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Several applicable models exist, including TR-55,  WRENSS (USEPA 1980), unit hydrograph approach models, and others (Troendle and Leaf 1980, Troendle and Nankervis 2000). This assessment step also considers operational hydrology from reservoirs, diversions and other flow modifications that influence the magnitude, duration and timing of stream flow. The input variables for most models are precipitation data, a vegetation alteration map by aspect and elevation, drainage area computations, percent impervious, and similar data specified based on the specific model being selected. The output on these models needs to be in the form of flow-duration curves. Flow-duration curves need to represent reference condition (full hydrologic utilization or recovery) and existing departure from reference. Because there are few stream gauges located on smaller watersheds being assessed, then the use of dimensionless ratio procedures for data extrapolation in flow models becomes essential.
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