Water located below the ground surface in soil pores and the fractures of lithologic formations is often withdrawn for agricultural, municipal and industrial use and is replenished by precipitation, river flow and irrigation return flow. Usage of ground water has some obvious advantages over surface water — better protection against pollution; little treatment required before supplying to user; constant temperature; small distances between source and user; and a fairly steady supply due to large storage capacities. These advantages have stimulated the interest in exploiting the ground water and its conjunctive use with surface water.
The total annual ground water potential for a unit refers to the available annual recharge after allowing for natural discharge in the monsoon season through base flow and subsurface inflow/outflow. According to a recent estimate (2004), the total annual replenishable ground water resources of the country are 433 bcm. Keeping 34 bcm for natural discharge, the net annual ground water availability for the entire country is 399 bcm. The annual ground water draft is 231 bcm out of which 213 bcm is for irrigation and 18 bcm is for domestic and industrial use. Among the states, Uttar Pradesh has the highest ground water potential of 76 bcm, followed Madhya Pradesh and Andhra Pradesh (37 bcm each). Although the national scenario of ground water availability is favourable, certain pockets face scarcity because the ground water development over different parts of the country is not uniform.
Ground Water Development Scenario
Ground water is assessed in India block wise, except Andhra Pradesh, Gujarat, Karnataka, Goa and Maharashtra where assessment is done on the basis of mandals and talukas. The assessment units have been categorised for groundwater development based on two criteria — stage of groundwater development (SGWD) and long term trend of pre and post-monsoon water levels. SGWD is the ratio of annual groundwater draft to the net annual groundwater availability. On all India basis, SGWD is 58 per cent. Among the states, SGWD varies from 0.04 per cent in Arunachal Pradesh to 170 per cent in Delhi; it is more than 100 per cent in Punjab, Rajasthan and Haryana. Clearly, ground water is under serious pressure in these States.
Ground water has contributed vastly to agricultural development in India, particularly during the last four decades. It has helped attain food security through green revolution and commercialisation of farming practices. However, its exploitation is largely in the hands of private individuals.
Estimates show that 70 to 80 per cent of the value of irrigated production in India now depends on ground water irrigation. Thus, nearly two fifths of India’s agricultural output comes from the areas that are irrigated with ground water and the contribution of ground water to India’s GDP is substantial. It has been noted that agricultural yields are generally 30 to 50 per cent higher in the areas irrigated with ground water compared to areas irrigated by water from other sources. As some profits from ground water irrigation were invested back into ground water development, it has led to a ‘tubewell explosion’ in many parts of India. This ‘explosion’ was further facilitated by government’s efforts to promote rural electrification, availability of cheap diesel, and institutional credit support from nationalised banks.
Regulation of Ground Water Use
Ownership of ground water in India is governed under section 7 of the Indian Easement Act, 1882, which refers to the right of every owner of land to collect and dispose all water under the land, which does not pass in a defined channel. These property based rights provide virtually unlimited opportunity to an individual having some property to exploit ground water. Consequently, in many areas, ground water development far exceeds annual renewable recharge. Provision of free electricity or cheap diesel to farmers to withdraw ground water has aggravated the situation. Lowering of water tables leads to wells running dry. The remedy lies in drilling deeper wells, consequently more energy is needed to pump out water. Thus there is a need to redefine the custodianship of ground water and implement workable regulatory mechanism. Subsidies (such as free electricity) should be limited and provided only to the targeted groups to avoid hardships to the farmers with limited resources. Besides quantity, ground water quality should also be given adequate attention.
To check, control and regulate the ground water from over exploitation, Government of India circulated a Model Ground Water Bill in 1990s to the states to constitute the regulatory bodies. It also constituted in 19 Central Ground Water Authority (CGWA) 97 under the Environmental Protection Act, 1986. The rules to check the over exploitation are still to be finalised and notified. Unfortunately, only a few states have proceeded to make legislation for ground water regulation. Sustainable use of ground water would require formulation of appropriate legislation, its strong implementation, and artificial recharge (wherever practical).
Climate Change and Ground Water in India
The occurrence of groundwater depends primarily on soil, geology, and rainfall — both pattern and volume. The inter relationships between these factors create complex patterns. Climate change will superimpose on this by modifying rainfall and evaporation patterns.
Changes in precipitation and evaporation translate to shifts in soil moisture deficits, surface water runoff and ground water recharge. Quantifying these relationships is complicated. Annual rainfall volume is important, but timing, intensity and duration of rainfall events, temperature, soil-vegetation conditions and land use are also important, as is the storage potential of aquifers. If intensity of rainfall increases, there will be less opportunity for recharge and large withdrawals will be made if frequency and length of dry spells increases.
Many of the impacts of climate change described in the IPCC Fourth Assessment Report (2007) relate to agriculture. A key concern is whether ground water-based irrigation could provide climate resilience. The ground water resources most at risk from over exploitation and climate change are in areas with intense farming of water guzzling crops and arid climate. In these environments regional depletion of ground water levels is more likely, particularly where farmers have large resources and regulation is (nearly) absent.