CLIMATE VARIABILITY, AGRICULTURE AND REMOTE SENSING - Food Security, Food Prices and Climate Variability - page 58

due to the action of chlorophyll decreases reflected red energy, while the reflected near-
infrared energy increases. The near-infrared energy is preferentially reflected as a result of the
strong scattering processes of healthy leaves within the canopy so that it won't damage the
ability of the chlorophyll molecules to function. When the amount of reflected red and/or
near-infrared radiation are combined into a vegetation index (VI), the signal representing live,
photosynthesizing leaves is boosted and the information becomes more useful ( Jordan, 1969).
Vegetation indices can then be used as surrogate measures of vegetation health and abundance,
including crops, from field to continental scales (Tucker, 1979; Tucker et al. , 1985).
Figure 3.4 shows the time series of vegetation from Niamey Niger and from Mogadishu,
Somalia from the AVHRR and from the Moderate Resolution Imaging Spectroradiometer
(MODIS), both of which collect information in the red and near-infrared regions of the
spectrum. Both sensors are commonly used for monitoring growing conditions by food
security organizations. Note how the seasonal green-up of the vegetation during the summer
wet season and then senescence during the dry season months is clearly seen. Niamey has only
one growing season during the summer, whereas Mogadishu has two, one in the spring
months and the other in the fall. These data can be used in models that estimate changes in
the start and end of the growing season dynamically, as well as identifying periods of
drought.

40

35

30

Blue Green Red Near-infrared

25

20

15

10

5

0

400 450 500 550 600 650

Wavelength, nm

700 750 800 850 900

FIGURE 3.3 The NDVI uses the red and near-infrared portions of the spectrum, maximizing the
difference between the absorption in the 600-700 nm (red) portion and the reflect-
ance in the 800-1,000 nm (NIR) of the spectrum of healthy vegetation.