SURFACE RUNOFF ESTIMATION USING GRID-BASED CURVE NUMBER METHOD IN THE UPPER LAM PHRA PHLOENG WATERSHED, THAILAND

摘要

Upper Lam Phra Phloeng watershed in the lower northeastern part of Thailand, covering rolling and hilly area about 786 km2, is classified to be one of the most active agricultural areas in the country. Severe erosion of the area thus becomes the big problem. The more accurate surface runoff information of the area is required. This study employed grid-based Curve Number (CN) method through the GIS drainage analysis operating on DEM to estimate surface runoff depth for each grid cell in the watershed. The total depths of runoff in grid cells were accumulated from upstream to downstream along the flow path. Eighteen storm events in rainy season of the year 2008 were observed. Ten events were used for the model calibration with surface runoff data measured at two observation stations (M.171 and M.145). Other eight were for the model validation. The calibration results show that the coefficient of efficiency (E) is 0.94 and the coefficient of determination (R~2) is 0.95 at the M.171 station. While E is 0.87 and R~2 is 0.91 at the M.145 station. The validation results show that E is 0.87 and 0.68 and R~2 is 0.89 and 0.75 at the M.171 and M.145 stations, respectively. This indicates that the calibrated grid-based CN method working under GIS can be applied with satisfactory accuracy to the surface runoff estimation.