An Alternate Method of Finding the USDA Soil Conservation Service Runoff Curve Number for a Small Watershed
Currently, one of the most widely-used methods for estimating losses for a watershed is the Soil Conservation Service (SCS) runoff curve number method (Chow et al, 1988; Viessman et al, 1989). This methods requires the identification of the hydrologic soil group, cover type, treatment, hydrologic condition, and antecedent runoff condition of a watershed (McCuen, 1982; USDA, 1986). These factors are used to select a representative curve number (CN) which, in turn, is used to estimate runoff for a given rainfall event.
Selecting a value for all of the watershed parameters listed above involves individual judgment and therefore introduces errors that might otherwise be avoided. The primary focus of this chapter was to test the hypothesis that a more accurate assessment of the curve number could be found by measuring multiple rainfall events using a recording flow meter and a rain gauge.
HEC-1 was used to analyze the hydrographs and determine an SCS runoff curve number and lag time for the watershed being studied. The work was conducted using a watershed with an area of 49 km2 (19 mi2) and an estimated time of concentration of 4.8 hours. Although a probable range of curve numbers (78 to 84) and lag times (3.4 to 3.8 hours) was identified, the variation in curve numbers among the five storms studied was 14. The variation in lag times was 4 hours.
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