Physiological control of transpiration and scaling evapotranspiration at the Bosque del Apache, a riparian forest

Evapotranspiration (ET) by riparian species has become a matter of interest for ecologists and water managers at the Middle Rio Grande. As a multidisciplinary project, several methods to estimate this important and uncertain component of the hydrologic budget have been tested at different sites along that region. Together with three micrometeorological methods to estimate ET at the stand level, a biophysical model to predict ET has been tested at the Bosque del Apache, near Socorro, New Mexico. Our overall interest was to develop a mechanistic model of transpiration ( E) and assimilation (A) capable of being scaled up from the leaf to the stand scale, and transferred among species and sites. Therefore, it was necessary to establish the physiological and ecological mechanisms and patterns controlling the fluxes of water and CO₂ at each scale. The second chapter addresses this need by testing that our experimental data on stomatal conductance and CO₂ assimilation can be fitted to a complete model of stomatal control. Our model captures the main physiological mechanism of control at the leaf scale. The third chapter presents tests of our ability to model light distribution on leaves as an important environmental factor controlling the fluxes at the branch to tree scales. The last chapter describes the whole model and presents an evaluation of its performance by comparing measured and predicted fluxes at the tree and stand levels. It also presents an analysis of other environmental mechanisms controlling the fluxes at the tree to stand scale. The model performed well in predicting branch, tree, and stand transpiration. A number of common approximations in process modeling were shown to affect accuracy in predicting fluxes; more rigorous process models such as we used are needed.