Observation and estimation of photosynthetic photon flux density in Southern China

Abstract

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.