Rain Water Harvesting Techniques To Augment Ground Water
- GS Mains paper III, Optional- Agriculture, Geography
- Environment, Agriculture, Water harvesting
Fresh water is scarce
- Of the total water on earth, only 3% constitutes freshwater. Rest is saline water in the oceans.
- 11% of the total freshwater on earth is groundwater available upto a depth of 800m which can be extracted for use.
- Mindless extraction and over exploitation of very small quantity of this precious nature resource has caused a rapid depletion and deterioration in its quantity and quality both.
Ground water recharge in Rural areas
- In rural areas, rain water harvesting is taken up considering watershed as a unit.
- Surface spreading techniquesare common since space for such systems is available in plenty and quantity of recharged water is also large.
- Following techniques may be adopted to save water going waste through slopes, rivers, rivulets and nalas:
- Gully plug: Gully plugs are built using local stones, clay and bushes across small gullies and streams running down the hill slopes carrying drainage to tiny catchments during rainy season.
- Contour bund: Contour bunds are effective methods to conserve soil moisture in watershed for long duration. Flowing water is intercepted before it attains the erosive velocity by keeping suitable spacing between bunds.
- Gabion structure: This is a kind of check dam commonly constructed across small streams to conserve stream flows with practically no submergence beyond stream course.
- Percolation tank: Percolation tank is an artificially created surface water body, submerging in its reservoir a highly permeable land, so that surface runoff is made to percolate and recharge the ground water storage.
- Check dams / cement plugs / nala bunds: Check dams are constructed across small streams having gentle slope. The site selected should have sufficient thickness of permeable bed or weathered formation to facilitate recharge of stored water within short span of time.
- Recharge shaft: This is the most efficient and cost effective technique to recharge unconfined aquifer overlain by poorly permeable strata.
- Dugwell recharge: Existing and abandoned dug wells may be utilized as recharge structure after cleaning and desilting the same. The recharge water is guided through a pipe from desilting chamber to the bottom of well or below the water level to avoid scouring of bottom and entrapment of air bubbles in the aquifer.
- Ground water dams or sub-surface dykes: Sub surface dyke or under-ground dam is a subsurface barrier across stream which retards the base flow and stores water upstream below ground surface. By doing so, the water levels in upstream part of ground water dam rises saturating otherwise dry part of aquifer.
Ground water recharge in urban areas
- In urban areas, rain water available from roof tops of buildings, paved and unpaved areas goes waste.
- This water can be recharged to aquifer and can be utilized gainfully at the time of need.
- The rain water harvesting system needs to be designed in a way that it does not occupy large space for collection and recharge system.
- A few techniques of roof top rain water harvesting in urban areas are described below.
- Recharge pit: In alluvial areas where permeable rocks are exposed on the land surface or are located at very shallow depth, rain water harvesting can be done through recharge pits. The technique is suitable for buildings having a roof area of 100 sq.m.
- Recharge trench: Recharge trenches are suitable for buildings having roof area of 200-300 sq. m. and where permeable strata is available at shallow depths.
- Tube wells: In areas where the shallow aquifers have dried up and existing tubewells are tapping deeper aquifer, rain water harvesting through existing tubewell can be adopted to recharge the deeper aquifers.