## 3 Results
### Annual Ecosystem Water Use Efficiency, Trends and Drivers
#### 3.3.2 Water Use Efficiency and Drought Trends, and Correlation with Climate Variables  
Figure 8a shows the annual eWUE linear trend during 2000-2014. Overall, the slope of the WUE increased at an average rate of 9.6 \(\times\) 10\({}^{-3}\) g C kg\({}^{-1}\) H\({}_{2}\)O y\({}^{-1}\). The annual eWUE trend increased significantly at a higher rate in larger parts of Kenya and Ethiopia. On the contrary, the annual eWUE showed a decreasing trend in Eritrea especially in southwestern parts of the country, and the average increasing eWUE trend was the smallest in the country (\(-3.7\times\) 10\({}^{3}\) g C kg\({}^{-1}\) H\({}_{2}\)O y\({}^{-1}\)). The annual eWUE based on agroecological zones showed the maximum increasing trend in the tropical warm-semiarid zone, however, all other agroecological zones showed a more or less similar increasing trend. Likewise, the annual eWUE showed the highest increasing trend in the grassland from all other land cover types.  
We also evaluated SPEI-12 and generated trend map for the last 15 years to assess the degree of drought severity in the Horn of Africa and associated it with the ecosystem water use efficiency patterns. Figure 8b shows the SPEI-12 trend during 2000-2015. The temporal drought trend using Mann-Kendal showed an overall increasing trend with the mean slope value of 0.05867 y\({}^{-1}\). The most severe drought was observed in the year 2009 with a minimum of \(-1.7\) SPEI scale of the annually averaged values. There is about 19.4% (404,787 km\({}^{2}\)) of the study area showing a decreasing SPEI-12 trend. This indicates the severity of the drought had slightly increased in recent years as the decreasing trend in the SPEI time scale corresponds to the increasing drought pattern especially in the northern latitudes between 10\({}^{\circ}\) N and 18\({}^{\circ}\) N.  
Figure 7: (**a**) Mean annual distributions of GPP (**a**, in g C m\({}^{-2}\)), of ET (**b**, in mm) and of eWUE (**c**, in g C kg\({}^{-1}\) H\({}_{2}\)O) during 2000–2014 in the Horn of Africa.  
Moreover, we assessed the pixel-based correlation coefficient to understand the relationship of the annual eWUE with climatic drivers (LST and Precipitation) and drought (SPEI-12) during the period 2000-2014. About 42.6% (940,015 km\({}^{2}\)) of the area in Figure 9a shows a negative correlation between annual eWUE and mean LST of which 3.26% (30,645 km\({}^{2}\)) is statistically significant (P < 0.05). There is also about 49.28% (1,095,745 km\({}^{2}\)) area in Figure 9b showing a strong and positive correlation between annual eWUE and total annual precipitation of which 4.1% (44,926 km\({}^{2}\)) is significant. Similarly, about 36.54% (723,523 km\({}^{2}\)) of the study area demonstrates a negative correlation between annual eWUE and SPEI-12 with only 1.3% significant pixels (9406 km\({}^{2}\)). The correlation of eWUE was the most positively correlated with SPEI-12 in the tropical cool-arid zone, and it had a strong and negative correlation in the Tropical warm-subhumid zone.  
Figure 8: (**a**) Ecosystem water use efficiency annual trend; (**b**) drought trend using SPEI-12 in the Horn of Africa during 2000–2019.  
Figure 9: The correlation coefficient of annual eWUE versus mean annual land surface temperature (**a**), total annual precipitation (**b**), and SPEI-12 during 2000–2014 in the Horn of Africa (**c**).