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Parameter Sensitivity in Hydrologic Modeling - page 58 / 163

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CRWR-PrePro calculates losses using the SCS curve number method or the initial plus constant loss method. To calculate curve number, soils data must be obtained. This may be done easily free of charge by downloading the data from the USDA-NRCS website (Appendix A). Attribute tables that relate to the soils data include the mapunit.dbf and comp.dbf tables, which should be added to the working directory in ArcView. The tables are modified and the CRWR- PrePro function Soil Group Percentages may be executed. This creates a table called muidjoin.dbf. Next Land Use/Land Cover (LULC) data must be obtained. Similar to the soils data, these data may be downloaded free of charge from the USGS website (Appendix A). By combining the LULC data and the soils data, a curve number grid (CN) may be generated using the Curve Number Grid command in CRWR-PrePro. An additional table that may be obtained off the CRWR-PrePro webpage, rcn.txt, is used as a look-up table to link curve number

values to land use and soils data. Table wshpar.txt plus constant loss method is used (fields in

C o n s t _ L o s s R a t e , a n d W s h _ V e l o c i t y ) .

The final

calculated as the average of the curve number values (Olivera et al., 1999).

is necessary when the initial table include Initial_Loss, CN for each sub-basin is within the sub-basin polygon

Once the data are in vector format, hydrologic parameters of sub-basins and reaches such as reach length, reach routing method (Muskingum or lag), and

either number of routing), or the parameters such

sub-reaches into flow time (for as flow velocity

which the reach is subdivided (for Muskingum

pure lag) are computed. and Muskingum X cannot

The other reach be computed from

48

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