Analysis of photosynthetically active radiation in Northwest China from observation and estimation.

Photosynthetically active radiation (PAR) observed at Fukang, China from 2005 to 2012 was used to investigate PAR variability and its relationship with global solar radiation (G) under various sky conditions in Northwest China. Clearness index (K t) was used for characterizing the sky conditions and model development; it was discovered that daily PAR/G (F p) generally increased from 1.79 ± 0.016 mol MJ(-1) in November to 1.95 ± 0.004 mol MJ(-1) in July with annual average being about 1.89 mol MJ(-1); F p generally decreased with sky conditions changing from overcast skies to clear skies in each month. Meanwhile, an efficient all-sky PAR model under any sky conditions has been proposed by investigating the dependence of hourly PAR on K t and cosine of solar zenith angle μ. The model was assessed through the statistical indices: mean bias error (MBE), mean absolute error (MAE), and root-mean-square error (RMSE) whose values were only -0.34% (-1.75%), 3.62% (3.47%), and 5.52% (4.79 %), respectively, at hourly (daily) basis. The model has also been tested at three other four sites with distinctly different climates in China. Finally, PAR values during 1961-2012 in Northwest China were reconstructed and annual mean daily PAR was 29.25 mol m(-2) d(-1). PAR decreased at 0.18 mol m(-2) d(-1) per decade during 1961-2012, and the decreases were sharpest in spring (0.63 mol m(-2) d(-1) per decade). The possible reasons for the long-term variability have also been analyzed, which will lay foundations for cloud effect study in the near future.