Spatial mapping and testing the applicability of the curve number method for ungauged catchments in Northern Ethiopia

Understanding the spatial variability of land and water resources has significant importance for its planning, management, and utilization. It is also significant in understanding the response behavior of a catchment in order to model the basic physical processes. In this study, a weighted overlay analysis technique using ArcGIS was implemented for developing a geo-database of the standard curve number (SCN) in a catchment around Northern Ethiopia. The spatial data were used to investigate a 'standard curve number method' for the simulation of the direct runoff at the outlet of the catchment. Both spot based rainfall and runoff measuring techniques were adopted for deriving an instant observed flow measurement, and to make a comparison with the simulated flow values. The results showed that the model underestimated most of the simulated values with a coefficient of regression of R2 = 0.52, with a proportion of higher variances between the simulated and observed runoff events. The result suggests that the accuracy of the model leaves room for significant improvement and the method could not be easily adopted in the catchment and other similar catchments in the semi arid regions of Ethiopia. For improving the prediction capacity of the model, further research in adjusting loss factors in the method is recommended. It is also suggested for developing a localized and modified SCN values by considering geologic, climatic and seasonal variation. The results of this study and the maps generated can be used for improving the hydrological understanding of the catchment. The study is useful for further investigation of the SCN methodology in other un-gauged catchments around the world.