How to calculate Catchment Area Step By Step


How to calculate Catchment Area Step By Step Drainage basin or watershed or catchment area is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point at a lower elevation, usually the exit of the basin, where the waters join another water body, such as a river, lake, reservoir, estuary, wetland, sea, or ocean. Large catchment areas like river drainage basins are difficult to measure and hence can be assessed using Geographical Information System (GIS) techniques. GIS works on images received from Remote Sensing satellites.

In closed drainage basins, the water converges to a single point inside the basin, known as a sink, which may be a permanent lake, dry lake, or a point where surface water is lost underground. The drainage basin includes both the streams and rivers that convey the water as well as the land surfaces from which water drains into those channels, and is separated from adjacent basins by a drainage divide.

The drainage basin acts as a funnel by collecting all the water within the area covered by the basin and channeling it to a single point. Each drainage basin is separated topographically from adjacent basins by a perimeter, the drainage divide or watershed (outside North America) making up a succession of higher geographical features (such as a ridge, hill or mountains) forming a barrier.


It is simple. Just use your geometrical skills to calculate rooftop area or any other catchment area of small size. Using some simple multiplication and additions you can come up with the catchment area value in square meter. Measure length and width of the roof in meters and multiply both to get the area in square meters. For different kinds of roof shape, use receptive formula.


We collect the data from FDA phase#5 from Engineer Muhammad Salman

Warmola dam is 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  kilometer.the main purpose of this dam is to irrigate the down stream are which include  Ghazikhel ,Suliman kheil , masti kheil , Zangi kheil, warmola balae while the inflow of the water is imported by difeerent stream warmola stream ,sarkai algad and the small canal flowing into the warmola stream and sarkai stream  .This dam will used for irrigation and water supply to the down stream areas however the dam is not yet constructed but it will be constructed in the future soon

How to calculate Catchment Area? We calculate the catchment area of warmola dam by

  1. Auto CAD
  2.  QGIS

NOW  the procedure we adopted is as follow


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 scan of the map of which we want to find the catchment area and load it into the AutoCAD by dragging into the AutoCAD Canvas.


Now 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.

How to calculate Catchment Area

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.

How to calculate Catchment Area

How to calculate Catchment Area

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 scan image of Warmola dam in QGIS.

Step 2:

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

How to calculate Catchment Area

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.

How to calculate Catchment Area

Step 5:

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

How to calculate Catchment Area


Area calculated in QGIS=23.5km2

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