Complete Updated Workflow Stream and Catchment Delineation in QGIS

Introduction

This tutorial guides you through the process of deriving streams and catchments using SRTM DEM tiles in QGIS 3.16 LTR. It utilizes various plugins such as QuickMapServices, SRTM-Downloader, PCRaster Tools, and GRASS tools. Following this step-by-step guide will help you effectively conduct hydrological analysis and catchment delineation in QGIS.

Step 1: Define Study Area Extent

• Open QGIS and create a new project.
• Use the "Add Vector Layer" option to import any existing shapefiles that represent your study area.
• If you don’t have a shapefile, you can draw a polygon using the "Create Layer" tool.
• Ensure that the extent is correctly set by checking the coordinate reference system (CRS).

Step 2: Download SRTM Tiles

• Install the SRTM-Downloader plugin if you haven't already.
• Go to the plugin's interface and specify the area of interest.
• Select the appropriate SRTM tile resolution (1 arc-second is recommended for detailed analysis).
• Start the download process for the selected tiles.

Step 3: Mosaic SRTM Tiles

• After downloading, use the "Raster" menu.
• Navigate to "Miscellaneous" and select "Build Virtual Raster (Catalog)".
• Add all the downloaded SRTM tiles and create a virtual raster to combine them into a single raster layer.

Step 4: Reproject and Subset DEM

• Right-click on the virtual raster layer and choose "Export" followed by "Save As".
• Select your desired coordinate reference system for the DEM and specify the output file name.
• Use the 'Clip raster by extent' option to focus on your study area.

Step 5: Style the DEM

• Open the layer properties by double-clicking the DEM layer.
• Go to the "Symbology" tab and apply a grayscale color ramp for better visualization.
• Adjust the contrast and brightness as needed to highlight terrain features.

Step 6: Convert DEM to PCRaster Format

• Ensure that the PCRaster Tools plugin is installed.
• In the menu, select "Raster" and then "PCRaster Tools".
• Choose "Raster to PCRaster" and select your DEM layer to convert it into PCRaster format.

Step 7: Fill Sinks and Calculate Flow Direction

• Use the PCRaster Tools to fill any sinks in the DEM.
• Then, calculate the flow direction by selecting the appropriate option in the PCRaster menu.
• Save the flow direction raster for further analysis.

Step 8: Style Flow Direction with Directional Ramp

• For better visualization, style the flow direction raster using a directional color ramp.
• This will help in understanding the flow patterns across the study area.

Step 9: Calculate Strahler Orders

• Utilize the PCRaster Tools to compute Strahler orders from the flow direction data.
• This will help classify the streams based on their hierarchy.

Step 10: Calibrate Strahler Orders

• Review the calculated Strahler orders and adjust any anomalies manually if necessary for accuracy.

Step 11: Derive Channel Network

• Use the Strahler order output to delineate the channel network.
• Apply a threshold to define which streams are significant enough to be included.

Step 12: Style Channels with Strahler Orders

• Style the derived channel network using the Strahler orders for enhanced visualization.
• Different colors can represent different orders for clarity.

Step 13: Smooth Channel Network

• Implement smoothing techniques to refine the channel network lines.
• This will improve the aesthetic and functional representation of the streams.

Step 14: Define Outlet of Catchment

• Identify and mark the outlet point of your catchment area on the channel network.
• Create a point layer to represent the outlet.

Step 15: Derive Catchment of Outlet

• Use the defined outlet to delineate the catchment area.
• Utilize the "Watershed" tool available in the PCRaster plugin.

Step 16: Polygonize Catchment

• Convert the raster catchment area into a polygon layer.
• Use the "Raster to Vector" tool in QGIS to achieve this.

Step 17: Style Catchment Boundary Polygon

• Open the properties of the catchment polygon layer.
• Apply an inverted polygon shapeburst fill for better visibility.

Step 18: Clip Channels with Catchment Boundary

• Clip the channel network to the catchment boundary to focus on relevant streams.
• This helps in isolating the streams that are part of the defined catchment.

Step 19: Store Results and Project in a GeoPackage

• Save all layers into a GeoPackage for better organization and portability.
• Use the "Export" function to create a GeoPackage file containing the DEM, channel network, and catchment areas.

Conclusion

By following these steps, you can successfully derive streams and catchments from SRTM DEM tiles in QGIS. This workflow enhances your capabilities in hydrological analysis and modeling. For further learning, consider exploring the additional resources provided by GIS OpenCourseWare.