# Modelling High-Resolution Actual Evapotranspiration through Sentinel-2 and Sentinel-3 Data Fusion
## 2 Materials and Methods
### Validation Sites  
Year 2017 measurements from eleven eddy covariance (EC) sites were used in this study, covering a wide range of land cover types and climates. Sites are summarised in Table 1 and data used in validation included the four components of net radiation \(R_{n}\)(shortwave/longwave downwelling/upwelling), soil heat flux \(G\), sensible heat flux \(H\), and latent heat flux \(\lambda E\). In addition, friction velocity, Monin-Obukhov length, and wind direction data from the EC system was used to estimate the satellite pixel footprint contribution [61; 62] to the turbulent fluxes at the satellite overpass. Validation sites comprise 5 agricultural sites, both irrigated and rainfed, including row crops (e.g., Sierra Loma vineyard) and an olive grove (Taous). In addition, two sites over grassland, one humid meadow (Grillenburg) and a semi-arid steppe (Kendall Grassland), one in conifer (Hyltemossa) and one in broadleaved forests (Harvard Forest) are also included in the validation list. Finally, complex heterogeneous landscapes are represented by two wooded savannas (Dahra and Majadas de Tietar). From all these sites, 3 are in Mediterranean climate, and two more in semi-arid climates, whereas the rest of the sites are located in temperate climates.  
Error metrics included mean bias error (\(\sum\left(Obs.-Pred.\right)/N\)), root mean squared error (\(RMSE=\sqrt{\sum\left(Obs.-Pred.\right)^{2}/N}\)), relative RMSE (\(RMSE/\overline{Obs}\)), and Pearson correlation coefficient between  
\begin{table}
\begin{tabular}{l l l l l l l} \hline \hline
**Site (Abrevation)** & **Land Cover** & **Climate** & **Location** & **Z (m)** & **PI/Reference** \\ \hline Choptank (CH) & Cropland, irrigated (rotation of corn and soybean) & Temperate & United States & 5 & [PERSON] (ARS-USDA) \\ Dahra (DA) & Savanna & Semi-arid & Senegal & 9 & [PERSON] \\  & & & 15.40278 & N & & [PERSON] (Univ. Copenhagen) [63] \\ Grillenburg (GR) & Grassland, meadow & Temperate & Germany & 3 & [PERSON] (T.U. Dresden) \\ Harvard Forest (HF) & Broadleaved forest & Temperate & United States & 30 & [PERSON] (Harvard Univ.) \\  & & & & 27.127578 & N & \\ Hyltemossa (HTM) & Conifer forest (spruce) & Temperate & Sweden & 27 & [PERSON] (Lund Univ.) \\  & & & & 56.097584 & N & \\ Kendall Grassland (KG) & Grassland, steppe & Semi-arid & United States & 6.4 & [PERSON] (ARS-USDA) [64; 65] \\ Klingenberg (KL) & Cropland (spring barley and catch crops) & Temperate & 109.94185 W & & 3.5 & [PERSON] (T.U. Dresden) \\ Majadas de Tietar (MT) & Savanna & Mediterranean & Spain & 15.5 & [PERSON] (CEAM) \\ Selhausen (SE) & Cropland (sugar beets and winter barley) & Temperate & Germany & 2.3 & [PERSON] (Julich) \\ Sierra Loma (SL) (previously known as Borden) & Cropland, irrigated (vineyard) & Mediterranean & United States & 5 & [PERSON] (ARS-USDA) \& \\ Taous (TA) & Cropland, rainfed & Mediterranean & Tunisia & 9.5 & [PERSON] (CESBIO) \& Dalenda \\  & & & N 10.60153 E & & [PERSON] (Institut de l’Olivier) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Description of eddy covariance sites used for validation. Sites are listed in alphabetic order. \(Z\) shows the EC measurement height in meters, while the contact person for the EC tower is credited in PI column.  
observed and predicted. Due to the lack of energy closure in the eddy covariance data, unless otherwise stated all metrics were computed after adding the energy balance residual (residual = \(R_{n,EC}-G_{EC}-\lambda E_{EC}-H_{EC}\)) to the latent heat flux, taking the assumption that errors in measurements of \(\lambda E\) are larger than errors in the measurements of \(H\)[68].