EROS Teams Step in to Fill Gap in Drought Monitoring Data Stream

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Each week, the USGS Earth Resources Observation and Science (EROS) Center releases VegDRI and QuickDRI maps that show the moisture content of the Nation’s vegetation. Those maps help build the U.S. Drought Monitor map, which in turn helps guide agriculture decisions and policy across the U.S.

Color photo of vegetation health maps from VegDRI project

Comparison of Vegetation Drought Response Index (VegDRI) results from EROS expedited Moderate Resolution Imaging Spectroradiometer (eMODIS) (a) and transformed EROS expedited Visible Infrared Imaging Radiometer Suite (eVIIRS') (b) for week 32, 2020 (August 3, 2020 to August 9, 2020). (c) relationship between eMODIS and eVIIRS’ VegDRI values, associated statistics are presented in Table 2, week 32 column. (d) VegDRI classification differences pixel by pixel.

EROS delivers crop health and evapotranspiration (ET) data to the Famine Early Warning Systems Network (FEWS NET), which uses them to inform food insecurity alerts and direct aid in developing countries.

It’s no stretch to say that those monitoring efforts have an impact on the lives of millions of people.

Which is why the impending loss of the satellites long used by EROS to collect those data points has been such an important topic at EROS in recent years. When NASA’s Terra and Aqua satellites reach their end in 2022 after nearly two decades of service, the daily stream of imagery from the Moderate Resolution Imaging Spectroradiometer (MODIS) will cease to flow.

“Aqua is going to get to a place where there just won’t be enough fuel to keep it going,” said Mike Budde, an EROS geographer and technical lead for FEWS NET’s vegetation monitoring. 

Now, research teams at EROS have developed a tool from a newer satellite source that will allow the EROS-based work to continue without pause and extend the historical record of crop and vegetation health in the U.S. and across the globe.

The data stream, called the expedited Visible Infrared Imaging Radiometer Suite (eVIIRS), is built from Earth observation data acquired by the VIIRS instrument onboard the Suomi National Polar-orbiting Partnership (S-NPP) satellite, a NASA-built spacecraft operated by the National Oceanic and Atmospheric Administration (NOAA) since 2011.

The eVIIRS data, which is now available through EarthExplorer, will fill the MODIS void for VegDRI, QuickDRI, and FEWS NET, and could do the same for other satellite-based vegetation monitoring projects. Research Geographer Jesslyn Brown, who led VegDRI and QuickDRI development at EROS, expects that the scientists who’ve come to rely on the EROS-modified version of MODIS (eMODIS) will benefit from eVIIRS well into the future.

“We anticipate that a similar user community will be adopting this, because everyone’s going to be facing the fact that MODIS is going away,” Brown said.

eMODIS: A User-Friendly Source of Satellite Imagery

MODIS data, which covers the entire planet every day, didn’t fit squarely into EROS’ drought monitoring needs at first.

What those operational programs needed from any research-grade satellite, Brown explained, was a source of Normalized Difference Vegetation Index (NDVI) data—data that serve as a stand-in for plant greenness—that would be available quickly, reliably, and frequently.

color graphic of vegetation health in Somalia

eMODIS NDVI for Somalia as a percent of 2003-2017 median: October 1-10, 2020 (left) and February 11-20, 2021 (right). eMODIS data from the USGS Earth Resources Observation and Science (EROS) Center informs food secruity reports for the Famine Early Warning Systems Network (FEWS NET).

VegDRI, for example, needed a regular seven-day NDVI composite, a sort of rolling daily average that smooths out the interference from clouds and other noise that might appear in any given daily observation, at MODIS’ highest resolution—250 meters per pixel. NASA offered composites, just not quickly or frequently enough for the monitoring job, and at 500 meters or 1 kilometer, not 250 meters.

That’s why EROS teams created the eMODIS data stream in 2010, and it’s proven quite popular. The initial eMODIS publication has been cited more than 100 times in peer-reviewed literature since 2010, and eMODIS products have been downloaded 320,173 times since their addition to EarthExplorer in 2012.

For VegDRI and QuickDRI, the ever-ready source of 7-day NDVI composites made it possible for Brown to meet her weekly deadlines for the U.S. Drought Monitor.

“Our requirement is to get them something as fresh as possible each week, as quickly as possible, so they can make their map and publish it every Thursday,” Brown said. “It’s a weekly publication, so they need the data in hand each week by Monday.”

For FEWS NET, NDVI data “has been the standard workhorse” since the 1980s, said EROS geographer James Rowland. FEWS NET was interested in MODIS early on, as its NDVI would offer higher resolution than the 1-kilometer datasets it had used in the program’s early years. A finer resolution would help pinpoint the anomalies in NDVI that can signal potential food insecurity with more detail.

“What we want to know is what the anomaly looks like,” Rowland said. “Where’s it worse than normal? Where is it less than normal? That’s the importance to FEWS.”

As with VegDRI and QuickDRI, however, FEWS NET needed to be sure it could report its metrics at regular intervals—dekadal (10-day) intervals, in this case. The eMODIS product made that possible.

Beyond NDVI, eMODIS surface temperature data were “a game changer,” Budde said, enabling the production of the ET estimates that now help FEWS NET track water use around the world. Since their initial development for FEWS NET, satellite-based ET estimates have become a huge part of remote sensing-based water use research in the U.S.

“It was developed for use in Afghanistan, then brought to the U.S. to use. It’s really flourished in the U.S., the Western U.S. especially,” Budde said.

Agencies outside the USGS benefit from the reliability of eMODIS as well. The U.S. Forest Service, for example, uses eMODIS in a vegetation tracking system called ForWarn, a project that earned the Federal Laboratory Consortium for Technology Transfer’s (FLC) Interagency Partnership Award in 2013.

William Hargrove, the Forest Service's lead researcher for ForWarn, credited the data teams at EROS for taking the initiative to produce a data stream that's proven to be such a reliable and consistent resource.

“I really think that eMODIS was a great example of a real case where there was an unfilled need, and it was the people in the trenches who saw that need and acted on it,” Hargrove said. “The ForWarn project completely depended on eMODIS for many, many years, and they never let us down.”

Preparing for a post-MODIS World

An operational, VIIRS-based update for MODIS has been in the works at EROS for nearly two years. The goal was to produce eMODIS-like NDVI composites using the similar data source and allow projects that needed them to continue when Terra and Aqua were no longer available.

The first step was to create a low-latency eVIIRS product. The next was to apply a transformative algorithm to eVIIRS that creates NDVI composites that mirror the eMODIS versions used by EROS’ drought monitoring projects.

As with eMODIS, EROS-produced NDVI composites from VIIRS will come with a faster turnaround time and have a higher resolution—375 meters per pixel—than the standard composites available from NASA through the Land Processes Distributed Active Archive Center (LP DAAC). The replacement comes with a slight loss in resolution compared to MODIS’ 250-meter data, but the accuracy of the final product is nearly indistinguishable.

“If you look at the comparisons in the VegDRI product, they’re visually identical,” said Trenton Benedict, an EROS contractor and lead author on an eVIIRS research paper. “There are some differences here and there, but you have to dive in pretty deeply to see them on the pixel level.”

What’s more, the compatibility of eVIIRS with eMODIS means “we’re looking at having a longer historical record,” Benedict said, as the old MODIS data can be folded into the new eVIIRS product to establish baselines that reach back to the 1999 launch of the first MODIS sensor.

Hargrove expects that eVIIRS will be of interest to researchers whose applications previously relied on data from the MODIS sensor.

“I think the EROS Center and the eVIIRS project are very forward-thinking, Hargrove said. “It’s another example of scientific leadership. I think people will be looking around to see what can suit their needs (after MODIS), and this will be there.”

eVIIRS (EROS Visible Infrared Imaging Radiometer Suite)

eVIIRS (EROS Visible Infrared Imaging Radiometer Suite) CONUS Sub-Sample

eVIIRS is already used to produce weekly VegDRI and QuickDRI maps, a swap-over hastened by an anomaly on the Aqua satellite in August of 2020.

The unbroken record made possible by eVIIRS is especially important to VegDRI and QuickDRI users who study drought in more far-flung parts of the U.S., Brown said.

“For the remote areas of the U.S., VegDRI and QuickDRI become very important—those underserved communities, where there are fewer people and there might not be a long-term weather station there providing detailed drought information,” Brown said.

The next step for eVIIRS NDVI will be transforming it to fit the operational needs of FEWS NET, a step Budde and Rowland foresee taking place by early 2022.

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