Did You Know?

Satellite-Based Drought Indicators

The most accurate assessment of drought and climatic conditions can be made by taking measurements of the climatic conditions where the climate is happening — on the ground or in the atmosphere. These are called in situ observations. There are several valuable in situ observational networks around the world that provide crucial data to support weather forecasters, industries and economies, and government and private sector decision makers and policy makers. To get the best information, networks with many stations spaced close together (high spatial density) are needed, but this kind of dense spatial coverage can be expensive. In their high earth orbits, satellites can supplement in situ data by providing consistent observations at high spatial density with global coverage.

Satellites measure energy intensities (radiances) at several wavelengths of the electromagnetic spectrum. This information is useful because everything — the ground, the oceans, the atmosphere, clouds, rain, vegetation, cities, people, etc. — absorbs energy at certain wavelengths and emits energy at other wavelengths. You may be familiar with some of these: visible satellite imagery of clouds showing the movement and strength of storms and fronts; infrared imagery which measures the temperature of clouds and weather systems; water vapor maps generated from a spectroscopic analysis of satellite data.

Two commonly-used satellite-based drought indicators are the Vegetation Health Index (VHI) and Vegetation Drought Response Index (VegDRI).

  • The VHI is a NOAA product that contains several indices derived from polar-orbiting NOAA satellites. The data and images have global coverage and represent average conditions over a 7-day period. The VHI monitors the health of vegetation regardless of the cause. Poor vegetation health, as indicated by the VHI, may be due to stress caused by drought, stress caused by too much water (e.g., flooding), or some other cause (such as insect infestation).
  • The VegDRI product is produced by the National Drought Mitigation Center (NDMC) in collaboration with several other partners. It is a national map covering the contiguous U.S. produced every two weeks and provides regional to sub-county scale information about drought's effects on vegetation. It is a unique hybrid product, in that the VegDRI calculations integrate satellite-based observations of vegetation conditions with in situ climate data and other information such as land cover/land use type, soil characteristics, and ecological setting. The VegDRI monitors the health of vegetation as it is specifically related to drought.