Applying multiple land surface temperature products to derive heat fluxes over a grassland site
Jiménez C, Michel D, Hirschi M, Ermida SL, Prigent C
Remote Sensing Applications: Society and Environment, 6, 15-24, https://doi.org/10.1016/j.rsase.2017.01.002
Land heat fluxes are essential components of the water and energy cycle and important variables in the management of agronomy and forestry resources. The estimation of the heat fluxes can be done with a number of methodologies, with some of them having the land surface temperature (Ts) as one of their key inputs to derive the fluxes. Here the production of Ts-driven surface heat fluxes over a grassland site in Switzerland is demonstrated by running a specific heat flux methodology (SEBS) fed by a number of satellite Ts estimates (from the instruments AATSR, MODIS, SEVIRI, AMSR-E, and SSMIS). The Ts estimates are compared with an in situ estimate derived from radiometric observations at the station, and the satellite latent heat flux (LE) estimates with the station Eddy Covariance (EC) measurements. The satellite Ts products include estimates at different spatial resolutions (from ~1 to ~25 km) and time samplings (from 2 overpasses per day to 1/2 hourly observations). Root Mean Square Differences (RMSDs) between the daytime satellite and station Ts are 2.72 (AATSR), 4.41 (MODIS), 3.59 (SEVIRI), 3.81 (AMSR-E), and 2.79 (SSMIS), but given the different time samplings and spatial resolutions it is difficult to be conclusive about the accuracy of the Ts estimates. Concerning the flux estimates, for those sensors with midday overpasses, a RMSD of ~25% are found when comparing the instantaneous latent flux (LE, or evaporation expressed as an amount of water) at satellite overpass with the EC observations, which compares well with the accuracy reported elsewhere for similar landscapes. Given that both Ts and LE are evaluated at the station, a link between Ts and LE accuracy is investigated, but it is not apparent for this specific comparison. This could be related to SEBS accuracy also depending on other variables, apart from Ts, but also to the representativeness of the metrics used for the evaluation given the spatial miss-matches existing between the satellite estimates and station observations. Discrepancies were observed between the EC fluxes, the measured surface available energy, and the evaporation estimates from a lysimeter also present at the station, illustrating also the difficulties of the ground observations to provide accurate heat fluxes for satellite evaluation.