U.S. Drought Monitor

Overview

Established in 1999, the US Drought Monitor (USDM) is a weekly report and map series that displays current drought conditions on a country-wide, state-wide, and regional scale. The USDM is generated by a collaboration of scientists from the National Drought Mitigation Center at the University of Nebraska-Lincoln, the United States Department of Agriculture, and the National Oceanic and Atmospheric Administration. The USDM provides an archive of weekly maps, drought summaries, and key data sets. “The map is based on measurements of climatic, hydrologic and soil conditions as well as reported impacts and observations from more than 350 contributors around the country. Eleven climatologists from the partner organizations take turns serving as the lead author each week. The authors examine all the data and use their best judgment to reconcile any differences among sources.” (US Drought Monitor website)

Below is an example of what the US Drought Monitor might look like.

To view a more comprehensive overview of the USDM, you can view their tutorials: US Drought Monitor Tutorial, US Drought Monitor Webinar Training, and Drought Impact Reporter Tutorial.

What it will tell you

Provides a “big picture” look at drought in the US and should not be used to infer local conditions. While the USDM uses information collected from local entities, it does not necessarily represent all areas accurately. It should be used to identify likely areas of drought impacts over large areas.

In order to better understand changing drought conditions, the USDM provides several ways to make comparisons between current and past conditions: 1) side-by-side comparison, 2) change map, 3) comparison slider, and 4) animated maps. Custom maps can also be requested. The USDM also offers a wide range of options for raw data comparison: 1) standard data tables and data downloads, 2) GIS Files, 3) Metadata, and 4) Time Series. The Farm Service Agency Eligibility Tool is also available for farmers and ranchers to assess their eligibility for financial aid during drought.

Definitions

Drought– while there are rmany different defitions of drought, a basic one would be a moisture deficit bad enough to have social, environmental, or economic effects. There are also multiple types of drought including meteorological, hydrological, agricultural, and socioeconomic. Learn more about the types of drought here.

Indices- an indicator, sign, or measure of something. For example, the results from the indices are a measure of drought conditions.

Summary Map– Identifies general areas of drought and labels them by intensity from D1 (least) to D4 (most). D0 areas are not in drought, but are either experiencing abnormally dry conditions leading toward drought or recovering from drought and are not yet back to normal. Also indicate whether effects will be short (S) (<6months, agricultural and grasslands) or long (L) term (+6months, Hydrology, ecology).

Drought Severity Classification Index (DSCI) – experimental method for converting drought levels from the map to a single value for an area. 1(D0) + 2(D1) + 3(D2) + 4(D3) + 5(D4) = DSCI

Statistics Explanation

Drought Intensity-Drought is quantified through a number of factors including precipitation, wind, streamflow, and soil moisture measurements. Then it is categorized into drought intensity designations based on percentiles, e.g. D2, D3, D4. For example, D4 drought occurs when precipitation, streamflow, and soil moisture measurements are at percentiles of 0-2, meaning that these are extremely low measurements and should only occur 0-2 times in 100 years.

Short and Long Term Indicators– Short-term drought indicator blends focus on 1-3 month of precipitation(<6months, agriculture and grasslands). Long-term blends focus on 6-60 months (+6months, Hydrology, ecology).

Additional indices used in the USDM, mainly during the growing season, include the USDA/NASS Topsoil Moisture, Keetch-Byram Drought Index (KBDI), and NOAA/NESDIS satellite Vegetation Health Indices.  Indices used primarily during the snow season and in the West include snow water content, river basin precipitation, and the Surface Water Supply Index (SWSI). Other indicators include groundwater levels, reservoir storage, and pasture/range conditions.

USDA/NASS Topsoil Moisture– The moisture of the top level of the soil which is defined as the top six inches.

Keetch-Byram Drought Index (KBDI)– A continuous index specifically for fire potential assessment. Its number represents the net effect of evapotranspiration and precipitation in producing cumulative moisture deficiency in deep duff and upper soil layers. I.e. the flammability of organic matter in the ground.

NOAA/NESDIS satellite Vegetation Health Indices– derived from radiance observed by a satellite. The results can be used as proxy data for monitoring vegetation health, drought, soil saturation, moisture and thermal conditions, fire risk, greenness of vegetation cover, crop and pasture productivity, etc.

Snow Water Content or Snow Water Equivalent (SWE)– The amount of water contained within the snowpack or the depth of water that would happen if the entire snowpack was melted instantaneously. “For example, say there is a swimming pool that is filled with 36 inches of new powdery snow at 10% snow water density. If you could turn all the snow into water magically, you would be left with a pool of water 3.6 inches deep. In this case, the SWE of your snowpack would equal 36″ x 0.10 = 3.6 inches.” (NRCS Website)

Surface Water Supply Index (SWSI)– A predictive indicator designed for places that rely on snow melt as the primary source of surface water and takes into account snowpack, precipitation, streamflow, reservoir storage, and soil moisture to create an idea of total surface water availability for spring and summer. The SWSI is calculated by combining pre- runoff reservoir storage with forecasts of spring and summer streamflow. The values are then scaled from +4.1 (abundant) to -4.1 (extremely dry) with zero indicating median water supply as compared to historical analysis. The benefit of this index is being able to compare it to similar years to help determine strategies for dealing with current years water supply.

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