What is Evaporative Demand?
- A measure of how much water the atmosphere ‘wants’ to pull from land and vegetation.
- Depends on temperature, humidity, wind speed, and solar radiation.
- Increasing due to global warming, making the air “thirstier”.
Relevance: GS 3(Environment and Ecology)
Concept of ‘Thirstwave’
Coined by Kukal & Hobbins (2025):
- A thirstwave = ≥3 days of extreme evaporative demand.
- Found to be more frequent, intense, and longer over time.
- Different from heatwaves; includes moisture stress along with heat.
Agricultural Implications
- Even with adequate irrigation, crop water use now depends on atmospheric demand.
Higher evaporative demand leads to:
- Increased water loss
- Drier soils and crops
- Potential stress on food security and irrigation planning
Measurement Tool: Standardised Short-Crop Evapotranspiration
- Represents the water used by 12 cm high grass under ideal conditions.
- Simplifies complex evapotranspiration dynamics by assuming constant vegetation.
- Rising values reflect more intense weather parameters (heat, wind, radiation).
India’s Historical Data & Contradictions
- Earlier (pre-1997) studies (e.g., by Chattopadhyay & Hulme) found decreased evaporation, despite warming — due to high humidity.
- Future warming, however, expected to override humidity’s dampening effect, increasing evaporative demand.
Recent Indian Research
- Studies (e.g., IIT Roorkee 2022) noted:
- Increased evapotranspiration in North India, Western & Eastern Himalayas.
- Could indicate vegetation growth or agri-expansion.
- Yet, data on extreme thirstwaves in India is lacking.
Research and Policy Gaps
India lacks:
- Real-time data on evaporative demand extremes.
- Crop-wise sensitivity studies to thirstwaves.
- Policy frameworks to integrate thirstwave risk into agri-planning.
- Current efforts (e.g., by Kukal & Shailza Sharma) aim to fill this void for South Asia.
Global South Vulnerability
- Greater exposure and weaker resilience in countries like India.
- Worst thirstwaves may occur in unexpected regions — necessitating re-evaluation of climate adaptation strategies.
Way Forward
- Incorporate thirstwave tracking in IMD & agri advisories.
- Train farmers and water managers on new irrigation demands.
- Boost climate-resilient research funding, especially in Global South.
