Compound Drought and Heatwave (CDHW) Events

A compound drought and heatwave event is when a place suffers a drought (a long shortage of water) and a heatwave (unusually high temperatures) at the same time. Each alone is harmful. Together, they make each other worse, so the damage to crops, forests, water supply and human health is far greater than the two would cause separately. Scientists call such linked disasters "compound events".

Why are two hazards together worse?

Think of the ground as a sweating body. When soil is wet, the sun's energy is used to evaporate water, and this keeps the surface cool, just as sweat cools our skin. When soil dries out, there is little water left to evaporate. The sun's energy then heats the ground and the air directly. Hotter air dries the soil even more. This loop is called the soil moisture-temperature feedback. Drought feeds heat, and heat feeds drought.

How do they hurt forests and farms?

• Plants close their pores: In heat and drought, leaves close their tiny pores (stomata) to save water. This stops them taking in carbon dioxide, so growth slows.
• Hydraulic failure: If drought goes on, air bubbles form in the water pipes inside the tree trunk, and the tree can die of thirst.
• Fire risk: Dry leaves and branches act as fuel, so fires spread easily.
• Crops: Heat during flowering and grain filling cuts yields, and drought leaves no water to cool the plant.
• People: Heat stress, water shortage and loss of income come together, hitting the poor and outdoor workers hardest.

How are they measured?

• Drought is measured by indicators such as rainfall shortfall, soil moisture, and indices like the Standardized Precipitation Index (SPI) and the Standardized Precipitation-Evapotranspiration Index (SPEI), which also counts water lost through heat.
• Heatwave in India is defined by the India Meteorological Department (IMD). A heatwave is considered when the maximum temperature reaches at least 40°C in the plains, 37°C at coastal stations or 30°C in hilly regions, and it is 4.5°C to 6.4°C above normal; more than 6.4°C above normal is a severe heatwave. In the plains, an actual maximum of 45°C or more is a heatwave and 47°C or more is a severe heatwave, whatever the normal.
• A CDHW event is recorded when both conditions overlap in the same place and time.

Where did the idea come from?

Scientists long studied droughts and heatwaves separately. Disasters like the 2003 European heatwave (tens of thousands of deaths, with very dry soils) and the 2010 Russian heatwave (with drought and huge wildfires) showed that the combination was the real killer. The Intergovernmental Panel on Climate Change (IPCC) gave compound events special attention. Its Sixth Assessment Report (AR6, 2021) found that human influence has likely increased the chance of compound extreme events since the 1950s, including more frequent concurrent heatwaves and droughts at the global scale (high confidence), and that they will become more frequent with further warming.

Why are they becoming more common?

• Global warming raises the baseline temperature, so any drought now comes with more heat.
• More evaporation: Warmer air pulls more water out of soil and plants, turning a dry spell into a drought faster ("flash droughts").
• Climate swings: Events like El Niño add heat and cut rainfall over the tropics, stacking on top of warming.
• Land use: Cutting forests reduces local moisture recycling, so rain falls less and land heats more.

India's position and examples

India often faces heat and dry spells together in the pre-monsoon months (March to June), and when the monsoon is weak, as in many El Niño years. India's framework includes:

• NDMA (National Disaster Management Authority) heatwave guidelines, first issued in 2016 and revised later, which guide states to prepare Heat Action Plans. Ahmedabad's Heat Action Plan (2013) was the first in South Asia.
• The Manual for Drought Management (2016, updated 2020) of the Ministry of Agriculture, which sets how states declare drought using rainfall, crop, soil moisture and water indicators.
• Heatwave is not in the list of notified disasters for the State Disaster Response Fund, but states can use part of the fund for local disasters they notify.

Commonly confused concepts

• Meteorological vs agricultural vs hydrological drought: Meteorological drought is a rainfall shortfall. Agricultural drought is when soil moisture is too low for crops. Hydrological drought is when rivers, reservoirs and groundwater fall. One can lead to the next.
• Heatwave vs high temperature: A heatwave depends on both the actual temperature and how far it is above normal for that place, so 40°C may be a heatwave in one region and not in another.
• Compound event vs cascading event: A compound event is several hazards at once (drought plus heat). A cascading event is when one hazard triggers a chain of others (heat causes power failure, which stops water pumps).

Issues, criticism and the way forward

• Planning in silos: Drought and heat are often handled by different departments and plans, though they strike together. Experts suggest joint early warnings and combined action plans.
• Weak local data: Many districts lack fine-scale soil moisture and temperature data, so warnings come late.
• Forests left out: Disaster plans focus on people and crops; forest drought and fire risk get less attention.
• Way forward: Combined risk indices, crop insurance and water storage planning for hot dry years, protecting forests that recycle moisture, and cutting emissions to limit how often such years happen.

Concepts to Know

• Drought: A long period with less water than normal, from low rain, high heat, or both.
• Stomata: Tiny openings on leaves through which plants take in carbon dioxide and lose water vapour.
• Evapotranspiration: The total water that moves from land to air, through evaporation from soil and transpiration (water loss) from plants.
• Feedback loop: When the result of a change makes that same change stronger (a positive feedback) or weaker (a negative feedback).
• IPCC: The UN body of scientists that assesses climate change research and publishes reports that governments use.