Ganga’s worst dry spell in 1300 years

The Ganga river has endured acute droughts over the last millennium, marked by dry decades in the 14th and 16th centuries. But, the river is experiencing its worst dry spell, more severe than at any point in history¹.

Using paleoclimate reconstructions and hydrological modeling spanning 700 to 2012, scientists at the Indian Institute of Technology (IIT) Gandhinagar and the University of Arizona show that the decline in streamflow since the 1990s surpasses even the devastating 16th-century drought that triggered widespread famine across northern India.

"The drying from 1991 to 2020 is unmatched in the past millennium," the researchers write.

Their analysis identified 1991 as a critical changepoint when mean annual streamflow persistently shifted and settled to a new low ---- approximately 620 cubic metres per second less than before — equivalent to 620,000 one-litre water bottles every second, or roughly 16 Olympic-sized swimming pools every hour.

Natural variations alone cannot account for this trend. “While past droughts were within the range of natural climate variability, the recent drying appears to exceed this natural range, indicating a shift toward human-influenced hydroclimatic conditions,” says Dipesh Singh Chuphal, a PhD research scholar at IIT Gandhinagar.

The drivers are both global and local. Weaker monsoons driven by Indian Ocean warming, aerosol pollution, and sustained groundwater pumping have depleted the 2520-kilometre river.

The primary dataset used was the Monsoon Asia Drought Atlas, which provides 1,300+ years of drought severity reconstructed from tree-ring observations across the Asian monsoon region. This was validated against historical famine records, cave isotopic oxygen records, and instrumental data.

Only five of the 25 CMIP6 global climate models captured the ongoing drying trend. Analysis of 100 ensemble climate simulations showed none reproduced the observed patterns, suggesting the crisis exceeds the range of natural climate variability these models can simulate.

Most global climate models project increasing rainfall over South Asia in recent decades under climate warming, primarily due to enhanced evaporation. “However, they often oversimplify the complex interactions between atmospheric circulation and land surface processes, resulting in an incomplete and inaccurate representation of rainfall patterns,” says Chuphal.

The models fall short because they downplay the impact of anthropogenic aerosols and large-scale cooling from irrigation, both of which suppress summer monsoon rainfall in the Ganga basin.

The same models that failed to predict the current drying also indicate that more rainfall under climate warming could reverse the drying trend by 2040. “Currently, low-flow years tend to happen about every two years, but projections suggest they might only happen once every six years in the future. However, we can’t be sure because these models are plagued by uncertainties,” says Chuphal.

Groundwater stress

Climate models must be refined to better represent monsoon–ocean–atmosphere interactions and human interventions such as irrigation and aerosols. Without this, water management strategies risk poor underpinning, the authors say.

Groundwater depletion, already critical in the basin, creates a dangerous feedback loop, as a substantial portion of the river’s flow depends on baseflow from aquifers. “Groundwater levels in some parts of the Ganga basin, according to our analysis, have dropped by more than 10 cm annually since the 1980s – faster than the decades it takes to refill an aquifer,” says hydrologist, Abhijit Mukherjee, at the Indian Institute of Technology, Kharagpur. He was not associated with the study.

In the last 30 years, groundwater input to the Ganga has fallen by 50% during summer, when baseflow can provide up to 70% of the river’s water.

Projections forecast that by mid-century, this could decline by as much as 75% compared to the 1970s, severely reducing surface water supply for irrigation. Receding flows could restrict food security for 115 million people in the coming decades, as crop yields decline and streamflow capture reverses the movement of water from rivers to aquifers.

Agricultural inefficiency aggravates the problem. Farmers in the region use nearly all of the available water for agriculture, but a significant portion of it goes to waste. Out of all the water used, only about one-third actually benefits the crops. The rest is lost through leakage, evaporation, or poor irrigation methods.

The scale of the drought

The stark conditions laid out in the reconstruction include 15 individual drought years between 1991 and 2020, with rainfall deficits during El Niño years, such as 1997, 2002, 2009, and 2015. The La Niña years — 2007 and 2010 — which typically bring above-average rainfall to India, failed to deliver adequate rain over the Indo-Gangetic plains.