Radiometric Temperature of Plant Canopies

14.3 Radiometric Temperature of Plant Canopies

Vegetative canopies are exceedingly complex, being composed of many leaves, branches, stems, soil, etc. Even though the aerodynamic temper- ature can be defined by Eq.

its relation to true thermodynamic temperature is virtually impossible to discover. However, another canopy temperature is easily measured. This is the radiometric temperature. The radiometric temperature of a blackbody (unity emissivity) surface is estimated

a direct measurement of thermal radiant flux density by inverting an integral of Eq. (10.4) over the wavelength band of sensitivity of the infrared radiometer. If a surface is not a blackbody, then adjustments must be made for emissivity. Norman and Becker (1995) discuss radiometric temperature and thermal emissivity in detail. Infrared radiometers that are used to measure radiometric temperature are called infrared thermometers. Because infrared thermometers are intended to estimate the temperature of a surface and be minimally influenced by the intervening atmosphere, usually they are sensitive only to wavelengths where the atmosphere is relatively transparent (between 8 and 13 wavelengths, see Fig. 10.6). From satellites, atmospheric

of

C are not uncommon even in the most transparent wavelength bands. We know that the integral of Eq. (10.4) over all wavelengths is equal to

3 to

If we assume the radiant flux density in the 8 to 13 wavelength band is proportional to

(a good approximation, but not perfect) we can work with

instead of complicated functions of the blackbody integral. Unfortunately the thermal emissivity of natural sur- faces between 8 and 13

may not be equal to the broad-band (4 to

80 thermal emissivity (particularly for soils), and the 8 to 13 emissivity must be known to obtain radiometric temperatures. Fortunately most full-cover vegetative canopies have thermal emissivities in the 8 to

13 wavelength band of to 0.99. The reason for this is discussed near the end of Ch. 15. Even for a blackbody, the radiometric temperature, the thermodynamic temperature, and the aerodynamic temperature resulting from a surface energy balance (Eq. (14.8)) will all be equal only if the surface and its surroundings are in thermodynamic equilibrium (they have a constant, uniform temperature). Since this rarely occurs in nature, in general these temperatures are not expected to be interchangeable. The aerodynamic temperature depends on the areodynamic conductance between the atmo- sphere and various parts of the surface that are at different temperatures. The radiometric temperature depends on the fourth power weighting of the absolute temperature of the parts of the surface that make up the view of the infrared thermometer. Because radiometric

can depend on radiometer view angle and aerodynamic temperature does not, the two will generally be different. Consider a partial-cover canopy with hot dry soil (50" C) and cool transpiring

(25" C), a com- mon occurance. If an infrared thermometer pointed at this surface from

Transpiration and the Leaf Energy Budget

directly overhead views 40 soil and 60 percent vegetation, assum- ing emissivity to be 1.0, the radiometric temperature would be 35.7" C ((0.6

+ 0.4 308.7 K). If the view zenith angle of the

infrared thermometer were changed to 85" , the view would be mainly vegetation and the indicated temperature would change to about

C. Since the aerodynamic temperature would have to be the same for the two infrared thermometer view angles, clearly the two temperatures can

be quite different. Since leaf and canopy temperature are determined, in part, by atal conductance, and conductance is determined, in part by availability of water to the plant, an effort has been made to sense plant water stress

airplanes or satellites using thermal imaging of vegetation tem- perature. For a dense, full-cover canopy, radiometric temperature may approximate aerodynamic temperature within C. However, Eq. (14.8) provides a means of finding canopy conductance

measurements of canopy and air temperature only if wind, radiation, and vapor deficit are known. Without measuring these confounding variables, or at least making the measurements during times when they are relatively constant (such as midday on clear days with high vapor deficits), determining water stress very accurately using this technique is difficult. Even when water stress can be determined from a canopy temperature measurement, additional information is needed to determine whether a crop needs ir- rigation. Stomata may close and canopy temperature increase for many reasons, only one of which is a soil water deficit.