Investigation of the variability of photosynthetically active radiation in the Tibetan Plateau, China

Photosynthetically active radiation (PAR) is fundamental to most ecological and biophysical processes because it plays a key role in biochemical processes and in the local and global energy budget; however, PAR that is measured in situ is scarce. To obtain high temporal and spatial resolution PAR values for ecological studies, the development of a reconstruction model to estimate PAR from more routinely measured data is important and useful. In this study, we develop an efficient model for estimating PAR under various sky conditions based on in situ measurement data. The relative error between the measured and calculated PAR using this model was within 5.6%. Then, we combine this reconstruction model with the hybrid model to obtain the historical dataset of daily PAR at 37 stations of the China Meteorological Administration (CMA) in the Tibetan Plateau (TP). Based on the historical dataset, the spatial distribution and temporal variation trends of PAR in the TP are discussed. The dimming and brightening period of PAR in the TP is significantly different from the variation trends of the average PAR over all of China. The PAR trends in the TP increased from 1961 to 1983 and then decreased from 1983 to 2003. Since 2003, PAR in the TP has presented an increasing trend. The average values of PAR in brightening and dimming periods are 31.07mol−2m−2d−1 and 30.86molm−2d−1, respectively. Generally, the aerosol optical depth (AOD) can explain 6.2% of the variation of PAR in this region, and water vapour may play an important role in the PAR change. This result may be useful for studies on ecological process modelling in the TP.