Evapotranspiration assessment in agriculture: a case study in the hazelnut orchards of Viterbo Province, Italy.

Evapotranspiration (ET) is a major component of the global water cycle and provides a critical nexus between terrestrial water, carbon and surface energy exchanges. Climate change influences ET through the combined effects of rising temperatures and increased net radiation, both of which tend to increase ET rates. Reliable estimation of ET across different spatial and temporal scales is therefore crucial for sustainable water resource management and for improving hazelnut productivity. This study integrates multi source satellite observations from Sentinel-2 and Sentinel-3 to estimate high-resolution ET fluxes over 40 hazelnut orchards in Viterbo Province, Italy, from 2016–2024. The objectives of this study were to: (i) estimate and compare ET fluxes in hazelnut orchards using two methods: The Priestley–Taylor approach and the Time Domain Triangle method; and (ii) examine the relationships between ET, climatic variables, and vegetation indicators. The results showed a statistically significant increasing trend in latent heat flux (λET) across 38 hazelnut fields from 2016 to 2024. λET exhibited a strong and significant relationship with temperature in all fields (with R² 0.59 to 0.85) as well as good correlation with NDVI in 13 orchards (with R² > 0.5). Notably, from June to August, NDVI values tended to be negatively correlated with ET, thus suggesting potential water stress. The relationship between λET and cumulative precipitation was generally weak across all hazelnut orchard fields. These findings demonstrate the potential of synergistic Sentinel-2 and Sentinel-3 observations for monitoring field scale ET dynamics in hazelnut orchards.