In hydrology, the friction force counterbalancing the downslope gravitational force (L. B. Leopold et al. 1964). Resistance is a function of the surface area in contact with the flow, and so flow becomes more efficient when the ratio of the flow cross-sectional area A to the wetted perimeter P is high (defining the hydraulic radiusR). See Smith et al. (2007) PPG31, 4 on applying flow resistance equations to overland flows.
Subjects: Earth Sciences and Geography.