WMS:OC Unit Hydro Effective Precipitation

Orange County Precipitation Wizard – Step 1 of 2

Effective precipitation is the rainfall that actually contributes to runoff. Compute effective precipitation for a basin by first selecting the basin and then choosing the Effective Precipitation button from the Edit Orange County Unit Hydrograph Parameters dialog. Effective precipitation is developed by:

1. Weighting point precipitation values for mountainous and non-mountainous terrain – There are different point precipitation values for mountainous (above 2000 ft) and non-mountainous terrain. Enter a decimal value that describes the percentage of mountainous terrain in the sub-area to automatically calculate weighted point precipitation values. Click on the Compute Mountainous button if a DEM or TIN exists to have WMS calculate the amount of mountainous terrain in the sub-area.
2. Applying depth-area reduction factors to get adjusted point precipitation – Point precipitation values are reduced as sub-area size increases. Enter an area to determine the depth-area reduction factors and view the adjusted point precipitation values.
3. Computing unit rainfall according to the time interval specified in the Job Control dialog
4. Subtracting unit losses from the unit rainfall – Two loss rates are used as a basis for computing unit losses in Orange County. There is a low loss rate (Ybar), which is derived from calculating SCS curve numbers, and a maximum loss rate (Fm) that is based on soil type. Click on the Compute Losses... button to computes losses using GIS data.

See the Orange County Hydrology Manual for more detailed descriptions of these steps.

The precipitation and losses data that are displayed for multi-day events correspond to the day selected in the combo box. Toggle on the Edit precipitation parameters check box to edit the point precipitation, depth-area reduction factors, and adjusted precipitation.

Click the Next button to view the effective rainfall in a graphical and tabular format.

Select the Done button to write the effective precipitation to the PI card for analysis using HEC-1.

Related Topics

• Design Storms
• Using GIS Data