CATCHMENTSIM: A NEW GIS TOOL FOR TOPOGRAPHIC GEO-COMPUTATION AND HYDROLOGIC MODELLING

This paper outlines the capabilities of, and describes the algorithms employed within a freely available GIS software package specifically tailored toward hydrologic applications. The algorithms employed within CatchmentSIM are designed to overcome many of the problems associated with the simplified hydrologic algorithms adopted by conventional GIS packages. CatchmentSIM is based on a raster Digital Elevation Model (DEM) which may be interpolated internally from vector contour and stream alignment data, or imported from external applications. A Priority First Search (PFS) breaching algorithm is utilised to remove flats and pits throughout the entire DEM, which avoids the parallel stream problems that plague flat and pit removal in more common techniques. Following this, subcatchments and watercourses may be accurately delineated using a vector flow routing algorithm that has been shown to be superior to the D8 method employed by most conventional GIS applications. The CatchmentSIM software also includes a range of tools that enable topographic analysis on a scale not practicable by hand and conventional map interpretation techniques. In particular, Strahler / Horton geomorphologic analysis has been incorporated, allowing subcatchment bifurcation and drainage density relationships to be accurately determined. These techniques have been demonstrated to be more resistant to grid scale and rotation effects than comparable raster approaches in conventional GIS software packages. Following analysis of a catchment with the software, an internal macro language may be applied to export project parameters to any existing hydrologic modelling software (of a known data format) a user may wish to apply. The adopted algorithms within CatchmentSIM, enable users to build on the increasingly comprehensive information available in today?s GIS, while avoiding the traditional pitfalls of conventional raster GIS techniques and maintaining tight coupling with existing 'industry standard' modelling approaches.