Estimation of Aerodynamic and Crop Resistances from Canopy Temperature1
- J. C. O'Toole and
- J. G. Real2
Previous reports illustrated two methods of interpreting canopy minus air temperature (Tc — Ta). The first was empirically based on linear regression of Tc — Ta on air water vapor pressure deficit (VPD) and appeared to illustrate crop specific characteristics. The second approach was an energy balance equivalency for Tc — Ta in which the crop parameters aerodynamic (ra) and canopy resistance (rc) were the primary crop specific components. From a crop physiology perspective, the previous reports provided an opportunity to couple crop specific physiological and micrometeorological parameters. Using data obtained from irrigated field plots in Arizona, California, and Philippines, we evaluated both approaches and tested a solution for the merger of the empirical and energy balance concepts. Briefly, the regression statistics from the regression of Tc — Ta on VPD were substituted into the energy balance allowing a mean value of ra(r̄ap) and rc (r̄cp) to be estimated given conditions of potential evapotranspiration by the crop canopy. Estimation of the mean resistances are illustrated for nine crop species with r̄apan r̄cp ranging from about 5 and 14 s m−1 for rice (Oryza sativa L.) to 16 and 69 s m−1 for fig tree (Fiscus Carica L.). Trends in r̄ap and r̄cpwere related to ecophysiological adaptation of the different species as well as phenological changes. Four growth stages of a rice crop showed increased r̄cp associated with postanthesis senescence of panicles and leaves in the canopy. The combination of empirical regression and energy balance approaches assists in our understanding and interpretation of Tc — Ta. It also provides a means of estimating species specific values of rc and ra necessary to more fully exploit the energy balance based assessment of Tc — Ta.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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