An intercomparison of a ‘bottom-up' and ‘top-down' modeling strategy for estimating canopy transpiration and carbon assimilation fluxes over a wide variety of C3 and C4 plant species

An accurate quantification of energy and carbon fluxes is of great importance for a wide range of ecological, agricultural, and meteorological applications. Two contrasting modeling strategies are currently used widely to retrieve this goal. ‘Bottom-up' models of land-atmosphere carbon exchange are based on detailed mechanistic descriptions of leaf-level photosynthetic processes scaled to the canopy whereas ‘top-down' scaling approaches neglect the behavior of individual leaves and consider the canopy response to its environment in bulk. The objective of this study is to compare the analytical, light-use efficiency (LUE)-based model of canopy conductance embedded in the Atmosphere-Land Exchange (ALEX) surface energy balance model with a commonly used mechanistic model of leaf photosynthesis-stomatal response employing a two-leaf scaling strategy. ALEX is a simplified version of John Norman's detailed soil-plant-atmosphere model Cupid, specifically developed for operational applications. For the purpose of inter comparisons, the mechanistic canopy sub-model was embedded in ALEX. The ability of the two canopy sub-models to reproduce observed diurnal patterns in energy and carbon fluxes measured in a variety of natural and agricultural ecosystems was then effectively evaluated.

Comparisons were done using several years of field data compiled from AmeriFlux sites across the US. The specific advantages and disadvantages associated with each modeling strategy are discussed and a sensitivity analysis employed to establish their susceptibility to uncertainties in tunable parameter specifications. Finally, the usefulness of the two approaches for regional-scale flux evaluations is discussed.