Albedo
Albedo is a measure of the reflectivity of a surface, in this case the ground around the PV plant under analysis. A value of 1.0 indicates perfect reflectivity. A value of 0 indicates complete absorption.
Typical values range from 0.15 for grass to 0.8 for fresh snow.
Specify albedo as either a single annual value or 12 monthly values (if your site has strong seasonal variation).