The partial differential momentum and continuity equations governing two‐dimensional overland flow on a plane surface with vertical inflow are developed and normalized.Using extensive data from the literature on such subjects as rainfall, topographic characteristics, and unsteady overland flow, the individual terms in these normalized equations are compared on an order‐of‐magnitude basis. This comparison results in a systematic and justifiable simplification in the equations and thus in the similarity criteria given by heir coefficients. The criteria obtained are realizable over a limited, but significant, range of prototype rainfalls and land forms.