New calculation methods of diurnal distribution of solar radiation and its interception by canopy over complex terrain

Abstract

The prescription of diurnal radiation distribution and the consideration of topographic impact on canopy radiation interception are often required in ecological modelling studies. The most commonly used methods in current models are the sine-curve assumption for diurnal radiation distribution and the topographic geometric projection for canopy radiation interception. In this study, the defects of these two methods are examined and two new methods are proposed. The new method for prescribing the diurnal radiation distribution is based on the assumption that the direct normal radiation and diffusive radiation follow the sine-curve of solar zenith angle. This improvement is particularly important to the modelling strategies of separating canopy leaves into sunlit and shaded. It can also be used to extrapolating daily data to hourly values so that the short time step models can be applied when only daily data are available. The new method for calculating canopy radiation interception over inclined surfaces is based on the hypothesis that the topographic variation of canopy radiation interception is caused by the variation of sunlit/shaded leaf area index. This new method gives the same amount of total radiant energy interception as the geometric projection method, but it may lead to very different impact of topography on ecological processes. The topographic variation of canopy photosynthesis was investigated by using this new approach and compared with that obtained by using the direct geometric projection method. The two new approaches proposed in this study are more physically realistic in regenerating the natural processes. The improvements can benefit ecological models on temporal integration studies as well as spatial scale analysis over complex terrain.