Theoretical relationship between stomatal resistance and surface temperatures in sparse vegetation

The relationship between stomatal resistance and foliage temperature in spa rse vegetation has been the subject of previous papers [Smith, R.C.G., Barr s, H.D., Fischer, R.A., 1988. Agric. Forest. Meteorol. 42, 183-198; Shuttle worth, W.J., Gurney, R.J., 1990. Q. J. R. Meteorol. Sec. 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. Sec. 111, 839-855]. In both studies, however, a major assumption exists concerning the contribution of the substrate to the evaporation pro cess. Using the same approach as these previous studies, an extended and up graded model is presented in the sense that it relates stomatal resistance to foliage and substrate temperatures (T-f and T-s)without any assumption o n substrate contribution. A comparison of stomatal resistances estimated fr om component temperatures (T-f and T-s) with values measured on fallow sava nnah 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 i nvolving various weather conditions and canopy characteristics. The sensiti vity of the calculated stomatal resistance to input variables and model par ameters is investigated. It is shown that the calculation of stomatal resis tance 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 sensiti vity of stomatal resistance to the two main coefficients involved in the pa rtitioning of available energy has also been investigated. (C) 2000 Elsevie r Science B.V. All rights reserved.