Planning :
If collection and storage are designed carefully, it is possible to cater water demand even in areas with rainfall as little as 400 mm / year. Analysis shows that between 80 to 85 % of all measurable rain can be collected and stored from the catchment area, which includes light drizzle and dew condensation. The run-off from a catchment area is computed (Raghunath,1995) by considering the spatial extent of a catchment (A), catchment coefficient (C, that depends on the land use pattern in the catchment) and rainfall intensity (I) and is given by Q= C * I * A [Where, Q = discharge (m3), C = co-efficient of run-off, I = total rainfall / annum (m) and A = catchment area (m2)].
The co-efficient of run-off depends upon the land use pattern in the region. The catchment coefficient for rocky and impermeable: 0.8-1.0; slightly permeable, bare: 0.6-0.8; cultivated or covered with vegetation: 0.4-0.6; cultivated absorbent soil: 0.3-0.4; sandy soil: 0.2-0.3; heavy forest: 0.1-0.2.
The co-efficient of run-off also depends on the shape, size, soil conditions, temperature, and geological conditions of the area of the catchment. However, on the basis of average annual rainfall and land use in the area, the co-efficient can be assumed, which are listed in Table 1.
Table 1: Catchment coefficient for various land uses.
| Catchment Type with rainfall and runoff |
Run off co-efficient |
| Dry tracts with annual rainfall 350 to 750 mm, run off 15 to 20 % |
0.15 to 0.2 |
| Intermediate Zones rain 750 to 1500 mm, run off 20 to 30 % |
0.2 to 0.3 |
| Higher Zone with rainfall above 1500 mm, run off 30 to 55 % |
0.3 to 0.55 |
| Roof and paved areas, run off 80 to 90 % |
0.8 to 0.9 |
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