Water Resources Management

, Volume 20, Issue 5, pp 643-660

First online:

An Improved I a S Relation Incorporating Antecedent Moisture in SCS-CN Methodology

  • S. K. MishraAffiliated withDepartment of Water Resources Development and Management, Indian Institute of Technology Email author 
  • , R. K. SahuAffiliated withDepartment of Civil Engineering, Indian Institute of Technology Bombay
  • , T. I. EldhoAffiliated withDepartment of Civil Engineering, Indian Institute of Technology Bombay
  • , M. K. JainAffiliated withNational Institute of Hydrology

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Employing a large dataset of 84 small watersheds (area = 0.17 to 71.99 ha) of U.S.A., this paper investigates a number of initial abstraction (I a )-potential maximum retention (S) relations incorporating antecedent moisture (M) as a function of antecedent precipitation (P 5), and finally suggests an improved relation for use in the popular Soil Conservation Service Curve Number (SCS-CN) methodology for determination of direct runoff from given rainfall. The improved performance of the incorporated M = α \(sqrt{P_5S}\) and I a = λ S 2/(S + M) relations, where λ is the initial abstraction coefficient, in the SCS-CN methodology exhibits the dependence of I a on M, which is close to reality; the larger the M, the lesser will be I a , and vice versa. Such incorporation obviates sudden jumps in the curve number variation with antecedent moisture condition, an unreasonable and undesirable feature of the existing SCS-CN model.

Key words

antecedent moisture curve number initial abstraction NEH-4 soil conservation service