# Performance of Satellite-Based Evapotranspiration
## 4 Results
### In-situ and Modeled Grassland Evapotranspiration
The higher-resolution PML_V2 product has a better performance than GLEAM 3.3b and follows more closely in-situ evapotranspiration through the year, although there is some systematic underestimation (\(<\)0.5 [mm day\({}^{-1}\)]) during summer (Figure 5a). The PML algorithm is likely overestimating moisture limitation to evapotranspiration during summer, when surface soil moisture typically falls below 20% (Figure 5b). Figure 5: Evaptranspiration estimates and daily surface meteorology at the Oromo calibration site (OCS) between August 2014 and December 2017. (**a**) Comparison of satellite-based and measured daily evapotranspiration. Measured ET\({}_{\text{c}}\) in Oromo (blue line) is compared with estimates by SEB models (points) and GLEAM 3.3b (yellow line), and PML_V2 (turquoise line) products for the nearest pixel to the station. (**b**) Daily rainfall (blue bars) and soil moisture (purple, turquoise lines) in Oromo. (**c**) Net daily radiation (purple) and surface temperature (orange) measured in Oromo. Large differences between SEBAL and METRIC models during spring and autumn were observed in Oromo when soil moisture peaked following rainfall events, causing an underestimation of H at the anchoring pixels and thus increasing the uncertainty of estimated \(\rm ET_{a}\). During summer 2017, a strong rainfall event increased soil water availability and increased \(\rm ET_{a}\) at Oromo from 2 to 5 [mm day\({}^{-1}\)] (Figure 5a,b). In addition, this event was also revealed by SSEBop, which matches in-situ estimates and other models such as SEBS and METRIC. The rest of the models did not show the impact of rainfall during summer under drought conditions. The relative performance of SEB and global models against Oromo measurements is shown in Figure 6. SSEBop and SEB are by far the best models in terms of error and correlation with observations, followed by the global evapotranspiration product PML_V2 (Figure 6a). These products are able to explain between 63 and 79% of the variance (\(R^{2}\)) of the in-situ observations (Figure 6b). The SEBS model accounted for 74% of the variance in observed evapotranspiration (\(\rm ET_{a}\)), with relatively low error (RMSE 1.08 [mm day\({}^{-1}\)] and MAE of 0.85 [mm day\({}^{-1}\)]; Figure 6b) but slight underestimation when estimated instantaneous \(\rm R_{n}\) is lower than 300 [W m\({}^{-2}\)] (bias: 0.41 [mm day\({}^{-1}\)]). SSEBop performed slightly better than SEBS (\(\rm R^{2}\): 0.79, RMSE: 0.68 mm day\({}^{-1}\), MAE: 0.55 [mm day\({}^{-1}\)]) with the lowest bias (0.09 [mm day\({}^{-1}\)]). In contrast, SEBAL presents a low statistical fit (\(\rm R^{2}\): 0.24) in addition to high error, with a RMSE of 2.46 [mm day\({}^{-1}\)] and MAE: 2.01 [mm day\({}^{-1}\)] mainly due to its high bias (1.22 [mm day\({}^{-1}\)]) during summer. METRIC had the poorest performance, with maximum RMSE of 2.57 [mm day\({}^{-1}\)], high dispersion of estimated values (\(\rm\sigma\): 2.11 [mm day\({}^{-1}\)]) and close to null correlation with Oromo measurements (\(\rm R^{2}\): 0.05). Therefore, the values estimated by METRIC are not representative over the study area. GLEAM shows an overall overestimation, with a bias up to 0.75 [mm day\({}^{-1}\)] and consequently low \(\rm R^{2}\) (0.35) and high RMSE 1.45 [mm day\({}^{-1}\)]). In contrast, PML_V2 shows a better agreement with in situ observations (\(\rm R^{2}\): 0.63, RMSE: 0.88 [mm day\({}^{-1}\)]) than GLEAM but it is the only model that underestimates (bias \(-\)0.28 [mm day\({}^{-1}\)]) high evapotranspiration values (Figure 6b). Figure 6: Performance of SEB models and global evapotranspiration products in Oromo. (**a**) Taylor diagram of evapotranspiration models in relation to the in-situ observations. The dashed green line denotes the root mean standard deviation (RMSD), while the dashed blue line indicates the correlation coefficient with the observations. (**b**) Scatterplots comparing estimated (\(\rm ET_{a}\)) and measured (\(ET_{c}\)) evapotranspiration in Oromo. The worst performing models SEBAL and METRIC are not shown.