Variation of runoff source areas under different soil wetness conditions in a semi-arid mountain region, Iran

Runoff source areas can serve as focus areas for water quality monitoring and catchment management. In this study, a conceptual form of the Soil Conservation Service Curve Number method (SCS-CN) is used to define variable-source runoff areas in a meso-scale catchment in the Zagros mountain region, southwest of Iran. The analysis indicates that for the average and dry antecedent soil wetness (the dominant soil moisture conditions), the original SCS-CN criterion that assumes the initial abstraction of rainfall to be equal to 20% of the maximum potential soil water retention fails to predict runoff source areas correctly, suggesting the ineffectiveness of the original form of the method for the study region. However, the determination of the initial abstraction based on hydrograph-hyetograph matching technique provides reasonable results, outperforms the original form of the SCS-CN based method and, more importantly, highlights the significance of having localized rainfall and runoff data. Under wet soil conditions, both techniques provide similar results; the robustness of the SCS-CN based method is thus supported only for wet soil conditions.