Explain Rain Water Harvesting, and Methods of Rain Water Harvesting & Maintenance Tips for Rainwater Harvesting
Rainwater Harvesting
Rainwater harvesting is the simple process or technology used to conserve rainwater by collecting, storing, conveying, and purifying rainwater that runs off from rooftops, parks, roads, open grounds, etc. for later use. For our water requirement, we entirely depend upon rivers, lakes, and groundwater. However, rain is the ultimate source that feeds all these sources. Rainwater harvesting means optimum use of rainwater at the place where it falls, i.e. conserving it and not allowing it to drain away and cause floods elsewhere.
Rainwater harvesting may be defined as collecting and storing rainwater at the surface or in a sub-surface aquifer before it is lost as surface runoff. The augmented resources can be harvested whenever needed. Rainwater harvesting may date back to 6,000 years ago in China. Evidence is available for rainwater collection at least to 4,000 years ago. Water harvesting was used in China from the 3rd millennium BC.
Need for Rainwater Harvesting
Water is one of the most essential requirements for existence of living beings. Surface water and groundwater are two major sources of water. Due to overpopulation and higher usage levels of water in urban areas, water supply agencies are unable to cope with demand from surface sources like dams, reservoirs, rivers, etc. which has led to the digging of individual tube wells by house owners.
Even water supply agencies have resorted to groundwater sources by digging tube wells to augment the water supply. Replenishment of groundwater is drastically reduced due to the paving of open areas. Indiscriminate exploitation of groundwater results in lowering of the water table rendering many bore-wells dry. To overcome this situation bore wells are drilled to greater depths.
Harvesting rainwater allows the collection of large amounts of water and mitigates the effects of drought. Most rooftops provide the necessary platform for collecting water. Rainwater is mostly free from harmful chemicals, which makes it suitable for irrigation purposes.
This further lowers the water table and in some areas, this leads to higher concentrations of hazardous chemicals such as fluorides, nitrates, and arsenic. In coastal areas like Chennai, over-exploitation of groundwater resulted in seawater intrusion thereby rendering groundwater bodies saline. In rural areas also, government policies on the subsidized power supply for agricultural pumps and piped water supply through bore wells are resulting in a decline in the groundwater table. The solution to all these problems is to replenish groundwater bodies with rain water by manmade means.
Advantages of rainwater harvesting
a) Promotes adequacy of underground water
b) Mitigates the effect of drought
c) Reduces soil erosion as surface run-off is reduced
d) Decreases load on the stormwater disposal system
e) Reduces flood hazards
f) Improves groundwater quality/decreases salinity
g) Prevents ingress of sea water in subsurface aquifers in coastal areas.
h) Improves ground water table, thus saving energy (to lift water)
i) The cost of recharging subsurface aquifer is lower than surface reservoirs
j) The subsurface aquifer also serves as a storage and distribution system
k) No land is wasted for storage purposes and no population displacement is involved
l) Storing water underground is environmentally friendly.
Methods for Harvesting Rainwater
There are three methods of harvesting rainwater are given below:-
i. Storing rainwater for direct use
ii. Recharging groundwater aquifers, from rooftop runoff.
iii. Recharging groundwater aquifers with runoff from the ground area.
i. Storing rainwater for direct use
In places where the rains occur throughout the year, rainwater can be stored in tanks. However, at places where rains are for 2 to 3 months, a huge volume of storage tanks would have to be provided. In such places, it will be more appropriate to use rainwater to recharge groundwater aquifers rather than to go for storage. If the strata are impermeable, then storing rainwater in storage tanks for direct use is a better method. Similarly, if the groundwater is saline/unfit for human consumption or the ground water table is very deep. This method of rainwater harvesting is preferable.
ii. Recharging groundwater aquifers, from rooftop runoff
Rainwater that is collected on the rooftop of the building may be diverted by drain pipes to a filtration tank (for bore well, through settlement tank) from which it flows into the recharge well. The recharge well should preferably be shallower than the water table. This method of rainwater harvesting is preferable in the areas where the rainfall occurs only for a short period of a year and the water table is at a shallow depth. The various methods of recharging groundwater aquifers from rooftop runoff.
iii. Recharging groundwater aquifers with runoff from ground areas
The rainwater that is collected from the open areas may be diverted by drain pipes to a recharge dug well/bore well through filter tanks. The abandoned bore well/dug well can be used cost-effectively for this purpose. The various methods of recharging groundwater aquifers with runoff from ground areas.