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Introduction
Physically based models for simulating the dynamics of a system are the ones that are based on physical laws. For given initial and boundary conditions these laws, represented by governing equations, are solved mathematically to obtain the system variables in space and time. In the realm of geohydrology, a number of such models are in use, or have been used. These range from simulation of the entire watershed process to the ones limited to a single aspect of the hydrological cycle process for rainfall-runoff simulation. These models vary in their representation of process dynamics with those solving the "exact" equations to the ones based more on conceptual representation of the processes.

During its initial phases, the physically based models were not developed with the objective of using remotely sensed data or geographic information system (GIS) as the availability of such data was, generally, scarce. With the gradual availability of such data and increased computing power, hydrological models being based on more and more spatially distributed data. However, with time, remotely sensed data and geographic information system data have become almost a necessity for spatially distributed models and such databases are also being made available to the public more conveniently. Even in developing countries, like India, attempt is being made to disseminate geographically distributed data in an electronic format to regional level planners (for example, the Natural Resources Data Management System programme of the Department of Science and Technology, Government of India). It is therefore natural that a need has arisen to develop computer models that may facilitate local planners in taking a decision by analysing the relevant regional databases and coming up with a desired result. The present model is one such, which can be linked to the spatial physical characteristics of an ungauged watershed, and for a given rainfall generate the possible discharge hydrograph at desired locations within the watershed. A user-friendly interactive interface has also been developed which would help the user in modifying some changes, if necessary, to the database. Effort is on to develop a post-processing screen which would visually display the flood hydrograph at user-selected locations in the catchment. The flood hydrograph, or more importantly the flood peak, is required for hydraulic design of spillways of small dams, for determining the waterways for bridges and culverts, construction of flood protection works, estimation of erosion potential, site planning for micro hydels etc. within a catchment.

In the remaining sections of this paper, a brief description of the model is presented followed by an example showing the application of the model. The paper concludes with brief guidelines regarding the usage of the model in taking decisions on some specific tasks.

Desctiption of the model
The requirements for the application of the proposed model to a catchment are an elevation contour map of the area, corresponding land use and soil maps preferably in a digital format, and a guideline for estimating the design storm. The elevation map, if available in a digital form, is processed by GIS software to create a Triangulated Irregular Network (TIN) form of digital elevation model (DEM). Some care must be taken during its creation (with the use of break line, etc.) such that the obvious drainage and ridgelines are recognized. If a GIS package is not available or if the one available does not have the capability to generate a TIN, the same task may be done manually although it may be quite time consuming and prone to errors.

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