Remote Sensing of Terrestrial Photosynthesis
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which can disrupt the relationship between electron transport and CO2
uptake, and midday wilting.
Improved spectral indices may allow more general and robust estimates
of photosynthesis based on remote sensing of the xanthophyll cycle. The
relatively small magnitude of the spectral signal, the requirement for narrow
band spectra, and complications from factors like water stress, however,
make it likely that practical applications of this approach will be limited to
scales ranging from the leaf to the landscape for the next several years.
24.4.3 Canopy Temperature
The difference between canopy and air temperature increases as canopy
conductance decreases. This effect creates the potential for monitoring
canopy conductance with a measure of canopy temperature. Infrared
temperature measurements are increasingly used to schedule crop irrigation,
and Peftuelas et al. (1992) showed that even mild water stress can lead to
significant elevations in canopy temperature. At larger scales, Nemani and
Running (1989a) demonstrated that the ratio of canopy temperature to
NDVI increases with decreasing canopy conductance.
24.5 Remote Sensing of Environmental Factors
Models of photosynthesis and production typically require several kinds of
environmental data, including incoming PAR, temperature, humidity, soil
moisture, and CO2. For large-scale assessments, remote sensing offers
increasingly attractive options for obtaining some of these data.
Downwelling PAR can be calculated from satellite-based estimates of
cloud cover. Bishop and Rossow (1991) developed a method that determines
surface solar radiation by combining data from the International Satellite
Cloud Climatology Project (ISCCP, which uses data from a number of
geostationary and polar orbiting satellites) with calculations from a general
circulation model. The method of Dye (1992) uses data from an ultraviolet
channel on the Total Ozone Mapping Spectrometer (TOMS) to calculate
cloud optical thickness.
Several sources yield temperature information. The microwave sounding
units on the NOAA weather satellites yield middle troposphere temperatures
with a monthly precision approaching 0.01°C (Spencer and Christy 1990).
Several sensors yield surface temperature, but the interpretation of radiometric surface temperature is difficult enough that few ecological models use
it as a source of temperature data.
The polarization of passively emitted microwaves is sensitive to soil
moisture (Schmugge et al. 1986), but the strong effect of vegetation makes
523
which can disrupt the relationship between electron transport and CO2
uptake, and midday wilting.
Improved spectral indices may allow more general and robust estimates
of photosynthesis based on remote sensing of the xanthophyll cycle. The
relatively small magnitude of the spectral signal, the requirement for narrow
band spectra, and complications from factors like water stress, however,
make it likely that practical applications of this approach will be limited to
scales ranging from the leaf to the landscape for the next several years.
24.4.3 Canopy Temperature
The difference between canopy and air temperature increases as canopy
conductance decreases. This effect creates the potential for monitoring
canopy conductance with a measure of canopy temperature. Infrared
temperature measurements are increasingly used to schedule crop irrigation,
and Peftuelas et al. (1992) showed that even mild water stress can lead to
significant elevations in canopy temperature. At larger scales, Nemani and
Running (1989a) demonstrated that the ratio of canopy temperature to
NDVI increases with decreasing canopy conductance.
24.5 Remote Sensing of Environmental Factors
Models of photosynthesis and production typically require several kinds of
environmental data, including incoming PAR, temperature, humidity, soil
moisture, and CO2. For large-scale assessments, remote sensing offers
increasingly attractive options for obtaining some of these data.
Downwelling PAR can be calculated from satellite-based estimates of
cloud cover. Bishop and Rossow (1991) developed a method that determines
surface solar radiation by combining data from the International Satellite
Cloud Climatology Project (ISCCP, which uses data from a number of
geostationary and polar orbiting satellites) with calculations from a general
circulation model. The method of Dye (1992) uses data from an ultraviolet
channel on the Total Ozone Mapping Spectrometer (TOMS) to calculate
cloud optical thickness.
Several sources yield temperature information. The microwave sounding
units on the NOAA weather satellites yield middle troposphere temperatures
with a monthly precision approaching 0.01°C (Spencer and Christy 1990).
Several sensors yield surface temperature, but the interpretation of radiometric surface temperature is difficult enough that few ecological models use
it as a source of temperature data.
The polarization of passively emitted microwaves is sensitive to soil
moisture (Schmugge et al. 1986), but the strong effect of vegetation makes