749
DIRECTIONAL INFRARED TEMPERATURE AND EMISSIVITY
OF VEGETATION: MEASUREMENTS AND MODELS
J.M. Norman, S. Castello, and L.K. Balick
Department of Soil Science, Univ. of Wisconsin, Madison, WI 53706 USA
and E G & G Energy Measurements, Las Vegas, NV 89125 USA
ABSTRACT
Directional thermal radiance from vegetation depends on many factors, including the architec
ture of the plant canopy, thermal irradiance, emissivity of the foliage and soil, view angle,
slope, and the kinetic temperature distribution within the vegetation-soil system. A one
dimensional model, which includes the influence of topography, indicates that thermal emis
sivity of vegetation canopies may remain constant with view angle, or emissivity may increase
or decrease as view angle from nadir increases. Typically, variations of emissivity with view
angle are less than 0.01. As view angle increases away from nadir, directional infrared canopy
temperature usually decreases but may remain nearly constant or even increase. Variations
in directional temperature with view angle may be 5 C or more. Model predictions of direc
tional emissivity are compared with field measurements in com canopies and over a bare soil
using a new method that requires two infrared thermometers. One infrared thermometer is
sensitive to the 8-14 pm wavelength band and a second to the 14-22 pm band. After correc
tion for CO, absorption by the atmosphere, a directional canopy emissivity can be obtained
as a function of view angle in the 8-14 pm band to an accuracy of about 0.005. Modeled and
measured canopy emissivities for com varied slightly with view angle (0.990 at nadir and
0.982 at 75° view zenith angle) and did not appear to vary significantly with view angle for
the bare soil. Canopy emissivity is generally nearer to unity than leaf emissivity. High spec
tral resolution, canopy thermal emissivity may vary by 0.02 with wavelength even though leaf
emissivity may vary by 0.07. The one-dimensional model provides reasonably accurate pre
dictions of infrared temperature and can be used to study the dependence of infrared tempera
ture on various plant, soil, and environmental factors.
KEY WORDS: Emissivity, Brightness Temperature, Thermal Infrared, Thermal Radiance
1 - INTRODUCTION
Thermal infrared measurements of surface temperature offer a possibility of remotely monitor
ing quantities related to the surface energy and water budgets. In particular, the difference
between canopy temperature and air temperature has long been known to be related to évapo
transpiration. Numerous attempts have been made to relate canopy temperature measurements
from infrared thermometers to fluxes of heat and water from vegetation. The relationship
between canopy temperature and évapotranspiration has been well established for more than
40 years (Penman, 1948). Unfortunately a precise definition of the term "canopy temperature"
has proven to be difficult. Penman, in fact, eliminated surface temperature from his famous
