Paucity of water is a significant problem in the hills where it is not available consistently as most springs are not perennial. Substantial time is wasted collecting water, especially during summers.

Moreover, the villagers report that the water level has been receding since last 4-5 years as the yield from the sources has been on a decline.

The pipe water supply is not able to cope with the demands of the existing population. Increasing population adds to the woe. Irrigation is also a serious issue during the dry summer months and cold winter period.

Rain is the primary sources of water. However, when it rains, either some of the water evaporates or run-offs and goes unused into the ocean or stands for days in drains and sewer lines to later flow away as sewage water.

Rainwater is the deliberate collection and storage of rainwater that runs off as wastage from catchments like rooftops, paved areas and bare ground. The problem of large variations in water availability associated with growing water demand leads to the need for rainwater harvesting. Water harvested after rains can either be used directly or for recharging the underground water.

Methods of harvesting rainwater
Rainwater harvesting can be done through two methods, storage (surface and sub surface) and through artificial ground water recharge.

Rainwater harvesting though surface/subsurface storage
In all rooftop rainwater-harvesting systems for direct use, the basic components are:

Design of storage tanks
The volume of the storage tank is determined by five factors.

Number of persons in the household: The number of members in a family determines the size of the storage tank. Bigger the family, larger the storage capacity required to achieve the same efficiency of fewer people under the same roof area.
Per capita water requirement: This varies from household to household based on habits and from season to season. Consumption rate has an impact on the storage systems design as well as the duration to which stored rainwater can last.
Average annual rainfall
Period of water scarcity: Apart from the total rainfall, the pattern of rainfall -whether evenly distributed through the year or concentrated in certain periods will determine the storage requirement. The more distributed the pattern, the lesser the size.
Type and size of the catchment: Type of roofing material determines the selection of the runoff coefficient for designs. Size can be assessed by measuring the area covered by the catchment, i.e., the length and horizontal width. Larger the catchment, larger the size of the required cistern (tank).

Rainwater harvesting through artificial ground water recharge
In places where the withdrawal of water is more than the rate of recharge, an imbalance in the groundwater reserves is created. Aquifers are recharged for various reasons.

To maintain or augment natural groundwater as an economic resource
To conserve excess surface water underground
To combat progressive depletion of groundwater levels
To combat unfavourable salt balance and saline water intrusion

Amount of rainwater harvest annually
The annual rainwater harvesting potential is calculated in terms of area of catchment, annual average rainfall, runoff coefficient, and filter efficiency.

Annual rainwater harvesting potential (cubic metre) = Area of catchment x annual average rainfall x runoff coefficient x filter efficiency

Area of catchment = roof area (sq.mt)= width x length of the roof.
In a sloped roof, only the section of the roof that is used for collection is measured.

Annual rainfall - average
Delhi = 611.0 mm = 0.61 mt Mumbai = 2,170 mm= 2.17 mt
Chennai = 1200 mm =1.2 mt Cochin = 3099 mm =3 mt
Darjeeling = 3200 mm = 3.2 mt

The Run-Off coefficient allows for the slope of the roof, water loss through wind evaporation, and the ability of the roofing material to hold onto water.

Typical Run-Off Coefficients

For pitched tiled roofs/asbestos sheet roof –approx. 0.75 to 0.80
For flat roof –approx. 0.50 to 0.65

Determinants for designing rainwater harvesting systems

End use/purpose
The purpose for harvesting rainwater may vary between ground water recharging, to augmenting available supply or to arrest runoff.

Climatic conditions: rainfall pattern & rate of evaporation
Rainfall is the most unpredictable variable in the calculation and, hence, to determine the potential rainwater supply for a given catchment, reliable rainfall data are required, preferably for a period of at least 10 years. In addition, it would be far better to use rainfall data from the nearest station with comparable conditions.

The number of annual rainy days also influences the need and design for
rainwater harvesting; the fewer the annual rainy days or longer the dry period, the more the need for rainwater collection in a region.

However, if the dry period is for long durations, big storage tanks would be required to store rainwater. Hence, in such regions, it is more feasible to use rainwater to recharge groundwater aquifers rather than for storage.

Catchment size and characteristics
All calculations relating to the performance of rainwater catchment systems involve the use of runoff coefficient to account for losses due to spillage, leakage, infiltration, catchment surface wetting, and evaporation. These contribute to reducing the amount of runoff.

Runoff coefficient for any catchment is the ratio of the volume of water that runs off a surface to the volume of rainfall that falls on the surface.

Runoff coefficients for various catchment surfaces

Cost
Rainwater harvesting methods are site specific and, hence, it is difficult to give a generalized cost. The major components of a rainwater harvesting system—rain and catchment area—are available free of cost.

A good proportion of the expenses incurred would be for the pipe connections. By judiciously fixing up the pipes on the slopes of roofs and location of rainwater outlets, the cost can be brought down considerably. However, the rainwater harvesting costs varies widely depending on the availability of existing structures like wells and tanks, which can be modified and used.