The U.S. Drought Monitor (USDM) is widely used to show where and how extensive drought conditions are across the country. For those interested in agriculture production, it is a widely anticipated and used map to help determine not only conditions in our own specific region, but how other areas of the country are doing as well. But how is the USDM determined? And how much rainfall will it take to get out of it? Those questions are not as easy to answer as we may think.

HOW THE US DROUGHT MONITOR IS PRODUCED

The USDM is a collaboration of authors from the National Drought Mitigation Center, NOAA and USDA. Together, these authors take a wide variety of information to create a single map of where drought exists and how intense it is. It is released every Thursday morning with data up until the Tuesday before the release. This gives authors two days to assess the information and draw drought classifications.

There are six classifications on the map: one for no drought, four categories of drought intensity showing moderate (D1), severe (D2), extreme (D3), and exceptional (D4) drought, and one category to indicate abnormally dry conditions (D0), where an area may be going into or out of drought. They also use the letters S (short-term impacts) and L (long-term impacts) on the map to help describe both how long drought has been an issue in an area and which sectors may be more affected by the drought. Short-term impacts are generally used for agriculture and surface-water interests while long-term impacts are generally ecological or socio-economical sectors that are more sensitive to ground water and reservoir levels.

Factors that go into determining the classifications are wide-ranging but come from a convergence of evidence in these categories: precipitation, streamflow, reservoir levels, temperature and evaporative demand, soil moisture, vegetation health, impact reports from media outlets and private citizens, and the input from a network of more than 450 experts. Sometimes these factors are at odds and it is up to the author to determine the overall impact drought may have in the area. And that may be different depending on the area of the country being considered. For example, mountain snowpack has more of an influence in the West than it does for the East. Authors try to get the best picture available and produce a map that can be a tool in helping make decisions.

IMPROVEMENT VERSUS DEGRADATION: HOW IS DROUGHT DETERMINED?

As noted above, more than just rainfall is used to classify drought and not a single piece of data can tell the whole story. Authors use a "convergence of evidence" approach to pick what drought level makes the most sense. Therefore, it's not easy to look at a rainfall map during the previous week and see where drought areas have improved or degraded.

Even a heavy rainfall event may not make a difference. A 4- to 6-inch rainfall event may sound like a lot, but not if it comes in a heavy thunderstorm where much of the rain runs off and into local streams and rivers. Soil moisture may not improve much if the rain does not soak in. Conversely, areas that have not seen a drop of rain in more than a month may not be in drought either, because of recent snowmelt boosting soil moisture, or cool conditions that have not allowed much evapotranspiration. That is why the impacts of drought are considered as well, a factor that cannot be easily quantified.

Though some on social media are eager to point out rainfall deficits, there is no specific amount of rainfall to get out of a drought. Many other factors must be considered and weighed.

MOMENTUM MORE IMPORTANT THAN CLASSIFICATION

The USDM map is important, but the momentum of the changes is probably more important, especially for agricultural uses. A farm in eastern Arkansas may be in D2 drought, but it matters more if the situation is improving (moving down from D3 drought), or degrading (moving up from D1 drought). Therefore, the change maps on the USDM website are extremely helpful in determining the direction the drought is moving. Comments from the authors attached to the map are also helpful to find areas that may see improvement or degradation. And other sources of information may be best to supplement the map to help determine if the situation is improving or degrading.

LIMITATIONS OF DROUGHT MONITOR

Though extremely useful, the USDM does have limitations. Drought is a lagging indicator, a snapshot of the conditions during the previous weeks, months and sometimes years. That can give a false impression of the drought situation depending on how someone uses it. A farmer in central Illinois may wonder why his area is still labeled in a drought classification after a recent 6-inch rainfall event where his fields were flooded, even though his long-term rainfall deficits are large and his crop is still suffering from the previous dryness. Conversely, another farmer in eastern Nebraska may wonder why there is no drought indicated for her area even though her crop is starting to wither from extreme heat and lack of soil moisture. Rapidly changing conditions do not show up as quickly on the USDM.

The map is only as good as the data coming in. Precipitation is a highly variable parameter; radar and satellites may not be able to get precipitation down to field level, especially in some extremely rural parts of the country. Soil moisture data is largely modeled or inferred by satellites, which may be blocked by extensive cloud cover and only get the top layers of soil to use. Deep soil moisture is notoriously difficult to determine and is also highly variable based on soil type, which can lead to errors.

The map itself is not a declaration of drought, but only a tool to use to help determine impacts and make decisions. It may be useful to look at more maps and data than just the USDM to help determine the overall situation as it stands. Also, weather forecasts going forward may lessen or intensify the impacts of drought.

Although the authors do a good job of staying consistent between themselves, the USDM is authored by humans and is subject to the biases of those humans -- like any other forecast. In general, with some exceptions, authors change classifications by one category every week. However, conditions may change more rapidly than the authors determine. Some input factors may be considered higher than they should be, or there is not an expert near your location that can confirm the recent changes in the precipitation or soil moisture impacts in time before the map is published. All of that can lead to a misclassification at the point in time you look at it.

But, despite some limitations and biases, the USDM is a very good tool for determining drought and impacts.

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John Baranick can be reached at john.baranick@dtn.com

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