If the water harvesting system includes a storage tank, the tank needs to be sized correctly in order to give adequate storage capacity and at the same time minimize capital investment. The storage requirement will be determined by a number of interrelated factors such as the local rainfall data and weather patterns, catchment area, runoff coefficient, user numbers and consumption rates. The calculation of the storage capcity can either be based on the actual demand or the possible supply of rainwater (Hartung, 2002).

Demand side approach:

storage requirement R = C * n * d

with:

C:Consumption per capita per day

n:number of people

d:longest average dry period

Supply side approach:

annual available water W = A * C * R

with:

A:catchment/roof area

C:runoff coefficient

R:average annual rainfall

demand D = R as calculated above in the demand side approach

The storage requirement can be derived as maximum difference between the cumulative harvested water and the cumulative demand.

Storage tanks should be watertight with a solid, secure cover, a screened inlet, an overflow pipe and a covered manhole. The extraction system should be designed in such a way that the water quality is protected. The choice of a specific system depends on several factors such as for example:

• space availability

• option available locally

• local traditions for water storage

• cost - of purchasing new tank and of materials and labour for construction

• materials and skills available locally

• ground conditions

• patterns of usage