Advances in thermal infrared remote sensing for land surface modeling

Over 10 years ago, John Norman and co-authors proposed a thermal-based land surface modeling strategy that treated the energy exchange and kinetic temperatures of the soil and vegetated components in a unique “Two-Source Model” (TSM) approach. The TSM formulation addresses key factors affecting the convective and radiative exchange within the soil–canopy–atmosphere system, focusing on the relationship between radiometric and aerodynamic temperature. John Norman's contribution came at a time when thermal-based techniques applied to standard “One-Source Model” (OSM) for large scale land surface flux and evapotranspiration (ET) estimation were generally considered unreliable and not viable for operational remote sensing applications. Others have subsequently modified OSM schemes to accommodate the radiometric–aerodynamic temperature relationship for partial canopy cover conditions, approaching accuracies achieved with the TSM. In this study, Norman's TSM and two current OSM schemes are evaluated over a range in canopy cover and moisture conditions simulated by the Cupid model—a complex soil–vegetation–atmosphere transfer (SVAT) scheme developed by Norman that simulates the complete radiation, convection/turbulence and hydrologic processes occurring at the soil/canopy interface. The use of SVAT simulations permitted the evaluation of TSM and OSM approaches over a greater range of hydrometeorological and vegetation cover conditions than typically available from field observations. The utility of the TSM versus OSM approaches in handling extremes in moisture/vegetation cover conditions simulated by the SVAT model Cupid is presented. Generally the TSM approach outperformed the OSM schemes for the extreme conditions. Moreover, the ability of the TSM to partition ET into evaporation and transpiration components provides additional hydrologic information about the moisture status of the soil and canopy system, and about the vertical distribution of moisture in the soil profile (surface layer vs. root zone). Examples for actual landscapes are presented in the application of the TSM as incorporated within in the Atmosphere Land EXchange Inverse/Disaggregation ALEXI (ALEXI/DisALEXI) modeling system, designed for operational applications at local to continental scales using multi-scale thermal imagery. This strategy for utilizing radiometric surface temperature in land surface modeling has converted many skeptics and more importantly rejuvenated many in the research and operational remote sensing community to reconsider the utility of thermal infrared remote sensing for monitoring land surface fluxes from local to regional scales.