Photosynthetically active radiation: measurements and modelling

Photosynthetically-active radiation is a necessary input in applications dealing with plant physiology, biomass production and natural illumination in greenhouses. Unfortunately, a worldwide routine network for the measurement of photosynthetically-active radiation is not yet established and it is often calculated as a constant ratio of the broadband solar radiation. This ratio is affected by many parameters. This work describes the variations of the ratio of photosynthetically-active radiation to broadband solar radiation with sky conditions, characterized by means of variables that could be accessible in common radiometric networks. For this purpose, data recorded at a radiometric station located in the University of Almeria, a seashore location (36.83° N, 2.41° W) are used. The data base includes hourly values of the relevant variables that cover the period from June 1990 to December 1992. We have studied the seasonal and daily patterns of the ratio photosynthetically-active radiation to broadband solar radiation. We explained these patterns through the influence of sky conditions, represented by means of broadband solar radiation ratios, solar elevation and dewpoint temperature. Additionally, we have developed weather-dependant functions of this ratio in order to obtain an empirical model that could be used to estimate the photosynthetically-active radiation in those radiometric networks that register solar broadband irradiance. This model can explain over 65% of the variability exhibited by the ratio of photosynthetically-active radiation to broadband solar radiation, avoiding the necessity of local calibration of this ratio.