Photosynthetically active radiation and its relationship with global solar radiation in Central China.

Photosynthetically active radiation (PAR) and other solar components were observed for a period of 3 years at Wuhan, China to determine for the first time the temporal variability of PAR fraction [PAR/G (G here stands for global solar radiation)] and its dependence on different sky conditions in Central China. PAR, G and PAR/G showed similar seasonal features that peaked in summer and reached their lowest values in winter. The seasonal PAR/G ranged from 1.70 E MJ(-1) (winter) to 2.01 E MJ(-1) (summer) with an annual mean value of 1.89 E MJ(-1). Hourly values of PAR/G increased from 1.78 to 2.11 E MJ(-1) on average as sky conditions changed from clear to cloudy. Monthly mean hourly PAR/G revealed a diurnal variation, with highest values observed around sunrise and sunset, slightly higher PAR fractions were also found around noon for most months. The effect of daylength on PAR/G was also studied and no significant impact was found. Three models were developed to estimate PAR from G. These models consisted of atmospheric parameters that were found to cause substantial changes of PAR/G, such as sky clearness, brightness, path length and the sky clearness index. The estimations obtained from different models were very close to the measured values with maximum relative errors below 8 % (hourly values) in Wuhan. The models were not only tested at seven radiation stations in Central China, but also verified in six stations with different climates in China. The models were found to estimate PAR accurately from commonly available G data in Central China; however, the results also implied that the models need to be modified to account for local climatic conditions when applied to the whole country.