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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 Q (p) 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) from 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) .