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How to calculate potential supply of rainwater from catchment area?

Water is essential for the Environment, for ensuring Food Security and to ensure Sustainable Development. Rainwater harvesting is the accumulation and storage of rainwater. We all know that fresh water resources available on earth are limited and falling rain drops can provide some of the purest naturally occurring water that is available anywhere.   We have experienced rapid population growth that reached  7 billion this year, combined with growing industrialization, urbanization, and agricultural intensification and water intensive lifestyles is resulting into a global water crisis. In this situation, , rainwater harvesting is one of the crucial options that everybody should use on a personal as well as national level.

The first question that arises while taking a decision for implementation of rainwater harvesting system is what will be the harvestable quantity of water? As the quantity of water is very important to effectively plan a rainwater harvesting system, the first step is to quantify it. Following steps illustrate how to calculate potential supply of rainwater from the catchment area. In short, we need values of three main parameters i.e. Mean Annual Rainfall, Catchment area and Rainfall run-off coefficient.

What is Catchment area?

A Catchment area is an area of land where surface water from rain gets collected and drains towards the down slope usually towards the exit of the basin and joins another water body like stream, river, and lake.  To assess roof-top rainwater harvesting potential, the catchment area is taken to be the roof. 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.

How to calculate Catchment area?

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 area in square meters. For different kinds of roof shape, use receptive formula.

How to calculate catchment area using Google earth images?

Google earth images can also help you to calculate catchment area. Just use ‘Google earth pro’ (You need to buy Google earth software for prolong use. Please click here to visit Google earth website for more details). Following pointers help to illustrate it;

  1. Mark catchment area using polygon option.
  2. Click on the measurements tab on the polygon option. It will show area in different units.
  3. Select unit of your interest.
Natural catchment area calcualtion using Google earth pro
Figure 1: Image shows natural catchment area of the stream (Watershed).
Roof top catchment area calcualtion using Google earth pro
Figure 2: Image shows catchment area of the roof top (Building) in square meters

What is Mean Annual Rainfall? How can I get it?

To assess harvestable quantity of rainwater, we need average rainfall intensity over some time period and hence the mean annual rainfall figure is commonly used. For simple understanding, mean annual is the statistical average calculated on the basis of measured rainfall over many years (usually 10 years time period is used). However, there is no guarantee that the accurate rainfall value will be achieved but the mean annual is generally accepted as the basis. Mean Annual Rainfall value is also useful to estimate runoff.

What is run-off? What is run-off coefficient?

It is the water that flows away from a catchment after falling on its surface in the form of rain.  Runoff depends upon the area and type of the catchment over which it falls as well as surface features. For example, normal concrete roof has highest run off value than the area with heavy vegetation.  The amount of rainfall contributing to the runoff of a given area must be known.  The runoff coefficient is the value that represents the ratio of runoff to rainfall.

How to calculate run-off coefficient?

There are many part of the hydrologic cycle affects the runoff coefficient. These includes interception, evaporation, infiltration, slope, soil types (not applicable to roof top systems), etc. Therefore to estimate run-off coefficient, following parametersneed to be assessed – Roof type; slope; soil type; land use; degree of imperviousness; surface roughness and duration and intensity of rainfall.

Following table shows default values for the run-off coefficient;

Type of Catchment


Roof Catchments


0.8 – 0.9

Corrugated metal sheets

0.7- 0.9

Ground surface coverings


0.6 – 0.8

Brick pavement

0.5- 0.6

Untreated ground catchments

Soil on slopes less than 10 per cent

0.0 – 0.3

Rocky natural catchments

0.2 – 0.5

Reference: Pacey, Arnold and Cullis, Adrian 1989, Rainwater Harvesting: The collection of rainfall and runoff in rural areas, Intermediate Technology Publications, London

Calculate potential supply of rainwater from the catchment area

Mean rainwater supply in m3 = Mean annual rainfall in mm/year (Need to convert this value in ‘m’)  X  Surface area of catchment in m2 X Run-off coefficient

Example – Rooftop catchment area made of concrete;

Mean annual rainfall in = 700 mm/year (0.7m)

Surface area of catchment = 2629 m2 (See Figure 2)

Run-off coefficient = 0.6 (Lowest value in this category)

Mean rainwater supply = 1104 m3(1104180 Litres)

Disclaimer: Catchment area calculation using satellite imagery depends on correct knowledge of ground data. Lack of ground information might result in a wrong value.

Image source: Google earth pro trial version


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