Rising "evaporative demand" spotlights India’s data and research gap

24 Jun 2025 GS 3 Environment
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Evaporative Demand and Thirstwaves

  • Evaporative Demand (ED): Measures how ‘thirsty’ the atmosphere is — i.e., the maximum water that can evaporate from land if water is available.

  • Thirstwave (Term by Kukal & Hobbins): Three or more consecutive days of high evaporative demand, now found to be increasing in intensity, frequency, and duration.

  • It differs from a heatwave as it also includes humidity, solar radiation, and wind speed, not just temperature.

Scientific Observations & Global Context

  • New US Study (2025) by Kukal and Hobbins:

    • Found thirstwaves are growing worse during crop seasons.

    • Evaporative demand is now a better indicator for irrigation planning than temperature alone.

  • Standardised Short-Crop Evapotranspiration (ET₀):

    • Proxy used to simplify evapotranspiration estimates.

    • Assumes constant crop conditions; isolates the role of weather.

    • Increasing ET₀ = Rising temperature, solar radiation, wind speed; falling humidity.

Indian Research Findings

  • Contradictory Past Trends:

    • A 1997 study based on 30 years of IMD data showed decrease in ET due to higher humidity, despite warming.

    • But projected climate models suggested future warming will override humidity effects, increasing ET and ED.

  • IIT-Roorkee-led study (2022):

    • Found actual ET is rising in many Indian sub-basins, notably in:

      • Northern India

      • Western and Eastern Himalayas

    • This may indicate increased vegetation or agricultural expansion.

Gaps in Indian Data & Research

  • Lack of data on extreme ED or thirstwaves in India.

  • No national-level assessment of how different crops or regions respond to rising evaporative demand.

  • India needs integrated tracking of ED for:

    • Irrigation management

    • Drought prediction

    • Crop sensitivity analysis

    • Water budgeting policies

Current Initiatives and Way Forward

  • Insight: Worst thirstwaves don’t always occur in areas with the highest ED, suggesting the need to reassess regional vulnerability and resource allocation.

  • Experts stress the urgent need to:

    • Monitor and report thirstwaves.

    • Build farmer awareness.

    • Integrate ED into climate resilience strategies.



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