Improving Two-Source Energy Balance Modeling of Evapotranspiration in Complex Terrain: Validation at Alpine Eddy Covariance Sites

In recent years, the Alpine region has experienced an increasing frequency of drought events, leading to periods of reduced water availability and consequent impacts on both agricultural and hydropower production. Evapotranspiration (ET) is a key variable for detecting drought conditions and optimizing water resource management, yet accurate estimates of ET at high spatial resolution remain scarce in mountainous regions. Remote sensing has become a valuable tool for generating spatially distributed ET maps using thermal infrared data. Among the existing methods, the Two-Source Energy Balance (TSEB) model has demonstrated robust performance across diverse land types and climates. In this work, we run TSEB simulations forced by input data optimized for complex terrain to assess the model's behavior in the Alpine region. Key datasets include topographically corrected high-resolution solar irradiance derived from a radiation product based on Meteosat Second Generation data (0.05° spatial resolution) and a high-resolution (5-m) land-cover map specific to the Alpine region. Vegetation height was obtained from a 30-m canopy height map derived from the Global Ecosystem Dynamics Investigation (GEDI) dataset, while biophysical parameters were estimated using distinct algorithms for forested and non-forested areas. We present a validation of the TSEB model at eddy covariance (EC) sites distributed across the Alpine region, representing a wide range of elevations and diverse land cover types. The model's performance was assessed using four configurations: (1) observed input variables from EC sites, (2) the standard Sen-ET implementation of TSEB using coarse resolution data, (3) high-resolution inputs as described above, and (4) a configuration incorporating meteorological data from a high-resolution analysis dataset. This work contributes to the PNRR project RETURN (Multi-risk science for resilient communities under a changing climate) and to the Italian National Drought Hydrological Monitoring System (NatDHMS).