The global direct flux of clear-sky incoming solar radiation over a given portion of terrain (e.g., the surface of a drainage basin) is modeled by using the Stokes' formula. Accordingly, line integration is substituted for surface integration to describe topographic variability. This allows for separating the effects of solar radiation geometry and terrain configuration. Application to a small alpine basin shows this model to provide accurate results also when a major portion of the terrain is in shadow. It can be applied to estimate radiative fluxes in hydrological models at the scale of small catchments and at the GCM scale when topography is complex. ¿ American Geophysical Union 1993