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/

Introduction to the SAGA Environment

Throughout your work in SAGA GIS, the tools used, progress, errors, and results can be tracked in different ways. The figure below shows what the SAGA GIS workspace looks like. The main control sections are the Manager, Data Source, Properties, and Messages.

• For accessing folders and data files refer to Data Source | File System (left of the screen). The imported data will appear under Manager; once the data layer is double clicked, it will appear in the right part of the screen as a Map layer.
• Under Properties, information about the layer can be viewed, as well as settings, symbology, attributes, and history of processing.
• The bottom part of the screen has Messages which will show in the General tab every single command ran in SAGA with the time of execution and if the method succeeded or not.
• The Execution tab shows the inputs assigned to the module.
• If any errors arise during running, they appear under the Errors tab.
• To track the used tools as well, SAGA keeps a record of the last used tools in order under Geoprocessing.

Data & Study Site

This exercise is done using data from Gatineau Park, Quebec (Canada). The used data:

• Digital Elevation Model (DEM): LiDAR-derived, 5 meter resolution
• Gatineau Park boundary: polygon layer

The data used is acquired from a non-public source, however, any DEM can be used to execute the same workflow demonstrated by this tutorial.

Setting Environment

Data Import

As mentioned above, to access and import layers into the SAGA workspace, the user’s working directory is displayed at the bottom left of the screen under File System. There, polygon, grid, and table data can be shown and imported if double clicked on. Other SAGA projects (*.sprj) can also be shown and launched with a double click.

• The DEM used in this project was acquired as a text file (*.txt) in ASCII format. Given that, double clicking on it will import it as a table and not a raster grid.
• To import the DEM as a raster grid, the geospatial data acquisition library GDAL is used. GDAL is a translator library that is usually coupled with another library OGR. GDAL is used to translate raster data formats into spatial software whereas OGR is for vector data translation and design.

• To import the ASCII DEM used, GDAL can be reached through: Geoprocessing |File |GDAL/OGR | GDAL: Import raster
• Locate your ASCII text file, and set the Interpolation' to Nearest Neighbour.