Topography-adjusted solar radiation indices and their importance in hydrology

Solar radiation, direct and diffuse, is affected by surface characteristics, such as slope, aspect, altitude and shading. The paper examines the effects of topography on radiation, at multiple spatiotemporal scales, using suitable geometrical methods for the direct and diffuse components. Two indices are introduced for comparing the direct radiation received by areas at the same and different latitudes, respectively. To investigate the profile of direct radiation through the Greek territory, these are evaluated from hourly to annual basis, via GIS techniques. Moreover, different approaches are examined for estimating the actual global radiation at operational spatial scales (sub-basin and terrain), according to the available meteorological data. The study indicates that the errors of typical hydrometeorological modelling formulas, ignoring the topographic effects and the seasonal allocation of direct and diffuse radiation, depend on the spatial scale and they are non-uniformly distributed in time. In all cases, the estimations are improved by applying the proposed adjusting approaches. In particular, the adjustment of the measured global radiation ensures up to 10% increase of efficiency, while the modified Angström formula achieves slight (i.e. 2-4%) increase of efficiency and notable reduction of bias.