Rain water harvesting is a technique of recharge of ground water by capturing and storing rain water. This is done by constructing special water-harvesting structures like dug wells, percolation, lagoon, check dams etc. Rain water harvesting is not only proving useful for poor and scantly rainfall regions but also for the rich ones.
The annual average rainfall in India is 1,200 mm. It is astonishing fact that Cherrapunji (Assam), the place receiving the second highest rainfall is 11,000 mm still suffers from water scarcity.
Rain water harvesting has the following objectives:
(i) To reduce run-off loss.
(ii) To avoid flooding of roads.
(iii) To meet the increasing demand of water.
(iv) To raise the water table by recharging ground water.
(v) To reduce ground water contamination.
(vi) To supplement ground water supplies during lean season.
Rain water can be mainly harvested by any one of the following methods.
(i) By storing in tanks or reservoirs above or below the ground.
(ii) By constructing pits, dug-wells, lagoons, trench or check-dams on small rivulets.
(iii) By recharging the ground water.
Traditional rain water harvesting:
In India it is an old practice in high rainfall areas to collect rain water from roof tops into storage tanks. Rajasthan is known for its underground tanks and khadins (embankments) for harvesting rain water.
In our ancient times we had adequate Talaabs, baawaris, Johars, Hauz etc. in every city, village and capital cities of our kings and lords, which were used to collect rain water and ensured adequate water supply in dry periods.
Modern techniques of rain water harvesting:
In arid and semi-arid regions artificial ground water recharging ‘is done by constructing shallow percolation tanks. Check- dams made of any suitable native material (brush, poles, rocks, plants, loose rocks, wire nets, stones, slabs, sacks, etc.) are constructed for harvesting run-off from large catchments areas. Ground water flow can be intercepted by building ground water dams for storing water underground.
As compared to surface dams, ground water dams have several advantages like minimum evaporation loss and reduced chances of contamination.
In roof top rain water harvesting, which is low cost and effective technique for urban houses and buildings, the rain water from the top of the roof is diverted to some surface tank or pit through a delivery system which can be later used for several purposes. Also, it can be used to recharge underground aquifers by diverting the stored water to some abandoned dug-well or by using a hand pump.
All the above techniques of rain water harvesting are low-cost methods with little maintenance expenses. Rain water harvesting helps in recharging the gainers, improving ground water quality by dilution, improves soil moisture and reduces soil erosion by minimizing run-off water.