## 4 Discussion
### Water Use Efficiency Variations and Drivers  
The multi-annual eWUE estimates (Figure 7c) showed high temporal and spatial variation across the Horn of Africa which could be influenced by different factors including climate, drought, vegetation cover and physical characteristics of the region [5]. For instance, the highest eWUE in Ethiopia might be due to the presence of forests, increasing temperature and availability of large croplands in the country. The lowest mean annual eWUE in the shrublands was observed probably due to its low vegetation productivity relative to the other land cover types in the region. The annual eWUE showed an overall increasing trend in the Horn of Africa, and the result matches with the findings of [21] who concluded an overall increased mean and trend of eWUE for different land cover types in Africa. However, our result showed the most increasing trend for the grassland rather than the forest land cover which could be due to differences in the spatial extent, GPP and ETdata sources and their spatial resolutions. The observed increasing trend of eWUE in the grassland might be attributed to the growing intensity and fluctuations of precipitation, water stress and drought conditions. Several studies indicated that different vegetation types tend to increase in eWUE due to water stress conditions [45; 50].  
The main driving forces behind the eWUE spatial and temporal variations in the Horn of Africa can be understood from the correlation of eWUE to LST, precipitation, and SPEI-12 (Figure 9). It can be concluded that an increasing precipitation intensity had the most significant influence on the eWUE variations in the region as the significance percentage of the correlation between eWUE and precipitation was the highest compared to the influences of LST and SPEI-12. The precipitation and eWUE had the most positive and significant correlation for the Shrubland and tropic cool-humid ecosystems, while there were negative and significant correlations for the grassland and tropic warm-subhumid ecosystems. The negative correlation between eWUE and precipitation was common in the semi-arid regions of Kenya, Ethiopia, and Eritrea with grassland cover in the region, this could be largely due to different rates of ET and GPP in the last two decades.  
A recent study by [PERSON] et al. [51] confirmed that there was a significant increase in the drought frequency, duration and severity across the Greater Horn of Africa. Especially, a higher magnitude of the drought was reported in Somalia, Ethiopia, and Kenya. The effect of the drought could be immense on different biomes. We specifically investigated the drought in the region using SPEI-12 to understand the recent drought trend (Figure 8b), and its relationship with eWUE (Figure 9c). In large part of the Horn of Africa, the SPEI-12 drought trend showed an increasing trend, however, there were specific areas which showed insignificant decreasing drought trend. Similar non-significant decreasing trends were also identified by [PERSON] et al., [51] and [PERSON] et al., [52] in the Greater of Horn of Africa which covers six other countries apart from our study area. This indicates that the severity of the drought had slightly increased in the past two decades. The decreasing trend in the SPEI time scale corresponds to the increasing hydrological drought especially in the northern latitudes between 10\({}^{\circ}\) N and 18\({}^{\circ}\) N and below the equator where the location and climatic factors had contributed the pattern. The most significant negative correlation between eWUE and SPEI-12 was observed for the shrubland and other biomes in the tropical warm-subhumid zone. The main reason could be probably the decrease in GPP was higher than for ET which led to an increase in eWUE during drought conditions as highlighted by [PERSON] et al. [50].