Estimation of half-hourly diffuse solar radiation over a mixed plantation in north China

Diffuse solar radiation plays important role in carbon exchange between forest ecosystems and the atmosphere. Based on a 2-year (2016–2017) dataset of global and diffuse solar radiation, the annual mean diffuse fraction (the proportion of diffuse solar radiation to global solar radiation, DF) was approximately 0.45 over the plantation canopy. A proposed model (BRL-1), including six predictors (half-hourly clearness index CI, solar elevation angle, apparent solar time, daily CI, a persistence of global solar radiation and relative humidity), was developed using the ground-based measurements data of 2016. The DF values estimated by the proposed model were in good agreement with the measured ones. The values of the coefficient of determination (R2), the root mean squared error (RMSE), the mean bias error (MBE) and the tstat for the proposed model were 0.82, 0.12, 0.002 and 1.16, respectively, lower than the results derived from other models. Therefore, the performance of the proposed model estimating half-hourly DF was better than the other models. Moreover, half-hourly diffuse solar radiation in cloudy skies estimated by the proposed model was more consistent with the measured one than that under clear skies. This study aims at helping us understand the change of diffuse solar radiation with clouds.