Drought basics: the types, the D0 to D4 scale, and the worst US droughts

Drought is a slow onset disaster and is caused by below normal rainfall over a prolonged period. Besides rainfall, other factors such as low soil moisture, irregular stream flow, less accessibility of groundwater or canal water supply and higher evapotranspiration (ET) increase the intensity of drought. Droughts are recurring events that can disturb large areas, continuing for either a period of a few weeks or lasting up to several years. Drought affects vegetation, crops, livestock, humans, and eventually the economy.

Types of drought.

Drought is classified into four types based on how water availability impacts the environment and society, i.e., Meteorological, Hydrological, Agricultural and Socioeconomic. The first three deal with ways to measure drought as a physical phenomenon. The last deals with drought in terms of supply and demand, tracking the effects of water shortfall as it ripples through socioeconomic systems.

• Meteorological drought is a prolonged period of significantly below-average precipitation. In regions with year-round precipitation, it may be defined by the number of days when precipitation falls below a specified threshold. Other definitions compare actual precipitation with long-term averages over monthly, seasonal, or annual periods.
• Hydrological drought is associated with the effects of periods of precipitation shortfalls (including snowfall) on surface or subsurface water supply (i.e., streamflow, reservoir and lake levels, groundwater). Hydrological droughts are usually out of phase with or lag the occurrence of meteorological and agricultural droughts. It takes longer for precipitation deficiencies to show up in components of the hydrological system such as soil moisture, streamflow, groundwater and reservoir levels.
• Agricultural drought links various characteristics of meteorological (or hydrological) drought to agricultural impacts, focusing on precipitation shortages, differences between actual and potential evapotranspiration, soil water deficits, reduced groundwater or reservoir levels needed for irrigation.
• Socioeconomic drought occurs when the supply and demand of goods or access to the public water supply exceeds the available supply due to weather-related shortages. This can lead to water rationing, higher food prices or energy shortages from diminished hydroelectric power.

More recently a new drought designation has been added. Ecological drought is a temporary water shortage that pushes natural ecosystems past their limits of survival, harming wildlife and disrupting the services people get from nature. Unlike traditional droughts that only measure a lack of rain or low river levels, ecological drought focuses on how water deficits damage plants, animals, and entire landscapes. Some impacts of ecological drought are dry conditions fuel severe wildfires and spark massive insect outbreaks, rivers run too warm or slow, killing fish and driving local species to extinction, and damage to nature reduces the clean water, food security, and carbon storage that human communities rely on.

How drought is measured.

Meteorological, hydrological, and agricultural droughts are often represented with drought indices, some examples are Standard Precipitation Index (SPI), Palmer Drought Severity Index (PDSI), Palmer Hydrological Drought Severity Index (PHDI), Streamflow Drought Index (SDI), Enhanced Vegetation Index (EVI), and Normalized Difference Vegetation Index (NDVI)

Drought severity is typically measured on a five-tier classification scale (ranging from D0 to D4) based on the U.S. Drought Monitor framework, which is also adapted internationally.

D0 (Abnormally Dry)

Short-term dryness that slows plant and crop growth or causes lingering water deficits, technically it’s a pre-drought rather than a full drought.

D1 (Moderate Drought)

Some damage to crops and pastures occurs; streams, reservoirs, or wells are low, and voluntary water restrictions may be requested.

D2 (Severe Drought)

Crop or pasture losses are likely, water shortages become common, and mandatory or standard water restrictions are imposed.

D3 (Extreme Drought)

Major agricultural losses occur alongside widespread water shortages or strict restrictions.

D4 (Exceptional Drought)

The most intense level, causing widespread, exceptional crop/pasture destruction and severe water emergencies in streams, wells, and reservoirs.

The worst droughts in US history.

The four worst droughts in US history were:

• The Dust Bowl (1930s) which covered most of the Western states and lasted at least four years.¹²
• The Texas/Southern Plains Drought (1950-1957) was the worst drought in Texas history and led to many farm foreclosures and bankruptcies.³⁴
• The Midwest Drought (1988-1989) destroyed 30% of the US corn crop, parts of the Mississippi shrank to a “trickle.”⁵
• The Plains/Midwest Drought (2011-2012) where 80% of US agricultural land experienced drought conditions and over 70% of the nation's crop and livestock production sat in moderate or worse drought zones.⁶

Climate change and drought.

Climate change increases the odds of worsening drought in many parts of the United States and the world.⁷⁸ Regions such as the U.S. Southwest, where droughts are expected to get more frequent, intense, and longer lasting, are at particular risk.⁹¹⁰