Abstract
Global climate projections expect an intensification of hydrological and agricultural droughts triggered by increasing temperatures in recent years. Due to changing climatic conditions, many areas are prone to deviations in water availability and water use that can be observed from evapotranspiration (ET) anomalies. Land surface temperature (LST) plays an essential role in regulating the exchange of water and energy between land and atmosphere, which directly relates it to ET. In this work, we exploit Sentinel-3 (S3) LST imagery for two-source energy balance (TSEB) modeling of ET at different spatial scales, including both original S3 dataset at 1 km spatial resolution and its downscaled products with 20-m and 100-m pixel size [Kustas & Norman, 1999]. In this regard, LST maps derived at multiple spatial scales together with multispectral Sentinel-2 (S2) instrument and ERA5 climate reanalysis dataset are used as main forcings for the TSEB model. First, we derive reflectance and biophysical parameters corresponding to different resolutions of the S2 datasets for thermal downscaling and then for ET estimation afterwards. In this context, we exploit relationships between 1-km LST grids and fine-resolution explanatory variables (both 20 m and 100 m) using decision trees algorithm [Gao et al., 2012]. Due to reduced capabilities of univariate models, multi-source predictors are considered, including time-coincident S2 observations to Sentinel-3 overpass along with slope and aspect derived SRTM DEM product. Consequently, meteorological forcings and solar radiation from ERA5 are generated for estimating instantaneous energy fluxes and daily ET at different spatial levels. This work will help to understand practical utility of the improved LST product for estimating TSEB outputs at higher spatial resolution and studying its performance against coarse-resolution data and in-situ measurements from eddy-covariance towers. Additionally, time-series of the improved LSTs are intended to temporally complement planned high-resolution thermal missions, including the ESA LSTM, TRISHNA, and SBG-Thermal as alternatives to the 1-km thermal observations from Sentinel-3 and MODIS instruments.