Determination of ‘S’ Using Volumetric Concept

In practice, the SCS-CN parameter, the potential maximum retention or postinitial abstraction retention (McCuen, 2002), S, is determined from scaled values of CN derived from the tables of National Engineering Handbook (NEH) for λ = 0.2 (Chapter 2). For computing S from rainfall-runoff data, equation (2.14) is used. Equation (2.14) is a specific form of equation (3.109). The existing SCS-CN method with λ = 0.2 is, therefore, a one-parameter model. In Chapter 3, the curve number CN is defined as the percent degree of saturation of the watershed soil by the 10-inch rainfall amount. As shown in Chapter 3, the generalization of the Mockus method leading to the SCS-CN method yields a relation between the Mockus parameter ‘b’ and S as: b =1/}S ln(10)}. Parameter ‘b’ depends on the antecedent moisture condition (AMC), vegetative cover, land use, time of the year, storm duration, and soil type; which describe CN. While attempting to theoretically justify the basis of the SCS-CN hypothesis, Yu (1998) described S as the product of the spatially averaged infiltration rate and the storm duration. In the previous chapter, the SCS-CN parameter S is distinguished from parameter s_av in the expression given by Yu (1998), for the former represents the volumetric capacity of the soil to retain water [L or L3] and the latter represents the ratio of S to the time duration of the storm [LT−1]. The description of S by Mockus (1964), given in Chapter 2, appeals most for S to be equivalent to the maximum difference of (P-Q), which corresponds to the maximum possible infiltration capacity of the soil, if other losses including initial abstractions are ignored. His further explanation is, however, ambiguous in that it compares the infiltration rate with the volumetric space available in the soil profile; the former represents the rate and the latter the volume, which are not comparable. In this chapter, S is distinguished from infiltration rate to represent the volumetric retention, consistent with the definition of McCuen (2002), and it is supported by the observed infiltration data. In addition to these, the runoff factor (C) = degree of saturation (S_r) concept (Chapter 3) is used to signify S and derive a hierarchy of the SCS-CN-based methods.