Evaluating models of shortwave radiation below Eucalyptus canopies in SE Australia

In this study we examine the performance of four models that simulate radiation under forest canopies. The models are different in terms of how transmittance of radiation is conceptualised and the data sources used to obtain parameters. Two models (PAlMS and PAI(NC) models) use plant area index (PAI) to represent transmission. A third model (PL model) represents transmission as a function of the path length (L) of a directional beam as it crosses the canopy. The fourth model (LPI model) is based on a light penetration index obtained directly from lidar. The PAI(MS) and PL models were calibrated using 6 months of radiation data at 4 independent sites. The LPI and PAI(NC) models are uncalibrated. Performance was assessed using sub-daily and daily radiation measurements during summer (December March) at 10 sites in forests ranging from open dry forests (PAI = 1.6) to tall temperate forests (PAI = 4.6). Mean annual precipitation ranged from 760 to 1750 mm across the domain. The PL model with a calibrated extinction coefficient (k(2) = 0.033) was the most accurate model within sites (R-square = 0.32-0.89 for sub-daily radiation) and across all sites (R-square = 0.82 for daily and sub-daily radiation). The LPI model performed well at most sites, but displayed some systematic bias in dense forests resulting in lower performance (R-square = 0.61 across all sites) while PAI(NC) was negatively biased and a poor predictor across all sites (overall R-square = 0). The PAlMS model, with a calibrated extinction coefficient (k(1) = 0.48), produced good results, but inspection of sub-daily radiation patterns show that the model tends to underestimate (overestimate) sub-canopy radiation during times of high (low) radiation. This is because transmittance is highest when the sun elevation is high (i.e. short path length), resulting in sub-canopy radiation that varies non-linearly with the intensity of above canopy radiation. Models that explicitly include path length in the transmission term therefore provide more flexibility for capturing sub-daily, seasonal and latitudinal variation due to sun position and vegetation structure. Other advantages of the path length approach include i) simple parameterisation using tree height as input and ii) explicit representation of diffuse and direct radiation, which can be important when accounting for terrain-effects on energy balance at the forest floor. We therefore recommend path length based modelling approaches for investigating hydrological processes below the canopy in temperate Eucalyptus forests.