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Parameter Sensitivity in Hydrologic Modeling - page 60 / 163





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Once the watershed of interest has been clipped, the user is prompted to select a time step. The time step is the interval that determines the resolution of model results during a run in HEC-HMS. Gauge data is linearly interpolated to the time interval, which may be chosen to range from 1 minute to 24 hours (USACOE-HEC, 2000). The Muskingum and lag methods are used for flow routing in the reaches, depending on the travel time. If the longest flow path (LFP) divided by the reach velocity is less than the time step the pure lag equation is used; otherwise Muskingum routing is used. It is apparent that time step selection and velocity have a very large effect on the outcome and should be modified accordingly (Olivera et al., 1999).

In addition to the fact that losses may only be calculated using the SCS curve number or the initial plus constant loss rate method, and lag time with only the SCS lagtime formula or length over velocity, another constraint of CRWR- PrePro is the slope calculation. CRWR-PrePro calculates slope based on the DEM cell elevations at 1% and 99% of the longest flow path divided by their distance along the longest flow path, and allows the user no flexibility in calculating watershed slope unless the user changes the Avenue code.

The user may want to calculate the slope at 10% and 85% of the longest flow path, not 1% and 99% percent of this flow path. In this way the high slope of the shallow concentrated flow is not included in the calculations for lag time. CRWR-PrePro calculates the longest flow path of a sub-basin in the set of cells


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