One Metric, Many Meanings: Reconciling Water Use Efficiency Definitions

Abstract Purpose of Review Water Use Efficiency (WUE) is widely used to describe carbon-water coupling, but its meaning varies among disciplines, measurement approaches and spatial and temporal scales. We review commonly used WUE definitions and formulations and clarify their conceptual differences. Recent Findings Ratio-based metrics, including WUE ( A /E) and intrinsic WUE ( A /g s w), describe realized carbon–water exchange but remain sensitive to environmental conditions. Optimality-based approaches distinguish these ratios from the marginal water cost of carbon gain (∂E/∂ A ), providing a mechanistic perspective on water-use strategy. We also highlight differences among leaf, plant, ecosystem, isotope, and model-derived estimate. Summary WUE is best understood as a context-dependent metric rather than a universal functional trait. We propose a framework linking each formulation to its carbon and water variables, spatial and temporal scale, environmental dependencies, and underlying assumptions. This approach supports transparent metric selection and improves comparisons across studies, scales, and methods.