Abstract. Key components in the climatology of the Earth are incoming and net solar radiation. Next to clouds, the major modulator of solar radiation at the surface is topography. Variability in elevation, slope, aspect, and shadowing can lead to large gradients in incoming and net solar radiation fields. The response of vegetation to these gradients can often be dramatic, as in the distribution of vegetation on south- and north-facing slopes. Recently much progress has been made in modeling the effects of topography on incoming and net solar radiation. Such models produce fields of radiation that have been adjusted for topography derived from digital elevation data. This paper presents a topographic solar radiation model that combines digital elevation data with surface and satellite measurements. Specifically, a monthly topoclimatology for the Rio Grande River Basin in Colorado is constructed for the hydrological years 1987 - 1990. Using digital elevation models with 30 m x 30 m grid spacing, representing a mosaic of 39 U.S. Geological Survey 1:24 000 Quadrangles, a digital representation of the watershed is created. Hourly pyranometer measurements taken nearby the basin are then used with satellite reflectances to drive the solar radiation model. The results are monthly maps at 30 m x 30 m grid spacing covering the entire basin that show considerable variability by location and season. Such maps may be useful for vegetation modeling, especially for pattern analysis and ecosystem process modeling.