Development of a generally applicable rainfall-runoff model and identification of associated model parameters require understanding of connections between physical processes at disparate scales and hydrological similarities between catchments. In this study, we test the hypothesis that understanding of geomorphometric scaling relations can reduce uncertainty when transferring model parameters between catchments when applying the TOPMODEL concept. Scaling relations on contributing area, slope, and contour length were successfully used to scale the topographic index distribution in watersheds located in vastly different regions of the world: Japan, Nepal, Panama, and the United States. Model parameters were identified through calibration of TOPMODEL in the 210-km(2) Kamishiiba catchment in Japan. These parameters were transferred to two Sun Koshi River subcatchments in Nepal, namely, the 850-km(2) Likhu and 620-km(2) Balephi catchments, the 414-km(2) Upper Rio Chagres catchment in Panama, and the 37-km(2) Town Brook catchment in the United States. Results show how a priori estimates of the most sensitive model parameters can be used to make predictions in poorly gauged or ungauged basins with some degree of confidence provided that scale effects are considered. This result hints at the potential universality of the topographic index distribution scaling relations in catchments where runoff is dominated by subsurface flow.