One-dimensional models of radiation transfer in heterogeneous canopies: a review, re-evaluation, and improved model

Despite recent advances in the development of detailed plant radiative transfer models, large-scale canopy models generally still rely on simplified one-dimensional (1-D) radiation models based on assumptions of horizontal homogeneity, including dynamic ecosystem models, crop models, and global circulation models. In an attempt to incorporate the effects of vegetation heterogeneity or clumping within these simple models, an empirical clumping factor, commonly denoted by the symbol Omega, is often used to effectively reduce the overall leaf area density and/or index value that is fed into the model. While the simplicity of this approach makes it attractive, Omega cannot in general be readily estimated for a particular canopy architecture and instead requires radiation interception data in order to invert for Omega. Numerous simplified geometric models have been previously proposed, but their inherent assumptions are difficult to evaluate due to the challenge of validating heterogeneous canopy models based on field data because of the high uncertainty in radiative flux measurements and geometric inputs. This work provides a critical review of the origin and theory of models for radiation interception in heterogeneous canopies and an objective comparison of their performance. Rather than evaluating their performance using field data, where uncertainty in the measured model inputs and outputs can be comparable to the uncertainty in the model itself, the models were evaluated by comparing against simulated data generated by a three-dimensional leaf-resolving model in which the exact inputs are known. A new model is proposed that generalizes existing theory and is shown to perform very well across a wide range of canopy types and ground cover fractions.