Drought is a complex natural phenomenon characterized by a prolonged period of unusually dry weather. This prolonged lack of precipitation causes serious water shortages, affecting crops and water supplies. Unlike sudden hazards like floods, drought often develops slowly and can persist for months or even years, making its onset difficult to pinpoint.
Meteorological Triggers
Drought begins with a significant precipitation deficit. This is often influenced by large-scale atmospheric conditions. Persistent high-pressure systems, for instance, can settle over a region, suppressing the formation of clouds and rain-bearing weather systems, thereby blocking moisture-laden air. Unusual wind patterns in the upper atmosphere can also divert typical storm tracks, preventing rainfall from reaching an area.
Higher than normal temperatures frequently accompany these dry conditions, exacerbating the moisture deficit. Warmer air increases evaporation from land and water bodies, and also enhances evapotranspiration. This increased atmospheric demand further dries out soils and vegetation, intensifying water scarcity even with slightly below-average precipitation. These combined meteorological factors establish the initial conditions for drought development.
Progression of Drought Types
As a meteorological deficit persists, different types of drought emerge in a sequential manner. Meteorological drought, the initial stage, is defined solely by a prolonged period of less than average precipitation for a specific region. This is measured by comparing current rainfall amounts to historical averages over time.
Following a meteorological drought, an agricultural drought typically develops. This type focuses on the impact of insufficient soil moisture on crop health and agricultural production. It occurs when soil water is inadequate for crop and pasture needs, often exacerbated by increased evapotranspiration from plants due to higher temperatures.
If the precipitation deficit continues, a hydrological drought will eventually manifest. This stage affects water levels in rivers, lakes, reservoirs, and groundwater. Hydrological drought tends to develop more slowly than meteorological or agricultural droughts because it involves the depletion of stored water, which takes time to show significant impacts. The final stage, socioeconomic drought, occurs when the physical water shortage begins to affect human activities and economic systems, such as disruptions to water supply for industries or increased food prices due to crop failures.
Monitoring and Classification
Scientists and agencies use various indicators and indices to identify, measure, and classify droughts once they are occurring. The Palmer Drought Severity Index (PDSI) is a commonly used regional index that incorporates both precipitation and temperature data to model moisture supply and demand. It is particularly effective for tracking long-term drought conditions, typically over several months.
Another widely used tool is the Standardized Precipitation Index (SPI), which characterizes meteorological drought based solely on precipitation data. The SPI can be calculated for various timescales, from one month to several years, allowing it to reflect short-term soil moisture conditions or longer-term groundwater and reservoir levels. Its standardization allows for comparison across different climatic regions.
The U.S. Drought Monitor, a collaborative effort, integrates multiple indicators, including precipitation, soil moisture, and streamflow, with expert input to classify drought severity. This system uses categories ranging from D0 (abnormally dry) to D4 (exceptional drought), providing a comprehensive assessment of drought presence, intensity, and spatial extent. These monitoring tools are important for understanding the progression and impacts of drought, enabling timely responses and planning.