A measure of the resistance offered by a material, such as vegetation, to a flow of water. The calculation of flow velocities within stream channels and over hillslopes cannot be made without accounting for the hydraulic resistance experienced by flow over different morphologies (Smith et al. (2007) PPG 31, 4). Moore and Burch characterize hydraulic resistance by choosing an appropriate value of the Manning roughness coefficient, and it can also be characterized using the Darcy–Weisbach friction factor, expressed as a function of the Reynolds number of the flow; see Prosser and Rustomji (2000) PPG 24, 2 for the equations. Fisher et al. (2007) Glob. Change Biol. 13, 11 recommend the introduction of hydraulic resistance to likely flow paths in the management of forests.