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A GIS - Remote Sensing compatible rainfall-surface runoff model for regional level planning
The model considers only Hortonian flow, that is the flow which occurs when the rainfall exceeds the infiltration rate. Thus, a rainfall input applied to the model routs the net precipitation (after deducting the infiltrated amount) over the watershed surface to generate flow hydrographs at the outlet of the catchment and at various locations within. The discharge and the corresponding water depths are used for taking decisions about the salient features of the different structures, as would be shown in the subsequent section. The selection of the rainfall, however, requires some judgment. The philosophy is a risk-based design, implying that for the structures across a river or a stream a decision has to be taken regarding the worst rainfall event that may be expected to occur within the life of the structure. It is expected that the structure would be able to withstand the resulting flood without any loss of performance. The return period of the design rainfall thus has to be chosen judiciously. Several guidelines exist for selecting the design storm, which mainly depend on the importance of the structure and the risk associated with it. Discussions regarding the usual practice followed in India and foreign countries may be had from the report National Institute of Hydrology. As such, for the Indian context, the Central Water Commission may be referred to. However, since the proposed model is mainly applicable to small catchments, of size around 25 km2 to 1500 km2, the design of only small water resources projects to be constructed by local authorities are considered. Normally, a design frequency of 1 in 50 years may be found to be sufficient.
The flood estimation using a design storm and a physically based routing model has perhaps never been tried in any Indian catchment. This is partly because other methods are available which, though quite empirical, have been correlated from past many years' observed data and also because they are less computation intensive than the physically based deterministic models.
It is the intention of this paper to demonstrate the use of a physically based distributed deterministic model for predicting design flood for small water resources structures, as it can be directly linked with GIS and RS data at the local level. Instead of empirical formulae or regression equations, a physically based model should give the decision managers a more realistic picture of the flow parameters within the watershed.
Developing graphical links between ARC/INFO, microsoft visual basic VER. 6.0 and the catchment
Currently the major stumbling block to progress in hydrologic modeling is not the ability to generate realistic models to simulate physical processes, or the ability to solve the equations associated with these processes, but it is the time consuming and costly process of considering the spatial variation of model parameters during the modeling process. However spatially explicit data and the ability to manipulate these data are essential for detailed hydrologic modeling.
GIS could offer those capabilities for the efficient integration of spatial variation within hydrologic model. Hydrologic parameters could be generated with in GIS, but the procedures are no sufficiently straight forward to be handled by the non-GIS operators. Many GIS data remain underutilized due to the absence of a bridge between the raw GIS data and the end user.
While the need for integration of GIS with hydrology is understood, an interface system has been developed to improve the link between GIS and hydrologic modeling by integrating GIS software (here Arc/Info) and the model.
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