India is a good example of a country that faces the paradox of water shortages and monsoons in the same year. There is an acute water scarcity in several urban and rural areas within India, despite the pounding rains and flooding brought on by the monsoon season. This kind of flooding can overwhelm the groundwater supply for a region and threaten water quality. It is not enough for a region to receive good annual rainfall if that precipitation is concentrated in only a few intense spells each year. This kind of rain runs off rapidly, and is not effective at replenishing groundwater reserves. For their parts, cities often have not maintained the infrastructure necessary to harvest water.
Some large Indian cities, such as Bangalore and Delhi, practice rainwater harvesting, as do other locations around the world, including Germany, the U.S., Singapore and Japan. These locations are motivated to harvest rainwater because:
- Rainwater is an ideal adjunct to inadequate groundwater and surface resources.
- Diverting rainwater from sewers and storm drains lessens the loads experienced by treatment plants.
- Harvested rainwater reduces flooding.
- Rainwater, when allowed to replenish existing aquifers, helps increase the quality of groundwater.
Scientists call the total amount of rainfall received in an area the rainwater endowment. Only a portion of this amount, the water harvesting potential, can be effectively harvested. To estimate the water harvesting potential of an area, scientists must first quantify the annual rainwater endowment, using preferably at least ten years of past local data. In addition, planners must know the patterns of rainfall: if there are prolonged drought periods throughout the year, the need for rainwater collection capacity will be higher than in cases where periods of rain are more evenly distributed. Regions with long drought periods usually cannot build enough water storage facilities, and so need to rely on techniques that help to recharge groundwater aquifers. But this decision is also dependent on the local sub-surface geology. What is needed to recharge groundwater is a permeable substratum. Lacking this, locations may be forced to opt for surface storage.
Typically, rainwater can be harvested from such features as rooftops, paved and unpaved areas, and natural bodies of water such as lakes and ponds. Also, well-maintained storm-water drains can provide a cost-effective way to harvest rainwater. Surface tanks can be used to store rainwater for reuse by residents. These can be situated in housing developments, hotels, and public buildings. Surface areas, or catchments, receive rainfall and direct it through a system of gutters, pipes and conduits to storage locations or allow it to drain into the groundwater supply. Some systems use a charcoal, pebble or sand water filter to keep collected water clean. Roofs can be configured with concentric chambers containing filter material. There are commercial companies that produce small multi-staged water treatment devices that are often quite affordable.