Membuat Peta DAS dan Menghitung Luas Catchment Area || Watershed Delineation on QGis

Introduction

This tutorial guides you through the process of creating a watershed map and calculating the catchment area using QGIS (Quantum GIS). Understanding watershed delineation is crucial for environmental management, hydrology studies, and land-use planning. By the end of this guide, you will be equipped with the skills to analyze and visualize hydrological data effectively.

Step 1: Prepare Your QGIS Workspace

• Download and Install QGIS
  • If you haven't installed QGIS yet, follow this tutorial: Download and Install QGIS.
• Add Online Basemap
  • To visualize your data effectively, add an online basemap using this tutorial: Add Online Basemap to QGIS.
• Open a New Project
  • Launch QGIS and create a new project to start your watershed analysis.

Step 2: Import Elevation Data

• Acquire DEM Data
  • Obtain Digital Elevation Model (DEM) data for your area of interest from trusted sources.
• Import DEM into QGIS
  • Drag and drop the DEM file into your QGIS workspace or use the ‘Add Raster Layer’ option to load the data.

Step 3: Fill Sinks in DEM

• Open the Processing Toolbox
  • Navigate to the Processing Toolbox by clicking on the 'Processing' menu and selecting 'Toolbox'.
• Fill Sinks
  • Search for the 'Fill Sinks (Wang and Wu)' tool.
  • Select your DEM layer and run the tool to ensure all depressions are filled, creating a more accurate representation of the terrain.

Step 4: Generate Flow Direction

• Access Flow Direction Tool
  • In the Processing Toolbox, locate the 'Flow Direction' tool.
• Run Flow Direction Analysis
  • Select the filled DEM as the input layer and run the analysis to create a flow direction raster.

Step 5: Create Flow Accumulation Layer

• Use Flow Accumulation Tool
  • Find the 'Flow Accumulation' tool in the Processing Toolbox.
• Generate the Flow Accumulation Raster
  • Input the flow direction layer from the previous step. This layer will show areas collecting runoff.

Step 6: Define Watershed Area

• Identify Pour Point
  • Place a point on the flow accumulation layer where you want to define the watershed boundary.
• Run the Watershed Tool
  • Locate the 'Watershed' tool in the Processing Toolbox.
  • Input the flow direction layer and the pour point to generate the watershed boundary.

Step 7: Calculate Catchment Area

• Use the Field Calculator
  • Select the watershed polygon layer and open the attribute table.
  • Use the Field Calculator to create a new field for area calculation.
• Calculate Area
  • Input the following expression to calculate the area:

    $area
    

  • This will give you the area of the catchment in the unit of your project’s CRS (Coordinate Reference System).

Conclusion

You have successfully created a watershed map and calculated the catchment area using QGIS. This process not only enhances your skills in hydrological analysis but also allows you to apply these techniques in various environmental studies. For further learning, consider exploring additional QGIS tutorials, such as layout design and advanced mapping techniques. Happy mapping!