A basin is defined as a topographic region in which all water drains to a common area. Identifying basins within a study area can be beneficial for land and water management because priority areas can be defined and the hydrology of the area can be visualized. Delimiting a basin can be done by using Geographic Information Systems such as QGIS and SAGA GIS. To do so, a Digital Elevation Model (DEM) is required since the drainage network is determined by the elevation and slope of the terrain. This tutorial demonstrates how to reproject a DEM raster in QGIS, how to fill the sinks of a DEM, to calculate flow accumulation, to obtain the channel network and the basin limits with SAGA GIS based on the DEM using the Upslope Area interactive tool.

Tutorial

Methodology

A DEM will be uploaded to SAGA GIS. The model is from the state of Tlaxcala Mexico, previously downloaded from the EarthData by NASA website on this tutorial.

Then, the SAGA-GIS will be used, starting with the Fill Sink (Wang & Liu) module. This module uses an algorithm proposed by Wang & Liu to identify and fill surface depressions in digital elevation models (SAGA-GIS).

After the sinks are filled, the accumulated flow will be calculated by using the Flow accumulation (Top-down) module. Top-down processing of cells for calculation of flow accumulation and related parameters. This set of algorithms processes a DEM downwards from the highest to the lowest cell (SAGA-GIS).

Next, the channel network will be obtained by using the Channel Network Module. The module derives said network based on gridded digital elevation data (SAGA-GIS).

The basin limits will be obtained by using the UpSlope Area (Interactive) module. A random point within the study area will be selected and a basin will be created. This module allows you to specify target cells, for which the upslope contributing area shall be identified (SAGA-GIS).

Finally, the obtained basin will be converted to a shapefile in QGIS. The polygonize tool will be used to complete this task. The tool allows us to convert raster files to vector files.