(Icon-Farm, February 12-16, 2008, pp. 10)
Land and water are the most vital natural resources, which must be conserved as intelligently as possible for environmental and ecological imbalance. In the world more than two billion hectares of land is affected by soil erosion against the present arable land of about 1.5 billion hectares. The research in water resources management and related watershed planning issues form a strong component of the environmental activities. With population explosion the demand of land, water and food is increasing a lot. This has led to the scientific approach to the watershed planning, development and management. Amongst these, hydrological modeling of watersheds is an important aspect. Intensive study of individual watershed is therefore necessary for developing management plan and also transforming the results of one watershed to another with similar characteristics. Surface hydrological modeling of watershed mainly includes processes like runoff and transport of sediment as well as pollutants from watershed. Estimation of runoff and sediment yield is necessary for the design of soil conservation structures for implementation of management program with minimal resources. State of the art GIS systems allow for the creation, administration, handling and analysis of spatial data in order to investigate hydrologic problems. GIS helps to link land cover data with topographic data and other information concerning processes and properties related to their spatial distribution when applied to hydrologic systems. With respect to preparing the scientific management and development plan for watersheds hydrological models like WEPP (Water Erosion Prediction Project) (Laflen et al, 1991), SWAT (Soil Watershed Assessment Tool) (Arnold et al., 1995), AGNPS (Agricultural Non Point Source) (Young et al., 1989), ANSWERS (Areal Non Point Source Watershed Environment Response Simulation) (Beasley et. al., 1980) etc. are available.
The starting point for development of Physically Based Spatially Distributed (PBSD} models could be Systeme Hydrologique Europeen (SHE) development in 1986. Later in 1996 Wicks and Bathurst worked on SHE and added bulk sediment transport system as SHESED. This SHESED was used for estimation of water flow and sediment transport from the watersheds. In recent past MIKESHE system came which can among other things, model advection and dispersion of conservative solutes in multilayer aquifers, nitrogen transformations in the root zone, soil erosion and irrigation. Though PBSD models are available for over 10 years but they still are not free from the criticism. One of the common criticisms of PBSD models is that they give unphysical results as a consequence of scale problem and the large size of the grid used. The GIS technology allows modeler to acquire, organize, analyze and display model input and out put data (Burrough, 1986). GIS is a new technique (McVicar and Jupp, 1998) and can provide a common platform for managing complex spatial data (Launen and Dupuis 1996). Tiwari et al, 1997 has extracted all watershed and stream characteristics using remote sensing and collateral data in GIS environment. They develop a monthly rainfall runoff model using these parameters to predict monthly runoff depth, which gave prediction, reasonably well within a four percent variation. Due to recent advances in computation technologies, RS and GIS it is possible to use hydrological models especially physically distributed models with reasonable accuracy and ease for the development of watersheds. Even recent techniques can be used for assessment of impact of developmental schemes on environment of the region.
