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Soil Moisture Depletion and Its Impact
Soil moisture depletion contributed to sea-level rise, finds study
Context: Between 1979 and 2016, soil moisture (SM) depletion contributed to a 10.78 mm rise in global mean sea level (GMSL), corresponding to a loss of 3,941 gigatonnes (Gt) of land water.
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- Since 2003, SM depletion has been deemed irreversible, intensifying to add an additional 2.76 mm rise in GMSL, equating to a loss of 1,009 Gt of water.
- To visualise: 1 Gt of water = 1 cubic km. The 3,941 Gt loss would cover an area 2.6 times the size of Delhi (1,484 sq km), with a water depth of 1 km.
Deterioration of Water Resources
- Sources such as groundwater and surface water bodies are declining due to:
- Reduced rainfall, rising temperatures (increasing evapotranspiration), and rapid urbanisation.
- More frequent and intense droughts, with drier spells now 1.7 times more frequent compared to the 1850–1990 average.
Link Between SM, Groundwater, and Drought Formation
- The relationship between soil moisture and groundwater in drought formation remains under-researched.
- Dr. Milind Mujumdar highlighted:
- Variability in monsoon patterns, droughts, and floods directly impacts SM, weather systems, crop yields, and groundwater recharge.
- Urges need for precise surface and sub-surface SM measurements with high spatial and temporal resolution.
- Limited in-situ SM data restricts comprehensive climate impact studies.
- His research indicates surface SM significantly influences temperature in strongly coupled regions like north-central India.
- Groundwater decline is linked to SM, precipitation, agricultural practices, and urbanisation.
- Geological factors also affect groundwater–SM relationship:
- High SM: promotes recharge.
- Low SM: reduces infiltration and increases depletion.
- Excessive SM: can lead to waterlogging, preventing recharge.
- Arid areas: deplete groundwater faster due to insufficient SM.
- Maintaining optimal SM levels is essential for groundwater sustainability.
Key Data Sources and Observations
- The study relied on data from:
- ERA5-Land (1979–2016).
- GRACE (2002–2017) and GRACE Follow-On (2018–present).
- Satellite altimeters and Earth’s polar motion data.
- Severe SM depletion was observed in Central Asia, Central Africa, East Asia, and North/South America.
- Partial recovery was noted in parts of India, Australia, and North America, though insufficient to reverse the global trend.
Challenges and Scientific Caution
- Benjamin Cook from NASA’s Goddard Institute expressed concerns:
- The study heavily relies on the ERA5 dataset, which other datasets do not fully support.
- The dataset’s time span may be too short to determine permanent changes, suggesting further research on natural climate variability.
Insights for India
- Dr. Mujumdar emphasised:
- Wet SM areas (e.g., Western Ghats, northeast India) are energy-controlled evaporation regimes.
- Dry SM regions (e.g., northwest India) exhibit weak evaporation variability.
- Moderate SM zones significantly affect evaporation variability and temperature dynamics.
- Modelling experiments showed:
- A 20% increase in SM perturbation could reduce extreme temperature event frequency by 60–70% and duration by 20–30%.
- A 20% decrease could increase frequency and duration by 60–100% and 15–40%, respectively.
Drought Resilience and Solutions
- Key themes at the 16th Conference of the Parties to the UN Convention to Combat Desertification (2024) included: Restoring degraded land and building drought resilience.
- Suggested strategies for India:
- High rainfall areas: Focus on flood management, better drainage, and waterlogging-resistant crops.
- Arid regions: Implement efficient irrigation (e.g., drip/sprinkler), rainwater harvesting, and SM conservation (e.g., mulching, agroforestry, drought-resistant crops).
- Advanced IoT-based observational networks are crucial for effective decision-making.
