Impact of hydropower dam operation and management on downstream hydrogeomorphology in semi-arid environments (Tekeze, Northern Ethiopia)

Due to renewed interest in hydropower dams in the face of climate change, it is important to assess dam operations and management in combination with downstream impacts on rivers in (semi-)arid environments. In this study, the impacts of the Tekeze hydropower dam on downstream hydrology and river morphology were investigated, including impacts under normal and extreme reservoir operation conditions. Field observations, in-depth interviews, repeat terrestrial photographs, multi-year high-resolution satellite images, daily reservoir water levels and data on hourly to daily energy production were collected and studied. The results show that high flows (Q(5)) have declined (with factor 5), low flows (Q(95)) have increased (with factor 27), seasonal flow patterns have smoothened, river beds have incised (up to 4 m) and locally aggraded near tributary confluences. The active river bed has narrowed by 31%, which was accelerated by the gradual emergence ofTamarix niloticaand fruit plantations. A new post-dam equilibrium had been reached until it was disrupted by the 2018 emergency release, caused by reservoir management and above-normal reservoir inflow, and causing extensive erosion and agricultural losses downstream. Increased floodplain occupation for irrigated agriculture consequently provides an additional argument for reservoir operation optimization to avoid future risks for riparian communities

Tree line dynamics in the tropical African highlands: identifying drivers and dynamics

Questions: What are the potential drivers of tree line change in the tropical African highlands? Are the temperature-sensitive tree lines in these highlands shifting as a result of climate change? Significance: The high-altitude forests provide important ecosystem services for the vulnerable environment of the tropical highlands. Climate change is expected to have pronounced effects on the tree line limit of these forests. Afro-alpine tropical tree lines are therefore potentially valuable as a proxy of climate change and the related response of ecosystems in the tropical highlands. Location: Tropical African highlands. Results: The influence of climatic factors in the African tropical highlands is significantly different compared to other regions. The potentially determining factors for tree line distribution in tropical Africa are temperature, precipitation and cloudiness, carbon balance, fire and anthropo-zoogenic impacts. Despite recent temperature increase, tree lines have not risen to higher altitudes in the tropical African highlands. Instead, high human pressure has caused stabilization and even recession of the tree lines below their natural climatic limit, particularly through livestock herding. But, even neglecting human pressure, there might be a lag in response time between temperature and tree line change. Conclusions: The actual drivers of tree line change in the African tropical highlands are mainly fire and anthropogenic pressure rather than climate change. But long-term drought periods can be a trigger for fire-induced deforestation of the tree line vegetation. Additionally, in volcanic active mountains, volcanic activity is also a potentially limiting factor for the tree line distribution. Tree line dynamics can thus not be used as a proxy of climate change for the African tropical highlands

Water balance components of the potential agricultural grabens along the Rift Valley in northern Ethiopia

Region: Ethiopia's Rift Valley. Focus: matching agricultural water demand and supply is a growing policy challenge in drylands. We investigated the water balance components in Raya (3507 km(2)) and Ashenge (80.5 km(2)) grabens. The rainfall depth, river discharge, abstraction, climate and soil data (2015-2017) were used to address the research question. New hydrological insights: the average annual rainfall of the graben's escarpment and its bottom was 806 +/- 162 and 508 +/- 110 mm, respectively. Heavy rains produce floods up to 732 m(3) s(-1) in the rivers that flow into the Raya graben. Moreover, greater runoff and river discharges volumes were recorded at the graben escarpments than at the graben bottom outlets (p < 0.001) due to the greater contributing area (p < 0.001, R-2 = 0.98) and headwater elevation (p < 0.001, R-2 = 0.98). About 24% of the water entering both graben bottoms comes from the runoff from the adjacent slopes, and about 40% of the runoff reaching the Raya graben bottom flowed out at the outlet. About 76% and 77.5% of the annual rainfall was lost through evapotranspiration from the Raya and Ashenge grabens, respectively. So about 16% and 33% of the average annual inflows infiltrated into the sediments in the Raya and Ashenge grabens, respectively. These insights provided by this study into the water balance in grabens along the Rift Valley can be used to help achieve sustainable agricultural development

The Impact of River Regulation on Downstream Socio-Hydrologic Systems in Ethiopia

In recent years, a renewed interest in large-scale hydraulic interventions has developed, frequently justified by the premise of making the agricultural and energy sectors climate-resilient. Despite this important climate effort, hydraulic interventions are controversial and have far-reaching impacts on river-dependent communities and the environment. Drawing on gis analyses of remote sensing images and qualitative and quantitative empirical evidence from the field, this PhD dissertation focused on the impact of two large dams and one inter-basin water transfer (ibwt) on downstream socio-hydrologic systems (coupled human-water systems) in Ethiopia. The results indicated that (i) downstream hydrogeomorphic systems drastically altered after the implementation of the hydraulic interventions, (ii) small-scale farmer-led irrigation systems more efficiently increased crop productivities than several largescale irrigation projects, (iii) the newly induced hydrologic regimes strongly altered downstream social interactions due to impeded river crossing and (iv) ill-prepared land redistributions and resettlements left thousands of households with a high risk of impoverishment

Impact of river regulation on downstream socio-hydrologic systems in Ethiopia

In recent years, a renewed interest in large-scale hydraulic interventions has developed, frequently justified by the premise of making the agricultural and energy sectors climate-resilient. Despite this important climate effort, hydraulic interventions are controversial and have far-reaching impacts on river-dependent communities and the environment. Drawing on gis analyses of remote sensing images and qualitative and quantitative empirical evidence from the field, this PhD dissertation focused on the impact of two large dams and one inter-basin water transfer (ibwt) on downstream socio-hydrologic systems (coupled human-water systems) in Ethiopia. The results indicated that (i) downstream hydrogeomorphic systems drastically altered after the implementation of the hydraulic interventions, (ii) small-scale farmer-led irrigation systems more efficiently increased crop productivities than several largescale irrigation projects, (iii) the newly induced hydrologic regimes strongly altered downstream social interactions due to impeded river crossing and (iv) ill-prepared land redistributions and resettlements left thousands of households with a high risk of impoverishment