Soil moisture (SM) depletion between 1979 and 2016 has contributed to a 10.78 millimetre rise in global mean sea level (GMSL), equating to a loss of 3,941 gigatonnes (Gt) of water from land, according to a new study published in the journal Science.
The study highlighted that this depletion has been irreversible since 2003 and has only intensified, resulting in an additional 2.76 mm rise in GMSL, corresponding to 1,009 Gt of lost land water.
To put this in perspective: 1 gigatonne of water occupies 1 cubic kilometre and the 3,941 Gt loss would cover an area 2.6 times the size of Delhi (1,484 sq km), assuming a water depth of 1 km.
Soil moisture and its link to groundwater, droughts
Water storage sources — groundwater and surface water bodies — are deteriorating. The study noted that declining soil moisture, rainfall deficits and increased evapotranspiration (due to rising temperatures) are contributing to more frequent and intense droughts. Rapid urbanisation further exacerbates the pressure on these natural resources. Overall, this has resulted in drier spells becoming 1.7 times more frequent compared to the 1850-1990 average.
The study also pointed out that the roles of soil moisture and groundwater in drought formation are still not fully understood.
Dr Milind Mujumdar, retired senior scientist at the Indian Institute of Tropical Meteorology, told DTE that variability in monsoon patterns, droughts and floods directly affect soil moisture and land-atmosphere interactions, influencing weather systems, crop yields and groundwater recharge.
He stressed the need for precise measurements of soil moisture at both surface and sub-surface levels, with high spatial and temporal resolution, to improve land surface hydrological processes in climate models, agricultural planning and hydro-meteorological monitoring. However, due to the sparse availability of in-situ SM observations, limited studies exist that show its impact on climate.
Mujumdar added that his team’s studies show surface soil moisture has a large impact on temperature increases in strongly coupled regions of north-central India. Groundwater decline, he said, is influenced by soil moisture levels, along with factors such as precipitation, agricultural practices and urbanisation.
He also noted that geological characteristics play a major role in the groundwater-soil moisture relationship. High SM levels promote recharge, while low levels reduce infiltration. In areas with excessive SM, waterlogging can prevent proper recharge, while arid regions with insufficient SM experience faster groundwater depletion. Hence, maintaining optimal SM levels is vital for sustainable groundwater management.
Key data sources and observations
The study utilised multiple data sources, including ERA5-Land (January 1979-December 2016), GRACE (May 2002-May 2017), GRACE Follow-On (June 2018-present), satellite altimeters and Earth’s polar motion data (for periods before GRACE). The table below summarises the key sources and findings:
The study also used various land surface models and climate indices, with each data source having limitations. Comparisons among datasets helped formulate the overall conclusions.
The study found that regions such as Central Asia, Central Africa, East Asia and North and South America have experienced severe SM depletion. Some recovery was noted in parts of India, Australia and North America, though not sufficient to counterbalance the global trend.
However, some scientists expressed caution. Benjamin Cook, an Earth system scientist at the National Aeronautics and Space Administration’s Goddard Institute for Space Studies and Columbia University, told Down To Earth (DTE) that the findings are highly dependent on the ERA5 dataset, which other SM datasets do not fully support.
Cook also noted that the dataset’s time span is too short to confidently label changes as “permanent”. Any real change is more likely a consequence of internal, natural climate variability, though much longer observations are needed to make any definitive conclusion.
Building drought resilience
Restoring degraded land and investing in drought resilience were key themes at the 16th Conference of the Parties to the United Nations Convention to Combat Desertification in Riyadh, December 2024. The focus was on both preventing further damage and restoring degraded ecosystems. While the Science study offers global insights, findings for India vary, with differing interpretations among researchers.
In the Indian context, Mujumdar told DTE that wet SM areas like the Western Ghats and northeast India are energy-controlled evaporation regimes, whereas dry SM regions like northwest India have weak evaporation variability. Moderate SM zones, where SM controls evaporation, show significant evaporation variability, which in turn strongly impacts surface energy fluxes and temperature variations.
Modelling experiments indicate that a 20 per cent increase in soil moisture perturbation — applied in both historical (1951-2010) and future (2051-2100) climate simulations — could reduce the frequency and duration of extreme temperature events in north-central India by 60-70 per cent and 20-30 per cent, respectively. Conversely, a 20 per cent decrease could lead to 60-100 per cent and 15-40 per cent increases, respectively.
Surface and sub-surface SM also influence convective rainfall activity, depending on rainfall deficits or excesses. Mujumdar stressed the need for terrain-specific strategies — for high-rainfall areas, interventions include flood management, better drainage and waterlogging-resistant crops. In arid regions, efficient irrigation (drip/sprinkler), rainwater harvesting and soil moisture conservation methods like mulching, agroforestry and drought-resistant crops are essential.
Additionally, real-time SM monitoring through remote sensing and Internet of Things or IoT-based observational networks is critical for informed decision-making.
