Monday, 20 February 2012

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, to determine the potential rainwater supply for a given catchment, reliable rainfall data are required, preferably for a period of at least10 years. Also, it would be far better to use rainfall data from the nearest station with comparable conditions.

ii) Pattern: The number of annual rainy days also influences the need and design for rainwater harvesting. The fewer the annual rainy days or longer the dry period, the more the need for rainwater collection in a region. However, if the dry period is too long, big storage tanks would be needed to store rainwater. Hence in such regions, it is more feasible to use rainwater to recharge groundwater aquifers rather than for storage.

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

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