In many terrestrial ecosystems, vegetation experiences limitation by different resources at different times. These resources include among others light, nutrients and water, all of which may affect leaf level stomatal conductance. Frequently, however, leaf-level modeling frameworks that unite these limitations rely on empirical functions to scale stomatal conductance as a function of water stress. This body of research presents a novel framework for calculating vegetation water stress that

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... ders ecophysiology in addition to soil and atmospheric conditions. In doing so, I define a threshold of vegetation water stress that represents the balance between stomatal conductance required for biochemical activity and stomatal conductance required to satisfy the steady-state requirements of whole-plant hydrodynamics. This balance point attempts to unify these two oftentimes divergent concepts in the science of plant-water relations. I demonstrate that this threshold of vegetation water stress is functionally dependent upon local environmental conditions (light, temperature, and atmospheric vapor pressure), parameters representing different vegetation types, and nutrient status, and demonstrate, that as environmental conditions become more favorable for assimilation, the likelihood of water stress increases. This model of vegetation water stress is applied to a simple crop canopy in Virginia using flux tower data, and to two ecosystems in the northern Rocky Mountains using leaf-chamber data. Finally, the model of vegetation water stress is integrated into a soil-vegetation-atmosphere transfer model to evaluate the effects of heterogeneous water stress on catchment-scale fluxes of water vapor in a small (300 ha) watershed in the northern Rocky Mountains. iii