ESTIMATION OF SCS RUNOFF CURVE NUMBER FOR AGRICULTURAL RESERVOIR WATERSHED BY ANALYSIS OF RESERVOIR OPERATION RECORD

摘要

The rainfall-runoff potential of a agricultural reservoir watershed was studied based on SCS (Soil Conservation Service; now the NRCS, Natural Resources Conservation Service, USDA) runoff curve number (CN) technique. The observed precipitation and the reservoir operation data such as irrigation amount, reservoir storage, river maintenance requirement, and flood control discharge were collected for ten years period and analyzed. The runoff of each storm event from the watershed of the reservoir was estimated by reservoir water balance model using reservoir operation records. The maximum retention, S, for each storm event from rainfall-runoff pair was estimated for selected storm events. The estimated S values were arranged in descending order, then its probability distribution was determined as log-normal distribution, and associated CNs were found about probability levels of Pr=0.1, 0.5, and 0.9, respectively. CNs determined from observed rainfall-runoff estimated by reservoir water balance analysis was compared to the suggested CNs by the method of SCS-NEH4. The CN-Ⅱ estimated from water balance analysis in this study were 80. However, the CN-Ⅱ, which was determined based on hydrologic soil group, land use was 67 indicating that actual runoff potential of the reservoir watershed is higher than that evaluated by SCS-NEH4 method. The results showed that watershed runoff potential for large scale agricultural reservoirs needs to be examined for efficient management of water resources and flood prevention.