The Vegetation Drought Response Index, or VegDRI, is a bi-weekly depiction of vegetation stress across the contiguous United States. VegDRI is a fine resolution (1-km2) index based on remote sensing data, but unlike other satellite-based measurements, VegDRI also incorporates climate and biophysical data to determine the cause of vegetation stress.
VegDRI began in 2006 with coverage available to only seven states in the Northern Great Plains (CO, KS, MT, NE, ND, SD, and WY). In 2007 that coverage expanded to include eight additional states in the Great Plains and Upper Midwest. VegDRI coverage expanded westward in 2008, and eastward in 2009. It now covers all states in the lower 48.
Development of the VegDRI map and associated products is a joint effort by the National Drought Mitigation Center (NDMC), the U.S. Geological Survey’s National Center for Earth Resources Observation and Science (EROS), and the High Plains Regional Climate Center (HPRCC).
Remote sensing data is combined with climate and biophysical data to create VegDRI. This integrated approach provides benefits over satellite-derived data alone. Multiple factors such as climate, pests, land use change, fire, and extreme weather events can influence vegetation conditions, so including climate and biophysical data helps distinguish stress due to drought.
Two variables related to general vegetation conditions – the Percent Average Seasonal Greenness (PASG) and Start of Season Anomaly (SOSA) – are calculated from satellite-based observations and incorporated into the VegDRI. Both variables are calculated from normalized difference vegetation index (NDVI) data acquired by NOAA’s Advanced Very High Resolution Radiometer (AVHRR). Climate-related variables incorporated into VegDRI include the Palmer Drought Severity Index (PDSI) and the Standardized Precipitation Index (SPI). Information about soils, land cover, land use, and the ecological setting are the types of biophysical data incorporated into VegDRI. This information is critical because the climate-vegetation response can vary depending on these different environmental characteristics.
Evaluating the Map
No single measure can be used to assess the accuracy (both quantitatively and qualitatively) of the VegDRI because of the varying definitions of drought. As a result, a variety of information sources are reviewed to evaluate VegDRI’s performance and accuracy.
Periodic feedback from experts, agricultural producers, and others in the general public are used to characterize the general strengths and weaknesses of VegDRI and highlight specific locations or trends that might be in error.
VegDRI is quantitatively compared USDA crop yield data as well as biophysical and soil moisture measurements. The spatial and temporal patterns in the VegDRI maps are qualitatively compared to drought patterns depicted in the U.S. Drought Monitor maps and to the spatial distribution, type, and frequency of drought impacts being reported in the Drought Impact Reporter.