Drought is a prolonged period of unusually low water availability relative to normal conditions for a particular region, season or environmental system. It is not caused by one universal mechanism because drought can arise from insufficient precipitation, excessive evaporation, altered atmospheric circulation, depleted soil moisture, reduced river flow, declining groundwater or combinations of these factors.
Meteorologists commonly distinguish meteorological drought, which concerns precipitation deficits, from agricultural drought, which concerns inadequate soil moisture for crops and vegetation, hydrological drought, which concerns reduced streamflow, reservoir storage and groundwater, and ecological or socioeconomic drought, which concerns consequences for ecosystems and human water demands. Drought therefore describes a condition of water shortage rather than simply a lack of rain.
A region can experience drought even when some rain continues to fall if evaporation is exceptionally high or demand for water exceeds available supplies. The duration, spatial extent, seasonality and intensity of drought vary greatly, making it a complex phenomenon that connects atmospheric processes with land, rivers, groundwater, ecosystems and human water use.
The most immediate meteorological cause of many droughts is persistent precipitation deficit. Large scale atmospheric circulation can divert storms away from a region or prevent moist air from reaching it. Persistent high pressure can suppress cloud development and precipitation, while shifts in jet stream position can redirect weather systems towards other areas. Atmospheric blocking can cause weather patterns to become unusually stationary, allowing dry conditions to persist.
Ocean temperatures can influence atmospheric circulation and moisture transport over large distances, while phenomena such as El Nino and La Nina can alter precipitation patterns in different parts of the world. The exact effect of these climate oscillations is regionally dependent and can vary by season. Drought can therefore emerge from a combination of atmospheric circulation patterns, ocean conditions and land atmosphere interactions rather than from a single missing rainfall event. A short dry spell becomes a significant drought when the deficit persists long enough, affects water availability or produces consequences relative to the normal climate of the region.
Temperature and evaporation are central to understanding drought because water can be lost rapidly even when precipitation deficits are moderate. Warmer air can increase the atmosphere's capacity to contain water vapour, and high temperatures can increase potential evapotranspiration. When evaporation and plant transpiration exceed the amount of water supplied by precipitation or irrigation, soil moisture declines. This process can intensify agricultural drought and contribute to hydrological drought if the deficit persists.
Climate warming can therefore worsen drought conditions in regions where increased evaporative demand is not compensated by additional precipitation. The relationship is not uniform worldwide because changes in rainfall patterns, humidity, vegetation and seasonal conditions differ geographically. Soil type also matters because sandy soils drain quickly and clay rich soils can retain water differently. Vegetation can influence local moisture recycling through transpiration, while depleted vegetation cover can reduce the movement of water back into the atmosphere and alter land surface energy balance.
Droughts can be amplified by land atmosphere feedbacks. As soils dry, evaporation decreases and a greater fraction of incoming solar energy can be converted into sensible heat, raising surface temperatures. Vegetation may reduce transpiration during water stress by closing stomata, further decreasing evaporative cooling. Warmer and drier surface conditions can increase atmospheric demand for moisture, accelerating soil moisture loss.
In some regions, vegetation changes can also affect local rainfall by altering moisture recycling, although the importance of this process varies substantially. Dry soils can therefore contribute to a feedback in which drought produces hotter conditions that increase atmospheric moisture demand and deepen the water deficit. Wildfires can also become more likely in drought affected ecosystems, while loss of vegetation can increase erosion and damage soil structure. These interactions demonstrate why drought cannot be understood solely by counting rainy days. The hydrological response depends on the balance among precipitation, evaporation, runoff, infiltration, groundwater recharge, plant water use and human extraction.
Human activities can contribute to or intensify drought impacts even when the original meteorological cause is natural. Irrigation, industrial water use, domestic consumption, groundwater pumping, reservoir management and changes in land use can alter the amount of water available to ecosystems and downstream users. Groundwater can provide an important buffer during dry periods, but excessive extraction can lower water tables and cause long term depletion.
Deforestation, soil degradation and urban development can change infiltration and runoff, affecting how efficiently rainfall becomes available for later use. Conversely, careful water management, efficient irrigation, groundwater regulation, reservoir storage and ecosystem restoration can reduce vulnerability to drought. It is important to distinguish drought itself from water scarcity caused primarily by human demand. A naturally dry period can become a severe societal crisis when water systems have little storage, demand is high, infrastructure is inadequate or governance cannot allocate limited supplies effectively.
Climate change affects drought risk in ways that differ by region and drought type. Global warming increases atmospheric moisture demand and can intensify drying where precipitation does not increase sufficiently to offset enhanced evapotranspiration. At the same time, a warmer atmosphere can hold more water vapour and can contribute to heavier precipitation when conditions favour rainfall, meaning that some regions may experience an increased alternation between very wet and very dry conditions.
Changes in atmospheric circulation can alter the frequency and persistence of drought in particular regions. Scientists therefore assess drought using multiple indicators rather than assuming that global warming produces identical drying everywhere. Measures include the Standardised Precipitation Index, which focuses on precipitation anomalies, the Standardised Precipitation Evapotranspiration Index, which includes atmospheric water demand, soil moisture measurements, streamflow records, reservoir levels and groundwater observations. Drought attribution requires examining the specific event, region, season and physical processes involved.
The consequences of drought extend across agriculture, ecosystems, water supplies, energy, health and economies. Crop yields can decline when soil moisture becomes inadequate, especially during sensitive stages of plant development. Pastures can deteriorate, reducing livestock feed. Rivers and reservoirs can fall to low levels, affecting drinking water, navigation, hydropower and aquatic ecosystems. Groundwater depletion can threaten wells and cause land subsidence in some areas. Drought can increase wildfire risk because vegetation and organic material become dry and more flammable.
Aquatic ecosystems can suffer from warmer water, lower oxygen concentrations and reduced habitat. Economically, drought can increase food prices, reduce agricultural income and create conflicts among competing water users. The severity of impacts depends on drought duration and intensity as well as infrastructure, water storage, agricultural practices, ecosystem condition, economic resources and preparedness.
Drought monitoring and management combine meteorological observations, hydrological measurements, satellite data, soil moisture observations, vegetation indices, climate models and water resource records. Early warning systems can identify emerging precipitation deficits and unusually high evaporative demand, allowing farmers, water managers and governments to adjust decisions before shortages become severe. Adaptation measures include improving irrigation efficiency, diversifying crops, conserving soil moisture, restoring wetlands, protecting groundwater, increasing water storage where environmentally appropriate and reducing unnecessary demand.
Drought resilience also depends on planning for uncertainty rather than assuming historical water availability will always continue. A common misconception is that drought means an area receives no rain. In reality, drought is defined relative to expected water availability, and significant drought can occur with intermittent rainfall if total water supply remains substantially below normal or losses and demand are unusually high.
