How to Calculate the Water Harvesting Potential of a Site

The Potential

The total amount of water that is received in the form of rainfall over an area is called the rainwater endowment of that area. Out of this, the amount that can be effectively harvested is called the water harvesting potential.
Influencing factors

Among the several factors that influence the rainwater harvesting potential of a site, eco-climatic conditions and the catchment characteristics are considered to be the most important.

a.    Rainfall

i)Quantity: Rainfall is the most unpredictable variable in the calculation and hence, reliable rainfall data are required, preferably for a period of at least10 years. ii) Pattern: The number of annual rainy days also influences the need and design for rainwater harvesting. Fewer the annual rainy days, more the need for rainwater collection.

b. Catchment area characteristics

Runoff depends upon the area and type of the catchment over which it falls as well as surface features.

All calculations relating to the performance of rainwater catchment systems involve the use of runoff coefficient to account for losses due to spillage, leakage, infiltration, catchment surface wetting and evaporation, which will all contribute to reducing the amount of runoff. (Runoff coefficient for any catchment is the ratio of the volume of water that runs off a surface to the volume of rainfall that falls on the surface).

Runoff coefficients for various catchment surfaces

Type of Catchment	Coefficients
Roof Catchments  - Tiles  - Corrugated metal sheets	0.8- 0.9 0.7- 0.9
Ground surface coverings  - Concrete  - Brick pavement	0.6- 0.8 0.5- 0.6
Untreated ground catchments  - Soil on slopes less than 10 per cent  - Rocky natural catchments	0.0 - 0.3 0.2 - 0.5
Untreated ground catchments  - Soil on slopes less than 10 per cent  - Rocky natural catchments	1.0 - 0.3 0.2 - 0.5

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

Based on the above factors the water harvesting potential of a site could be estimated using the formula given below.

Water harvesting potential       = Rainfall (mm) x Area of catchment x Runoff coefficient