Rainwater harvesting tanks that have been abandoned for modern canals and groundwater wells could become an important water source for people in southern India, according to a University of Waterloo researcher.

India’s ancient rainwater harvesting system forms one of the largest man-made wetland systems ever built, says Nandita Basu, a professor in the Department of Earth and Environmental Sciences.

Climate change and the overuse of groundwater have governments concerned there won’t be enough water to support agriculture by 2050.

ancient well

Tanks used as far back as 150 B.C.

There is evidence that the storage “tanks,” constructed from natural depressions in the landscape, were used in the southern state of Tamil Nadu as far back as 150 B.C. The tanks fill with water during the annual monsoons and then sluices and shutters control the outflow of water to fields.

Michael Steiff, a graduate student in Civil and Environmental Engineering, is developing a computer model that shows the tanks can be part of a complex system that also includes groundwater, rivers, lakes and estuaries.

“It’s clear that a basin-wide approach that maximizes water availability while simultaneously managing demand is important if we are to maintain a stable water resource for everyone,” says Basu, who is cross-appointed to the Department of Civil and Environmental Engineering and a member of the University’s Water Institute.

Tanks are in disrepair

Today, these tanks are in a serious state of disrepair as local farmers were encouraged to abandon the tradition for modern canal networks, dams and groundwater wells. But with more than 1.8 million groundwater wells in Tamil Nadu, water is being pumped out faster than it can recharge. Approximately 12 per cent of wells are dried up, and in some areas this failure rate is as high as 40 per cent.

Kim Van Meter, a graduate student in Earth and Environmental Sciences, has developed a Coupled Human and Natural Systems (CHANS) framework to explore this complicated system. The CHANS framework, which was described in a recent paper, shows that the tanks can help with water distribution, but only if there is enough water to go around.

If both the groundwater and the rainwater harvesting systems become overexploited, it could lead to water disputes and additional degradation of other habitats like estuaries, which also support local fisheries.

Basu says traditional rainwater harvesting systems should be part of an integrated water management plan beyond the village scale. More regional hydrological data is also needed in order to calculate the optimal numbers of tanks a watershed can support.

The article’s co-authors include University of Iowa Professor Eric Tate and graduate student Joseph Wyckoff.

This work was funded by the US National Science Foundation.