USING REMOTE SENSING & MODELING FOR EVALUATING HYDROLOGIC FLUXES, STATES, & CONSTITUENT TRANSPORT PROCESSES WITHIN AGRICULTURAL LANDSCAPES
Title: Using leaf chlorophyll to parameterize light-use-efficiency within a thermal-based carbon, water and energy exchange model
Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 21, 2011
Publication Date: April 5, 2011
Citation: Houborg, R., Anderson, M.C., Daughtry, C.S., Kustas, W.P., Rodell, M. 2011. Using leaf chlorophyll to parameterize light-use-efficiency within a thermal-based carbon, water and energy exchange model. Remote Sensing of Environment. 115:1694-1705.
Interpretive Summary: A standard technique for remote sensing models of carbon uptake by vegetative canopies uses nominal light-use efficiency (LUE) values for broad plant-functional type categories to constrain carbon assimilated per unit radiation absorbed by the canopy. LUE is related to the canopy chlorophyll content, which can also be diagnosed using remote sensing techniques. This paper explores a new method for mapping land-surface carbon fluxes by combining these approaches, working towards algorithms that can be robustly applied over large agricultural areas using primarily data that can be collected by satellite. These methods will have utility in forecasting yield and monitoring the land-atmosphere exchange of carbon dioxide. We find that incorporation of physiological controls on stomatal resistance constrained by the retrieved light use efficiency values also serves to improve estimates of canopy transpiration, making this a potentially valuable tool for monitoring regional water use as well.
Chlorophylls absorb photosynthetically active radiation and thus function as vital pigments for photosynthesis, which makes leaf chlorophyll content (Cab) useful for monitoring vegetation productivity and an important indicator of the overall plant physiological condition. This study investigates the utility of integrating remotely sensed estimates of Cab into a thermal-based Two-Source Energy Balance (TSEB) model that estimates land-surface CO2 and energy fluxes using an analytical, light-use-efficiency (LUE) based model of canopy resistance. The LUE model component computes canopy-scale carbon assimilation and transpiration fluxes and incorporates LUE modifications from a nominal (species15 dependent) value (LUEn) in response to short-term variations in environmental conditions. However LUEn may need adjustment on a daily timescale to accommodate changes in plant phenology, physiological condition and nutrient status. Day to day variations in LUEn were assessed for a heterogeneous corn crop field in Maryland, U.S.A. through model calibration with eddy covariance CO2 flux tower observations. The optimized daily LUEn values were then compared to estimates of Cab integrated from gridded maps of chlorophyll content weighted over the tower flux source area. The time continuous maps of daily Cab over the study field were generated by fusing in-situ measurements with retrievals generated with an integrated radiative transfer modeling tool (accurate to within ±10%) using at-sensor radiances in green, red and near-infrared wavelengths acquired with an aircraft imaging system. The resultant daily changes in Cab within the tower flux source area generally correlated well with corresponding changes in daily calibrated LUEn values derived from the tower flux data, and hourly water, energy and carbon flux estimation accuracies from TSEB were significantly improved when using Cab for delineating spatio-temporal variations in LUEn. The results demonstrate the synergy between thermal infrared and shortwave reflective wavebands in producing valuable remote sensing data for operational monitoring of carbon and water fluxes.