Applicability of SCS curve number method for a California oak woodlands watershed

The curve number (CN) method deveseped by the Soil Conservation Service (now NRCS) for predicting peak runoff from Watershed has not been extensively tested in western regions of the United States. We used a 17 year rainfall and runoff record from a California Oak woodland watershed to compare the accuracy of CN as prescribed in the SCS National Engineering Handbook (NEH-4) with two alternative methods. Each method predicted mean annual peak runoff that was not significantly different from observed runoff and correlation between estimated and observed runoff from each of the three methods was statistically significant. However, the highest correlation coefficient showed that only 50% of the variability in the data were accounted for by any of the methods. The NEH-4 method under predicted maximum flows for the highest flow years. The more conservative Hjelmfelt method more frequently over-predicted peak flow. Overprediction provides a measure of safety when using the CN technique.