One-minute assessment of photosynthetic active solar radiation (PAR) models in Uruguay.

Abstract

Photosynthetically Active Radiation (PAR) is the spectral portion of global solar radiation (between 400 and 700 nm) relevant to a plant's growth processes. The PAR fraction (fp) is the ratio at horizontal surface between the photon’s flux per square meter (Qp) and the global solar irradiance (Gh). In this work, the first assessment in Uruguay of PAR fraction empirical models is presented using 4 years of 1-minute measured data for one site representative of the Pampa Húmeda region. The chosen models have been developed for the 1-hour time scale and use the clearness index and/or the solar altitude as predictive variables. Original and locally adjusted versions of these models are evaluated and compared with the utilization of a constant value for PAR fraction (frequently used in agronomical practice). It is found that polynomial kt models with original coefficients have acceptable performance, but they cannot be used with locally adjusted coefficients at the 1-minute timescale.