WMS:OC Unit Hydro Using GIS Data

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GIS data is especially useful in WMS for delineating watersheds and computing Orange County loss rates.

Sub-area delineation can be automated when digitial terrain data (DEMs or TINs) exists within WMS. GIS data may be used to identify geometric features such as sub-area boundaries, streams, and concentration points.
Computing Orange County Losses 
Soil type and land use data are required in order to compute Orange County losses (Fm and Ybar). Compute losses by choosing the Compute GIS Attributes command on the Calculators menu or by clicking on the Compute Losses button in the Orange County Precipitation Wizard. Losses are computed by overlaying either triangles on a TIN or basin polygons (from a drainage coverage) divided into small squares with the GIS data to calculate an Fm and Ybar value for each square or triangle. Composite loss values for each sub-area can be calculated using the triangles or squares located within the sub-area. Follow these steps to compute losses:
  1. Import shapefiles with soil type and land use data
  2. Map GIS data to feature objects, if necessary
  3. Select Orange County Losses for the computation type in the Compute GIS Attributes dialog
  4. Choose the coverages, grids, or GIS layers to use for computations
  5. Import a mapping table – this table needs to have percent impervious information for each land use in addition to SCS curve numbers
  6. Select OK to compute losses for all drainage basins

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