GEOGRAPHICAL INFORMATION-SYSTEM APPROACH TO DISTRIBUTED MODELING

A rainfall and run-off model has been developed which is based on a rectangular grid cell network. The run-off in each cell is estimated empirically and is routed through the network to the principal drainage lines. Information on the land cover in each 50 x 50 m cell is interpreted from remotely sensed data, gathered by an orbiting satellite. Values of topographic parameters for each cell are derived from a digital model of the terrain. Other more conventional data are acquired by digitizing maps. The large amounts of data required in this approach are handled by a geographical information system (GIS), which is a management system specially designed for data with spatial characteristics. A GIS has functions which allow the user to combine the many layers of data within the GIS arithmetically or logically to provide fresh information. At the current stage of development the GIS provides estimates of the United States Department of Agriculture Soil Conservation Service curve number for estimating the run-off in each cell. An analysis of the digital terrain model defines the pathway taken by run-off from each cell and the run-off is routed from cell to cell as far as the principal drainage lines, where a conventional hydraulic analysis is used. The model benefits from a simple and robust approach to run-off estimation distributed on a very fine grid. An important feature of the model is its ability to use historical satellite images to estimate changes in the catchment's response over a period of time. Future changes in land use may also be easily incorporated into the GIS. The model is being tested on data from the Tywi catchment in West Wales, UK, which had undergone large changes in land cover due to forestation.