A published physical algorithm designed for estimating total solar and photosynthetically active radiation (PAR) fluxes at the Earth's surface from GOES visible imagery has been modified for the Boreal Ecosystem-Atmosphere Study (BOREAS) applications at high space and time resolutions (1 km/half-hourly). Substantive changes to the algorithm are described, along with descriptions of various additional features needed to apply the algorithm over the boreal forest. Because of the propensity of forest fire smoke to impact the BOREAS study area during the summer period, particular attention has been given to the treatment of aerosol effects. To validate the algorithm, instantaneous estimates of downwelling total solar and PAR fluxes at half-hourly time steps obtained from a GOES 7 data set are compared with 15-min averaged in situ radiometer measurements obtained during the second summer intensive field campaign of 1994 from the array of BOREAS automatic meteorological stations. The validation results have been stratified according to sky conditions to help understand the detailed nature of algorithm performance and to identify weaknesses in the current algorithm design. A variety of sensitivity tests have also been conducted to help evaluate the algorithm's strengths and weaknesses. In addition, a large-scale analysis of the retrievals over the five 1994 field campaigns has been carried out to provide background information for modelers on the nature of the solar component of the surface radiation budget across the boreal forest zone. The overall accuracies of the algorithm are 1.6% and 6.5% for total solar and PAR fluxes, with relative precisions of ~20% considering all days, including those with extensive cloud cover and/or high concentrations of forest fire smoke. Such precisions are consistent with current published expectations at an hourly timescale. Better precisions of around 7% are found for clear, relatively aerosol-free days, which represents noteworthy algorithm performance in terms of expectations at these timescales. ¿ 1997 American Geophysical Union