The Stanford Watershed Model was used to simulate the effects of urbanization on the discharge from five drainage basins in the upper Santa Ana Valley, an area with an average annual precipitation of 15 inches. The drainage basins ranged in size from 3.72 to 83.4 square miles. Using the model, synthetic records of streamflow for each basin were generated to represent various degrees of urban development. Examination of the synthetic records indicated that urbanization has the following effects on streamflow in the area:
- Average annual runoff from a drainage basin with an effective impervious area of 10 percent of the drainage area is approximately 2 inches, and increases by 1 inch for each increase in effective impervious cover equal to 10 percent of the drainage area. About 30 percent of a fully urbanized area is effectively impervious.
- Urbanization can increase the magnitude of peak discharge and daily mean discharge with a recurrence interval of 2 years by a factor of three to six.
- Peak discharges and daily mean discharges that have recurrence intervals greater than a limiting value ranging from 50 to 200 years or more are little affected by urbanization.
