This happens in open areas as well as in congested cities through the installation of required equipment. The collection and storage of rainwater from run-off areas such as roofs and other surfaces has been practised since ancient times in India. It is particularly useful where water supply is inadequate. If the collection and storage equipment is designed carefully it is possible for a family of four persons to live for a year in areas where annual rainfall is as little as 100 mm. Observations in some other countries like Zimbabwe, Botswana and Israel have shown that up to 85 per cent of all measurable rain can be collected and stored from outside catchment areas.
This includes light drizzle and dew condensation which occurs in many parts of the country during drier months. A study has shown that if the rainfall is 635 mm per year, the run-off from a suitable catchment area can be as high as 500 mm. An area covering one hectare may yield 5000 kilolitres of water in the year-enough for 500 heads of cattle for six months. One ml of rain falling on one sq m of area will yield approximately one liter of water. For a family of five persons the daily requirement is about 100 liters per day. If we assume that the longest period without rain to be six months, the volume of required water can be worked out at 18 kilo liters. This requirement can be met through rainwater harvesting at the time of rainy season.
Nature has its own systems of recharging groundwater. In forests water seeps gently into the ground as vegetation breaks the fall and flow of water. This groundwater in turn feeds wells, ponds, lakes and rivers. Protecting the forests, therefore, means protecting catchment areas. However, with the cutting down of trees, these systems are being hindered.
In urban areas, the construction of houses, roads and footpaths has left little exposed parts of earth for water to soak in. Most of the water, therefore, runs wastefully through drains. In rural areas, the water quickly takes the form of flood and flows into rivers which dry up as soon as the rain stops as the bulk of water flows away. If this water can be held back, it can seep into the ground and recharge the groundwater supply.
A sample urban installation of roof rainwater collection in a metropolitan city can be like this: If you live in a single dwelling unit or a multi-tenant apartment complex you already have 80 per cent of rainwater harvesting system. Only small re¬orientation of the plumbing design needs to be done. The existing designs in the cities make all the rainwater from the roof and all the groundwater areas surrounding the house flow towards the street, from where it goes to the drains and runs off wastefully as sewage water.
What we need to do is to bring the rainwater down using closed PVC pipes and direct it to a sump. Then include a simple three-part filtration unit consisting of sand, brick jelly and broken mud bricks. If you do not have a sump, use a well/construct a baby well about 2 ft in diameter and about 16 ft deep based on soil structure. Another type of rainwater harvesting system collects groundwater and stops its flow at the gate. A concrete slab is put with holes in it, a 2 ft deep pit is built across the full width of the gate. Then a pipe is connected and water is flowed to a well or baby well as the case may be.
Rainwater harvesting has become a very popular method of conserving water, particularly in the urban areas. Collecting rainwater on the roofs of building and storing it underground for later use has several advantages. It conserves water as a valuable source and stops it from running off wastefully as sewerage water. It provides water during dry season. It also recharges the aquifers or the reservoirs of water below the surface of the earth, thus raising the level of underground water table. This is highly beneficial for trees and other vegetation cover which draw mainly from underground water.
When vegetation is dense and strong there are other benefits like checking of soil erosion, soaking in of water when rain falls directly on the open area. Raising the underground water table makes it easier for pumping sets and bore wells to draw water out of irrigation and other uses. Arresting the decline of water table also checks water in grass, i.e. prevents sea-water from moving landward.
Rainwater harvesting can also stop groundwater contamination. During rainy season, water tends to collect in puddles in low lying areas, in unattended pits on the ground. At times it gets collected in empty tins, containers, tires or other waste material. Such water gets contaminated within a few hours and becomes a breeding ground for insects and vectors like mosquitoes which spread malaria, dengue, meningitis and chikungunya. With rainwater harvesting all these problems can be solved to some extent if not entirely.
Agriculture contributes 22 per cent to India’s GDP and provides employment to about 58 per cent of our population. With almost 60 per cent of the cropped area dependent on rains, monsoon continues to have a major bearing on our agriculture growth and on GDP as a whole. The only solution is to free agriculture from the clutches of rain-fed growth. One way is to bring more and more area under irrigation. For this massive investment both public and private sector is needed. Another alternative is the adoption of rainwater harvesting.
Availability of sufficient water in an area does not mean that water supply will be there forever. If the favourable conditions disappear because of natural causes or human activities, there can be a shortage. Cherrapunji was famous for receiving the largest volume of rainfall in the world, and it still is, but experiences acute water shortage as a result of extensive deforestation and due to the reason that water conservation methods are not used. Free flow of water along the slopes of hills has caused heavy erosion of top soil. There are now many stretches devoid of trees and greenery. People have to walk long distances to collect water. If rainwater harvesting had been used, things would have been different.
Similarly, if there is an acute water shortage in some region, it does not mean that it will continue to experience such shortage forever. If proper water conservation methods are adopted, the shortage can be reversed or at least reduced. This actually happened in the surrounding area of river Rupee in Rajasthan. It does not receive even one-fourth for rainfall received in Cherrapunji but has more water available then the Meghalaya city. When drought-like conditions developed in Rupee area during the 1980s, some Non-Government Organisations (NGOs) guided the women living in the area to take initiative in building small, round ponds and dams to hold back rainwater.
Gradually water began coming back as proper methods of conserving and harvesting rainwater were followed. The revival of the river has transformed the ecology of the place and consequently, of the people living there. If such efforts by the people are supplemented by proper plans and implementation by the government, water shortage in India can be overcome.