Rainwater is a precious resource that is seldom effectively captured and used. Rainwater harvesting is a decentralized technology, which requires individual buildings or group of buildings to collect and utilize the rainwater during the monsoon period. The water can be stored in storage tanks and the overflow is usually used to charge the ground water by means of various techniques such as percolation pits, trenches or wells. The objective of a rainwater harvesting system is to collect and store the maximum possible rainwater falling on the site. If planned in the design stage of a building, the costs of implementing it are minimal. Rainwater is not only free from high calcium and pH levels but it is also good for activating chemicals used on farms. Farmers can set up mini catchment areas on farms, harvest the water and use it later to feed crops which will in turn give a higher yield. Farmers may also build underground storage tanks on farms and use storage roof to catch and channel the water to the source of storage. This method is suitable in hilly areas where water is scarce because of a lack of access to the national water system.





Ways to harvest rainwater

Rain gardens are bowl-shaped gardens designed to absorb stormwater runoff from roofs and parking lots.  They range from small formal home gardens to large commercial bio-retention gardens. They have a ponding area planted with perennial plants and maybe grasses, trees and shrubs.  Stormwater is routed to the ponding area where it is absorbed and filtered.  Many of the plants used are native to the region and have extensive, deep roots.  To increase its effectiveness, the garden bed is prepared or replaced to a depth of two feet in order to decompact the soils and increase the rate of absorption. Some of their benefits include: being attractive landscaping features, creating wildlife habitat, having no need to be fertilized or sprayed, only weeded and mulched, absorbing a lot of rain and tackling stormwater pollution, removing many common pollutants, being low maintenance etc.

A rain barrel is perhaps the most straightforward way to harvest water on-site. Any household with outdoor space can install a rainwater catchment system that fills a barrel with water that can then be used to nourish plants. Rainwater is naturally soft, and can be used for a number of different uses, including gardening and washing. Usually fed by a downspout off a gutter, rain barrels should be elevated above the ground for greater water pressure when using a garden hose. For every inch of rain that falls on a catchment area of 1,000 square feet, you can expect to collect approximately 600 gallons of rainwater. Ten inches of rain falling on a 1,000 square foot catchment area will generate about 6,000 gallons of rainwater. 

 A helpful design principle is the efficient use of the power of gravity. For example, when rain
falls on a slope, the runoff can be trapped through the application of swales, a landform that utilizes a raised mound and an immediately uphill ditch to slow downhill runoff. This runoff moisture is then absorbed by the large surface area of the mound, where it is accessible to plant roots. Swales must be built along contour lines to ensure a balanced system. On the mound, growers can plant fruit or nut trees, annual and perennial vegetables, berry bushes, herbs and more to create a food-producing ecosystem that prevents erosion and harvests water through its root networks. In principle, shaping or treatment of catchment surface boost the runoff yield collected by the ponds. However, the pond catchments are largely covered by various forms of vegetation, which results in enhanced infiltration of rainwater and reduced run-on to the ponds. To tackle this, mulch, or mulching, is used referring to a general practice of covering the soil with an organic layer of matter that suppresses weeds and retains soil moisture by shielding soil from the sun.

An interesting case is the one of village Kotla which is situated at the foothills of Aravallis from which streams carry the precipitated rain water to a vast catchment area uphill. With no obstruction in way, the water used to flow through the village streets bringing tonnes of small rocks from the hills and, because of high gradient, no water percolated under the ground and run off used to flow very fast. At the same time, the high velocity of flow caused erosion to top fertile soil in agricultural fields. In the absence of recharging, ground water was depleting at more than 300-500 mm every year. To encounter this situation, the concept of storing and percolation of the same in the ground with ridge to valley approach was adopted. Ridge to valley approach was unique as it was able to reduce the flow velocity and minimize the chances of erosion in the agricultural fields and silting in storage basins helping to increase the recharging potential. This was carried out through a series of check dams wherever possible with the idea that backwaters of a check dam touch the preceding check dam in the stream such that the whole length of this stream becomes storage and percolation basin. There was only a 200 meter wide stretch under the hills where fresh ground water was available; otherwise rest of the village had saline ground water. The fresh water pockets were depleting year by year due to high exploitation but, now, villagers for the first time noticed a rise of water table by 7 feet in the open wells. During the last few years, it was observed that harvested rain water was more than 10 times of the storage capacity of a check dam.

Sources:

Rainwater harvest and management

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