Abstract
Long-duration rainfall totals are sometimes used to calculate runoff volumes for sizing and design of storm water containment structures. Rainfall totals are also used to determine runoff-volume-based fines for illicit storm water dischargers. The objective of this project is to evaluate how well the Soil Conservation Service Method estimates annual runoff based upon annual accumulated rainfall. In particular, calculations of annual runoff using a single value of annual rainfall with daily and storm event runoff coefficients were performed to test how well calculated annual runoff volume compares to measured annual runoff volume. The Black Creek watershed located in the Central California foothills was selected for the current study because it is gaged, and its land use has not changed much over the data record under evaluation. Land use and soil type data from Calaveras County were used to produce an area-weighted runoff coefficient (one of four cases analyzed). Topography data from the United States Geological Survey (USGS) quadrangles were used to delineate the watershed. Sixteen years of overlapping runoff and rainfall data from the USGS and the California State Climatologist were utilized in the project. The USGS data provided measured runoff volumes, and the California State Climatologist data provided measured rainfall totals for the same set of years. Additionally, 38 individual Black Creek hyetographs and their corresponding hydrographs were selected from the California Data Exchange Center (CDEC). The CDEC data allowed for the empirical development of minimum, average, and maximum runoff coefficients (the other three cases analyzed). The CDEC data provided less data overlap than the other two sources and was used only to derive runoff coefficients. Based upon the data and the assumptions used in this project, the calculated annual runoff volumes overestimate the actual measured annual runoff at the 99.99 percent significance level for all four cases investigated. The average theoretical runoff volumes were 77 to 135 percent higher than the measured runoff volumes over the 16 water years of data for the four cases investigated. A CN value was calculated based upon the 16 years to data by minimizing the sum of the squared residuals (the difference between the calculated and measured runoff volumes). This CN value is much lower than those of the four cases analyzed and may represent an “annual” runoff coefficient for the Black Creek watershed. These results appear consistent with the work of Critchley et. al. (1991) and they could serve as a warning for those considering using event-based calculations and runoff coefficients for estimating long-duration runoff from long-duration rainfall totals. These results could be incentive to the hydrologic industry to investigate and develop better volume-based hydrologic methods to estimate long-duration runoff volumes.