Runoff, initial abstraction, SCS, partial source area


Despite its wide use, the Soil Conservation Service runoff Curve Number (SCS-CN) equation is not trusted by many hydrologists due to its fitness gap to different areas. In the process of finding a simple method for estimating runoff contributing area within a watershed and assessing the runoff mechanisms, we have used the original concept of SCS-CN approach in a 113.4ha Anjeni Watershed in the headwaters of the Blue Nile Basin, Northern Ethiopian highland. Analysis was done at event, daily and weekly base on 10 years of hydrological data (1988-97) by classifying the rainfall seasons in to six based on the seasonal cumulative of effective rainfall (Pe). The initial abstraction (Ia) for event based is first calibrated and the calibrated value is used in runoff amount determination. For daily and weekly, it is computed by Thornthwaite-Mather water balance in replacement of the 20% of the potential storage (S). Effective rainfall (Pe) is the difference of total rainfall and Ia. The model performed more as the seasonal cumulative Pe is increased indicating that runoff responses occurred as the watershed saturated. The proportion of runoff contributing area (Af) increased linearly until the cumulative Pe up to nearly 500mm and then the watershed reaches in equilibrium for addition increase of Pe, which is in line with the concept of partial source area hydrology.