal of linking the understanding to decisionmaking tools is usually approached using simulation models, computer programs encapsulating our knowledge in a form useful to managers. These simulation models are especially important in making the connection between science and management because they are placed where the two actually meet and intertwine with each other in an interactive way. Table 5.1 is a summary of selected computer simulation models used in watershed research and development activities and in watershed management. These models were sponsored/developed by EPA, the U.S. Army Corps of Engineers Hydrologic Engineering Center (HEC), ARS, and the U.S. Geological Survey (USGS). These agencies, developed and support these models in cooperation with other federal agencies, universities, state and local agencies, and consulting companies. Therefore, these models are widely used within these agencies and by other agencies and private organizations throughout the world. However, the models described below were selected to show a range of technology and of applications. No attempt was made to include all agencies, technologies, or applications and thus, the compilation is illustrative rather than comprehensive.

The development, testing, and parameterization of simulation models is not a trivial task, because the models are extremely complex; there is a lack of data for internal model verification, validation, and parameterization; objective criteria for model evaluation are not standardized; and model output (point or distributed) is often difficult to interpret. Wurbs (1994) summarized a large number of available water resources models. Most of these models are used to evaluate components of the watershed -- for instance, there are ground water models, urban stormwater runoff models, water distribution system models, and nonpoint pollution models. The discussion of water resources models can be divided into the following categories (Wurbs, 1994):

• demand forecasting and balancing supply with demand
• water distribution system models
• ground water models
• watershed runoff models
• stream hydraulics models
• river and reservoir water quality models
• reservoir/river system operation models

Water resources handbooks edited by Maidment (1993) and Mays (1996) contain extensive information on water resources models. A contemporary watershed model should include simulation of the following features:

1. water quantity and quality
2. single even and continuous simulation