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What is drought, and how is it measured? 
Anywhere that has water can experience drought
September 29, 2023

Drought is something most places on earth will experience at some point: Anywhere that has water can have drought. But what makes a dry period a drought? It’s about the balance of moisture in a given place, driven by the supply and demand of water.

What is drought?

Drought is a temporary state of dryness that represents an aberration from the historical norm. Drought can result from low supply of precipitation (less rain and snow) as well as high demand for moisture, resulting from high evaporation due to high temperatures, or more sunlight.

What causes drought?

Every place has unique patterns of supply and demand of water that keep local moisture levels within a range that plants, animals, and even people are used to.

Low supply—resulting from less precipitation—is usually the main driver of drought. Because all local plant and animal life is adapted to a certain amount of precipitation, when that amount drops, the demand for moisture is greater than the supply. High demand can also play an important role in driving drought. If high temperatures and increased evaporation, as well as sunny and windy conditions, lead to extra demand for moisture, supply won’t be able to keep up. Both these scenarios cause the moisture deficit that can initiate a drought. 

Drought only ends once that moisture balance is restored. A single rainstorm after a dry period typically does not end a drought: Depending on the stage and severity of the drought, it may take a lengthy period after a return to local precipitation patterns to bring levels back to normal. 

How is drought measured?

Because drought is a local phenomenon, it’s measured against local historical moisture levels. To detect a drought, scientists calculate local moisture levels and compare that with the norms for the area to look for a deficit. How badly a drought impacts a locality depends on the size of that moisture deficit, as well as the duration of the drought. The two most common ways to measure drought are drought type and drought severity.

Types of drought

As drought progresses in an ecosystem, it will affect the moisture levels wherever moisture is stored. Drought types are categories that tell you where drought is manifesting. 

The first type of drought is meteorological drought, where weather upsets the balance of water supply and demand.

As a meteorological drought persists, moisture levels in the soil decrease. If these levels drop to a critical point, the drought has progressed into ecological (or soil moisture) drought. 

When ecological drought continues long enough with sufficient severity, water levels in rivers, lakes, and groundwater can deplete. When these water sources are significantly drier than usual, the drought has progressed into hydrological drought.

Severity of drought

While drought type is a category that tells you where drought is impacting an ecosystem, drought severity is a scale that tells you how dry the drought is.

Drought severity is calculated by measuring current moisture levels and comparing those levels to the norm for that region. The difference between them tells us which drought severity category the region is in and the impacts we can expect to see on its ecosystem. 

The U.S. Drought Monitor (USDM) labels areas in drought by severity, alongside likely impacts. D0 is the lowest severity and D4 is the highest severity.

Drought severity is different from the stage of drought: It describes the impacts of the drought. This is a scale from the U.S. Drought Monitor used to classify and describe drought severity. Source: The U.S. Drought Monitor

Here’s a breakdown of what happens in each drought severity category:

  • D0 Abnormally dry. When the region is going into drought, it will experience short-term dryness that slows planting and growth of crops and pastures. Coming out of drought, there will be some lingering water deficits. Pastures and crop growth may not be back to regional norms.
  • D1 Moderate drought. There will be some damage to crops and pastures. Streams, reservoirs, or wells are low; water shortages may be developing or are imminent; and voluntary water use restrictions are often requested of the community.
  • D2 Severe drought. Crop or pasture loss is likely; water shortages are common; and water restrictions are required in the community. 
  • D3 Extreme drought. There are major crop and pasture losses and widespread water shortages or restrictions.
  • D4 Exceptional drought. There are exceptional and widespread crop and pasture losses. Shortages of water in reservoirs, streams, and wells create water emergencies.

Drought type and severity are not always linked because the makeup of the region, such as vegetation or bodies of water, plays a role in determining drought type, while drought severity is just a measure of dryness.

How long can drought last? 

Drought can last for weeks, months, or years. The longer a drought lasts, the more damaging it can be—in part because when patterns start to vary from the norm, they can reinforce themselves, worsening dry conditions. 

For example, imagine a region sees less rainfall than usual, and the local forests dry out. These abnormally dry forests transpire less moisture, leading to air that produces minimal rain or snow. With less precipitation, the forests dry out further. If the forested region continues to get drier and drier, the land itself might change, becoming too dry to absorb precipitation and losing the trees that previously would have transpired moisture into the air. 

What if drought doesn’t end?

If it continues long enough, drought can irreversibly alter the water cycle, soil, and ecosystem of a place. Since drought is by definition temporary, the transition of a region into permanent dryness goes by a different name: aridification. 

Aridification is the long-term transition of a region into a permanently drier, more arid condition, as measured by decreasing moisture levels. In some places, climate change is leading to aridification by creating hotter, drier conditions that decrease precipitation, increase evaporation, and deplete groundwater. Scientists believe any region’s aridification is likely irreversible, at least on human timescales.

This map depicts the annual likelihood in percentage of year-plus extreme drought at two different degrees of warming.

Climate change and drought

Climate change is increasing the frequency, intensity, and duration of droughts. Warmer air temperatures accelerate evaporation, drying out soil, rivers, and lakes, and even pulling water from the leaves of plants. This warmer air also holds more moisture—7% more per degree Celsius increase in temperature—meaning precipitation like rain or snow falls less frequently, and when it does, the parched soil cannot absorb the deluge. In the example of snowfall, higher temperatures also melt mountain snowpacks earlier and more quickly than usual, shortening the period in which plants and soil benefit from the runoff.

Want to see how drought type and severity could change in places you care about? Read Part 3: Land in our climate handbook to learn about the future of drought.