Characterizing ecosystem responses to water stress remains challenging due to uncertainties in upscaling plant traits and in disentangling confounding environmental factors. We reduce the complexity of describing water fluxes through the soil-plant-atmosphere continuum by combining plant traits with soil and climate variables into non-dimensional parameter groups that define key eco-physiological tradeoffs independently of scale. We implement this framework using data from flux tower sites and quantify ecologically successful water use strategies that are consistent with the observed long-term soil water balance as well as ecosystem water and carbon flux measurements. These findings support the hypothesis that plants are adapted to optimally balance the benefits of water uptake and long-term risks of water stress. We use this emerging link between ecohydrological performance and non-dimensional parameter groups as a parsimonious eco-evolutionary criterion to infer optimal carbon costs of water use necessary to constrain the sensitivity of transpiration and photosynthesis to water status. We also link results to water use efficiency and show that they provide insights into the plausible range of CO₂ fertilization effects on global photosynthesis.