Modifications to SCS-CN Method for Long-Term Hydrologic Simulation

Abstract

The original soil conservation service curve number (SCS-CN) technique is primarily used to transform daily rainfall into surface runoff by assuming the proportionality between retention and surface runoff based on a parameter referred to as curve number (CN). The conventional method does not take into account the temporal and spatial variability of curve number. In this paper, an attempt has been made to modify the existing SCS-CN model in two ways by varying the CN using antecedent moisture condition (designated as Model I), and by using antecedent moisture amount (designated as Model II). The daily moisture storage is updated based on varying the curve number and other hydrologic abstractions. These two different models are constructed to compute streamflow components: Direct surface runoff, base flow, and hydrological abstractions. These methodologies were successfully applied to daily data of catchments of Cauvery, Narmada, Ganga, and Ulhas Rivers, lying in different climatic regions of India, and the results were analyzed. Application of Model I to Hemavati (a tributary of River Cauvery, Karnataka State) data yielded maximum efficiency of 84% in calibration, and minimum efficiency of 54% with Ramganga (a tributary of River Ganga, Uttaranchal State) data, whereas Model II showed maximum efficiency of 85% in Hemavati catchment and minimum efficiency of 64% in Kalu catchment (a tributary of River Ulhas, Maharashtra State). Model II performed better than Model I on all four catchments. It is found that the proposed models reasonably simulate the catchment response and these SCS-CN-based models are applicable to complex natured watersheds.