What is the U.S. Drought Monitor (USDM)?
The USDM is a national map that depicts drought severity and extent across the entire Country. It is a blended drought index that uses multiple data types to capture and depict all types of drought (such as meteorological, agricultural, hydrological, socioeconomic, and ecological). This means that while it is easy to see where drought exists in the United States and how severe it is, it may be difficult to understand why the USDM depicting drought conditions.
Who Creates the USDM?
There are multiple USDM authors, all of which belong to one of three agencies: the National Drought Mitigation Center (NDMC), the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Department of Agriculture (USDA). Each map is created by one USDM author, as authorship usually changes every one to two weeks.
What is the USDM Process?
Each week one of the authors updates the map, based on changes to the data. Since the USDM is a blended index, it looks at multiple data types. Along with the data, the USDM also incorporates input from local experts around the country to put the context to the data for any needed changes to the map. This use of input from local experts is a unique feature of the USDM.
Typically, the author starts to look at data on Monday, taking in recommendations from local experts and making changes as information comes in. During this process, the authors send out multiple drafts with changes so local experts and USDM contributors can suggest further changes. Recommended changes to the map are taken until noon on Wednesdays. Thursday morning is when the final map is released to the public.
What data are used for the USDM?
Multiple types of data are used as inputs for the USDM as the map depicts multiple types of drought. The data includes precipitation, soil moisture, vegetation health, temperature, streamflows, groundwater, evapotranspiration, and impact and condition reports. With each location being unique in terms of expected precipitation, soil composition and water holding capacity, and surface and groundwater availability, data used in the USDM is in the form of percentiles. This allows each data point to be ranked from 1 to 100 (1 being the driest and 100 being the wettest), which provide insight to how the current data compares to the historical past for a given location.
What are the USDM drought categories and how do they work?
The USDM has six categories. Two are non-drought categories: normal and abnormally dry (D0), and four are drought categories: moderate (D1), severe (D2), extreme (D3), and exceptional (D4). Since there are multiple types of data that are used in the drought monitor, a convergence of evidence approach is used to determine a location’s USDM category. This means that one data type or one indicator cannot drive the process. Rather, multiple data points and types of data need to show similar conditions to get a specific drought designation. Generally, a higher number of indicators need to converge for more severe USDM category designations.
The six categories of the USDM are based on percentile ranges. Normal is anything above the 30th percentile. Abnormally dry (D0) is the 21st-30th percentile range, moderate drought (D1) is the 11th-20th percentile range, severe drought (D2) is the 6th-10th percentile range, extreme drought (D3) is the 3rd-5th percentile range, and exceptional drought (D4) is the 1st-2nd percentile range. As an example, a D4 designation on the USDM in a location that is experiencing either the driest or second driest conditions on record across multiple data types and indicators.
It takes time for a location to change from one USDM category to the next as it takes time for data to change in the percentile rankings. It also takes time for multiple types of data to change in the percentile rankings. For these reasons, and because the map is updated weekly, a location can only change by one USDM category each week. Also, because of the nature of drought, it is unlikely that drought category will degrade or fluctuate for consecutive weeks.
For more information about the USDM as a national drought product, please visit
How does South Carolina participate in the USDM process?
Drought monitoring in the state is the responsibility of the SC SCO, and thus participates in the USDM process every week. Since drought affects multiple sectors, the SC SCO collaborates with multiple state and federal partners within South Carolina to provide recommendations to the USDM. These partners include the S.C. Department of Agriculture (SCDA), Clemson Extension, SC DNR Hydrology, the National Weather Service (NWS), the Farm Service Agency (FSA) and the Natural Resources Conservation Service (NRCS) (both within the U.S. Department of Agriculture), and U.S. Geological Survey (USGS).
Since South Carolina shares river basins with both of our neighbors, North Carolina and Georgia, the SC SCO communicates weekly with their respective USDM collaborators so that conditions match up across state lines, as drought rarely follows political borders.
After collaborating with our partners within South Carolina and discussing conditions with our neighbors, the SC SCO sends the state’s recommendations to the USDM author. The SC SCO typically sends recommendations to the USDM author on Tuesday afternoons. This process of multi-partner and multi-state collaboration is conducted every week.
It is important to note that while USDM is collaborative process with local input, the USDM author always gets the final decision on what the map will depict each week. Therefore, while the SC SCO provides input, the USDM author can reject those changes if he or she feels differently. While this occurrence is seldom, it has happened before and is a possibility in the future.