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, ecoclimatic 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 
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 
