Run off rainwater harvesting
Practical Action
The tank should not be in close proximity to the house or to paths /roadways as it is
possible for children and even negligent adults to fall in. As an additional security
measure, construct a fence around the tank.
The opening of the tank should be to the direction of the flow of rainfall. It is not
advisable to obstruct patterns of natural flow of water as there is a possibility of mud
and other waste getting into the tank. (The mud filters function only when the water
flows directly through them).
Things to be bare in mind
If a very strong current of water is flowing it could place the tank in jeopardy.
If by construction of the tank, the natural water flow is obstructed, soil erosion can
occur and crops can consequently be destroyed.
Constructing the rainwater harvesting tank
Clear the selected land thoroughly. Flattening the land is important for ease of taking
measurements.
It is advisable to construct a circular tank as it will withstand greater pressures.
Determine the quantity of water required for irrigation purposes.
In such instances, the following factors should be considered;
a.) The rainfall pattern of the area. (If the area experiences regular rainfall
throughout the year, a small tank of 4000-5000 litres would suffice, whereas
in particularly dry areas which experience dry spells for about 6 months of the
year, it would be beneficial to store as much water as possible.)
b) The extent of land, which is proposed to be cultivated
c) The amount of investment that can be made.
The tank should not be more than 1.75 m in depth in order to withstand the pressure of the
water. Low depth makes cleaning and use of the tank easier.
The chart given below can be used as a guide to determine the radius of the tank.
Capacity of tank (Litres)
Radius of tank (Meters)
5000
6000
7000
8000
9000
10000
11000
0.9
1
1.125
1.2
1.275
1.35
1.425
1.5
Table 2: Capacity according to the radius of the tank
Take a length of rope as long as the radius and tie it to a wedge. Plant the wedge at
the place you want to be the centre of the tank and draw a circle.
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