# Performance of Satellite-Based Evapotranspiration
## 4 Results  
### Estimated Surface and Meteorological Fields  
The spatial patterns of estimated \(\rm T_{s}\) for selected spring and summer days over the study domain are shown in Figure 2 along with a pixel-level validation against ground observations at the Oromo calibration site. The latitudinal temperature gradient is weaker than variations associated with vegetation cover and elevation. There is a clear contrast between warmer pastures in the interior lowlands and colder rainforests in the Andean and Coastal mountains to the eastern and western parts of the domain, respectively.  
The region experienced a pronounced warming during 2016, with most of the pastures exceeding 25 \({}^{\circ}\)C (Figure 2a). The estimated high temperatures (\(>\)300 K) for this period compare well with in-situ measurements at the Oromo calibration site (Figure 2b). Atmospheric moisture demand increased rapidly, and pasture productivity declined (Figure 1b), leading to many farmers to irrigate these historically rainfed systems to maintain productivity levels. Satellite-based temperature estimates show a good linear fit with in-situ measurements (\(r^{2}\) = 0.92) but have a cold bias of \(-\)1.26 K (Figure 2b).  
Cloud cover can significantly reduce data availability during the wet and even the dry summer season, resulting in substantial spatial gaps in satellite-based radiation and surface temperature fields (e.g., summer 2015 in Figure 3). The interpolated meteorological fields of air temperature, instantaneous solar radiation and relative humidity capture the large-scale patterns of variation reasonably well but show substantial uncertainties at the pixel level due to the sparseness of the underlying network of agrometeorological stations (Supplementary Materials Figure S1). Pixel-level validation at our Oromo calibration site shows RMSE values of 6.12 [%] for relative humidity, 248 [W m\({}^{-2}\)] for global radiation, 0.90 [K] for air temperature and 1.15 [m/s\({}^{-1}\)] for wind speed (Supplementary Materials Figure S2).  
Instantaneous net radiation for summer days averages 575.02 \(\pm\) 91.84 [W m\({}^{-2}\)]. As expected, R\({}_{\rm n}\) was homogeneous over grasslands during clear sky-days such as 11 Oct 2015 and 31 January 2016 (Figure 3a). Under cloudy conditions, such as in 13 February 2015 and 6 March 2017, R\({}_{\rm n}\) values are lower (\(\sim\) 300 [W m\({}^{-2}\)]) and more spatially variable. At the Oromo calibration site, the linear fit of satellite-based R\({}_{\rm n}\) is 0.70 with an RMSE of 208.38 [W m\({}^{-2}\)] and a substantial overestimation (BIAS of 175.25 [W m\({}^{-2}\)]) due to the retrieved surface albedo for grasslands (Figure 3b). Indeed, the estimated surface albedo (\(\alpha\) = 0.16) is 9% lower than the average albedo measured in Oromo (\(\alpha\) = 0.27). In addition, the presence of clouds, as indicated by the high variability of in situ measurements, is associated with greater departures between satellite estimates and in-situ measurements.  
Figure 2: Estimates of surface temperature and comparison with in-situ measurements. (**a**) Satellite-based estimates of T\({}_{\rm s}\) for the study area during selected spring and summer days. White gaps in the images represent regions with cloud cover. (**b**) Local comparison with in-situ measurements at the Oromo calibration site.