Observation and estimation of photosynthetic photon flux density in Southern China

Observations of photosynthetically active radiation (PAR) and global solar radiation (G) at Sanya, Southern China during 2005-2011 were used for investigating their relationships under various sky conditions. Monthly mean PAR generally increased from 24.73 mol m(-2) day(-1) in December to 40.97 mol m(-2) day(-1) in July with annual mean value being about 32.88 mol m(-2) day(-1). Monthly mean daily PAR fraction (F-P) changed from 1.82 mol MJ(-1) in March (or April) to 2.03 mol MJ(-1) in October with annual mean being 1.90 mol MJ(-1). Clearness index (K-t) was used for characterizing sky conditions and partly cloudy skies (41.45 %) were the dominated sky condition in this area. Meanwhile, an efficient PAR model has been introduced by investigating the dependence of PAR on K-t and cosine of solar zenith angle under any sky conditions. The model has been tested at Sanya, Lhasa, and Inner Mongolia, China through the statistical indices: mean bias error (MBE), mean absolute percentage error (MAPE), and root-mean-square error (RMSE), which were generally lower than -3.6, 8.24, and 10.26 %, respectively. PAR values during 1992-2012 were then reconstructed and it was discovered that annual mean PAR decreased at the rate of -1 mol m(-2) day(-1) per decade.