Quantifying Topographic Effects on Carbon and Water Fluxes Over Mountainous Areas

Abstract Relative to flat surfaces, mountain terrains modify solar radiation absorbed by vegetation on sloping surfaces, causing changes in mass and energy fluxes, including gross primary productivity (GPP) and evapotranspiration (ET). However, these changes are generally ignored in regional and global ecosystem models and their magnitudes have not been systematically evaluated. In this study, we first validated the Biosphere‐atmosphere Exchange Process Simulator (BEPS) model against measured GPP and ET over mountainous sites, and then applied it to a mountainous region (Fujian Province, China). In BEPS, the topographic effects are systematically considered in the following steps: (1) the satellite‐derived leaf area index (LAI) is projected to sloping surfaces, (2) canopy radiative transfer is modeled relative to the normal to the slope, and (3) the modeled fluxes are reprojected from sloping to horizontal surfaces. Step (1) decreases LAI as sloping surfaces are larger than the corresponding horizontal surfaces, but Step (3) increases fluxes in the opposite way. Because of the nonlinear relationships between fluxes and LAI, GPP and ET simulations without considering the topographic effects are always underestimated, especially on sunlit slopes. The underestimation increases with increasing slope, and for slopes greater than 40°, GPP is underestimated by 11% and ET by 33%, suggesting that existing global GPP and ET products could have been significantly underestimated in mountainous regions.