6.5.2: Plant Canopy Temperature
- Page ID
- 44421
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Since evaporation of water is a cooling process, the foliage of well-watered plants is usually cooler than the surrounding air, especially in arid climates. Plants that are experiencing water stress will have higher leaf temperatures than well-watered plants. With the advent of infrared thermometry, it is relatively easy to measure canopy temperature (Lo et al., 2018; Figure 6.15). In general, two approaches have been developed to use canopy temperature in irrigation scheduling, the crop water stress index method (CWSI) and the time-temperature-threshold (TTT) method.
The temperature difference (DT) between the air and the plant canopy depends on both the plant water status and the vapor pressure deficit (VPD). Jackson (1982) presented an excellent overview of plant water stress response to DT and VPD. Jackson et al. (1981) developed the CWSI method for quantifying plant stress. It relies on baseline values of canopy temperature of non-transpiring reference and a non-stressed canopy. Crop yield can then be related to CWSI as illustrated by Irmak et al. (2000). To apply this technique the base-line or reference lines must be established. Jackson (1982) suggested that the CWSI will be a very useful tool for irrigation scheduling because it is easy to use handheld infrared thermometers for measuring canopy temperature. He points out some of the problems, such as the effects of bare soil in the field of view and the establishment of the threshold stress indicators for various crops or plants. As pointed out by Stegman (1983), the effect of wind and cloud cover on the interpretation of DT data and how it relates to irrigation management must be considered.
Another way of using canopy temperature in irrigation scheduling is the time-temperaturethreshold method. Wanjura et al. (1995) defined TTT as “the amount of time accumulated above a specific temperature in one day by a crop.” If the time-temperature value exceeds a threshold, a temperature stress exists and irrigation is needed. Peters and Evett (2008) successfully automated the irrigation scheduling of a center pivot irrigation system using the TTT method. They used infrared thermometers mounted on the center-pivot to monitor canopy temperatures as it irrigated the field. The center-pivot was equipped with LEPA drops thus the canopy was not wetted during an irrigation event. The TTT method requires the establishment of both the temperature threshold and the time threshold.
Figure 6.15. Diagram of a hand-held infrared thermometer.
