The U.S. Drought Monitor map is not strictly objective, in the sense that its authors use expert judgement to reconcile what different streams of data are saying about drought. They also incorporate expert local interpretations of data, such as how conditions are affecting people and the environment. But objective data is the backbone of the process, and the mix, or blend, of objective data varies by region and season.
Since the U.S. Drought Monitor went operational in 1999, its authors have followed a similar blueprint for examining data they use to build the weekly map that shows the latest drought conditions across the country and its territories. They study key indices that provide information about precipitation, soil moisture, snowpack and other drought indicators to see where drought conditions could develop or persist, and they compare that information with on-the-ground reports. Even when there were fewer drought indicators, it was a lot to assimilate. So U.S. Drought Monitor authors sought to develop a tool that could help them. The author-led effort, which began during the first U.S. Drought Monitor Forum in 2000, led to the creation of a weekly product that would provide short-term and long-term depictions of drought conditions based on a weighted model that blended several sets of climate division data together. The “objective blends” were born. NOAA’s Climate Prediction Center, which developed the product based on the authors’ input, houses the blends online.
Over the past 22 years, more data have started feeding into the U.S. Drought Monitor process, and the network of observers has grown, leading to a higher resolution depiction of drought conditions across the country. But the objective blend maps remained largely unchanged. This year, the National Drought Mitigation Center is changing that with the release of newly developed short- and long-term objective blend tools.
NDMC climatologist and longtime U.S. Drought Monitor author Brian Fuchs said that the original objective blends served a unique and helpful purpose, in that they provided authors with two maps that they could glance at that showed both short-term and long-term drought conditions across the U.S. With only a few days to synthesize numerous datasets into a single map that shows both short- and long-term drought, the blends provide Drought Monitor authors with a helpful reference to glance at and build upon. But their usefulness diminished over time as the resolution of other data improved and the CPC-housed objective blends remained the same.
“Early on in the process of the objective blends, authors were able to use them as a ‘first guess’ to direct them to areas that may need to be investigated further,” Fuchs said.
The recently released objective blends developed by the NDMC, unlike the CPC originals, are produced with gridded datasets, which Fuchs said produce a detailed picture of drought conditions across the country. The short- and long-term blends are produced with different percentages and temporal scales of drought indices that include the Standardized Precipitation Index, the Standardized Precipitation-Evaporation Index and the NOAH Soil Moisture index. Fuchs said the NDMC’s three U.S. Drought Monitor authors have been using the blends internally for several months to help make the USDM maps and they were shared internally with the full USDM authoring group during the last quarter of 2021. The new blends went public at the start of 2022, and can be found at ndmcblends.unl.edu. Fuchs said there are several pending research questions that will change the makeup of the blends even further, and welcomed feedback and suggestions from the drought monitoring and climatology communities on the new blends.
“With the NDMC’s effort, we hopefully will go back to utilizing the blends [more] as they are providing an idea of areas of drought on both the short- and long-term scales at a finer resolution which could be helpful in digging into the data further in an area,” Fuchs said.
The work associated with the development of the new objective blends was funded by USDA’s Office of the Chief Economist.
-Cory Matteson, NDMC Communications