Rising Evaporative Demand and India’s Climate Data Gap
Relevance
GS III Climate Change, Agricultural Planning, Water Management
Introduction
- Evaporative Demand: Measures the “thirst of the atmosphere” — the maximum potential water loss through evaporation and transpiration, assuming unlimited water availability.
- Rising global temperatures have intensified this demand, influencing agricultural water requirements.
- The new term “Thirstwaves” refers to three or more consecutive days of unusually high evaporative demand, highlighting a climate risk indicator under-researched in India.
Key Concepts
Evaporative Demand
- Depends on temperature, humidity, solar radiation, and wind speed.
- Increased solar radiation, wind, and heat drive more evapotranspiration, drying out land and vegetation faster.
Standardised Short-Crop Evapotranspiration (ET₀)
- Simplified model measuring water loss from a reference grass crop (12 cm tall) under ideal conditions.
- Helps plan irrigation scheduling and climate risk management.
Thirstwave (Kukal & Hobbins, 2025)
- Coined to describe extreme atmospheric water demand events during growing seasons.
- First characterised in the United States, showing:
- Increase in severity, duration, and frequency.
- Reduced likelihood of thirstwave-free crop seasons.
India’s Context
Lack of Extreme Event Data
- No established system in India to track thirstwaves or extreme evaporative stress.
- Existing studies (e.g., IIT Roorkee, 2022) have measured only general evapotranspiration trends over river basins.
Contradictions in Past Data
- Chattopadhyay & Hulme (1997): Despite rising temperatures, evaporation declined in India (due to humidity).
- However, future projections show temperature-driven rise in evaporative demand outpacing humidity effects.
Emerging Research
- Kukal is collaborating with Indian researchers to study thirstwave dynamics across South Asia.
- Early findings show worst thirstwaves may not occur in hottest regions, suggesting current vulnerability maps may be misleading.
Impact on Agriculture & Water Resources
| Area | Consequence |
| Irrigation | Farmers may under- or over-estimate water needs without considering evaporative demand. |
| Crop Yields | High atmospheric demand can cause crop stress even when irrigation is provided. |
| Groundwater | Rising ET₀ pressures irrigation, increasing groundwater overuse. |
| Water Management | Can lead to inefficient allocation and policy blind spots. |
Policy Gaps and Challenges
- No thirstwave monitoring in India by IMD or agricultural departments.
- Climate models focus on temperature and rainfall, ignore ET₀ or atmospheric dryness.
- Agro-advisories often neglect evaporative demand trends.
- Unequal vulnerability assessments, as non-hot regions may still face severe thirstwaves.
Way Forward
- Include ET₀ and thirstwaves in IMD’s climate forecasts and farmer advisories.
- Establish monitoring stations for real-time evaporative demand tracking.
- Encourage crop-specific studies on water stress under high ET₀.
- Integrate thirstwave data into state action plans (SAPCCs) and disaster risk frameworks.
- Support cross-institutional research collaborations to fill data gaps.
Conclusion
As India faces increasingly uncertain rainfall and rising temperatures, the atmosphere’s thirst — evaporative demand — could silently erode agricultural sustainability. Thirstwaves expose a critical blind spot in climate preparedness. With robust tracking and farmer-centered data systems, India can better manage water stress, secure food production, and build true climate resilience.
