Researchers: Eric E. Small, Carolyn J. Roesler, and Kristine M. Larson, University of Colorado, Boulder
Written by Linda Rowan
8 August 2018
The health of wild vegetation during the California drought of 2012 to 2014 was measured with microwave-wavelength reflected Global Positioning System (GPS) measurements from hundreds of sites that are part of the EarthScope Plate Boundary Observatory. The geodetic data is compared to optical-wavelength measurements at collocated sites. The observations and comparisons show that three years of drought stunted growth and reduced the growing season with the late precipitation in the third year of the drought being particularly devastating to vegetation. These methods can help gauge future conditions and inform environmental impacts on humans and nature.
Natural vegetation, such as herbaceous ground cover and shrubs, are affected by prolonged droughts. Remote sensing, with extensive spatial and temporal resolution, can help to measure vegetation health as weather and climate changes to understand these affects. A relatively new and ingenious approach is to take advantage of the dense network of ground-based GPS sites in places like California to measure microwave-based estimates of the water content in ground-hugging vegetation. The technique uses the satellite signal that reflects from the ground into the GPS antenna. This reflected signal is sensitive to the amount of water in the vegetation and other factors such as soil moisture and vegetation roughness. The GPS interferometric reflectometry (IR) technique can measure water in vegetation in an area of about one hectare on a daily basis. In addition, the microwave-wavelength measurements called normalized microwave reflection index (NMRI) are not limited by weather such as clouds, so they can measure changes continuously, especially as the weather changes.
Optical-wavelength measurements of vegetation health can cover a wider area over a different temporal scale than GPS-IR. Such measurements determine plant greenness, called the normalized difference vegetation index (NDVI). The index measures the plant’s ability to absorb photosynthetically active radiation. There are other techniques to measure vegetation health, but the GPS-IR and optical are the most comparable and are used here to see how the California drought affected natural vegetation.
A comparison of measurements over ten years, shows that the greenness (optical NDVI) begins to increase earlier in the spring, peaks sooner and extends for a longer duration than the wetness (microwave NMRI) of the natural vegetation. As the major California drought of 2012 to 2014 begins, the greenness and wetness decrease as the rate and amount of precipitation decreases. The vegetation growth is stunted by the drought. Above average growth is observed when above average precipitation occurs from 4 to 12 months before the growing season. The two different data sets provide information at different spatial and temporal scales for tracking and understanding vegetation health related to varying precipitation rates. The new geodetic measurements add important details about water uptake in the vegetation and takes advantage of existing infrastructure for these unique observations of important environmental conditions.
Eric E. Small, Carolyn J. Roesler, and Kristine M. Larson, Vegetation Response to the 2012-2014 California Drought from GPS and Optical Measurements, Remote Sens. 2018, 10, 630, doi:10.3390/rs10040630.
vegetation health, precipitation, drought, reflectometry
Send questions or comments about this page to scienceunavco.org.
Last modified: 2020-01-28 22:16:20 America/Denver