Groundwater, as a natural resource has the major advantage of meeting the requirements of emergency supplies during times of scarce water. Dependence on groundwater to meet the needs of the industrial sector also has resulted in lowering the groundwater table. Overdrawal of water along the coastal areas has resulted in the intrusion of saline water from the sea.
This rapid and localized overdraw of groundwater has not been compensated adequately in terms of recharging the subterranean aquifers that store water. The widening imbalance between the groundwater depletion rate and depletion of the freshwater aquifers is resulting in the lowering of the freshwater table.
Such over-exploitation of the underground aquifers had hydrologically and geologically transported a variety of chemicals such as fluoride, chlorides, sulfates, iron, calcium, magnesium, arsenic, nitrates, phosphates.
One of the basic sources of pure water available on Earth is rainwater which can be effectively and economically collected as a potential resource. About 119 X 106 m3 of rainwater is precipitated annually on land out of which 44.5 X 1012 m3 is wasted as runoff to the oceans. Even if a small fraction of this wasted run-off is recovered. We will be provided with a new origin of a precious resource.
Large variation in the availability of potable water and growing demand has necessitated the need for rainwater harvesting. Rainwater harvesting is a technique of increasing the recharge of groundwater by capturing and storing rainwater. In other words, it can be defined as the method of storage or conservation of water by the use of water tanks during rainfall, so that dry seasons are not followed by a scarcity of water. Rainwater, wherever it falls, is captured and pollution of this water is prevented. Rainwater harvesting is not only proving useful for poor and scanty rainfall regions but also for the rich ones.
Importance of Rainwater Harvesting:
- This process is a means to preserve rainwater to be used in dry seasons and in dry areas.
- Rainwater, which flows into the drains without any use, can be used for mankind purposes by applying this method.
- Rainwater harvesting raises the water table by recharging groundwater.
- Industries can also use such water for their purposes and this water is also applicable in irrigation. Thus, water, a precious resource, can be conserved to some extent.
- Rainwater harvesting reduces groundwater contamination.
- It provides drinking water when all other sources have dried up.
Rainwater Harvesting Methods:
Rainwater can be mainly harvested by any one of the following methods-
- By storing in tanks or reservoirs above or below ground.
- By constructing pits, dug-wells, lagoons, trench or check-dams on small rivulets.
- By recharging the groundwater.
Before adopting a rain-water harvesting system, the soil characteristics, topography, rainfall pattern and climatic condition should be understood.
Traditional Rain Water Harvesting:
In India, it is an old practice in high rainfall areas to collect rainwater from rooftops into storage tanks. In foot-hills, water flowing from springs is collected by embankment type water storage. In Himalayan foot-hills, people use the hollow bamboos as pipelines to transport the water of natural springs. Rajasthan is known for its “tankas” (underground tanks) and “khadins” (Embankments) for harvesting rainwater. 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 rainwater and ensured adequate water supply in dry periods.
Modern Techniques of Rain Water Harvesting:
- Pit Recharge- To recharge the shallow aquifers, pits are constructed.
- Trenches- To collect water in the shallow depth trenches are constructed.
- Filtration tank- Rooftop water is carried with the help of a pipe to a filtration tank.
- Dug well- Rainwater, rooftop water collected in dug well.
- Ferrocement Tank- Type of storage made up of simple materials.
Advantages of Rain Water Harvesting:
- Rainwater is naturally soft, contains almost no dissolved minerals or salts and therefore, reduces water conditioning costs involved in softening, demineralising, reverse osmosis treatment etc.
- Rainwater harvest and localized utilization results in energy saving since the energy input required to operate a control water system declared to treat and purify water, a river, a vast service area is bypassed.
- Water accessibility is easier since it is harvested at the concerned site itself.
- Increased independence and water security.
- Groundwater recharging can be instrumental in reducing the concentration of salts in the groundwater by dilution. This recharge acts as a potential resource for future withdrawals.
- Reducing the demand for wastewater supply from an external source.
- It can be used in individual processes like cooling towers and steam generation, with reduced blowdown quantities.