Dark zones are human-made

Dark zones are human-made

Groundwater sustains almost 60 per cent of the irrigated land in India. Most districts in India today have larger shares of irrigated land under groundwater irrigation than under surface-water irrigation. This ominous change in water use patterns in India has been extremely rapid since the 1970s.

Extensive groundwater usage has caused depletion of water levels in many areas. This has had concomitant effects such as drying up of dug wells and shallow wells, springs and wetlands becoming arid and intrusion of seawater along coasts. The quality of groundwater has also deteriorated greatly following depletion in its levels.

The Maheshwaram watershed, located about 35 kilometres from Hyderabad in Andhra Pradesh, was extensively monitored between 1999 and 2002 by the Hyderabad based Indo-French Centre for Groundwater Research (ifcgr) -- a laboratory run jointly by the National Geophysical Research Institute, India and Bureau de Recherches Gologiques et Minires, France. The Maheshwaram basin is composed of hard rocks and its topography is typical of more than two thirds of India's landmass. The objective of the study was to enhance understanding of India's complex groundwater system. It also aimed to quantify reliably all parameters controlling dynamics of groundwater in India, including storage and renewable resource estimation.

ifcgr has developed a mathematical model adapted to hard rock aquifer that simulates the behaviour of the water table in Maheshwaram in the next 20 years. The equations of the model are solved according to the physical and hydraulic properties of soils and rocks of the area. The ifcgr model is specially adapted to the specific characteristics of fractured hard-rocks such as heterogeneity and anisotropy. It calculates groundwater levels according to different scenarios of recharging and pumping. The mathematical model projects that at the present rate of increase in borewells, the water table in the Maheshwaram basin will decline by about 25 meters in the next 20 years if there are no dramatic climatic changes such as drastic increase or reduction in rainfall.

The study showed that in the Maheshwaram basin, water level depletion is linked to the development of tube wells and hand pumps for irrigating paddy fields. It has exploded the commonly held theory that deficit rainfall during the last few years is responsible for water table depletion in the Maheshwaram basin.

A comprehensive study of the groundwater budget at the watershed scale at Maheshwaram demonstrates that even during a normal monsoon, groundwater utilisation in the area is higher than its natural recharge. This obviously induces water table decline. The situation got even worse after a dry 2002 when climatic effects combined with human activities lead to a deepening of the water table.

The alarming projections of the Maheshwaram might indeed be true for most of India. The rapid depletion of groundwater levels needs to be tackled at the watershed scale by the community. It is clear that offer-side solutions such as rainwater harvesting are not enough to mitigate the water scarcity. So it is imperative that demand-side actions such as diversification of crops and use of water-saving irrigation techniques be encouraged.

The need of the hour is to adapt exploitation of groundwater to its availability and also to create a concept of groundwater bank. For that, it is indispensable that policy makers be equipped with suitable decsion-making tools that enable them improve water resources management at the watershed level. New modeling and management tools developed by ifcgr can prove useful in this endeavour.

J C Marchal and S Ahmed are scientists at the Indo-French Centre for Groundwater Research, Hyderabad