Monitoring Vegetation Drought Stress

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Drought strikes somewhere in the United States every year, turning green landscapes brown as precipitation falls below normal levels and water supplies dwindle. Drought is typically a temporary climatic abnormality, but it is also an insidious natural hazard. It might last for weeks, months, or years and may have many negative effects. Drought can threaten crops, livestock, and livelihoods, stress wildlife and habitats, and increase wildfire risks and threats to human health.

Corn showing the effects of drought in Texas

Corn showing the effects of drought in Texas. 

(Credit: Bob Nichols, USDA. )

Drought conditions can vary tremendously from place to place and week to week. Accurate drought monitoring is essential to understand a drought’s progression and potential effects, and to provide information necessary to support drought mitigation decisions. It is also crucial in light of climate change where droughts could become more frequent, severe, and persistent.

No two droughts are alike. The complexity of drought phenomena underscores the need for multiple drought monitoring tools. Two drought monitoring and mapping tools developed by the U.S. Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center and the National Drought Mitigation Center (NDMC) are modeled though the integration of satellite-based observations of vegetation conditions, climate data, and other biophysical information. These tools are called the Vegetation Drought Response Index (VegDRI) and the Quick Drought Response Index (QuickDRI). VegDRI was developed with sponsorship from the USGS, the U.S. Department of Agriculture (USDA) Risk Management Agency (RMA) and from the National Aeronautics and Space Administration (NASA). Continued weekly operational production of VegDRI is supported by the USGS. QuickDRI is currently under development with support from NASA and USGS.

Research and methods for Drought Monitoring are developed in tandem with Remote Sensing Phenology.


The Vegetation Drought Response Index (VegDRI) is a weekly national drought tool in operation since May 2009. VegDRI maps portray vegetation conditions as plants respond to solar energy, soil moisture, and other limiting factors. The VegDRI data and models that underpin the relatively-detailed VegDRI maps indicate levels of drought stress on vegetation across the conterminous U.S. at a 1 km spatial resolution. The data are consulted each week by the U.S. Drought Monitor (USDM) map team and many other decision makers.

The VegDRI models incorporate a variety of input data including satellite observations of vegetation condition, traditional climate data, and other biophysical information about land cover, irrigation land use, elevation, and soils. These data provide crucial information on the geographic, physical, and ecological setting where climate conditions are causing drought stress in vegetation. It’s important to note that VegDRI is not an indicator of hydrological drought or low flow conditions in streams or rivers. VegDRI provides information on the condition of the vegetation growing on the land, including grasslands, forests, and agriculture.

Since the 1980s, satellite-based remote sensing has been widely used for many large-area vegetation characterization applications including drought monitoring. The Normalized Difference Vegetation Index (NDVI) has been the most commonly used index for large-area vegetation monitoring. NDVI is a simple, two-band mathematical transformation that capitalizes on reflected energy from vegetation in the visible red and near infrared (NIR) spectral regions, respectively. The NDVI has also been referred to as a measure of vegetation “greenness”. A large body of research has found that NDVI fluctuations over time are strongly linked with climate variations indicating that this index is an effective measure of climate-related vegetation changes.

Satellite-based observations from global imagers such as the Advanced Very High Resolution Radiometer (AVHRR), Medium Resolution Imaging Spectrometer (MERIS), Moderate Resolution Imaging Spectroradiometer (MODIS), and SPOT (Satellite Pour l’Observation de la Terra) Vegetation instruments are a source of near-daily, continuous coverage useful for monitoring vegetation drought stress. These data complement point-based weather station observations that have been used to generate traditional, climate-based drought indices such as the Palmer Drought Severity Index (PDSI) and the Standardized Precipitation Index (SPI). Both weather station observations and satellite-based data are powerful sources for describing drought conditions and impacts that become more powerful when merged together in VegDRI.

VegDRI has a moderately long (that is, seasonal) time horizon for characterizing drought conditions, and consequently is not as sensitive to short-term drying accompanied by heat waves in the climate signal. A second index, called QuickDRI, is designed to be more sensitive to emerging or rapidly changing drought events, sometimes called “flash” droughts.


Rapid-onset flash drought events can have devastating impacts on agriculture, natural resources, and the economy. The Quick Drought Response Index (QuickDRI) is our newest operational weekly drought indicator developed by a team including researchers at the NDMC and CALMIT at the University of Nebraska - Lincoln, the USGS EROS Center, the USDA, and NASA.

QuickDRI was designed to fill a critical need in operational drought monitoring for a tool that is sensitive to early onset and rapidly evolving drought conditions. QuickDRI incorporates multiple remote sensing- and model-based input variables that portray key components of the hydrologic cycle influencing drought-related vegetation stress (evapotranspiration [ET], soil moisture, and vegetation index-based plant health). These variables are integrated with traditional precipitation- and temperature-based climate indices and other general environmental descriptors (land use/land cover, soil water holding capacity, elevation, and irrigation status) to map and monitor early-stage drought and rapid-onset, known as "flash droughts."