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Recent approaches in two and three‐dimensional numerical models have used a drag force term in the momentum equations to model the stem drag imposed by plants which project through a significant amount of the water depth. This approach reduces the ability to subsume under‐represented processes and data uncertainties in the calibration process which are inherent in roughness closures using a bed friction term such as the Mannings n. This paper examines the data uncertainties when applying the drag force approach to flow through natural plants by an a priori determination of the projected area of the plants as a function of height and explores the implications of this relationship on the computed velocity profile. Using eight individual trees of species Salix fragilis, geometric parameters including diameter and volume were measured and the projected area as a function of tree height was determined. Velocity measurements were conducted in the Wienfluss river in Austria and the relationship of drag ...