## 1 Introduction
Time series of NDVI and LAI can provide very valuable information about changes in the terrestrial ecosystem and specifically detect trends in the vegetation dynamics and phenology [17]. The assessment of eWUE based on GPP and ET in addition to NDVI variations can be essential to understand the ecosystem carbon-water coupling under climate change in the region [18], and this may further increase the information to predict vegetation growth, ecosystem resilience analysis, and ecosystem management [19]. The vegetation dynamics studies in the African continent are focused more on the Sahel region and only a few are found in East Africa with a limited extent to the Horn of Africa. For instance, a study on trends of vegetation dynamics for the African Sahel showed positive greening trends in NDVI and a net increase in biomass production between 1982 and 2003 [20]. Although the reasons are largely attributed to rainfall variations, further analysis based on higher resolutions from MODIS and LANDSAT is recommended. A recent study on ecohydrological resilience for the whole of Africa using coarse spatial resolutions identified 31.22% of the terrestrial ecosystems being non-resilient to ecosystem shifts [21]. In East Africa, the vegetation dynamics based on LAI assessment between 1982 and 2011 showed LAI increased in the region at a rate of about \(4\times 10^{-3}\) units per year and it is believed the trend encountered series of breaks and variations [22]. Conversely, in the same period between 1982 and 2011, vegetation trend estimates based on LAI in Eastern Africa demonstrated a persistent decrease in areas from Southern Ethiopia through central Kenya to central Tanzania [23]. Moreover, [PERSON] et al. [24] found a positive linear trend between 1981 and 1998, while there was a negative linear trend during 1998-2015 over East Africa using the NDVI3g dataset. Likewise, [PERSON] et al. [25] assessed vegetation variations in the Horn of Africa based on GIMMS data and concluded that the mean NDVI slowly decreased from 1998 to 2013. This research aims to provide a detailed and higher spatial resolution assessment in the vegetation dynamics and ecosystem resilience analysis to climate variables (precipitation and land surface temperature), and the drought factor, considering the limited focus of such study in the region. The improved outputs can supply up-to-date regional spatial information for ecosystem management and planning. There are gaps and disagreement in research findings related to the vegetation trends in East Africa, and a lack of ecosystem resilience study to drought which is the main natural hazard in the region. Moreover, most of the vegetation dynamics studies in the Horn of Africa give less attention to interpretation based on agroecological zones, and it has not been possible to quantify ecosystem water use efficiency in the region. Therefore, the specific objectives of this study are three-fold: (i) To analyze the annual and seasonal MODIS NDVI trends in the last two decades; (ii) to find the main causes of vegetation change and water use efficiency variations based on main climate variables; and (iii) to assess ecosystem water use efficiency and ecosystem resilience to drought in the Horn of Africa.