Applications in hydrology

Although there are many influences on an areas water balance, the terrain is a key factor as water usually runs downhill in the direction of the steepest slope. DTM’s therefore, are often applied in hydrology specific computer models. Nevertheless we shouldn’t forget that water sometimes infiltrates into the soil, evaporates from the soil or is intercepted by plants without flowing downhill. Also the inertia of water may occasionally cause a somewhat atypical behaviour, e.g. it may flow uphill for a limited distance. However DTM’s have proved advantageous as a basis for hydrologic modelling. Further information like evaporation, infiltration or interception can always additionally be included into a hydrological model and contribute to an increased plausibility.

Rhinefalls in Schaffhausen: Try to spot some ridges and some flow channels in the picture. (Photo: Sabine Timpf)

As hydrology is closely linked to terrain features, the potential of hydrologic modelling goes beyond pure runoff prediction:

The extraction of drainage networks sometimes uncovers points and lines without any catchment at all. Those can be identified as ridges.
Other areas possibly have a very large catchment and can therefore be considered as part of a flow channel.

In such a way, hydrologic rules can help to extract geomorphologic information from a DTM.

Learning Objectives

You know how a local drain direction net (ldd net) can be extracted from a raster DTM.
You are able to use an ldd net to derive hydrologic indices such as; a wetness index or a stream transport index and you are able to compute topographic features such as ridges or stream channels.