DROUGHT: Degrees of Drought Reveal the True Picture
Scientists often refer to drought as a “creeping disaster” because its exact onset and end often cannot be identified until long after the event has come and gone. This presents a great challenge for forecasters, planners, and resource managers at all levels. To address that challenge and more easily describe conditions for the public, the U.S. Drought Monitor (USDM) established a numeric drought classification scale similar to the Fujita Scale for tornadoes and the Saffir–Simpson scale for hurricanes. This numeric scale allows scientists to convert a large volume of data into a number that represents a measure of severity, while also offering “apple to apple” comparisons over time.
The USDM bases its intensity scale on drought impact guidelines, indicators, and index and model thresholds. A percentile ranking approach serves as the objective scientific backbone allowing the weekly report’s rotating authors to compare different parameters having different units and lengths of record regardless of location. In addition, the scientists also take into account the duration and both regional and seasonal influences, as well as whether a given location is improving or getting worse in terms of drought condition.
The USDM’s drought intensity scale is composed of five different levels: D0, D1, D2, D3, and D4. The abnormally dry category, D0, corresponds to an area experiencing short-term dryness that is typical with the onset of drought. This type of dryness can slow crop growth and elevate fire risk to above average. This level also refers to areas coming out of drought, which have lingering water deficits and pastures or crops that have not fully recovered. It is also important to note that scientists only consider D0 level areas to be dry, and they are not necessarily experiencing drought conditions.
The moderate drought category, D1, corresponds to an area where damage to crops and pastures can be expected and where fire risk is high, while stream, reservoir, or well levels are low. The severe drought category, D2, corresponds to an area where crop or pasture losses are likely, fire risk is very high, water shortages are common, and water restrictions are typically voluntary or mandated. The extreme drought category, D3, corresponds to an area where major crop and pasture losses are common, fire risk is extreme, and widespread water shortages can be expected requiring restrictions. The highest category, exceptional drought, or D4, corresponds to an area experiencing exceptional and widespread crop and pasture losses, fire risk, and water shortages that result in water emergencies.
The USDM’s weekly report uses this classification scale in combination with a color-coded map to provide a tool for decision making and drought planning. It also plays a key role in heightening awareness of drought as a hazard through dissemination by various media sources and state and federal agencies. Linking indices, such as the USDM, to impacts also allows decision makers to subsequently develop threshold alerts for communities to take action in response to drought conditions.
This is part of a series of articles about monitoring and assessing drought conditions across the United States. Check back next week to learn more about monitoring the impacts of drought or check out the importance of drought indicators.