How to calculate Catchment Area Step By Step


Defined as: Drainage basin or watershed or catchment area is a boundary or area of ​​land where rain changes surface water and melts snow or ice at low altitudes, usually in the basin. Exits, 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 difficult to measure and can therefore 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, which is a permanent lake, dry lake, or area where surface water is lost underground. The drainage basin contains both streams and rivers that show water, as well as the land surfaces through which water flows into those canals, and are 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 or drainage outside North America.

How to calculate Catchment Area

It’s easy. Use your geometric skills to calculate the roof area or any other small watershed area. Using some simple multiplication and addition you can come up with a catchment area value of 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 that is located in frontier region lakki marwat (F.RLakki )                          

The  coordinates for the dam axis is 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 irrigate the downstream are which include  Ghazikhel, Suliman Khel, Masti Khel, Zangi kheil, warmola balae while the inflow of the water is imported by different stream warmola stream,sarkai algad and the small canal flowing into the warmola stream and sarkai stream. 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 constructed in the future soon

Now, How to calculate Catchment Area? We calculate the catchment area of warmola dam by suing 2 software:

  1. Auto CAD
  2.  QGIS

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


Taking and using Using scanned image of the map in AutoCAD we can calculate it’s 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 the AutoCAD by dragging into the AutoCAD Canvas.


In the next step, we will do scaling as the AutoCAD canvas in not aligned with the map, for that we need to do scaling in order to coincide the map units with the AutoCAD canvas. For this draw a line of unit dimension and the 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 area on the map for which you want to calculate the catchment area, after selection is done select 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


Calculation of Catchment Area of Warmola dam through QGIS:

Step 1:

First of all, I added the scanned image of Warmola dam in QGIS. you can use the insert image command or by dragging the image in the canvas of QGIS.

Step 2:

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

Step 3:

Then I captured the coordinates of the boundary of the catchment area through coordinate capture plugin. 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 catchment area of the dam.


Area calculated in QGIS=23.5km2


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

How to calculate Catchment Area, Catchment Area