The study estimates the initial abstraction coefficient for six watersheds in various agro-climatic zones of Karnataka from rainfall-runoff events by asymptomatic fitting of Curve Numbers calculated from observed rainfall-runoff data using Soil Conservation Service (SCS) model of United States Department of Agriculture (USDA). The SCS method assumes the initial rainfall abstraction as the fraction of potential maximum retention of soil.
Curve Number is a measure of the water retention in the watershed. The dimensionless Curve Number reflects runoff potential, which depends on hydrologic soil group, land cover type, hydrologic condition and antecedent moisture condition (AMC). The calculated Curve Number values have been used to estimate the initial abstraction coefficients for the study watersheds, and can be used for evaluating effects of changes in land use and land treatment on direct runoff. The study establishes the initial abstraction parameters in generating direct runoff from the watersheds in hard rock region under different physiographic and climatic conditions.
The SCS had, to begin with, developed the initial abstraction coefficient for small experimental watersheds in USA and the coefficient does not hold good for Indian conditions. The original method has been subjected to improvements and changes to suit the conditions in India under the study by using historical rainfall-runoff data. The value of initial abstraction as per the study is in the range of 0.15 to 0.3. These values may be used as representative value for the watersheds in the particular agroclimatic zone and made use of in further studies.
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