Watershed Delineation using DEM
**Watershed Delineation** is the process of identifying the boundary of a drainage basin — the area of land where all surface water flows to a common outlet — using elevation data. This is essential for hydrology, flood risk modeling, and water resource management. ---
📋 Prerequisites
- QGIS Desktop 3.22+ installed on your computer.
- Basic understanding of GIS data concepts.
Step-by-Step Instructions
Understand the Concept
Water always flows from higher to lower elevation. By analyzing a DEM, GIS software can trace the path water would take across the terrain, and group areas that drain to the same point into a **watershed** (also called a catchment or basin). ---
Prepare Your DEM
1. Download a DEM covering your study area (SRTM 30m works well for most cases) 2. Load it into QGIS: **Layer → Add Layer → Add Raster Layer** 3. Clip it to a reasonably sized study area if needed ---
Fill Sinks in the DEM
Raw DEMs often contain small errors ("sinks" or "pits") that would incorrectly trap water flow. This must be corrected first. 1. Install the **GRASS GIS** processing plugin (usually bundled with QGIS Processing Toolbox) 2. Open **Processing Toolbox → GRASS → Raster → r.fill.dir** 3. Run this on your DEM to produce a **filled/depressionless DEM** ---
Calculate Flow Direction
1. In the Processing Toolbox, search for **r.watershed** (GRASS) or use **Flow Direction** from SAGA tools 2. Input your filled DEM 3. This generates a flow direction raster, showing which way water moves from each cell ---
Calculate Flow Accumulation
Flow accumulation shows how much upstream area drains into each cell — high values indicate streams/rivers. 1. Run **r.watershed** with your filled DEM as input 2. This produces both flow accumulation and a preliminary stream network ---
Delineate the Watershed
1. Identify your **pour point** (the outlet location — e.g., a stream gauge or river mouth) 2. Use **r.water.outlet** (GRASS) with your flow direction raster and the pour point coordinates 3. The tool outputs a raster representing the watershed boundary for that outlet 4. Convert this raster to a polygon: **Raster → Conversion → Polygonize** ---
Visualize and Verify
1. Overlay the resulting watershed polygon on your DEM/hillshade 2. Check that the boundary follows ridgelines (high elevation) — this is the correct hydrological logic 3. Add the stream network layer to see how it fits within the watershed boundary ---
🧠 Quick Recap & Practice Questions
## Quick Recap
- Watershed = area draining to a common outlet point
- Workflow: Fill DEM → Flow Direction → Flow Accumulation → Delineate from pour point
- GRASS GIS tools (r.fill.dir, r.watershed, r.water.outlet) handle the heavy lifting in QGIS
- Always verify the output follows natural ridgelines
---
## Practice Questions
1. Why must sinks be filled in a DEM before delineating a watershed?
2. What does flow accumulation represent?
3. What is a "pour point" in watershed delineation?
---
*Next Tutorial: Change Detection using Multi-temporal Satellite Images*