Closing the demand supply gap through rainwater harvesting - A case study of Sargasan - Gujarat

This paper presented at the International Symposium on Artificial Recharge (ISAR-4) Adelaide, Australia describes the attempts made by VIKSAT at experimentation with artificial recharge technique in a peri-urban area called Sargasan in Gandhinagar taluka, Gujarat.

Ground water as a dependable source and its increasing extraction for various uses in India is reflected in the drastic lowering of water levels leading to “local” draw downs. The efficacy of surface water bodies such as tanks, lakes and canals as a means of natural recharge to the ground water has drastically reduced simply because the local water levels are too deep. The need of the hour therefore is for artificial recharge systems that convey the fresh rainwater to the “aquifer”.

The ground water level in Gandhinagar taluka has declined from around 30 meters to 80 meters below ground level over a period of two decades, with half of the decline occurring in the past 7 years. In other words, the dry aquifers above the present yielding zones offer opportunity for storing water. Underground storage of water, which has the additional advantage of avoiding evaporation losses as against surface water bodies, is increasingly seen as an option due to severe competition for land and changing land use. 

The major objective of the experiment was to augment local supplies through the recharge route. The artificial recharge technique comprised developing an existing tank, drilling a borewell and facilitating rainwater recharge through a filter bed constructed with the borewell as the center. Data on water levels was collected from 32 borewells located within a radius of 1.5  kilometer from the recharge borewell to
understand the effect on the water level build up.

The experiment helped to establish the following:
A total of 23 (gross 33.2) million liters of rainwater was recharged into the ground with about 70% of the average annual rainfall incident in the area in the monsoon of 2001. Analysis of data from the 32 observation borewells revealed the presence of a mound around the recharging borewell, which was observed to slope gradually towards southeast and southwest while it was steep towards northeast. The water level observations over a 4-month period (June-September 2001) indicated certain amount of dispersion, which is found to be due to some borewells under pumping.

When the pumping was significant, the dispersion too was significant. However, interestingly, the mound did continue to be present throughout the 3 month-monsoon period and over a 600-800 meter distance from the recharging borewell, in particular along southeast and southwest directions. In other words, this represents the zone of influence of recharge which depends upon, inter alia, the piezometric gradient, pumping density and intensity, and also the direction of the ground water movement. This coincides well with the fact that the general direction of ground water movement in Gandhinagar is towards southeast. 

The study estimates that recharge efforts, if properly planned, can effectively contribute to closing the existing gap between extraction and natural recharge. According to the Central Ground Water Board, Government of India, Gandhinagar taluka has an annual ground water draft of 130.35 MCM from 2382 borewells and a natural recharge of 111.57 MCM leaving a gap of about 19 MCM. This gap comprises approximately 15% of the extraction. Based on the field data, it is computed that the recharge system in Sargasan has absorbed a net 23 million litres when the incident rainfall was 70% of the annual average.

Calculations show that the demand-supply gap can be closed by having about 700 “Sargasan” type of medium recharge systems. Larger catchment tanks would reduce the number proportionately. Thus, in principle, the gap can be effectively addressed as there are 520 tanks existing today in Gandhinagar taluka. Appropriate demand management, especially in the agriculture sector and domestic drinking sector, needs to be adopted simultaneously to utilize the natural resource sustainably.

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Post By: rajshekar