Measurement and estimation of photosynthetically active radiation from 1961 to 2011 in Central China

Measurements of photosynthetically active radiation (PAR) and global solar radiation (G) at WHU, Central China during 2006-2011 were used to investigate the seasonal characteristics of PAR and PAR/G (PAR fraction). Both PAR and PAR fraction showed similar seasonal features that peaked in values during summer and reached their lowest in winter with annual mean values being 22.39 mol m−2 d−1 and 1.9 mol M J−1 respectively. By analyzing the dependence of PAR on cosine of solar zenith angle and clearness index at WHU, an efficient all-weather model was developed for estimating PAR values under various sky conditions, which also produced accepted estimations with high accuracy at Lhasa and Fukang. PAR dataset was then reconstructed from G for 1961-2011 through the new developed model. Annual mean daily PAR was about 23.12 mol m−2 d−1, there was a significant decreasing trend (11.2 mol m−2 per decade) during the last 50 years in Central China, the decreases were sharpest in summer (−24.67 mol m−2 per decade) with relatively small decreases being observed in spring. Meanwhile, results also revealed that PAR began to increase at a rate of 0.1 mol m−2 per year from 1991 to 2011, which was in consistent with variation patterns of global solar radiation in the study area. The proposed all-weather PAR model would be of vital importance for ecological modeling, atmospheric environment, agricultural processes and solar energy application.