Transpiration and the atmospheric boundary layer : progress in modeling feedback mechanisms

Graduation date: 1997Simple models of transpiration, e.g., the Penman-Monteith equation, treat\ud atmospheric conditions as driving variables. In fact, transpiration modifies temperature\ud and humidity throughout the convective boundary layer, creating feedbacks that stabilize\ud the water use of vegetation. This thesis concentrates on the new empirical relationships\ud proposed by Monteith (1995), for developing simple models of feedback, and then applies\ud these relationships to data from the Oregon Cascades. Monteith showed that there is\ud strong laboratory evidence to support a linear relationship between leaf transpiration rate\ud and leaf conductance. If this relationship holds for vegetation in the field, simple models\ud to explain the diurnal variation of canopy conductance can be developed. When this\ud model was applied to data from a Douglas fir forest, canopy conductance changed in\ud response to transpiration rate, rather than to saturation deficit, as has been previously\ud assumed. Monteith also reanalyzed data from McNaughton and Spriggs (1989) which\ud explored the dependence of the Priestley-Taylor coefficient alpha on surface parameters.\ud He showed that there is a linear relationship between alpha and surface conductance. By\ud combining this "demand function" with the physiological "supply function" described\ud earlier, the PMPT model is developed in which evaporation rate depends on physical\ud feedbacks in the convective boundary layer and physiological feedbacks within plants.\ud The thesis will focus on the results of the research done using this model. The PMPT\ud model will then be compared with other simple models of transpiration in order to\ud determine its applicability