The climatological characteristics of photosynthetically active radiation in arid and semi-arid regions of China

摘要

Photosynthetically active radiation (Q_p) is a key variable in models of net primary productivity and carbon cycle modelling. The relationship between broadband global solar radiation (R_s) and Q_p is investigated using 6 years of radiation data collected at 9 sites in arid and semi-arid regions of China. The dependence of Q_p/R_s on aerosol optical depth (AOD) and water vapour content are also discussed. A simple and efficient all-weather empirically derived model is developed to estimate Q_p from R_s. The annual average daily Qp in arid and semi-arid areas is 29.9± 11.7 and 27.3 ± 10.1 mol m~(-2) d~(-1), respectively. The highest value (31.9± 11.3 mol m~(-2) d~(-1)) appears at Linze in the arid area. The lowest value (24.3±9.7 mol m~(-2) d~(-1)) appears at Ansai in the semi-arid area. The results show that the monthly variation of the Q_p/R_s ratio ranges from 1.69±0.19 mol MJ~(-1) in Aksu to 1.91 ±0.08 mol MJ~(-1) in Fukang. There is a small decreasing trend of the ratio of Q_p to R_s (PAR fraction) in arid and semi-arid regions because of the recent increase in fine aerosols. A simple and efficient empirically model suit for all-weather condition was developed to estimate Q_p fromr R_s The slope a and intercept b of the regression line between estimated and measured values is close to 1 and zero, respectively. The application of the model to data collected from different locations also results in reasonable estimates of Q_p.