Gabriel Stecher

Flooding associated with rising inland lake levels represents a distinct and understudied hazard compared to conventional river or coastal flooding. Unlike riverine floods, lake inundation develops gradually and persists over extended periods, yet integrated flood risk assessments specifically addressing this phenomenon remain scarce. This study uses Lake Baringo in Kenya’s Rift Valley as a case study, a lake that has experienced exceptional water level rises of nearly 10 m since 2010, to assess flood risk through the integration of scenario-based flood hazard modelling, spatial exposure assessment, and household vulnerability analysis. Three lake level scenarios were developed, spanning from maximum observed conditions to a worst-case threshold at the lake’s sill point. Results show that under the worst-case scenario, the lake area could expand by approximately 64%, potentially exposing nearly 18,000 people and 156 km of road infrastructure. A Flood Vulnerability Index (FVI) ranging from 0.58 to 0.73 indicated consistently high socioeconomic vulnerability across all shoreline communities, with vulnerability accounting for 42–53% of overall flood risk across scenarios. Flood risk was highest along the flat southern shoreline, where topography amplifies lateral inundation extent. The findings highlight the importance of integrating lake level dynamics into spatial planning and flood risk management, and the need for improved monitoring, livelihood diversification, and institutional coordination in semi-arid inland lake regions facing increasing hydro-climatic variability.

Soil erosion poses severe threats to agricultural land in Sub-Saharan Africa, yet locally validated evidence to guide conservation planning remains scarce. Most catchment-scale assessments apply the Universal Soil Loss Equation (USLE) but often omit the support practice factor (P-factor), introducing a systematic bias into erosion estimates. This study introduces a participatory systems mapping approach (CoSMoS) to co-produce spatially explicit soil and water conservation practice (SWCP) inventories for the Mpanga catchment, Western Uganda, thus translating stakeholder knowledge into current and future spatially distributed P-factor layers, integrated into a USLE-based erosion modelling framework. Current conservation efforts, represented by a catchment-wide P-factor of 0.78, show a mean erosion rate of 7.8 t·ha⁻¹·y⁻¹ and reduce mean annual soil erosion by 28% relative to the baseline without SWCPs (10.8 t·ha⁻¹·y⁻¹), However, severe erosion hotspots persist, with over 25% of the catchment exceeding erosion of 11.4 t·ha⁻¹·y⁻¹. Future conservation pathways were developed by upscaling adoption rates of stakeholder-identified priority agricultural conservation practices by 50% and 100% under both general and spatially targeted allocation strategies. Targeted allocation scenarios consistently lower erosion compared with general upscaling at the same levels of SWCP adoption upscaling. The spatially targeted conservation with a 10 t·ha⁻¹·y⁻¹ threshold, achieved a mean catchment-wide erosion reduction of 35%, which was greater than the 22% achieved under 100% upscaling. Hotspot erosion is reduced by more than 60% under the most ambitious targeted scenario (5.1 t·ha⁻¹·y⁻¹). These findings provide Catchment Management Committees with spatially explicit, locally validated evidence directly addressing Uganda's Catchment Management Plan priorities and Land Degradation Neutrality commitments. The CoSMoS approach offers a replicable and cost-effective framework for spatial distributed P-factor estimation in data-scarce catchments across Sub-Saharan Africa.

The likelihood of unprecedented precipitation extremes is increasing across Sub-Saharan Africa (SSA), yet where and when future events may exceed the full range of historical experience remains understudied. While previous studies have documented historical trends and projected changes in precipitation extremes across sub regions of Africa, integrated SSA-wide assessments explicitly identifying hotspots of record-breaking precipitation extremes remain limited.Here, we present a sub-continental, SSA-wide assessment of the emergence of unprecedented precipitation extremes under future climate change. Unprecedented extremes are defined as future events (2030-2100) that exceed the observed and simulated range during a historical reference period (1950-2014). Using precipitation-based extreme metrics relevant to water security (e.g., maximum 1-day rainfall, maximum 5-day rainfall, consecutive wet and dry days, maximum and minimum seasonal rainfall amount), derived from Coupled Model Intercomparison Project – Phase 6 (CMIP6) multi-model large-ensembles, we explicitly assess the future time and regional hotspot of emergence of record-breaking precipitation conditions. We also examine changes in the probability of emergence of unprecedented extremes and their potential large-scale ocean-atmospheric drivers (e.g., El Nino-Southern Oscillation, Atlantic Multidecadal Variability, and Indian Ocean Dipole), while accounting for uncertainties associated with both model physics and internal climate variability.By systematically identifying where and when observed and retrospectively simulated precipitation limits are exceeded, this study offers a new sub-continental perspective on the emergence of unprecedented hydroclimatic conditions and provides a robust foundation for assessing future water security risks and supporting climate-resilient planning in Sub-Saharan Africa under increasing hydroclimatic uncertainty.

Interactions of humans with the environment are strongly related to land use and land cover changes (LULCCs). In the last decades, these changes have led to a degradation of ecosystem services, including water regulation and flood control. In the Alpine areas of Austria, land cover changes have increased flood risk since the middle of the 19th century. In this paper, we assess the influence of these long-term land use changes on the landscape’s ability to retain water using the qualitative Water Retention Index (WRI). The changes are thereby evaluated on the basis of the historical (1826–1859) and present (2016) land cover situation, which is to our knowledge the first high-resolution and regional application of the WRI. The results show that the water retention potential mimics the mountainous characteristic and features. Except for areas strongly dominated by settlement areas, the highest retention potentials are found in valley floors and the lowest values are depicted along the main Alpine complex. In low-lying areas, the retention decreased by over 10%. It was found that this decrease can be mostly attributed to settlement expansion. Above 1,250 m, land use transformations led to slightly increasing water retention values owing to the transformation of wasteland or glaciers to stagnant waters and to the expansion of forest and grassland in high elevations. This examination allows for a holistic and spatially distributed LULCC impact assessment on the landscape’s water regulation capacities and offers valuable high-resolution information for future land use planning and sustainable land development.

The Rift Valley lakes of Kenya are biodiverse ecozones, classified not only as RAMSAR wetlands of international importance, but also as UNESCO World Heritage. In the last decade, starting in 2010, several Kenyan lakes have experienced significant rises in water levels. The consequences have been severe. Inundations of the riparian areas have not only flooded homes, schools and hospitals, but also the basis for the local livelihoods and economy such as agricultural fields, national parks or tourism infrastructure has been destroyed. Nearly eighty thousand households with 400,000 people are affected according to a governmental report published in 2021.There is fear of an ecological catastrophe, should the water levels of the alkaline Lake Bogoria continue to rise. The result would be an overflow and mixing with the freshwater Lake Baringo, the basis of the local community supporting drinking water provision, agriculture, tourism and fisheries. Currently no analysis exists to understand the topographical conditions between the two lakes and the potential flow paths. It is unclear, what climatic rainfall conditions are necessary to lead to an overflow and sustained flows from Lake Bogoria towards Baringo. This analysis therefore assesses (i) the overflow or sill point location and potential flow paths towards Lake Baringo, (ii) the required lake water volume changes, until the sill point is reached and (iii) the mean rainfall conditions, which would be necessary to provide the required volume and a sustained flow towards Lake Baringo. The analysis relies on satellite altimetry-based lake levels and lake volume variations, remote sensing-based rainfall data and lake areas but also high-resolution space-borne and UAS-based DEMs. Field surveys and electrical conductivity measurements used as a proxy to understand flow paths in the flat and swampy Loboi plain between Bogoria and Baringo complement the data basis.To reach the sill point elevation, the lake level of Bogoria would have to increase by 3.5-4 m, compared to the water levels of 2020. This translates to an additional water volume of around 0.17 km³. To put this into perspective: From 2009-2014, lake levels increased by 3 m (or 0.11 km³), and after a recession phase, from 2017-2020 by 2.3 m (0.10 km²). If the sill point would be reached, the rainfall conditions in the last decade would have allowed for a permanent flow towards Baringo, at least in single years. These preliminary results suggest that it is feasible that the sill point can be reached and that a permanent flow from Bogoria towards Baringo can be sustained, if wetter conditions – as we have observed in the last decade - persist for a longer period.

Long-term land use and land cover changes (LULCC) are estimated to affect almost one third of the global land area (Winkler et al., 2021). This also alters hydrological processes and has implications on the ability of the landscape to retain water. In alpine areas of Austria, extensive LULCC have occurred since the mid-19th century, which also led to changes in flood hazard and an increase in flood risk, especially in the valley floors.This contribution analyses, how long-term LULCC from the 1820s until now affect the water retention potential of the Austrian catchments of the rivers Rhine, Salzach and Drava. The Water retention index (WRI) (Vandecasteele et al., 2017) was calculated at a high spatial resolution (100*100 m) for the past and present LULC situation. The WRI is a qualitative indicator and shows the water retention capacity on a relative scale (0-10) by a composition of the governing physical processes (e.g. interception, percolation) through proxy datasets.The resulting WRI maps of the historic and present state reveal that the general spatial features and characteristics exhibit similar WRI patterns. High values (WRI > 5) occur in valley floors and rather flat areas. Areas dominated by steep topography and alpine characteristics show low WRI values (< 3). The comparison of the two time periods shows a moderate to strong reduction (< -2) of the water retention potential especially in the alpine valleys and low elevations for the current state. This is largely explained by the expansion and development of settlement areas and soil sealing. Additionally, the draining of wetlands, river channelization and disconnection of flood plains and deforestation also strongly reduced the WRI values. In contrast, increasing WRI values occur primarily in areas at higher altitudes. Here, forest areas increased and wasteland transformed to grassland. In addition, new artificial water reservoirs have been constructed to produce hydropower, which have a positive effect on the retention potential. Generally, the spatial and altitudinal changes in the water retention capacity reflects the land and settlement development in the past 150 years. This development resulted in higher flood exposure but might have also reduced flood hazards due to higher water retention capacities.References: Vandecasteele, I., Marí i Rivero, I., Baranzelli, C., Becker, W., Dreoni, I., Lavalle, C., and Batelaan, O., 2017. The Water Retention Index: Using land use planning to manage water resources in Europe. Sustainable Development, 26 (2), 122–131. https://doi.org/10.1002/sd.1723Winkler, K., Fuchs, R., Rounsevell, M., and Herold, M., 2021. Global land use changes are four times greater than previously estimated. Nature Communications, 12 (1), 1–10. https://doi.org/10.1038/s41467-021-22702-2

Interactions of humans with the environment are strongly related to land use and land cover changes and the available ecosystem services. In last decades, these changes have led to a degradation of ecosystem services, including water regulation and flood control. In Alpine areas of Austria, land cover changes have increased flood risk since the mid of the 19 ^th century. In this paper, we assess the influence of these long-term land use changes on the landscape’s ability to retain water using the qualitative Water Retention Index (WRI). The changes are thereby evaluated using the historical (1826-1859) and present (2016) land cover situation, which is to our knowledge the first high-resolution and regional application of the WRI. The results show that the water retention potential mimics the mountainous characteristic and features. Except for areas strongly dominated by settlement areas, the highest retention potentials are found in valley floors and the lowest values are depicted along the Alpine main complex. In low-lying areas, the retention decreased by over 10 %. It was found that this decrease can be mostly attributed to settlement expansion. Above 1250 m land use transformations led to slightly increasing water retention values owing to the transformation of wasteland or glaciers to stagnant waters and to the expansion of forest and grassland in high elevations. This examination allows for a holistic and spatially distributed LULCC impact assessment on the landscape’s water regulation capacities and offers valuable high-resolution information for future land use planning and sustainable land development.

In this study the impact of hydropower reservoirs on floods is systematically assessed for eight heavily modified river basins in Austria. Since little discharge data from prior to the construction is available, unaffected annual peak discharges downstream of the reservoirs were estimated by transposing the peak runoff of an unaffected catchment, considering the spatial proximity, the catchment area and the temporarily corresponding precipitation conditions. Comparing the observed and estimated discharge, results show significant reductions not only at gauges directly affected by the reservoir but also farther downstream at receiving rivers. The propagation of the flood peak reduction downstream is also related to catchment area and reservoir characteristics. Extreme events with return periods greater than 30 years show a noteworthy flood peak reduction of over 33% on average. These findings show that reservoirs primarily constructed for hydropower generation also contribute significantly to flood control and flood mitigation.

Dams and hydropower reservoirs constructed in the headwaters of river basins alter the hydrological characteristics as well as other physical and biological conditions of rivers downstream. In this study the impact of reservoirs on floods is systematically assessed for 8 heavily modified river basins in Austria. Since discharge data prior to the construction is not available the natural unaffected annual peak discharges downstream of the reservoirs were estimated by transposing the peak runoff of an unaffected reference catchment by considering the spatial proximity, the catchment area and the precipitation conditions in both catchments. The potential impact is then calculated by comparing the estimated and the observed discharge peaks. The results show significant flood peak reductions not only at gauges directly affected by the reservoir but also further downstream at receiving rivers. The propagation of the flood peak reduction downstream is also related to the ratios (i) between the reservoir catchment and the total catchment area and (ii) between the storage capacity and the catchment area. This indicates the decrease of possible flood control further downstream. Considering the return periods, results indicated that flood peak reductions are most significantly for events with higher return periods. Extremer events with return periods larger than 30 years show a considerable flood peak reduction of over 33 % on average. These findings show that reservoirs foremost constructed for hydropower generation also contribute significantly to flood control and flood risk mitigation.

Study RegionThe Great Rift Valley lakes of Kenya have recently experienced significant increases in their water levels, negatively impacting the local communities. This has provoked renewed concerns about the causations, with various geological, anthropogenic and hydro-climatic influences hypothesized as potential causes of the water level rises. Study FocusThis study analyses and documents water level fluctuations in Lakes Baringo, Bogoria, Nakuru, Solai, Elementaita and Naivasha. Hydrometeorological analyses are undertaken to understand potential causes and lake volume data is used to derive the “Integrated Catchment Response” (ICR), a magnitude which allows to relate changes in water balance components to signals observed in the lake volume changes.New Hydrological Insights for the RegionCompared to 1984-2009, the recent increases in lake areas range from 21% for Lake Naivasha to an extraordinary 123% for Lake Solai. Mean annual rainfall for 2010-2020 increased by up to 30%. Actual evapotranspiration also increased, but to lesser extent compared to rainfall. The results illustrate that changes in catchment properties due to anthropogenic influences or changes in underground permeability are not necessary to explain the lake level rises. Based on the ICR only surprisingly minor changes in the water balance are necessary to explain the lake level rises, since an increase of only 0.4 - 2% of mean annual effective rainfall leads to the observed phenomena.