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Methodology
The method for evaluating the hydrological change due to land-use changes can be implemented by integrating remote sensing, GIS and HEC-1. The widely accepted, the SCS-CN technique is adapted here to compute the runoff from the several Hydrological Similar Units (HSUs) of the basin for the given rainfall. HSUs are the areas, which have same land-use, same soil type. The runoff from each individual HSUs are then routed and the total runoff resulted from the basin was computed for the given rainfall using HEC-I model.

The derived applicable characteristics from different sources are used to derive the HSU and the hydrological changes due to land-use changes are assessed as described in Fig.1. From satellite image data (LandsatTM), the visual image interpretation is carried out using an interpretation key generated through field survey. The ground checks were made for confirming the land-use units. The spatial information on land-use, soils type, topography, hydraulic characteristics and meteorological information are incorporated. The digital information is kept in different layers. The generated layers are used for overlay analysis and to derive the HSUs, based on its land-use, soil and topography. Appropriate CNs according to standard tables (SCS, 1975) are assigned to each HSUs considering antecedent moisture conditions (AMC). Then the direct runoff values from each HSU are estimated using SCS-CN method for rainfall events. The interflow is taken 5 to 15% of excess soil moisture from soil moisture retention capacity according to slope range of the each HSU (Shrestha, 2002). The runoff values from each HSU including interflow are added to subbasin outlet. Then the total subbasin outlet runoff is routed to outlet of the basin using HEC-1 model. The cumulative runoff value from the basin outlet is compared with observed runoff for that period. Effect of land-use change is evaluated for different periods by quantifying the runoff. In developing the relationship for Q (runoff depth), the SCS (1972) developed the expression as,

Where P = total precipitation, I_a = initial abstraction, S = potential retention. This is valid for P > I_a , that is , after runoff begins and Q = 0 otherwise. To remove the necessity for an independent estimation of initial abstraction, a linear relationship between I_a and S was suggested by SCS (1985) as,
I_a=lS          (3)
where l is an initial abstraction ratio. The values of l varies in the range of 0 and 0.3 have been documented in a number of studies encompassing various geographical locations in the United States and other countries. The value of l is estimated here in this study through sensitivity analysis. The variable S in mm, which varies with antecedent soil moisture and other variables, can be estimated as;