Summary

Gridded DTM’s can be used for a large variety of hydrologic modelling. In a first step, pits have to be removed by filling up or cutting through and flat spots have to be dealt with as the drain direction is undefined here. Then a local drain direction net (ldd net) can be extracted from the DTM using either a single flow algorithm or an algorithm that allows dispersion. The actual choice of one algorithm depends on the terrain data and the nature of the application. The ldd net can be used to calculate parameters like the upstream area of a grid cell or the length of a catchment. Some geomorphologic features like ridges or flow channels are closely related and can also easily be derived. These parameters may be used in hydrologic indices or they may be combined with other geographic information in order to make runoff predictions or to model occurrences of plants and animals.