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Curve Number Estimation for Watershed using Digital Image of IRS-1D LISS-III
Soil Conservation Service (SCS) Model
The SCS runoff equation (SCS, 1972) that is an empirical model was developed to provide a consistent basis for estimating the amounts of runoff under varying land use and soil types (Neitsch, 2001). 
where Qsurf is the accumulated runoff or rainfall excess (mm), Rday is the rainfall depth for the day (mm), Ia is the initial abstractions which includes surface storage, interception and infiltration prior to runoff (mm), and S is the retention parameter (mm). The retention parameter varies spatially due to changes in soils, land use, management and slope and temporally due to changes in soil water content. The retention parameter is defined as 
where CN is the curve number for the day. The initial abstractions, Ia, is commonly approximated as 0.2S and equation 1 becomes 
The SCS curve number is a function of the soil’s permeability, land use and antecedent soil water conditions. The daily curve number value adjusted for moisture content is calculated by rearranging equation 2 and inserting the retention parameter calculated for that moisture content: 
where S is the retention parameter calculated for the moisture content of the soil on that day. For Indian conditions this equation has been modified (Hand book of hydrology, 1972) as follows 
where Q is actual direct runoff (cm), P is the total storm rainfall (cm), S is the potential maximum retention (cm). The weighted value of CN for individual watershed was determined as by the following equation 
where CN is curve number if individual land use and soil group, A is the area of respective combination and TWA is the total area of watershed. The average condition regarding runoff potential (AMC II) was considered in the present study for determination of CN values. The CN values for AMC II can be converted in to CN values for AMC AMC III and I by using available conversion factors (Suresh, 1997). |