Mapping Evapotranspiration in the Alps through Two-Source Energy-Balance Models and Multi-Satellite Data Fusion: Scale Effects in Heterogeneous Landscapes

Abstract:

This work aims to assess a diagnostic approach which links evapotranspiration (ET) to land surface temperature (LST) measured by thermal remote sensing in the Alps. We estimate gridded ET, from field (30 m) to regional (1 km) scales. A specific study is performed on water- and energy-limited grassland ecosystems in a dry inner alpine valley in South Tyrol (Italy), to evaluate the model sensitivity to soil moisture, topography and canopy structure variations. The energy balance model TSEB ALEXI (Two Source Energy Balance Atmosphere Land EXchange Inverse) is first applied to Meteosat satellite data. Then ET is estimated by the flux disaggregation procedure DisALEXI driven by MODIS and Landsat LST retrievals. Finally, ET products based on MODIS and Landsat are fused by the algorithm STARFM (Spatial and Temporal Adaptive Reflectance Fusion Model), to obtain daily maps at Landsat ground resolution (30 m). We validate the model by eddy-covariance (EC) measurements from established stations in the Alps. In addition, for studying the scale representativeness of the satellite retrieved ET, we exploit a thermal camera installed on an unmanned aerial vehicle (UAV), addressing a similar spatial scale as EC measurements, which allows to study ET spatial patterns. Results show that in the Alps the fusion with MODIS-retrieved ET does not significantly improve retrievals based only on Landsat acquisitions. This is due to i) the low availability of clear-sky scenes and ii) the small scale (~10 m) changes in soil moisture, topography and canopy density, which control ET patterns in mountainous regions. Specific TSEB model runs driven by UAV-borne thermal sensor data confirm these results. In conclusion, current thermal satellites lack the temporal and spatial resolution required to characterize ET in the Alps. This limitation can be overcome only by developing new high resolution thermal-based remote sensing tools with a higher temporal frequency.