Rainwater Harvesting -Needs and Uses

Biology Presentation

By Atrayee Chatterjee

IntroductionRainwater Harvesting:

“Grab hold of water where it falls” Rainwater harvesting ( RWH) is a technique of collection and storage

of rainwater into natural reservoirs or tanks, or the infiltration of surface water into subsurface aquifers (before it is lost as surface runoff).

Rainwater Harvesting

Roof tops

Local Catchment

Seasonal floodwater

Watershed Management

Catchment Type

Catchment Size

Micro-catchment

Macro-catchment

Johads of Rajasthan

Earthen or masonry rainwater harvesting

structure,

for providing water for domestic use to the

communities.

Tankas of Bikaner, Rajasthan

Unique underground structures of various shapes and sizes to

collect rain water for drinking purposes

Bamboo drip irrigation:

200-year-old system.Used by tribal farmers

of Khasi and Jaintiahills in Meghalaya.

Bamboos divert water from perennial springs

on hilltops to the lower reaches by

gravity

Why is RWH needed?

1. To overcome the inadequacy of surface water to meet our demands.2. To arrest decline in ground water levels.3. To enhance availability of ground water for sustainable development.4. To improve ground water quality by dilution.5. To increase agriculture production.6. To improve ecology of the area by increase in vegetation cover etc.7. To mitigate floods.8. To reduce pressure on other water resources

Case Study: Tamil Nadu’s Success• Chennai gets an average of 1,300 mm of rainfall

every year, which is higher than the national average of 800 mm. But the rainfall occurs in short spells of a few days, and nearly 95% of it is lost due to surface runoff and evaporation.

• The Rain Water Harvesting movement launched in 2001 was the brainchild of the Honourable Chief Minister. It has had a tremendous impact in recharging the groundwater table all over Tamil Nadu.

• To consolidate the gains, various measures have been taken up for rejuvenation of RWH structures created already in both public and private buildings, besides creating new ones.

• There are 1821 water bodies maintained by Town Panchayats with an extent of 6286.84 acres.

• Over 95 % buildings under Tamil Nadu Town Panchayats have some form of RWH System.

Villapuram Vellore

Potentials And Advantages

Potential Areas:1. Where ground water

levels are declining on regular basis.

2. Where substantial amount of aquifer has to be de-saturated.

3. Where availability of ground water is inadequate in lean months.

4. Where due to rapid urbanization, infiltration of rain water into subsoil has decreased drastically and recharging of ground water has diminished.

Rainwater Harvesting TechniquesTwo main ways for rainwater harvesting: 1. Storage of rain water on the surface for future use. 2. Recharge of ground water.

Storage Type Recharge Type

Rainwater storage reservoir at Dholavira (Rann of Kutch) –

Harappan civilization (2500-1900 BC)

Artificial recharge to ground water is a process by which the ground water reservoir is augmented at a rate exceeding that under

natural conditions of replenishment.

Methods Popular in Urban Regions

• 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.

Roof Top Rainwater Harvesting through:

1. Recharge Pit

2. Recharge Trench

3. Tubewells

4. Recharge wells

Roof Top Rainwater Harvesting through Recharge Pit

a. In alluvial areas where permeable rocks are exposed on the land surface or at very shallow depth, roof top rain water harvesting can be done through recharge pits.

b. The technique is suitable for buildings having a roof area of 100 square metres.

c. Recharge Pits may be of any shape and size which are back filled with boulders (5-20 cm), gravels (5-10 mm) and coarse sand (1.5-2 mm) in graded form

d. A mesh should be provided at the roof so that leaves or any other solid waste/debris is prevented from entering the pit and a desalting/collection chamber may also be provided at the ground to arrest the flow of finer particles to the recharge pit.

e. The top layer of sand should be cleaned periodically.

Roof Top Rain Water Harvesting through Recharge Trench

a. Recharge trenches are suitable for buildings having roof area of 200-300 sq m and where permeable strata are available at shallow depths.

b. Trench may be 0.5 to 1 m wide, 1 to 1.5 m deep and 10 to 20 m long depending upon availability of water to be recharged.

c. These are back filled with boulders (5-20 cm), gravels (5-10 mm) and coarse sand (1.5-2 mm) in graded form

d. A mesh should be provided at the roof so that leaves or any other solid waste/debris is prevented from entering the trench

e. By-pass arrangement is provided before the collection chamber to reject the first showers.

f. The top layer of sand should be cleaned periodically to maintain the recharge rate.

Methods Popular in Rural Regions

• In rural areas, rain water harvesting is taken up considering watershed as a unit.

• Surface spreading techniques are 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:

1. Gully Plug

2. Contour Bund

3. Check Dams

4.Percolation tanks

5. Recharge Shafts

6. Dugwell Recharge

Rain water harvesting through Check Dams

A check dam is generally constructed on small streams and long gullies formed by the erosive activity of water. The ideally a check dam is located in a narrow stream with high banks.

A check dam serves many purposes.

• It cuts off the runoff velocity and reduces erosive activity

• The water stored improves soil moisture of the adjoining areas and allows percolation to recharge the aquifers

• While constructing a series of check dams on along stream course, the spacing between two check dams should be beyond their water spread. The height of the check dam should be such that even during the highest flood, water does not spill over the banks.

Rain Water Harvesting through Contour Bund

• Contour bunds are effective method to conserve soil moisture in watershed for long duration.

• These are suitable in low rain fall areas where monsoon run off can be impounded by constructing bunds on the sloping ground all along the contour of equal elevation.

• Flowing water is intercepted before it attains the erosive velocity by keeping suitable spacing between bunds.

• Spacing between two contour bunds depends on the slope of the area and the permeability of the soil. Lesser the permeability of soil, the close should be spacing of bunds.

• Contour bunding is suitable on lands with moderate slopes without involving terracing.

Comparison With Other Methods

Water Source Setup Cost Running Cost Yield Applicable in

Tubewells Medium Medium Medium Places with Deep Aquifers

Hand-dug wells Low Medium Low-Medium Places where water table is not lower than six metres

Reverse Osmosis

High High High Only in Urban regions

Rainwater Harvesting

Low-Medium Low-Medium High Only in regions which receive moderate-high rainfall once a year

Thus one of the main drawbacks of RWH is that is it is economically viable only in regions receiving regular rainfall of moderate-high level, i.e. around or greater than national average

Here are some important laws that’s have been passed: In Chennai: Rainwater harvesting has been made mandatory in three storied buildingsIn Ahmedabad: In 2002, the Ahmedabad Urban Development Authority (AUDA) had made rainwater harvesting mandatory for all buildings covering an area of over 1,500 square metres.In Himachal Pradesh: All commercial and institutional buildings, tourist and industrial complexes, hotels etc, existing or coming up and having a plinth area of more than 1000 square metres will have rain water storage facilities commensurate with the size of roof area.

It is evident that there is considerable scope for collection and utilisation of rainwater especially, in regions where groundwater is :

deep, inaccessible due to surface conditions

Groundwater is salty, acidic or unfit too use/consume

Water we actually tap is: 40% of precipitation

Any Questions? References: 1. http://cgwb.gov.in/documents/case-studies-rwh-ar-may2011.pdf2. http://www.yourarticlelibrary.com/water/rain-water-harvesting-in-india-

need-methods-and-other-details/20917/3. http://www.wateraid.org/~/media/Publications/Rainwater-

harvesting.pdf4. http://www.rainwaterharvesting.org/policy/legislation.htm