Proceedings of the Open source GIS - GRASS users conference 2002 - Trento, Italy, 11-13 September 2002 The solar radiation model for Open source GIS: implementation and applications Jaroslav Hofierka *,** , Marcel œri *,*** * GeoModel s.r.o., M. Marečka 3, 841 07 Bratislava, Slovak Republic, http://www.geomodel.sk ** Department of Geography and Geoecology, Faculty of Humanities and Natural Science, University of Preov, 17. novembra 1, 081 16 Preov, Slovak Republic, tel. +421 905 494487, fax +421 2 65315915, e-mail hofierka@geomodel.sk *** European Commission Joint Research Centre, Institute for Environment and Sustainability, 210 20 Ispra (VA), Italy, tel. +39 0332 789977, fax +39 0332 786394, e-mail marcel.suri@jrc.it. 1 Introduction Solar radiation, incident to the earths surface, is a result of complex interactions of energy between the atmosphere and surface. At global scale, the latitudinal gradients of radiation are caused by the geometry of the earth and its rotation and revolution about the sun. At regional and local scales, terrain (relief) is the major factor modifying the distribution of radiation. Variability in elevation, surface inclination (slope) and orientation (aspect) and shadows cast by terrain feature creates strong local gradients. The spatial and temporal heterogeneity of incoming solar energy determines dynamics of many landscape processes, e.g. air and soil temperature and moisture, snow melting, photosynthesis and evapotranspiration, with direct impact to the human society. Accurate and spatially distributed solar radiation data are desired for various applications (environmental science, climatology, ecology, building design, remote sensing, photovoltaics, land management, etc.). There are several hundreds of ground meteorological stations directly or indirectly measuring solar radiation throughout the Europe. To derive spatial databases from these measurements different interpolation techniques are used, such as spline functions, weighted average procedures or kriging [1, 2, 3]. In mountainous regions, the use of additional information gained from satellite images may improve the quality of solar radiation interpolation using co-kriging approach [4, 5]. Spatially continuous irradiance values can be also derived directly from meteorological geostationary satellites (e.g. METEOSAT). Processing of satellite data provides less accurate values (compared to ground measurements), but the advantage is a coverage over vast territories at temporal resolution of 0.5-12 hours [6, 7]. Other techniques of generating spatial databases are solar radiation models integrated within geographical information systems (GIS). They provide rapid, cost-efficient and accurate estimations of radiation over large territories, considering surface inclination, aspect and shadowing effects. Coupling radiation models with GIS and image processing systems improves their ability to process different environmental data and co-operate with other models. A significant progress has been made toward developing solar radiation models in the last two decades [8]. One of the first GIS-based solar radiation models was SolarFlux [8, 9] developed for ARC/INFO GIS. Similar initiative was made by implementation of solar radiation algorithms into commercially available GIS Genasys using AML script [10]. Another approach for computing all three components of radiation was realised in a standalone model Solei under MS Windows that was linked to GIS IDRISI via data format [11, 12]. All three mentioned models use rather simple empirical formulas. Some of their parameters are spatially-averaged (lumped) and therefore are not suitable for calculations over large areas. More advanced methods for ecological and biological applications are