Theoretical relationship between stomatal resistance and surface temperatures in sparse vegetation

The relationship between stomatal resistance and foliage temperature in sparse vegetation has been the subject of previous papers [Smith, R.C.G., Barrs, H.D., Fischer, R.A., 1988. Agric. Forest. Meteorol. 42, 183–198; Shuttleworth, W.J., Gurney, R.J., 1990. Q. J. R. Meteorol. Soc. 116, 497–519], in which the modeling is based upon the one-dimensional two-layer approach of Shuttleworth and Wallace [Shuttleworth, W.J., Wallace, J.S., 1985. Q. J. R. Meteorol. Soc. 111, 839–855]. In both studies, however, a major assumption exists concerning the contribution of the substrate to the evaporation process. Using the same approach as these previous studies, an extended and upgraded model is presented in the sense that it relates stomatal resistance to foliage and substrate temperatures (Tf and Ts) without any assumption on substrate contribution. A comparison of stomatal resistances estimated from component temperatures (Tf and Ts) with values measured on fallow savannah during the HAPEX–Sahel experiment confirms the good performance of the model. Numerical simulations show the general behavior of the relationship between stomatal resistance and foliage temperature in several scenarios involving various weather conditions and canopy characteristics. The sensitivity of the calculated stomatal resistance to input variables and model parameters is investigated. It is shown that the calculation of stomatal resistance exhibits a significant sensitivity to foliage temperature and a much lesser one to substrate temperature. Uncertainties in leaf area index have a relatively weak impact on the calculated stomatal resistance. The sensitivity of stomatal resistance to the two main coefficients involved in the partitioning of available energy has also been investigated.