Enhanced Wetness Modelling in SAGA GIS

Purpose

The purpose of this tutorial is not only to present an efficient workflow for modelling wetness, but also to explain the different options offered for module inputs throughout the way, as well as why the chosen ones were favoured. The SAGA documentations show that different options are available, yet only includes a reference to a paper that tells where the method was derived from with minimal description. This tutorial will explain how the used methods work and what differentiates them from others. Other methods that were options but not used will be briefly explained as well.

Preamble

This is a workflow that offers an enhanced method for modelling wetness of a terrain. A terrain’s wetness index is a secondary terrain attribute derived from a digital elevation model through different transformations to the raster grid. The method presented in this tutorial is rather more detailed and precise then others because the two defining parameters involved in calculating a wetness index (up slope area and slope) are calculated in particular ways that do not use the assigned defaults or the one step algorithms.

Attention: this workflow has been generated using SAGA GIS 2.2.2 thus all the headings and toolset names are specific to this version. Slight differences in module names are introduced between this version and the one before. SAGA offers Module Library Documentations for the last 5 version. Note that some instructions that yield the same result might have different names/options in different versions (e.g. Catchment Area tool in 2.2.2 was called Flow Accumulation in 2.1.2). Another helpful resource would be the SAGA Forum where a lot of resources and exchanges between GIS users are available.

SAGA GIS was used because of its high efficiency in processing spatial data, and its many offered options for dealing with environmental modelling. SAGA GIS is also a Free Open Source Software (FOSS) which means its source code can be manipulated to suit the user’s needs. In this tutorial no custom code or automation was used, but would be considered for a bigger workflow to increase productivity. To download SAGA GIS: http://sourceforge.net/projects/saga-gis/files/