AVHRR data
The Advanced Very High Resolution Radiometer (AVHRR), which orbits the
earth on NOAA meteorological satellites, is commonly used for monitoring
vegetation over large areas such as the Mid-Atlantic region of the United
States. The data collected by the AVHRR are often used to calculate an
indicator of vegetation condition known as the normalized difference
vegetation index (NDVI), which varies in range from -1 to 1. High
values (e.g., 0.6) are associated with green healthy vegetation, and
low values (e.g., 0.1) are associated with sparse or environmentally
stressed vegetation.
The NDVI can be used as an indirect indicator of crop growth, or net
primary production (NPP), which is the total production of biomass by
plants, less the cost of their maintenance. In other words, it is a
measure of carbon accumulation. Figure 1 shows the relationship of
NDVI (as measured by AVHRR) to NPP over the Mid-Atlantic. High NPP
levels are associated with high NDVI values, and vice versa.
Year-to-year rainfall differences impact NPP, and can be monitored
using satellite data. Figure 2 illustrates interannual variability in
NPP/NDVI that is characteristic of the Mid-Atlantic. The plot shows
the change in NPP against the change in NDVI for a drought year (1988)
compared to a normal year (1985). Large negative changes in NDVI due
to drought effects are associated with large negative changes in NPP.
That is, the productivity of the vegetation decreases as it declines
in amount and/or health.
The scatter of points about the general trend in Figures 1 and 2 indicates that factors in addition to the NDVI at one time in July influence NPP. As indicated above, the NDVI measures the amount of healthy, green vegetation. More specifically it measures the amount of sunlight that the leaves absorb for photosynthesis. However this absorbed energy may not be fully used, or may be used by the plant for its own metabolic processes, rather than contributing to the accumulation of carbon, which is what NPP measures. More formally, NPP depends on the following:
- Light absorption for the energy demanding processes of photosynthesis (NDVI and incident solar radiation are needed to estimate this over large areas such as the Mid-Atlantic)
- Soil moisture, the amount of water vapor in the air and the air temperature, all of which determine the plant's ability to take in carbon dioxide, the raw material of plant matter.
- The amount of the vegetation that is living, but not green and producing new biomass. This component (stems, trunks, roots etc) accounts for a significant proportion of the photosynthetic energy and results in a return of carbon dioxide to the atmosphere.
- The length of the growing season. In the Mid-Atlantic region, this is longest for evergreen trees and shortest for some spring crops like wheat. It is beat estimated by making NDVI measurements at frequent intervals throughout the year.
Thus we would not expect the NDVI for one short period in July to perfectly predict the NPP for the whole growing season, and this is the meaning of the scatter of points in Figures 1 and 2. Nevertheless, the strong linear trend in both figures indicates that the amount of green vegetation at this time is an important component of the overall NPP.
Futher information:
http://see.gsfc.nasa.gov/edu/SEES/globa/class/Chap_6/index.htm
http://www.inform.umd.edu/Geography/glopem/
Figure 1.
Mean annual net primary production (NPP) vs. mean
normalized difference vegetation index (NDVI) in late July for 1982-89
in the Mid-Atlantic, U.S., based on 8-km NOAA AVHRR data. The best fit
line through the data indicates a direct relationship between NPP and
NDVI.
Figure 2.
Difference in mean annual NPP vs. difference in NDVI
between July, 1988 (drought year) and July, 1985 (typical year) in the
Mid-Atlantic, U.S., based on 8-km NOAA AVHRR data. The best fit
line through the data indicates a direct relationship between the
NPP difference and the NDVI difference.
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