India's groundwater is depleting very fast posing a threat to the freshwater resources and food security in the region. North India, a food basket for more than a billion people, has been experiencing rapid groundwater depletion leading to reduction in cropped area and the production of wheat and rice. This is because, inspite of many major rivers and an extensive canal network, agriculture in the region continues to depend mainly on irrigation from groundwater drawn out from electric and diesel tube wells.
Groundwater withdrawal from deep/ confined aquifers contributes to most of the depletion, which accumulates over time in the absence of groundwater recharge. Both climate change and human interventions are found to influence groundwater storage variability. While summer monsoon season precipitation is the main contributor to groundwater recharge, climate change is suspected to have a major impact on the summer monsoon and winters in the region.
This study titled 'Summer monsoon drying accelerates India's groundwater depletion under climate change' published in Earth's Future aims at quantifying the role of summer monsoon drying and winter warming on irrigation demands and groundwater storage and recharge in the observed and projected future climate in north India. The study uses observations, climate model projections, and a hydrological framework to examine the combined impacts of climate change and human interventions on groundwater storage in India.
Drying summer monsoons and warming winters are increasing in the region
Groundwater storage in India depends on the summer monsoon rainfall and irrigation demands during the key crop growing seasons, primarily in the summer monsoon (Kharif) and winter (Rabi). Combination of summer monsoon season drying and winter warming can influence groundwater storage variability in north India, which has already witnessed a substantial decline in groundwater.
What does evidence from the past show?
A decline in the summer monsoon season rainfall and the warming in winter season has occurred in the region during 1951–2021 with an 8 percent reduction in rainfall. Frequent and severe droughts (summer rainfall deficit more than 10 percent) were experienced in the region during the recent four decades, mainly in 2002, 2004, 2009, 2014, 2015, and 2018.
The 2002 and 2009 droughts were major summer monsoon droughts that affected the entire country while the 2014–2015 drought was mainly centered in the Indo‐Gangetic Plains. The summer monsoon of 1979 was the driest during the entire period (1951–2021), with a deficit of ∼25 percent followed by the drought in 2009. Similarly, several warmer winters occurred during 1951–2021 in north India with air temperature anomalies exceeding 0.5°C.
The year 2009
The year 2009 faced the double whammy of monsoon season rainfall deficit (∼20 percent) followed by anomalously warm (1°C) winter season. Thus groundwater‐based irrigation experienced a considerable rainfall deficit and warming winter during 2009, which posed challenges for groundwater storage in the region.
Summer monsoon rainfall deficit affects rice and maize crops that are primarily grown during the Kharif season. And rice has higher irrigation water demands and summer monsoon failure makes people pump more groundwater to sustain crops during the deficit periods. At the same time, winter warming enhances soil moisture depletion and causes crop water stress during the Rabi season, implying more irrigation water requirements for wheat and other major crops. Warming winters also increases the rate of evapotranspiration (ET) thus increasing irrigation demands and decline in groundwater levels.
An observation of groundwater wells during 2009 also show a reduction in groundwater levels during the summer monsoon, winter, and entire water year. Thus entire north India faced a substantial reduction (∼10 percent) in groundwater storage during 2009, indicating the summer monsoon drying followed by the winter warming proved to be detrimental for groundwater storage in the region.
Other than 2009, the 2014–2015 summer monsoon season drought in the Indo‐Gangetic Plain also caused a decline in groundwater storage during the winter season, primarily due to reduced groundwater recharge and enhanced pumping.
Summer monsoon has an important role in influencing groundwater decline and recharge
The impact of top five driest and wettest summer monsoons during 1983 to 2021 showed that rainfall deficit of ∼17 percent led to a ∼20 percent rise in irrigation water demand during the summer monsoon season in the region and increase in groundwater pumping.
Thus, deficit rainfall during the summer monsoon season leads to a 15 percent decline in groundwater recharge, while normal rainfall leads to a 10 percent increase in recharge during the surplus years. Therefore, summer monsoon drying considerably impacts irrigation water demands, groundwater recharge, and groundwater storage in north India during the monsoon season.
Winter warming moderately affects annual irrigation water demand
Irrigation water demand during the winter season is more strongly impacted by winter warming instead of summer monsoon drying. For instance, irrigation water demand increases by ∼9 percent during the warm winters regardless of if the summer monsoon season was normal, dry, or wet. However, dry/wet summer monsoons reduce/increase groundwater recharge during the winter season indicating the summer monsoon rainfall is crucial for groundwater recharge not only in the monsoon season but also during winter.
Dry summer monsoon followed by warm winter reduces groundwater recharge more substantially than for normal‐warm or wet‐warm conditions and groundwater storage declines more substantially (∼9%) during the dry‐warm conditions.
What does the future show
A 10 to 15 percent rainfall deficit during the summer monsoon droughts combined with 1–5°C winter warming will lead to a rise in the annual irrigation water demands by 6 to 20 percent under climate change in different periods of the 21st century. Annual groundwater recharge will substantially decline by 6 to 12 percent due to summer monsoon drying and winter warming in north India. The summer monsoon drying, and winter warming will reduce groundwater storage by 6 to 12 percent in the future.
Thus increase in irrigation water demands in the future will accelerate groundwater depletion, especially during the monsoon droughts. The summer monsoon season precipitation will be more important for annual irrigation water demands than winter temperature and groundwater storage will largely be driven by the rainfall variability during the monsoon instead of warming during the winter season.
Thus sustainable management of groundwater and reducing groundwater abstraction and enhancing the groundwater recharge during the summer monsoon seasons will be crucial to ensure future agricultural production.
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