Modeling and analysis of the spatiotemporal variations of photosynthetically active radiation in China during 1961-2012

Photosynthetically active radiation (PAR) observed at 39 stations of Chinese Ecosystem Research Network (CERN) during 2006-2012 were reported for developing all-sky PAR models at hourly and daily scales in China. The models were proposed by investigating the dependence of PAR on cosine of solar zenith angle and clearness index (Kt) under various sky conditions. The models have been validated through various statistical methods and the results indicated that our models can produce satisfied estimations in China. The PAR dataset from 1961 to 2012 was then reconstructed based on global solar radiation (G) at 115 meteorological stations obtained from National Meteorological Center. Spatiotemporal characteristics of PAR in China were further analyzed, and it was discovered that annual mean daily PAR ranged from 18.95 to 35.48 mol m-2 day-1 with average value being about 27.61 mol m−2 day−1; the values were generally higher in Northwestern China and Qinghai-Tibet Plateau. Annual mean PAR decreased at the rate of 0.15 mol m−2 per decade in China during 1961-2012, but it still increased in spring months (0.08 mol m−2 day−1 per decade). The possible causes for the spatial patterns and temporal variations of PAR were further investigated by analyzing the annual and seasonal variations of G, PAR/G (Fp) and Kt during the whole study period.