A STOKESIAN MODEL OF AREAL CLEAR-SKY DIRECT-RADIATION FOR MOUNTAINOUSTERRAIN

The global direct flux of clear-sky incoming solar radiation over a gi ven portion of terrain (e.g., the surface of a drainage basin) is mode led by using the Stokes' formula. Accordingly, line integration is sub stituted 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 te rrain is in shadow. It can be applied to estimate radiative fluxes in hydrological models at the scale of small catchments and at the GCM sc ale when topography is complex.