Control of atmospheric fluxes from a pecan orchard by physiology, meteorology, and canopy structure: Modeling and measurement

We constructed and validated against eddy-covariance data a model of the fluxes of water vapor, sensible heat, CO2, and radiation in a substantially mature pecan orchard (Carya illinoinensis (Wangenh.)K. Koch) in an arid environment near El Paso, TX, USA. The detailed process-based model is designed for insights into major control points for photosynthetic gain and water use as exerted by canopy structure, leaf physiology, and micrometeorological drivers. Toward this end, it resolves extensive details of leaf micro environments (radiation and scalars) in realistic canopy structures, as well as photosynthetic and respiratory physiology, stomatal control, and water relations from roots to leaves. The model is for a static mid-season canopy, with the ability to link it to dynamics models of development and management. Field flux measurements agreed well with model estimates that were derived using measurable parameters rather than data-fitting. An exception was the measurement-model disparity in sensible heat flux under conditions of strong advection of dry air; the model diagnostics imply a marked insensitivity of pecan stomata to humidity that has not been reported earlier. Formulation and parametrization of most of the physical and physiological processes was robust, shared well between the study site and an alternate site, but gaps are evident in the knowledge of several important processes, primarily in responses to water stress. The study indicates limitations in simpler models, such as those based on constant canopy conductance or light-use efficiency, while offering leads to making more accurate simple models suitable for use in decision support systems, ultimately for stress management under limited water availability.