Rainwater harvesting (or rain harvesting) is an ecologically renewable source of drinking water, especially where this is in scant supply. The term actually refers to the accumulation and storage of rain water, for use in irrigation, groundwater recharge and for drinking purposes – of animals, birds and humans. In many areas ‘rain harvesting’ is the only viable or economical means of accessing potable water. As per some studies, catchment areas should have in excess of 200 mm rainfall per year to provide viable rain harvesting. Rainwater is usually collected from the roofs of buildings in rain water barrels and tanks or even on the ground itself, in specially prepared (and lined) ponds or lakes.
Rainwater harvesting can vary from a simple system to the very complex ones seen at the industrial level. How successful any system is depends on the intensity of the rainfall, planned catchment area and the efficiency of the system in storing the water. Ground storage is better suited for small communities rather than individual families, whereas storage in rain barrels would suit the latter more than the former. Water from the first rains (after the dry season) should be allowed to run off, carrying with it most if not all the accumulated dirt. If the volume of the first rains was not sufficient to do so, the second flush should also be allowed to run off to prevent future contamination of the water. Some things to remember when designing a rainwater collection system:
- Gutters should be strong and large enough to be able to handle peak flows and be correctly inclined to prevent water stagnation.
- Collection and storage tanks should be covered to prevent evaporation and the breeding of plants as well as water borne insects.
- For the reasons mentioned in point 2 above, hygiene has to be maintained through regular maintenance of these systems.
Another way to collect rainwater is to build dykes in aquifers that allow accumulation of groundwater – formed from the seepage of only rainwater (and not sea water). A similar method is groundwater recharge, where a system is built allowing excess rain water to be stored in a sump or pond or lake, from which it then seeps into the ground to raise groundwater levels.
Rainwater harvesting is an ecologically sound and economically beneficial activity even in urban areas. It can supplement town supply, reduce flooding, improve the quantity and quality of groundwater and in places where the water is ‘hard’ this water may be more suitable for use in the toilet, for washing and even for drinking (if required, after suitable treatment).
Families can store rain water in barrels made from local material (again an ecological plus), including those made from wood and earthenware pitchers and tanks. Plastic buckets, barrels and tanks of various sizes are also available. Rain barrels may be expensive to buy initially, but the cost pays off over time especially if mains water is charged. Size can vary from 50 litres to 100000 litres capacity and the smallest rain barrels for sale in the US can cost around $ 100.00.