How To Calculate Catchment Area Easy Way

A drainage basin or watershed or catchment area is defined as a boundary or area of ​​land where rain changes surface water and melts snow or ice at low altitudes, usually in the basin, where water meets other water sources such as a river, lake, reservoir, estuary, swamp, sea, or ocean.

Large catchment areas such as river drainage basins are challenging to measure and can be estimated using Geographic Information System (GIS) methods. GIS works on images obtained from remote sensing satellites.

In closed drainage basins, the water becomes a point inside the basin, called a sink, a permanent lake, a dry lake, or an area where surface water is lost underground. The drainage basin contains both streams and rivers that show water and the land surfaces through which water flows into those canals and is separated from adjacent valleys by the division of the canals.

The drainage basin acts as a funnel, collecting water from the area covered by the basin and circulating it to a single point. Each drainage basin is separated by a perimeter from the adjacent basin, creating a barrier of high geographical features (such as a peak, hill, or mountain) by drainage division or drainage outside North America.

How to calculate Catchment Area

It’s easy. Use your geometric skills to calculate the roof area or any small watershed area. Using simple multiplication and addition, you can develop a catchment area value per square meter. Measure the length and width of the roof in meters and multiply both to get the area in square meters. For different types of roof shapes, use the eclipse principle.


First of all, we need some data. For this demo, we have collected the data from FDA phase#5 from Engineer Muhammad Salman.

The dam we are taking is Warmola dam, a small proposed dam, located in the frontier region lakki marwat (F.RLakki).                          

The coordinates for the dam axis are as Longitude: 32.62°N, Latitude: 70.45°E.

The elevation of the dam above sea level is 330meter. The practically calculated catchment area is 23square kilometers. The main purpose of this dam is to rinse the downstream are which include Ghazikhel, Suliman Khel, Masti Khel, Zangi kheil, warmola balae. 

At the same time, the inflow of the water is imported by different warmola streams, the sarkai algad and the small canal flowing into the warmola and sarkai streams. 

This dam will be used for irrigation and water supply to the downstream areas; however, the dam is not yet constructed, but it will be built soon.

Now, How to calculate Catchment Area? We calculate the catchment area of the warmola dam by using two software:

  1. Auto CAD
  2.  QGIS

The detailed procedure is discussed step by step. The method we adopted is as follows:

How to calculate catchment area IN AUTO CAD:

Taking and using Using the scanned image of the map in AutoCAD, we can calculate its catchment area. So we use the following three simple tips:


Make a clear scan image of the map for which you want to find the catchment area and load it into AutoCAD by dragging it into AutoCAD Canvas.


In the next step, we will do scaling as the AutoCAD canvas is not aligned with the map; we need to do scaling to coincide the map units with the AutoCAD canvas. For this, draw a line of unit dimension and draw a line segment on the scale of the map covering one unit, Then use the command AL to align these two lines; finally, scaling will be done after executing the command.

Step 3

Now using PL or Spline command, select all the areas on the map for which you want to calculate the catchment area. After the selection is made, choose the whole area and type LI in the command bar. Finally, it will show up the area of the selected polygon or close box.

Result : Catchment area Actual = 23 sq km

             Though AutoCAD = 24 sq km

How to calculate catchment area IN AUTO CAD

How to calculate catchment area IN QGIS:

Calculation of Catchment Area of Warmola dam through QGIS:

Step 1:

First of all, I added the scanned image of the Warmola dam in QGIS. After that, you can use the insert image command or drag the image on the canvas of QGIS.

Step 2:

I georeferenced the image as a raster image in GIS by assigning four real-world coordinates to the picture.

Step 3:

Then I captured the coordinates of the catchment area boundary through the coordinate capture plugin. Finally, I prepared a Text file by adding the coordinates of the catchment area; thus, a well-known text file of a polygon is prepared.

Step 4:

I added that delimited text file to QGIS. Thus a polygon comes from the WKT file.

Step 5:

Finally, I measured the area of that polygon that represents the dam’s catchment area.


Area calculated in QGIS=23.5km2


From the above results, it is clear that the catchment area calculated by both methods is pretty accurate. However, QGIS gives a more precise result. For More Updates, visit our Civil Engineering blog section, drop comments below, or watch the video tutorial.

How to calculate Catchment Area, Catchment Area

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