ABSTRACTInteraction among plants is one of the major processes that drive the health and management of an ecosystem. Such plant–plant interaction, often expressed in terms of biomass yields observed from field or laboratory experiments, reveals the plants' competitiveness for resources such as water and nutrients. In computational modelling of the coupled changes in hydrology and plant growth, the interaction between plants can be expressed by the competition coefficients in model algorithms such as the Lotka–Volterra system of equations. The competition coefficients determine the plant dynamics and the hydrological processes because vegetation distribution affects the partitioning of water. Therefore, it is imperative to apply appropriate competition coefficients to understand water partitioning by the plants for their growth. For such evaluation of competition coefficients, in this study, we used a coupled groundwater–vegetation predictive model, in which competition coefficients chosen to represent different competitive scenarios were input to simulate the plants' biomass and water use. The simulated biomass and water use were then used to calculate competition coefficients. The results demonstrated that rather than merely confirming the occurrence of competition, neither biomass nor water use measures, by themselves, will be sufficient for describing the entwined relationship that drives the competitive interaction between the plants. We therefore recommend a combined evaluation of both water use and biomass yield for better parameterization of competition coefficients implemented in ecohydrological models. Copyright © 2013 John Wiley & Sons, Ltd.