Function to calculate the topographic solar radiation of a site as Mj* Cm^-2 * yr^-1
R function 'SolarRad' for the calculation of potential annual direct incident radiation and heat load as described in McCune B, Keon D: Equations for potential annual direct incident radiation and heat load. J Veg Sci 2002, 13(4):603-606.
written by: Francesco Maria Sabatini. Alma Mater Studiorum, University of Bologna
This program is free software: you can redistribute it and/or modify it under terms of the GNU General Public License http://www.gnu.org/licenses/
Usage: SolarRad(Slope, Aspect, Latitude, Exp)
Arguments
Slope and Aspect must be in degrees. Aspect can be NA only if slope=0
latitude must be in the form (degrees.decimal) and not in the form (degrees minutes' seconds" )
Exp must be TRUE/FALSE. When FALSE (default), SolarRad returns the logarithm of Pontential Solar Radiation