In many terrestrial ecosystems, vegetation experiences limitation by different resources at different times. These resources include, among others, light, nutrients, and water. Frequently, however, leaf-level modeling frameworks that unite these limitations rely on empirical functions to scale stomatal conductance as a function of water stress. These functions use prescribed values of soil water content to mark the transition between water-stressed and unstressed conditions without accounting for the dependence of such a water content threshold on atmospheric and hydrologic conditions and nutrient availability. To address the phenomenon of a variable threshold to water stress, we combine an existing water-limited stomatal conductance model with an existing assimilation (photosynthesis)-limited stomatal conductance model. In this manner, we simulate variable controls on stomatal conductance and use a combination of the two models to define the threshold at which soil water content becomes limiting to transpiration. Modeled plant processes are used to define this water stress threshold as functionally dependent upon local environmental conditions (light, temperature, and atmospheric vapor pressure), parameters representing different vegetation types, and nutrient status. Simulations demonstrate that as environmental conditions become more favorable for assimilation, the likelihood of water stress increases. Specifically, there exist ranges of leaf temperature, light, and atmospheric humidity for which water stress is maximized.