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Rain water harvesting
Rainwater harvesting is the accumulation and deposition of rainwater for reuse before it reaches the aquifer. Uses include water for garden, water for livestock, water for irrigation, and indoor heating for houses etc. In many places the water collected is just redirected to a deep pit with percolation. The harvested water can be used as drinking water as well as for storage and other purpose like irrigation.

Advantages Rainwater harvesting provides an independent water supply during regional water restrictions and in developed countries is often used to supplement the main supply. It provides water when there is a drought, can help mitigate flooding of low-lying areas, and reduces demand on wells which may enable ground water levels to be sustained. It also helps in the availability of potable water as rainwater is substantially free of salinity and other salts.

Quality The concentration of contaminants is reduced significantly by diverting the initial flow of run-off water to waste.[1] Improved water quality can also be obtained by using a floating draw-off mechanism (rather than from the base of the tank) and by using a series of tanks, with draw from the last in series. The stored rainwater may need to be analyzed properly before use in a way appropriate to ensure its safe use

The quality of collected rainwater is generally better than that of surface water. Contamination is always possible by airborne dust and mists, bird feces, and other debris, so some treatment may be necessary, depending on how the water will be used.

System setup Rainwater harvesting systems can be installed with minimal skills. The system should be sized to meet the water demand throughout the dry season since it must be big enough to support daily water consumption. Specifically, the rainfall capturing area such as a building roof must be large enough to maintain adequate flow. The water storage tank size should be large enough to contain the captured water

Design of storage tanks
The volume of the storage tank can be determined by the following factors: Number of persons in the household: The greater the number of persons, the greater 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 also 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 could 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). Dry season demand versus supply approach In this approach there are three options for determining the volume of storage: Matching the capacity of the tank to the area of the roof Matching the capacity of the tank to the quantity of water required by its users Choosing a tank size that is appropriate in terms of costs, resources and construction methods. In practice the costs, resources and the construction methods tend to limit the tanks to smaller capacities than would otherwise be justified by roof areas or likely needs of consumers. For this reason elaborate calculations aimed at matching tank capacity to roof area is usually unnecessary. However a simplified calculation based on the following factors can give a rough idea of the potential for rainwater colection.

Illustration Suppose the system has to be designed for meeting drinking water requirement of a five-member family living in a building with a rooftop area of 100 sq. m. The average annual rainfall in the region is 600 mm (average annual rainfall in Delhi is 611 mm). Daily drinking water requirement per person (drinking and cooking) is 10 litres. ''' Design procedure: ''' Following details are available: Area of the catchment (A) = 100 sq. m. Average annual rainfall (R) = 611 mm (0.61 m) Runoff coefficient (C) = 0.85 1. Calculate the maximum amount of rainfall that can be harvested from the rooftop: Annual water harvesting potential = 100 x 0.6 x 0.85 = 51 cu. m. (51,000 litres) 2. Determine the tank capacity: This is based on the dry period, i.e., the period between the two consecutive rainy seasons. For example, with a monsoon extending over four months, the dry season is of 245 days. 3. Calculate drinking water requirement for the family for the dry season = 245 x 5 x 10 = 12,250 litres As a safety factor, the tank should be built 20 per cent larger than required, i.e., 14,700 litres. This tank can meet the basic drinking water requirement of a 5-member family for the dry period. A typical size of a rectangular tank constructed in the basement will be about 4.0 m x 4.0 m x 1.0 m

Salient features of this approach:

Simplest approach to system design but is relevant only in areas where distinct dry seasons exist Provides a rough estimate of storage volume requirements This method does not take into account variations between different years, such as the occurrence of drought years. It also entirely ignores rainfall input and the capacity of the catchment to deliver the runoff necessary to fill the storage tank. This technique can be used in the absence of any rainfall data and is easily understandable to the layperson.These points are especially relevant when designing systems in the remote areas of developing countries where obtaining reliable rainfall data can be difficult.

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