An analytical model for estimating canopy transpiration and carbon assimilation fluxes based on canopy light-use efficiency

Abstract

We develop a simple, analytical model for canopy resistance to canopy–atmosphere gas exchange that is well suited for incorporation into regional-scale land-surface parameterizations. This model exploits the conservative nature of canopy light-use efficiency (LUE) in carbon assimilation that is observed within broad categories of plant species. The model paradigm assumes that under standard environmental conditions, a canopy will operate at the field-measured LUE, but will deviate from this standard efficiency as conditions change. Effective LUE estimates generated by the model respond to variations in atmospheric humidity, CO 2 concentration, the composition of solar irradiation (direct versus diffuse beam fractions), and soil moisture content. This modeling approach differs from scaled-leaf parameterizations in that a single estimate of nominal canopy LUE replaces both a detailed mechanistic description of leaf-level photosynthetic processes and the scaling of these processes from the leaf to canopy level. This results in a model that can be evaluated analytically, and is thus computationally efficient and requires few species-specific parameters. Both qualities lend themselves well to regional-and global-scale modeling efforts. For purposes of testing, this canopy resistance submodel has been embedded in the Atmosphere–Land Exchange (ALEX) surface energy balance model. The integrated model generates transpiration and carbon assimilation fluxes that compare well with estimates from iterative mechanistic photosynthetic models, and with flux measurements made in stands of corn, soybean, prairie grasses, desert shrubs, rangeland, and black spruce. Comparisons between modeled and measured evapotranspiration (LE) and carbon assimilation ( A c) fluxes yield mean-absolute-percent-differences of 24% (LE) and 33% ( A c) for hourly daytime fluxes, and 12% (LE) and 18% ( A c) for daily-integrated fluxes. These comparisons demonstrate robustness over a variety of vegetative and climatic regimes, suggesting that this simple analytical model of canopy resistance will be useful in regional-scale flux evaluations.