USING REMOTE SENSING & MODELING FOR EVALUATING HYDROLOGIC FLUXES, STATES, & CONSTITUENT TRANSPORT PROCESSES WITHIN AGRICULTURAL LANDSCAPES
Title: Combining observations in the reflective solar and thermal domains for improved mapping of carbon, water and energy fluxes
Submitted to: IEEE Transactions on Geoscience and Remote Sensing
Publication Type: Proceedings
Publication Acceptance Date: April 30, 2010
Publication Date: July 25, 2010
Citation: Houborg, R., Anderson, M.C., Kustas, W.P., Rodell, M. 2010. Combining observations in the reflective solar and thermal domains for improved mapping of carbon, water and energy fluxes. IEEE Transactions on Geoscience and Remote Sensing. July 25-30, 2010, Honolulu, Hawaii. 2010 CDROM.
The REGularized canopy reFLECtance (REGFLEC) modeling tool integrates leaf optics, canopy reflectance, and atmospheric radiative transfer model components, facilitating accurate retrieval of leaf area index (LAI) and leaf chlorophyll content (Cab) directly from at-sensor radiances in green, red and near-infrared wavelengths. Cab is particularly useful for monitoring vegetation productivity and is an important indicator of the overall plant physiological conditions. This study investigates the utility of REGFLEC retrievals of LAI and Cab for optimizing CO2 and energy fluxes simulated by a thermal-based Two-Source Energy Balance (TSEB) model that implements 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 (species-dependent) value (LUEn) in response to variations in environmental conditions. However LUEn needs
adjustment on a daily timescale to accommodate changes in plant phenology, physiological condition and nutrient status. Day to day variations in LUEn, assessed for a corn crop field in Maryland U.S.A. through model calibration with CO2 flux tower observations, were found to correlate well with daily changes in Cab derived from aircraft radiance observations, and hourly carbon and energy flux estimation accuracies were significantly improved when using Cab for delineating spatio-temporal variations in LUEn. The applicability of the established curvilinear relationship between LUEn and Cab was also tested for an agricultural area near Bushland, Texas. LUEn was distributed over the modeling domain using Cab retrieved from SPOT and Landsat radiance data whereas the thermal input to TSEB was taken from ASTER and Landsat data. The modeled carbon and energy fluxes were compared with eddy covariance measurements made in stands of cotton and wheat and with fluxes measured by an aircraft flying transects over irrigated and non-irrigated agricultural land and natural vegetation. The results demonstrate utility in combining remotely sensed observations in the reflective solar and thermal domains for estimating carbon and water fluxes within a coupled framework.