An alternative to define canopy surface temperature bounds

Canopy temperature, which may be estimated by infrared thermometry (IRT), can serve as an indicator of plant water status. Idso et al., 1981a] and Idso et al., 1986] proposed the nowadays much used concept of the crop water stress index, which relates observed canopy surface temperature (T_s) to maxima and minima temperature bounds. Jackson et al. (1981) defined those bounds on the basis of the energy balance. Those bounds vary with the meteorological situation. In this paper a chart is offered for general use with a fixed frame for the upper and lower bound. It relates canopy surface temperatures with r₁(=1 + r_c/r_a)-values (r_c the canopy resistance and r_a the aerodynamic resistance) as a function of a specifically defined temperature sum (S). It links the curved lower bound with the straight upper bound by a bundle of r₁-curves (the T_s-S-r₁-chart). The lower bound can be expressed by an equation, which approximates the energy balance solution with high accuracy. The sensitivity of the upper bound is also discussed. A comparison was made between bounds following Jackson et al. (1981) and the proposed alternative method, which, however, is limited by the short data-set available for this paper.