The groundwater level in north India declined at a rate of 2 cm per year during the period 2002-2013, while in north-central and south India it increased by 1-2 cm per year during the same period, says a study carried out by a team of researchers from the Indian Institute of Technology (IIT) Gandhinagar.
While changes in monsoon rainfall pattern during the period of study can largely explain the total variability of groundwater storage in north-central and south India, the usage of groundwater for irrigation purposes accounts groundwater variability in northwest India. The increased usage of groundwater for irrigation in northwest India is, in turn, linked to changes in monsoon rainfall pattern. The results were published in the journal Nature Geoscience.
“A 2009 study carried out in Punjab and Haryana showed that groundwater extraction was mainly responsible for declining groundwater storage. But our study shows that changes in groundwater storage are linked to changes in rainfall pattern in most parts of India,” says Prof. Vimal Mishra from the Civil Engineering Department, IIT Gandhinagar and the corresponding author of the paper. “In north-central and south India, rainfall is a major driver of groundwater storage change. But in northwest India, groundwater pumping is a major cause of groundwater storage variability.”
Reduced monsoon rainfall in north India due to Indian Ocean warming has led to reduced groundwater storage and increased usage of groundwater for irrigation. For instance, over the Gangetic Plain and other parts of north India monsoon rainfall has been declining since 1950, leading to reduced recharge of groundwater. As a result of declining monsoon rainfall and intensive agriculture, groundwater withdrawals in the country have increased over tenfold since the 1950’s — from 10-20 cubic km per year in 1950, to 240-260 cubic km per year in 2009.
A team led by Prof. Mishra used GRACE satellite data to estimate groundwater storage anomalies. They also used information on groundwater levels from observation wells. “The GRACE satellite data is very reliable. It shows consistent trend with the observation well data in India,” he says.
A majority of observation well in north indicate a 15-25 cm per year decline in groundwater table depth between 1996 and 2013. In the case of south India, there has been 5-20 cm per year increase in groundwater table depth in the observation wells. “The depletion in groundwater storage during the last 15 years might have been due to long-term decline in rainfall in north India,” he says.
To study the long-term changes in rainfall on groundwater storage, the researchers look at wells located in regions with significant increase or decrease in rainfall and those located in regions where more than 40 per cent of irrigation is by groundwater. Wells in areas that witnessed significant increase in rainfall showed increased groundwater level despite being in an area that is heavily irrigated using groundwater. However, wells in areas that witnessed significant reduction in rainfall showed decrease in groundwater level.
Despite groundwater-based irrigation increasing in northwest, north-central and south India during 2002-2013, the groundwater storage in these three regions show contrasting trends. This highlights the importance of rainfall in recharging groundwater.
“There has been a decline in rainfall since the 1950s and increase in groundwater pumping in north India. If this trend continues for another 10-15 years then it will be unsustainable,” says Prof. Mishra. “Unlike in south India where aquifers recharge quickly, north India has very high groundwater persistence — it takes a long time to recharge and deplete the aquifer.”
“Steps should be taken to artificially recharge the groundwater during the monsoon season so that groundwater can be used during dry season for irrigation in a sustainable manner. Irrigation demands may increase in response to warming climate in future. Sustainable use of groundwater for irrigation along with a better groundwater recharge approach is required. Moreover, there is a strong need to use highly-efficient irrigation measures to reduce the amount of groundwater used and ensuring future food and water security,” he says.