Percolation Tanks – A Component for Ground Water Recharging

Rainwater harvesting is being taken as a national level project in India. In fact, there were many groundwater recharge movements in India in the past. In the present, there are various government and non-government bodies who are researching in this area and work with their best competencies to design effective solutions for groundwater recharge. Among these, Percolation tank is the most traditional but still a popular method.

How does Percolation Tank look like?

Percolation tank is an artificial reservoir which are constructed across stream, submerging a land area with adequate permeability to facilitate sufficient percolation to collect surface water run-off and allow it to percolate within the permeable land. This is one of the effective methods of refilling groundwater table (also known as groundwater recharge).

These are earthen dams having masonry structures for spillway only and are preferred to be constructed over second or third order streams.

The ideal size of the percolation tank must be governed by its capacity of strata in the tank bed.  Usually, percolation tanks are designed for storage capacity of 0.1 to 0.5 MCM and a ponded water column should be generally between 3 & 4.5 m.

General Guidelines

• Percolation tanks should normally be constructed in a terrain with highly fractured and weathered rock for speedy recharge.
• Too high permeability may result in the percolated water escaping in the downstream as regenerated surface flow, hence it will defeat the purpose of water conservation.
• The aquifer to be recharged should have sufficient thickness (about 3m) of permeable Vadose zone to accommodate recharge.
• The benefitted area must have sufficient number of wells, hand pumps etc.
• It is advisable and benefitting to have the percolation tank in an area of good/ average catchment.
• A long-term evaluation of the pattern of rainfall for the benefitted area must be studied so that the percolation tank gets filled up fully during monsoon.
• To minimize the silting at the bottom of the tank, soils in the catchment area should preferably be of light sandy type.
• Except for outlets provided for surface irrigation and the depth of the cut-off trench, there is not much difference, in the construction between a percolation tank and a minor irrigation tank.

How geographic conditions of India affects the use of percolation tanks

India is a country of diverse climates and types of land conditions. The amount of water a percolation tank would be able to seep into the ground for compensating depleted groundwater, depends upon the following:

• The amount and nature of precipitation of the area,
• thickness of topsoil and weathered zone,
• type of vegetation,
• evaporation from the surface of wet soil,
•  profile of underlying hard rock,
• the topographical features of the sub-basin
• the status of soil,
•  and the water conservation activities adopted by villagers.

The semi-arid climate of peninsular India results in the loss of surface water due to high evaporation rate. Thus, the percolation tank design should be made to maintain the largest possible storage capacity where water can percolate to ground water reservoir by January/February.

Based on the soil texture and its capacity of water percolation, different practices are being employed in various states of India. For example, government lays the legislation to cover the percolation tanks in Maharashtra

In Tamil Nadu, there is an over-exploitation of ground water. Hence, the awareness programs are run, and farmers are now volunteering to spare land for percolation tanks.

In the Saurashtra region of Gujarat, the major need of these tanks is for recharging wells that could support peanut production.

Hard rock aquifers are the non-carbonate, fractured rocks like the crystalline basement complex and metamorphic rocks and it covers an area of about 800,000 sq. Kms. in central and southern India. In the region of hard rock land surface, the depth of ground water occurrence, in useful quantities, is usually limited to a hundred meters or so. In India, the ratio of recharge to rainfall in hard rock terrain is calculated to be between 3 to 15%.

Major challenges being faced  

The artificial methods of harvesting the rainwater are dependent majorly on natural rainfall conditions and soil texture. Hence, it becomes challenging to utilize the substantial amount of harvested water for percolation underground to feed the ground water table.

The regions where the soils are sandy and porous, example, lateritic soil in Konkan region, there is a rapid loss of rain water due to higher rate of percolation. The water holding capacity of such soil is less and even after a heavy rainfall, not sufficient amount of water can be retained on the surface of tanks.

This rapid percolation of water also leads to heavy loss of plant nutrients viz., Ca, Mg, S, K, etc., resulting in soil becoming acidic.

Silting is another major issue of concern. The muddy runoff from the watershed reduces the initial efficiency of a percolation tank due to silting of its bottom. In an average of 5 to 6 Monsoon seasons, the tank bed can accumulate about 0.20 to 1.00 meters of silt. Due to its permeability, silt reduces the storage capacity of the tank. It also impedes the rate of vertical flow of recharge.

Though, silting can be minimized if the watershed is well-forested and has a cover of grass, bushes and crops.

Some worthy statistics

Percolation tanks are found in abundance in state like Tamil Nadu, Andhra Pradesh, karnataka, Gujarat and MaharasHtra

As per the 4th Minor Irrigation Census that refers to the year 2006-2007, there are about 6 lakh tanks and storages that were created in the country; 58.9 lakh of them have irrigation potential out of which only 39.31 lakh has been utilized.

In Tamil Nadu, the state has completed 32 major dams, 178 medium projects and approximately 2274 minor tanks till the end of June 2005

BAGALKOTE district in Karnataka has around 677 percolation tanks/ponds.

In the state of Maharashtra in western India, over 10,000 percolation tanks have been constructed so far. (DIRD website, 2011). These will benefit the farmers and are of course, very popular with them.

Find your RWH solutions at Chaitanya

CHAITANYA RAIN HARVEST PRODUCTS & SYSTEMS PVT. LTD. is a trusted name in rainwater harvest services and products. The services include preparation of rainwater harvesting reports, designing rainwater harvesting layout, designing detailed engineering drawings, conducting hydrogeological and borewell surveys and providing feasibility reports.  The company’s RWH product line includes

• APOLLO Vee-Wire Filter Screen Kit – most modern filtration systems for rainwater harvesting product.
• Fibre-reinforced plastic (FRP) Roof rainwater drainage gutters
• Authorised distributorship of U Channels Pipes and fittings in Maharashtra for Hycount Group of Industries
• Gravels of size ranges from 5mm to 10mm used for filling the borewell pit and other RWH systems.
• Precasted RCC Hume Pipes collar rings  are available in 3 feet, 4 feet and 5 feet outer diameter sizes.