Kolar district, in India's southern state of Karnataka, has been known for centuries as a district of tanks: its landscape is dotted with nearly 3,000 surface reservoirs excavated by successive ancient kingdoms to secure water for household use, livestock, irrigation and groundwater recharge. These tanks, together with shallow open wells, were the predominant sources of irrigation water up until 1985.
Then the flow began to diminish, and farmers started extracting groundwater from deeper aquifers to irrigate fields. By 2000, farmers could no longer rely on Kolar's mostly dry ancient tanks; irrigation had shifted to borehole wells. Now a district of boreholes, Kolar has also become one of the most water-stressed districts in South India, with the gap between groundwater supply and demand growing by the year.
Why did an age-old system dry up? It's easy to point to a rising number of people competing for the same water: the district's population rose from 830,000 in 1972 to 1,540,000 in 2011. But no comprehensive studies investigated the hydrological shifts happening below the surface. Until 2020, that is, when a team led by researchers from the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) published the results of a methodologically advanced deep dive into Kolar's groundwater history in the Journal of Hydrology: Regional Studies. Their research, supported by the CGIAR Research Program on Water, Land and Ecosystems, shows that apparently flourishing agriculture and forestry on the surface can have serious consequences for what lies beneath.
A global groundwater problem in local focus
Granted, Kolar's water problems are by no means unique. Human enterprise extracts nearly a trillion cubic meters of groundwater annually around the world, with 70% of it flowing to agricultural uses. India ranks first among countries in the use of groundwater, and extraction has gushed ahead from 25 billion cubic meters in 1960 to 250 billion in 2015.
Not coincidentally, assessments suggest that annual groundwater recharge in India has decreased from 447 to 432 billion cubic meters. Today, extraction exceeds recharge in 17% of India, and equals recharge in another 5% of the country. There is an urgent need to understand how rapid economic development has reshaped hydrological processes.
Titled 'Impact of land use changes and management practices on groundwater resources in Kolar district, Southern India', the new study aims to identify the reasons and causes of the water gap by analyzing changes extending back to 1970 and connecting these into a water balance model.
More than 40 years of public data sources provided ample evidence. This included monthly observations from wells monitored by Karnataka's Department of Mines and Geology, weather data from the India Meteorological Department, and records of land and irrigation use from the Directorate of Economics and Statistics. Another powerful input was time series satellite data for the years 1972, 1992 and 2011, which the researchers used to visually analyze historical shifts in land use.
They backed this up with data collected by ICRISAT itself in a study of surface run-off from a 300 ha site in the district, where, for three years starting in 2006, an automatic recorder measured run-off every 30 minutes. The team also collected and analyzed soil samples, performed detailed surveys of some of the district's ancient tanks, and computed rates of evapotranspiration (how much water is sucked up by the roots of plants and eventually dissipated into the air) for the most widely grown crops.
Tipping the water balance at both ends
The data revealed that between 1972 and 2011 there was a major shift from grassland and rainfed fields, with irrigated crops only growing in patches near tanks, to eucalyptus tree plantations (proliferating to cover 17.7% of the district) and widespread irrigated cultivation (rising to 18% of the district).
The trees and the crops have tipped the water balance at both ends. Eucalyptus trees, adapted to dry landscapes, can develop roots more than 2 meters deep and suck up nearly all water that enters the soil around them. Plantations of the trees produce extreme evapotranspiration that returns the majority of rainfall to the atmosphere and allows next to none to percolate deeper into groundwater reserves.
However, the eucalyptus trees cannot take all of the blame. Another factor is needed to explain the growth of the water gap, which is the growth in borehole irrigation. The irrigated crops included maize, which typically needs three rounds of irrigation to get through a growing season in Kolar; vegetables and fruits under year-round irrigation; and mulberry, used to feed the silkworms that produce Karnataka's famous Mysore silk. These profitable but thirsty crops have come to replace staples like finger millet, which can easily get by on rainfall alone. As total irrigation demand has increased from 57 mm per year across the district to 140 mm, groundwater abstraction has shot up by 145%.
Towards a deeper awareness of groundwater
The process of agricultural intensification is not likely to reverse, but attending to the water balance can help in deciding permissible thresholds on water use as well as guide land use. This is a crucial concern for a majority of Indian farmers because the water gap is also an equity gap. Smallholder farmers who cannot afford to invest in ever-deeper boreholes are bearing the brunt of declining water tables. In fact, the researchers estimate that fewer than 30% of Kolar district's farmers are able to invest in groundwater extraction from deep aquifers, leading to harsh inequality in water access.
In the face of land use that is neither equitable nor sustainable, the study's results can help various stakeholders, including district and national authorities, develop water management strategies that will begin to heal the gap between water supply and demand -- in this district, and in many others. It shows that understanding the history of land use changes, together with their hydrological consequences, is key to deciding on the optimum allocation of land uses to protect the shared resources that lie beneath.
Thrive blog is a space for independent thought and aims to stimulate discussion among sustainable agriculture researchers and the public. Blogs are facilitated by the CGIAR Research Program on Water, Land and Ecosystems (WLE) but reflect the opinions and information of the authors only and not necessarily those of WLE and its donors or partners.