In this tutorial, we walk you through the process of generating multiple catchments/ watersheds using QGIS, which is a powerful open-source GIS tool.
A watershed or catchment is an area of land where all surface water converges to a single point, making its analysis crucial for hydrology, environmental management, and urban planning. We'll begin by discussing the importance of using GIS tools like QGIS or ArcGIS for precise watershed delineation. You'll learn how to obtain and work with a Digital Elevation Model (DEM), which is essential for accurately mapping terrain and water flow. We’ll cover key concepts like Strahler stream order to derive river networks and then guide you through the steps of generating catchments. Additionally, we’ll perform geometric computations, such as calculating catchment areas, to help you fully understand the spatial characteristics of the watersheds.
One of the tools that we are going to rely on during the delineation process is SAGA tools, which can be installed as an external plugin on QGIS. It has been brought to attention that some QGIS versions encounter issues when working with SAGA tools, and specifically when running an algorithm related to a tool called Upslope Area. During this tutorial, we are also discussing that issue, and we are offering a potential solution for that as well for those who are encountering this issue along the way.
We start this tutorial from scratch, and work our way up at a decent pace so that even if you are new to using these GIS tools, you will have plenty of time and applications through which you would be able to become proficient in using the tools by the end of this tutorial.
For your convenience, the structure of the video has been listed below. If you wish to skip over to a specific part of the video, feel free to use the time stamps.
00:00 – Introduction
01:28 – DEM data downloading
02:01 – Adding DEM data to QGIS workspace
07:29 – Checking the relevant UTM zone for DEM reprojecting
10:07 – Defining the area of interest using a polygon object and clipping the DEM
16:30 – Using SAGA fill tool for correcting the DEM irregularities
19:54 – Deriving stream order using Strahler Order method
31:27 – Deriving the river network in as a polyline type vector layer
34:12 – Deriving a single watershed using SAGA Upslope Area tool
41:33 – Discussing issues with errors when running Upslope Area tool, and the potential fix
46:09 – Generating multiple sub-catchments using batch processing
01:05:43 – Calculating areas of sub-catchments
Link to download the data for this tutorial – https://drive.google.com/drive/folders/1WbFz8rOr8q_3arGdjEvBzcP3E5hdbwL_?usp=sharing
Downloading a DEM for free using NASA EarthData tools – https://youtu.be/7LeGd6tuEr4
#hydrology #QGIS #delineation