MEANING:

Rainwater harvesting is the gathering, or accumulating and storing, of rainwater.Rainwater harvesting has been used to provide drinking water, water for livestock, water for irrigation or to refill aquifers in a process called groundwater recharge. Rainwater collected from the roofs of houses, tents and local institutions, or from specially prepared areas of ground, can make an important contribution to drinking water. In some cases, rainwater may be the only available, or economical, water source. Rainwater systems are simple to construct from inexpensive local materials, and are potentially successful in most habitable locations. Roof rainwater is usually of good quality and does not require treatment before consumption. Household rainfall catchment systems are appropriate in areas with an average rainfall greater than 200mm per year, and no other accessible water sources (Skinner and Cotton, 1992).
There are a number of types of systems to harvest rainwater ranging from very simple to the complex industrial systems. Generally, rainwater is either harvested from the ground or from a roof. The rate at which water can be collected from either system is dependent on the plan area of the system, its efficiency, and the intensity of rainfall

METHODS OF RAIN - HARVESTING:

In ancient days itself, people, especially Indians, know the methods of conservation of rainwater. There are evidences that, even during Harappan period, there was very good system of water management as could be seen in the latest excavation at Dholavira in Kachch. During independence period, the people use to manage water resources considering it as part of the nature which is essential for their survival. This could be seen from the rain water harvesting structures in the low rainfall areas of Rajasthan, harvesting springs in hilly areas and mountainous region and percolation ponds and tanks in southern India.

In Tamil Nadu, the ancient people stored rainwater in public placed separately one for drinking purposes and another for bathing and other domestic purposses and called them as Ooranies. They also formed percolation tanks or ponds, for the purpose of recharging irrigation or domestic wells. They periodically clean the water ways so as to get clean water throughout the year. These are instances in the history that people constructed crude rubble bunds across river courses either for diversion of water or for augmenting the ground water. The various methods of rainwater harvesting are classified below under two category, Traditional and Modern methods.

Traditional Methods :

Traditional rainwater harvesting, which is still prevalent in rural areas, was done in surface storage bodies like lakes, ponds, irrigation tanks, temple tanks etc. In urban areas, due to shrinking of open spaces, rainwater will have to necessarily be harvested as ground water, Hence harvesting in such places will depend very much on the nature of the soil viz., clayey, sandy etc.

Modern Methods :

The Modern methods of rainwater harvesting are categorised under two, they are Artifical Recharging and Rain Water Harvesting. The former is classified into Absorption Pit Method, Absorption Well Method, Well cum Bore Method and Recharge trench cum injection well. The later is categorised into Individual Houses and Grouped Houses which are further classified into Percolation Pit Method, Bore Well with Settlement Tank, Open Well Method with filter bed Sump and Percolation Pit with Bore Method.

RWH Techniques:

There are two main techniques of RWH

a) Storage of rain water on surface for future use

b) Recharge of ground water

The storage of rainwater on surface is a traditional technique and structure used in underground tanks, ponds, check dams, weirs etc.

Recharge of ground water is a new concept of RWH and the structures generally used are:

1. Pits Recharge: Pits are constructed for recharging the shallow aquifers.

2. Trenches: These are constructed when the preamble strata is available at shallow depths.

3. Dug wells: Existing dug wells may well be utilized as recharge structure and water should pass through filter media before putting into dug well.

4. Hand Pumps: The existing hand pumps may be used for recharging the shallow / deep aquifers, if the availability of water is limited. Water should pass through filter media before diverting it into hand pumps.

5. Recharge Shafts: With bore wells for recharging the upper as well as deeper aquifers, lateral shafts of 1.5 to 2m wide and 10 to 20m long depending upon availability of water with one or two bore wells is constructed. The lateral shafts are back filled with boulders, gravels and coarse sand.

6. Spreading Techniques: When permeable strata start from top then this technique is used. Spread water in streams by making check dams, cement plugs or a percolation pond may be constructed.

Benefits of RWH:

1. An ideal solution to water problems in areas having inadequate water resources.

2. The ground water level will rise.

3. Mitigates the effects of drought and achieves drought proofing.

4. Reduces the runoff which chokes the storm water drains.

5. Flooding of roads is reduced.

6. Quality of water improves.

7. Soil erosion will be reduced.

8. Saving of energy per well for lifting of ground water.

9. Prevention of sea water ingress.

DISADVANTAGES OF RWH:

#The annual rainfall is limited to a minimum of roughly 2000 mm/year and should be spread in at least two (long) rain-periods of three months without total drought in between.

#Cost per capita is higher for a rainwater harvesting system than for a communal pump or well.

#Personal systems do not improve social activities (nor do they necessarily spoil them).

#Some kind of organisation, structure or retailer should be and stay present after construction to supply the users of spare parts and repair materials.