# Performance of Satellite-Based Evapotranspiration
## 4 Results
### In-situ and Modeled Grassland Evapotranspiration  
Annual in situ precipitation and evapotranspiration in Oromo averaged 1178 and 517 mm over the study period, respectively (Table 1). This results in a positive net water flux into the surface on a yearly basis (661 [mm year\({}^{-1}\)]) as annual evapotranspiration amounts to 44% of annual precipitation. However, during dry summers with precipitation under 100 [mm] (e.g., 2015 and 2016), evapotranspiration can exceed precipitation for up to 100 and 200%. For instance, during the extremely dry summer of 2015 rainfall reached a historical low of only 46.7 [mm] and soil moisture fell to 14-15%, but evapotranspiration was maintained at near average levels and reached 148.2 [mm] (Table 1). Seasonally, evapotranspiration reaches a maximum during spring (216.4 [mm]) when soils are wet and radiation is near its seasonal peak, and then it decreases towards the summer (166.8 [mm]), autumn (60.6 [mm]) and winter (48.2 [mm]) as water and then energy become limiting. The evapotranspiration estimates of the PML_V2 global product also follow the same pattern than the observations in Oromo and provide a longer record (Table 1). The SEBS and SSEBop models produce similar spatial patterns of grassland evapotranspiration during summer days (Figure 4). In contrast, the SEBAL and METRIC models simulate substantially higher spatial variability in evapotranspiration. During the severe summer drought that affected the region in 2014-2015, soil moisture levels in the Oromo station by 13 February 2015 reached some of the lowest levels (0.15 [m\({}^{3}\) m\({}^{-3}\)]) on the 2014-2017 record and surface temperatures exceeded 295 [K] (Figure 5b-c). During this hot summer drought day, the SEBS and SSEBop models estimated an evapotranspiration flux of 4.38 \(\pm\) 1.0 and 4.03 \(\pm\) 0.9 [mm day\({}^{-1}\)], respectively (Figure 4). For the same day, the SEBAL and METRIC models estimated an evapotranspiration flux of 3.89 \(\pm\) 1.75 and 4.24 \(\pm\) 2.49 [mm day\({}^{-1}\)], respectively. The estimated evapotranspiration flux under drought conditions is about 30% lower than the estimates for a similar date (31 January) during the following summer in 2016, when SEBS and SSEBop suggest an evapotranspiration flux of 5.59 \(\pm\) 0.86 and 5.79 \(\pm\) 0.87 [mm day\({}^{-1}\)], respectively. Figure 5 shows a comparison between time series of evapotranspiration obtained from the SEB models and estimates from global ET products and in situ estimates at the Oromo calibration site. Persistent cloud cover precluded evaluating the performance of the models during winter. The  
Figure 3: Estimates of net radiation and comparison with local measurements. (**a**) Satellite-based estimates of instantaneous net radiation [W m\({}^{-2}\)] in pastures during selected spring and summer days. Note that the images correspond to the same dates than those shown in Figure 3 for surface temperature. (**b**) Local comparison against in-situ measurements at the Oromo calibration site. estimates from the SSEBop model closely follow in-situ evapotranspiration, particularly during autumn and summer (Figure 5a). The rest of the models strongly overestimate evapotranspiration during summer. The magnitude of the overestimation in SEBAL, METRIC and SEBS models exceeds 100% in January and February. However, the SEBS model matches SSEBop during autumn. The performance of the global evapotranspiration product GLEAM 3.3b at the Oromo site contrasts strongly during the spring-summer and autumn-winter periods (Figure 5a). The increase in local evapotranspiration with rising temperature and radiation during the spring and early summer is well captured, but evapotranspiration during late summer and autumn is systematically overestimated by around 2 [mm day\({}^{-1}\)]. The poor performance in autumn is likely related to a mixing of land cover types and misrepresentation of local vegetation phenology in this product due to its coarse resolution (\(\sim\)25 km).