Stefaan Dondeyne

In recent decades, the Upper Great Ruaha River Basin (UGRRB) has experienced severe water scarcity due to alterations in its river discharge. However, the extent to which these alterations may be attributed to climate change versus land use/cover changes, water withdrawals, etc., remain uncertain. This study aims to assess the historical impact of climate change in the water balance and river discharge using a modelling approach. The Soil and Water Assessment Tool Plus (SWAT+) model was developed, calibrated and validated for the basin. By forcing the model with both factual (including anthropogenic climate change) and counterfactual (excluding anthropogenic climate change) climate data for 1901–2019, the impact of climate change was quantified as the difference between the two model simulations. Furthermore, long–term trends and magnitudes from simulations were assessed by Mann–Kendall and Sen’s slope statistical approaches, respectively at 5% significance level. The SWAT+ model yielded satisfactory performance with a Nash-Sutcliffe Efficiency of 0.76 and 0.67 during monthly calibration and validation periods, respectively. Attribution results from factual and counterfactual model simulations revealed increases in annual precipitation, evapotranspiration, surface runoff, lateral flow and percolation, as well as simulated annual river discharge attributable to climate change. Conversely, analysis of observed annual river discharge data revealed a consistent and significant decline during 1963–2017 (–1.029 m ³ s ⁻¹ yr ⁻¹ , p < 0.05). This discrepancy highlights the influence of non–climatic factors such as land use changes and increased water use. These findings underline the urgent need of integrating both climatic and non–climatic factors into basin’s water management frameworks to mitigate emerging water challenges.

This study investigates how landcover change between 1935 and 2020 have shaped hydrological responses in the semi-arid highlands of Tigray, Ethiopia. Focusing on the Tsili catchment (27.5 km²), it examines links between landcover change, drainage network evolution, and river channel width under conditions of population growth and climate variability. Landcover and drainage maps were derived from historical aerial photographs and satellite imagery for four time steps, and surface runoff was simulated using the SWAT model with uniform meteorological forcing to isolate landcover effects. Results show a 37.6% increase in cropland and substantial declines in shrubland (−29.3%) and forest (−10.1%). River channel width at the outlet widened from 7.5 to 10.5 m, while drainage density increased 1.5-fold. These physical changes aligned with modelled increases in surface runoff. Strong correlations were found between runoff, channel width, drainage density, and landcover types. The findings highlight that cropland expansion—at the expense of natural vegetated land—has intensified runoff and erosion risks. As climate change is expected to bring more intense rainfall to East Africa, this underscores the need for land management strategies that reduce hydrological connectivity and support sustainable agriculture in data-scarce regions.

Land use/landcover (LULC) changes and climate variability impact soil erosion; however, their combined long-term effects are poorly studied. Using remote sensing data, this study investigates changes in LULC and rainfall from 1985 to 2022 and their implications for soil erosion in the Lake Tana Basin, Ethiopia. The Global Land Cover Fine Classification System (GLC_FCS30D) data were used to analyze LULC changes; Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS-v2) were used to assess rainfall trends; and the RUSLE was used to estimate potential soil erosion. The GLC_FCS30D proved to have an overall accuracy of 77.3% for 2005, 80.2% for 2014, and 80.3% for 2022. The cropland area increased slightly, from 32.9% to 33.3%, while tree cover initially decreased from 31.2% to 27.8% before recovering to 29.9%. Overall, annual rainfall increased by 2.92 mm yr⁻¹, though it exhibited strong spatial variability, and rainfall erosivity rose by 1.25 MJ mm ha⁻¹ h⁻¹ yr⁻¹. Despite seemingly modest changes in landcover and rainfall, the combined effect on potential soil erosion was substantial. Potential soil loss in the Lake Tana Basin showed significant spatial and temporal variation, with a slight increase of 0.9% from 1985 to 1995, followed by a slight decrease of 0.12% from 1995 to 2005, and a more substantial decrease of 2.3% from 2005 to 2015 before a notable increase of 8.2% occurred from 2015 to 2022. By elucidating the intricate interactions between landcover changes and rainfall variability, this study enhances our understanding of landscape dynamics in the Lake Tana Basin. The findings highlight the importance of considering the interaction between rainfall and landcover changes in climate change studies, as well as when targeting soil conservation efforts and promoting sustainable land management and ecosystem resilience in the tropics.

The soils of the high-elevation mountains along the East African Rift Valley are poorly understood. Assessing the potential of soils for agriculture, climate change mitigation, and environmental functioning requires insight into how they relate to the factors influencing soil formation. Between 3000 and 4120 m a.s.l., 85 soil profiles of Mount Guna were described and sampled. Standard physicochemical analyses were done on all pedons. Additionally, X-ray diffraction, Al_ox and Fe_ox content, and P fixation were performed on six selected profiles. Soils on Mount Guna included Andosols, Phaeozems, Leptosols, Regosols, Cambisols, Luvisols, and Vertisols. With increasing elevation, clay content, bulk density, and pH decreased while the C:N ratio remained constant. In contrast, sand, silt, silt-to-clay ratio, SOC, Ntotal, and SOCS increased. With a factor analysis, the soil-forming factors’ elevation/climate could be disentangled from the factor’s parent material as these affect topsoil and subsoil differently. In the ordination based on climate/elevation and parent material, Andosols and Vertisols stood out while other Reference Soil Groups (RSG) showed indistinct patterns. Soil erosion appeared as an additional soil-forming factor not accounted for by the factor analysis. The distribution of the RSG was significantly associated with elevation belts (p < 0.001), lithology (p < 0.001), and landcover (p < 0.003). On the summital ridge, the Andosols were crucial for groundwater storage due to high precipitation. Shallow and stony soils in the mid-elevation belt contributed to runoff generation. Average soil carbon stock ranged from 8.1 to 11 kg C m⁻² in the topsoil and from 29.2 to 31.9 kg C m⁻² in the upper meter, emphasizing the global importance of high-elevation areas for carbon sequestration.

The soils of the high-altitude mountains along the East African Rift Valley are poorly understood. Understanding the potential of soils for agriculture, climate change mitigation, and environmental functioning necessitates an understanding of their relationship to soil-forming factors. Therefore, this study focuses on the volcanic soils of Mount Guna. Eighty-five soil profiles, between 3000 and 4120 m a.s.l., were described and sampled along seven topographic transects. The samples were analyzed for physicochemical characteristics using standard methods and classified according to WRB 2022. The clay mineralogy of six profiles was analyzed with X-ray diffraction. The first four-factor axes, which are related to elevation, parent material, climate, and land use were found by factor analysis, explaining more than 60% of the total variation. The clay portions are primarily composed of trioctahedral chlorite, trioctahedral mica (phlogopite or biotite), vermiculite, kaolinite, some quartz, some amorphous silicates (most likely pyroclastic glass), and minor feldspar. The presence of weatherable minerals (biotite, amphibole, feldspars, and so on) suggests that these soils have not been weathered extensively. The dominant Reference Soil Groups found in the study area are Andosols, Phaeozems, Leptosols, Regosols, Cambisols, Luvisols, and Vertisols. As a result, our findings suggest that altitudinal variation, climate, lithology, and their contributions to the variability of soil characteristics and development along the toposequence cannot be separated in this study; more similar studies in other high-elevation/altitude mountains are required. There have been no other studies of high-altitude mountains in East Africa where so many soil profiles have been examined.

Deltas, being areas where river sediments accumulate, are sensitive to changes in climate and anthropogenic processes that affect sediment generation. The Omo Delta, on the Ethiopia–Kenya border, is constantly changing due to fluctuating lake levels, a variable climate, and rapidly changing land use. Due to field data scarcity and the region's limited accessibility, we relied on remote sensing (RS) data to investigate delta extent between 1990 and 2018 and attempted to disentangle the effect of climatic from anthropogenic variables. We obtained a time series of delta extent from Landsat imagery using Random Forest (RF) classification. Using parametric and non‐parametric regression techniques, we regressed the delta extent on Lake Turkana levels, Omo River discharges, Omo‐Gibe Basin rainfall, tree cover loss, and irrigation extent. The RF algorithm distinguished water from land with high (>90%) accuracies and revealed Omo Delta extent fluctuations ranging from 949 km2 in 1993 to 651 km2 in 2000. Lake water level, which depends on the rainfall over the Omo‐Turkana Basin, emerged as the best predictor of delta extent. However, the annual rainfall over the Omo‐Gibe Basin showed no correlation with delta extent. The regression models further show a connection between delta extent, irrigation extent, and tree cover loss. We conclude that rainfall indirectly influences delta extent across the Omo‐Turkana Basin. Regression models indicate additional cumulative effects of human activity in the Omo‐Gibe Basin but fall short in explaining delta dynamics.

Knowledge of hydrologic connectivity is important to grasp the hydrological response at a basin scale, particularly as changes in connectivity can have a negative effect on the environment. In the context of a changing climate, being able to predict how changes in connectivity will affect runoff and sediment transport is particularly relevant for land-use planning. Many studies on hydrology, geomorphology and climatology have been conducted in the Lake Tana Basin in Ethiopia, which is undergoing rapid development and significant environmental changes. This systematic literature review aims at assessing to what extent the hydrologic connectivity has been taken into account in such research, and to identify research gaps relevant to land and water management. On the Web of Science and Scopus databases, 135 scientific articles covering those topics were identified. Aspects of hydrologic connectivity were mostly implicitly taken into account based on process-based, statistical and descriptive models. Amongst the drivers of changing connectivity, the climate was covered by a large majority of publications (64%). Components of structural hydrologic connectivity were accounted for by considering geomorphology (54%) and soils (47%), and to a lesser extent, hydrography (16%) and geology (12%). Components of functional connectivity were covered by looking at surface water fluxes (61%), sediment fluxes (18%) and subsurface water fluxes (13%). While numerous studies of the Lake Tana Basin accounted for the hydrologic connectivity implicitly, these related predominantly to functional components. The structural components are given less attention, while in the context of a changing climate, better insights into their influence on the hydrologic seem most relevant. Better knowledge of the static aspect of connectivity is particularly important for targeting appropriate soil and water conservation strategies. Being able to explicitly assess the ‘structural connectivity’ is therefore of direct relevance for land management and land-use policy.