Seasonal Changes in Solar Radiation, Net Radiation and Photosynthetically Active Radiation in Khon Kaen, Northeast Thailand

Abstract

Continuous measurements of solar radiation (R_s), net radiation (R_n) and photosynthetically active radiation (PAR) were conducted in Kohn Kaen, Northeast Thailand characterized as dry-and-wet tropics over 4 years (R_sandR_n) and a year (PAR) . Monthly mean solar radiation was more than 15 MJ m^-2over a year with its minimum occurring in dry season. Monthly mean net radiation in the dry season was a half of that in the rainy season. On daily basis, PAR/R_s-ratio changed from 1.81μmol MJ^-1in January to 2.08μmol MJ^-1in July. Equations for converting instantaneous and daily solar radiation to PAR were empirically derived with daily mean water vapor pressure as a parameter.