Thermal-based modeling of coupled carbon, water, and energy fluxes using nominal light use efficiencies constrained by leaf chlorophyll observations

Authors

M. A. Schull, USDA-ARS
M. C. Anderson, USDA-ARSFollow
R. Houborg, King Abdullah University of Science and Technology
Anatoly A. Gitelson, University of Nebraska at LincolnFollow
W. P. Kustas, USDA-ARS

Document Type

Article

Date of this Version

2015

Citation

Biogeosciences, 12, 1511–1523, 2015

Comments

© Author(s) 2015. CC Attribution 3.0 License.

Abstract

Recent studies have shown that estimates of leaf chlorophyll content (Chl), defined as the combined mass of chlorophyll a and chlorophyll b per unit leaf area, can be useful for constraining estimates of canopy light use efficiency (LUE). Canopy LUE describes the amount of carbon assimilated by a vegetative canopy for a given amount of absorbed photosynthetically active radiation (APAR) and is a key parameter for modeling land-surface carbon fluxes. A carbonenabled version of the remote-sensing-based two-source energy balance (TSEB) model simulates coupled canopy transpiration and carbon assimilation using an analytical submodel of canopy resistance constrained by inputs of nominal LUE (β_n), which is modulated within the model in response to varying conditions in light, humidity, ambient CO₂ concentration, and temperature. Soil moisture constraints on water and carbon exchange are conveyed to the TSEB-LUE indirectly through thermal infrared measurements of landsurface temperature. We investigate the capability of using Chl estimates for capturing seasonal trends in the canopy β_n from in situ measurements of Chl acquired in irrigated and rain-fed fields of soybean and maize near Mead, Nebraska. The results show that field-measured Chl is nonlinearly related to β_n, with variability primarily related to phenological changes during early growth and senescence. Utilizing seasonally varying β_n inputs based on an empirical relationship with in situ measured Chl resulted in improvements in carbon flux estimates from the TSEB model, while adjusting the partitioning of total water loss between plant transpiration and soil evaporation. The observed Chl– β_n relationship provides a functional mechanism for integrating remotely sensed Chl into the TSEB model, with the potential for improved mapping of coupled carbon, water, and energy fluxes across vegetated landscapes.