Abstract
Abstract. The Soil Conservation Service Curve Number (SCS-CN) approach is widely used as a simple method for predicting direct runoff volume for a given rainfall event. The CN parameter values corresponding to various soil, land cover, and land management conditions can be selected from tables, but it is preferable to estimate the CN value from measured rainfall-runoff data if available. However, previous researchers indicated that the CN values calculated from measured rainfall-runoff data vary systematically with the rainfall depth. Hence, they suggested the determination of a single asymptotic CN value observed for very high rainfall depths to characterize the watersheds' runoff response. In this paper, the hypothesis that the observed correlation between the calculated CN value and the rainfall depth in a watershed reflects the effect of soils and land cover spatial variability on its hydrologic response is being tested. Based on this hypothesis, the simplified concept of a two-CN heterogeneous system is introduced to model the observed CN-rainfall variation by reducing the CN spatial variability into two classes. The behaviour of the CN-rainfall function produced by the simplified two-CN system is approached theoretically, it is analysed systematically, and it is found to be similar to the variation observed in natural watersheds. Synthetic data tests, natural watersheds examples, and detailed study of two natural experimental watersheds with known spatial heterogeneity characteristics were used to evaluate the method. The results indicate that the determination of CN values from rainfall runoff data using the proposed two-CN system approach provides reasonable accuracy and it over performs the previous methods based on the determination of a single asymptotic CN value. Although the suggested method increases the number of unknown parameters to three (instead of one), a clear physical reasoning for them is presented.
Highlights
Simple methods for predicting runoff from watersheds are important in hydrologic engineering and hydrological modelling and they are used in many hydrologic applications, such as flood design and water balance calculation models (Abon et al, 2011; Steenhuis et al, 1995; van Dijk, 2010)
The Soil Conservation Service Curve Number (SCS-curve number (CN)) method was originally developed by the SCS (US Department of Agriculture), to predict direct runoff volumes for given rainfall events and it is documented in the National Engineering Handbook, Sect. 4: Hydrology (NEH-4) (SCS, 1956, 1964, 1971, 1985, 1993, 2004)
The main weaknesses reported in the literature are that the SCS-CN method does not consider the impact of rainfall intensity, it does not address the effects of spatial scale, it is highly sensitive to changes in values of its single parameter, CN, and it is ambiguous considering the effect of antecedent moisture conditions (Hawkins, 1993; McCuen, 2002; Michel et al, 2005; Ponce and Hawkins, 1996)
Summary
Simple methods for predicting runoff from watersheds are important in hydrologic engineering and hydrological modelling and they are used in many hydrologic applications, such as flood design and water balance calculation models (Abon et al, 2011; Steenhuis et al, 1995; van Dijk, 2010). The SCS-CN method was soon adopted for various regions, land uses and climate conditions (Elhakeem and Papanicolaou, 2009; King and Balogh, 2008; Mishra and Singh, 1999; Romero et al, 2007). It was evolved well beyond its original scope and it became an integral part of continuous simulation models
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