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Watershed Delineation using DEM Data in QGIS:
Step 1: Load DEM Data
1. Open QGIS and add your DEM file (.tif format) by going to Layer, then Add Raster Layer.
2. Ensure the DEM data covers the area of interest and has sufficient resolution.
Step 2: Fill Sinks in the DEM
Sinks (depressions) can cause issues in flow direction analysis, so they must be filled.
1. Open the Processing Toolbox (Ctrl+Alt+T).
2. Search for Fill Sinks (under SAGA or GRASS GIS tools).
3. Select your DEM as input, then run the tool to produce a depression-free DEM.
Step 3: Generate Flow Direction
1. Search for Flow Direction in the Processing Toolbox.
2. Select the filled DEM as input.
3. Run the tool to produce a flow direction raster.
Step 4: Generate Flow Accumulation
1. Search for Flow Accumulation in the Processing Toolbox.
2. Use the flow direction raster as input.
3. Run the tool to generate a raster showing how much water accumulates in each cell.
Step 5: Define a Pour Point (Outlet)
1. Choose a point where you want to define the watershed outlet (e.g., river confluence).
2. Use Add Point Layer, then create a new point feature at the desired location.
3. Convert this to a raster using Rasterize (Vector to Raster) tool.
Step 6: Delineate the Watershed
1. Search for Watershed in the Processing Toolbox.
2. Select the flow direction raster and the pour point raster.
3. Run the tool to generate the watershed boundary.
Step 7: Convert Watershed Raster to Vector
1. Use Raster to Vector (Polygonize) tool to convert the watershed raster into a shapefile.
2. The final vector layer represents the watershed boundary.