On the Ia–S relation of the SCS-CN method

Abstract

The initial abstraction (Ia) versus maximum potential retention (S) relation in the Soil Conservation Service Curve Number (SCS-CN) methodology was revisited, and a new non-linear relation incorporating storm rainfall (P) and S was proposed and tested on a large set of storm rainfall-runoff events derived from the water database of United States Department of Agriculture-Agriculture Research Service (USDA-ARS). Employing root mean square error (RMSE), the performance of both the existing and proposed models was evaluated using the complete database, and for model calibration and validation, data were split into two groups: based on ordered rainfall (P-based) and runoff (Q-based). A specific formulation of the proposed model Ia=λS(P/(P+S))α with λ=0.3 and α=1.5 was found to generally perform better than the existing Ia=0.2S, and therefore was recommended for field applications. When evaluated using the observed Ia data, the proposed version performed significantly better than the existing one.

Research on the SCS-CN initial abstraction ratio using rainfall-runoff event analysis in the Three Gorges Area, China

“…Mishra et al (2006a,b), employing a large dataset of 84 small watersheds (area = 0.17 to 71.99 ha) of USA, investigated a number of initial abstraction (Ia)-potential maximum retention (S) relations incorporating antecedent moisture as a function of antecedent precipitation. Jain et al (2006) reviewed the Ia-S relationship, and proposed a new non-linear relation incorporating storm rainfall (P) and S. Ponce and Hawkins (1996) suggest that the fixing of the initial abstraction ratio at 0.2 may not be the most appropriate number, and that it should be interpreted as a regional parameter.…”

“…The selection of a proper Ia/S value is crucial to accurate estimation of direct runoff from the SCS-CN method (Jain et al, 2006). Thus, the objectives of this study were: (1) to determine the initial abstraction ratio (Ia/S), in an experimental watershed in the TGA, by analyzing measured rainfallrunoff events; (2) to compare the performance of the traditional and modified Ia/S values with observed rainfall-runoff data.…”

Performance Evaluation of Different Runoff Estimation Methods in North Western Tract of India

“…Studying the great variation in λ values for different watersheds, Jain et al, (2006) by using different mathematical treatment of Mishra and Singh (1999) reported that λ varied with rainfall and runoff. They further reported that λ is directly related with S and P, rather than S alone.…”

“…So, the performance of the proposed empirical equation was evaluated over the other methods proposed in literature i.e. original NRCS-CN, Crazier and Hawkins (1984), Woodward et al, (2003) and Jain et al, (2006) etc. With this background the objective of the study was to propose a simple and empirical approach using rainfall-runoff data, over the other approaches for the estimation of surface runoff and to evaluate the performance of proposed approach over the other approaches for goodness of fit procedures in the area.…”

Development of a Modified SMA Based MSCS-CN Model for Runoff Estimation

“…The SCS-CN model suffers from an inconsistent unstable theoretical foundation and has several disadvantages, such as the inability to flexibly display antecedent moisture conditions, unaccounted rainfall intensity and storm duration, discrete unrealistic representation of CN and antecedent condition, and fixing of initial abstraction coefficient (λ) (Ponce and Hawkins 1996;De Michele and Salvadori 2002;Michel et al 2005;Jain et al 2006 andShi et al 2009). The fixing of λ (=0.2) has frequently been questioned for its validity and applicability invoking many researchers for a critical examination of the I a -S relationship in pragmatic applications, where I a = initial abstraction before surface runoff occurs and S = potential maximum retention (Hawkins et al 2001).…”