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Madagascar is known for its high erosion rates in the central highlands, yet the role of human disturbance versus natural processes is not well understood and is a topic of ongoing debate. At present the necessary quantitative data to couple vegetation dynamics and sediment fluxes over time in Madagascar is scarce. This study aims to provide more insight in vegetation changes and sediment transfers over the last millennia in the Lake Alaotra region, and specifically on the role of human disturbances and natural processes. Our vegetation reconstruction is based on pollen records from two lake sediment cores, covering the last 2600 years. Sediment accumulation rates were calculated from cores obtained from the floodplains, from wetlands surrounding the lake, and from Lake Alaotra itself. Our data show an early opening in the landscape, between 2050 and 1700 cal a BP, with a transition from a wooded grassland or woodland/grassland mosaic towards open grassland and an increase in charcoal accumulation rates. (Indirect) human impact is suggested as the main driver for these vegetation changes. Floodplain and wetland sediment accumulation rates only increase in the last 1000 years and peak in the last 400 years. This increased accumulation can mainly be linked to the increased anthropogenic pressure (grazing and farming activities) that triggered increased lavaka (gullies) activity. No changes in accumulation rate were observed in Lake Alaotra, indicating that most sediments are buffered in the floodplains and wetlands. Overall, our pollen and charcoal data suggest an indirect effect of human disturbance on vegetation shifts whilst strong evidence was found for a direct effect of human disturbance on sediment accumulation through intensified use of the grasslands.

Cosmogenic nuclide analysis of river sediment provides insight into erosion and catchment dynamics. Studies on factors controlling spatial variations in long-term erosion rates have often focussed on tectonically active mountainous areas, where strong linkages with topographic variables like catchment gradient and normalized river steepness have been found. Less is known about rates and controls in tropical areas with deeper soils in tectonically less active regions which are often intensively used by people. Information on long-term erosion rates is crucial if human impact on landscape dynamics is to be understood in these areas.Here, we investigate spatial patterns and controls on 10Be-inferred erosion rates in Madagascar, a moderately seismically active island surrounded by passive margins, with considerable relief and a climate that varies from humid tropical to semi-arid. We use a dataset of 99 detrital in-situ 10Be measurements from a wide range of catchments (combining new measurements with data from the literature), covering more than 30% of the country and including a wide range of topographic, bioclimatic, and geological characteristics. Overall, 10Be erosion rates are very low (2.4 - 51.1 mm kyr-1) but clear differences were found between different geomorphic regions with some of the highest rates on the eastern escarpment while most catchments in the central highlands had extremely low erosion rates. The latter shows that, under (sub-) tropical climax vegetation catchments can be very stable, despite a pronounced topography with convex slopes exceeding 30° and the presence of a thick and erodible regolith mantle covered by a protective laterite. Statistical tests indicate that 27% of the observed variation in 10Be erosion rates is associated with elevation (lower rates for higher catchments) and an additional 18% of the variation is associated with river concavity, seismicity, and lithological erodibility (higher rates for more convex, more seismically active and more erodible catchments). An additional test using random within-between (REWB) analysis, in which different geomorphic regions are also considered as independent variables, shows that the main variations in 10Be-inferred rates between the different regions are linked to river concavity, seismicity and gully abundance, where additional variation within geomorphic regions is linked with seismicity only. The random within-between model explained 73% of the observed variation, suggesting that differences between regions are indeed important, yet are only partly explained by the environmental controls we considered in our analysis. The fact that the association between topographical controls and 10Be-inferred erosion rates is weaker in Madagascar in comparison to tectonically (very) active areas is likely to be related to the long time scale considered and the fact that sediment buffering as well as individual random events may have a more important impact on 10Be-inferred erosion in relatively stable environments such as the ones we studied in Madagascar.

Geo-hydrological hazards (landslides and floods) are often associated with significant damages on physical infrastructure like buildings and roads. Understanding the factors controlling the extent of damage is a prerequisite for quantitatively estimating risk and its spatial distribution, and advising on measures to reduce vulnerability. In this study we document the impact of 64 landslide and six flood events in four selected districts in western Uganda for the period May 2019 - March 2021 through extensive fieldwork. We quantify in economic value the physical damage of landslide and flood hazards on exposed buildings, roads and bridges. We then analyse the physical vulnerability based on damage ratios and determine the factors (building material, hazard characteristics and age of the building) that control the degree of damage using fractional logistic regression. Out of the 91 buildings affected by landslides, 54% were totally destroyed, and only 10% not or minorly damaged, for an average damage cost of 3,179 USD/building. For the 212 documented buildings affected by floods, 35% were totally destroyed, 28% had severe to moderate damage and the rest were minorly or not affected, with an average damage costs of 1,755 USD/building. The physical vulnerability of buildings to landslides depends on the size of the landslide, age of the building, type of building wall material and the steepness of the slope cut to establish an artificial foundation platform. On the other hand, the physical vulnerability of buildings to flood hazards is largely controlled by the flood depth, the distance from the river channel, slope, size of flooded area and type of floor material. The physical vulnerability functions developed in this study are being used as a new inputs into a regional quantitative model of geo-hydrological risks. Combining the hazard estimates with the most accurate information on exposure of physical infrastructure, will facilitate the identification of the types of events and the locations that require most attention for risk reduction.

Concurrent with an extensive population growth, the African continent has experienced a vast urbanization trend over the last decades. In 2000, around 35% of the population resided in urban areas. By 2020, this share has increased to around 44% and is projected to increase even further by 2050 following the Shared Socioeconomic Pathways (SSP) scenarios. Besides an important effect on local land use through urban expansion, this also affects food systems by shifting dietary patterns away from traditional diets towards imported or convenient goods. This is particularly the case for rice, which is predominantly imported from Southeast Asia, India, or Pakistan, and is gaining in popularity in African urban diets because of the low effort needed for cooking or storage – giving it a strong advantage over other staple crops. This dietary shift will alter trade dynamics, increase the pressure on local resources such as land, water, and fertilizer use, and subsequently also on biodiversity. In studies investigating the influence of urbanization, either the direct effect of urban expansion on land cover or the effects of dietary changes on demands are investigated, but rarely a combination or comparison of both. Particularly in impact studies or applications that focus on the synergy between water, land, and food-related issues, the dimension of human behavior, such as consumer preferences, is often overlooked.In this study, we provide an initial projection of the expected future effects of both sprawl and shifting preferences for rice caused by urbanization on rice availability, land – and input use, rice-specific emissions, and trade dynamics. By combining micro-level data from household surveys stemming from the Living Standards Measurements Study (LSMS) with the partial equilibrium Global Biosphere Management Model (GLOBIOM) at an African scale, we were able to identify the relative contribution of land cover effects stemming directly from urban expansion and indirectly from dietary shifts caused by rural-urban migration and a divergence in income between urban and rural areas.We indicate that while urban expansion only has a limited effect at the continental scale, the omission of any dietary shifts caused by urbanization substantially underestimates projections of African rice demand (by around 8% under an SSP2-scenario). This also results in subsequent underestimations of impacts on land use, trade dynamics, and rice-specific methane emissions. By this, our study exemplifies that consumer preferences are an essential component to understanding urbanization impacts, and that, by extension, human behavior is important to consider in impact and nexus studies.

Non-typhoidal Salmonella (NTS) ranks first among causes of bloodstream infection in children under five years old in the Democratic Republic of Congo and has a case fatality rate of 15%. Main host-associated risk factors are Plasmodium falciparum malaria, anemia and malnutrition. NTS transmission in sub-Saharan Africa is poorly understood. NTS bloodstream infections mostly occur during the rainy season, which may reflect seasonal variation in either environmental transmission or host susceptibility. We hypothesized that environment- and host-associated factors contribute independently to the seasonal variation in NTS bloodstream infections in children under five years old admitted to Kisantu referral hospital in 2013–2019. We used remotely sensed rainfall and temperature data as proxies for environmental factors and hospital data for host-associated factors. We used principal component analysis to disentangle the interrelated environment- and host-associated factors. With timeseries regression, we demonstrated a direct association between rainfall and NTS variation, independent of host-associated factors. While the latter explained 17.5% of NTS variation, rainfall explained an additional 9%. The direct association with rainfall points to environmental NTS transmission, which should be explored by environmental sampling studies. Environmental and climate change may increase NTS transmission directly or via host susceptibility, which highlights the importance of preventive public health interventions.

Landslide susceptibility, the spatial likelihood of occurrence of landslides in a specific geographical area, is the subject of countless scientific publications. Different authors use heterogeneous data, and apply many different methods, mostly falling under the definition of statistical and/or machine learning approaches, with the common feature of considering many input variables and a single target output, denoting landslide presence. It is a classification problem: given N input variables assuming different values, each combination associated with a 0/1 possible outcome, a model should be trained with some dataset, tested to reproduce the target outcome, and eventually applied to unseen data possibly of practical application.At variance with many fields of science, no reference data exist to comparatively assess the performance of a given method for landslide susceptibility classification and mapping. We propose a benchmark dataset in Italy, extracted from a larger dataset covering the whole country and based on slope units as basic mapping units. The selected 7,360 slope units encompass an area of about 4,100 km^2 in Central Italy. The attribute table contains 26 columns, corresponding to predictors, and a binary column containing the landslide presence/absence flag.We release the dataset, along with a ``call for collaboration'', aimed at collecting a number of different calculations peformed with common input data, and establish a benchmark for landslide susceptibility models. Contributions to this collaboration will be presented at the 2023 European Geoscience Assembly, and collected in a journal publication authored by all of the contributors.

Food stability – or the lack thereof has caused several issues on global food security in the past. Prime examples are the COVID-19 pandemic, the Ukraine-Russian war, and the 2008 African food crisis. Meanwhile, the occurrence of extreme meteorological events has been increasing rapidly (droughts, floods, …), resulting in substantial harvest losses putting food security under pressure with strong indications that these will become even more prevalent under climate change. Yet, the amount of studies assessing food stability or potential climate change effects is scarce – hindering purposeful policymaking. This makes a methodological framework to assess food stability urgent.

Human activity influences both the occurrence and impact of landslides in mountainous environments. Population pressure and the associated land-use changes are assumed to exacerbate landslide risk, yet there is a lack of statistical evidence to support this claim, especially in the Global South where historical records are scarce. In this work, we explore the interactions between population, deforestation and landslide risk in the Kivu Rift in Africa. To do so, we develop a holistic landslide risk model that evaluates 58 years of population and forest-cover trends. We show that the current landslide risk in the eastern Democratic Republic of the Congo (DRC) is twice as high as in neighbouring Rwanda and Burundi. Congolese households, on average, populate more hazardous terrain, probably as a result of conflicts and economic pull factors such as mining. Moreover, the recent large-scale deforestation of primary rainforest in the DRC has considerably exacerbated the landslide risk. Our analysis demonstrates how the legacy of deforestation, conflicts and population dynamics is reflected in the landslide risk in the Kivu Rift. A mapping study covering over 50 years finds that landslide risk is much higher in the Democratic Republic of the Congo, due to more recent deforestation and more people living in more susceptible areas, than in similar landscapes in neighbouring countries.

The extent to which the central highlands of Madagascar were once covered by forests is still a matter of debate: while reconstructing past environments is inherently difficult, the debate is further hampered by the fact that the evidence documenting land cover changes and their effects on carbon and sediment dynamics in Madagascar has hitherto mainly been derived from lake coring studies. Such studies provide an integrated view over relatively large areas but do not provide information on how land-use change affects hillslopes in terms of carbon and sediment dynamics. Such information would not only be complementary to lake inventories but may also help to correctly interpret lake sediment data. Carbon stable isotope ratios (δ13C) are particularly useful tracers to study the past dynamics of soil carbon over time spans ranging from years to millennia and thus to understand the consequences of land-use change over such time spans. We analysed soil profiles down to a depth of 2 m from pristine forests and grasslands in the Lake Alaotra region in central Madagascar. Along grassland hillslopes, soil organic carbon (SOC) content was low, from 0.4 % to 1.7 % in the top layer, and decreased rapidly to ca. 0.2 % below 100 cm depth. The current vegetation predominantly consists of C4 grasses (δ13C ∼ −13 ‰), yet topsoil δ13C-OC ranges between −23.0 ‰ and −15.8 ‰, and most profiles show a decrease in δ13C-OC with depth. This contrasts with our observations in the C3-dominated forest profiles, which show a typical profile whereby δ13C values increase slightly with depth. Moreover, the SOC stock of grasslands was ∼ 55.6 % lower than along the forested hillslopes for the upper 0–30 cm layer. δ13C values in grassland and forest profiles converge to similar values (within 2.0 ± 1.8 ‰) at depths below ∼ 80 cm, suggesting that the grasslands in the Lake Alaotra region have indeed developed on soils formerly covered by a tree vegetation dominated by C3 plants. We also observed that the percent of modern carbon (pMC) of the bulk OC in the top, middle and lower middle positions of grasslands was less than 85 % near the surface. This could reflect a combination of (i) the long residence time of forest OC in the soil, (ii) the slow replacement rate of grassland-derived OC (iii) and the substantial erosion of the top positions towards the valley position of grasslands. At the valley positions under grassland, the upper 80 cm contains higher amounts of recent grass-derived OC in comparison to the hillslope positions. This is likely to be related to the higher productivity of the grassland valleys (due to higher moisture and nutrient availability), and the deposition of OC that was eroded further upslope may also have contributed. The method we applied, which is based on the large difference in δ13C values between the two major photosynthetic pathways (C3 and C4) in (sub-)tropical terrestrial environments, provides a relatively straightforward approach to quantitatively determine changing vegetation cover, and we advocate for its broader application across Madagascar to better understand the island's vegetation history.

Tropical mountainous regions are commonly identified as landslide hotspots with particularly vulnerable populations. Anthropogenic factors are assumed to play a role in the occurrence and impact of landslides in these populated regions, yet the relative importance of these human-induced factors remains poorly documented. In this work, we explore the impact of forest cover dynamics, agricultural land management practices, urbanisation, roads, and mining/quarrying activities on the occurrence of landslides and their associated risk in the North Tanganyika–Kivu Rift, an area that stretches at the border of four countries in Africa. Using a holistic approach that combines extensive fieldwork, optical and SAR/InSAR satellite remote sensing, time-series analysis, UAS image acquisition, historical photograph processing, citizen science and geomorphic marker understanding, our study demonstrates the role of human activities on the frequency, size and deformation patterns of landslides in the region, in both rural and urban environments. The interaction between uplift associated with the continental rifting in the region, fluvial incision, and the (human-induced) landslides are also key elements that are considered in our analysis. Overall, we highlight the need to consider the human context when studying hillslope instabilities in regions under anthropogenic pressure.

The Rwenzori Mountains, in southwest Uganda, are prone to precipitation-related hazards such as flash floods and landslides. These natural hazards highly impact the lives and livelihoods of the people living in the region. However, our understanding of the precipitation patterns and their impact on related hazardous events and/or agricultural productivity is hampered by a dearth of in situ precipitation observations. Here, we propose an evaluation of gridded precipitation products as potential candidates filling this hiatus. We evaluate three state-of-the-art gridded products, the ERA5 reanalysis, IMERG satellite observations, and a simulation from the convection-permitting climate model (CPM), COSMO-CLM, for their ability to represent precipitation totals, timing, and precipitation probability density function. The evaluation is performed against observations from 11 gauge stations that provide at least 2.5 years of hourly and half-hourly data, recorded between 2011 and 2016. Results indicate a poor performance of ERA5 with a persistent wet bias, mostly for stations in the rain shadow of the mountains. IMERG gives the best representation of the precipitation totals as indicated by bias score comparisons. The CPM outperforms both ERA5 and IMERG in representing the probability density function, while both IMERG and the CPM have a good skill in capturing precipitation seasonal and diurnal cycles. The better performance of CPM is attributable to its higher resolution. This study highlights the potential of using IMERG and CPM precipitation estimates for hydrological and impact modeling over the Rwenzori Mountains, preferring IMERG for precipitation totals and CPM for precipitation extremes.

Between 1990-2018, annual rice consumption in Africa has quadrupled to around 40 Mt. This surge can be attributed to an interplay between rapid population growth and dietary shifts. Despite recent yield advances, rice production increases are lagging behind the growing demand, making the region increasingly import-dependent. Even after the 2008 food crisis, succeeding which many African policymakers presented rice as a flagship for food security by boosting domestic rice-production capacity, Africa’s reliance on the international market remains large. Currently, increases in production capacity are driven by acreage extensification rather than intensification, putting pressure on land and jeopardizing sustainable development. How the production of rice will further develop in Africa, and how sustainable this development will be in terms of food security and available land under changing socio-economic and climatic conditions is uncertain. This research assesses future developments of the African rice system under different socio-economic and climatic scenarios by combining biophysical crop model projections (EPIC) with a spatial economic partial-equilibrium model (GLOBIOM) through the Representative Concentration Pathways (RCP) and Shared Socioeconomic Pathways (SSP) frameworks.Our results suggest that by 2050, socioeconomic pressures will have a larger impact on future production levels than long-term climate changes on the African rice system. This is mainly explained by strong differences in population estimates between SSP scenarios and by a limited effect of climate change on yields as negative climatic effects including heat- and water stress are projected to be largely outbalanced by CO2-fertilization effects for rice in Africa. Our simulations do suggest that the expected increase in climatic variability will result in increasing fluctuations in annual yields and production levels in comparison to historical variability. Regions dominated by rainfed systems are particularly vulnerable to such climate shocks, leading not only to variations in production but also in import and consumption levels. Our results also show that disruptions in production have effects beyond the climate-affected region due to bilateral trade. Particularly production shocks in Southeast Asia could have a strong impact on rice availability in Africa because of the vast import dependency of Africa. The magnitude of the effect of a climate-induced production shock in Southeast Asia on rice consumption levels in some African regions is even comparable to the effect of a similar climate shock on domestic production. Since the robustness of the analysis is strongly linked to the performance of the biophysical crop model, we also present a comparison of these preliminary results to other existing biophysical global gridded crop models (GGCMs). In summary, we demonstrate that future socio-economic pathways have a more important impact on the African rice system than climate changes in the long term, but that increases in the short-term climate variability strongly affect production and consumption. While trade may partially offset a negative effect of a climate-induced production shock on rice consumption in Africa through increased imports, trade and Africa’s import dependence make the continent as vulnerable to climate-induced production shocks in Southeast Asia as in Africa itself.

Over the past decades advanced technology has become available, revolutionizing the assessment of surface topography. At smaller scales (up to a few km²) structure from motion (SfM) algorithms applied to uncrewed aerial vehicle (UAV) imagery now allow sub-meter resolution. On the other hand, spaceborne digital elevation models (DEMs) are becoming increasingly accurate and are available at a global scale. Two recent spaceborne developments are the 12 m TanDEM-X and 30 m Copernicus DEMs. While sub-meter resolution UAV-SfM DEMs generally serve as a reference, their acquisition remains time-consuming and spatially constrained. However, some applications in geomorphology, such as the estimation of regional or national erosion quantities of specific landforms, require data over large areas. TanDEM-X and Copernicus data can be applied at such scales, but this raises the question of how much accuracy is lost because of the lower spatial resolution.Here, we evaluate the performance of the 12 m TanDEM-X DEM and the 30 m Copernicus DEM to i) estimate gully volumes, ii) establish an area-volume relationship, and iii) determine sediment mobilization rates, through comparison with a higher resolution (0.2 m) UAV-SfM DEM. We did this for six study areas in central Madagascar where lavaka (large gullies) are omnipresent and surface area changes over the period 1949-2010s are available. Copernicus derived lavaka volume estimates were systematically too low, indicating that the Copernicus DEM is not suitable to estimate erosion volumes for geomorphic features at the lavaka scale (100 – 105 m²). The relatively coarser resolution of the DEM prevents to accurately capture complex topography and smaller geomorphic features. Lavaka volumes obtained from the TanDEM-X DEM were similar to UAV-SfM volumes for the largest features, while smaller features were generally underestimated. To deal with this bias we introduce a breakpoint analysis to eliminate volume reconstructions that suffered from processing errors as evidenced by significant fractions of negative volumes. This elimination allowed the establishment of an area-volume relationship for the TanDEM-X data with fitted coefficients within the 95% confidence interval of the UAV-SfM relationship. Combined with surface area changes over the period 1949-2010s, our calibrated area-volume relationship enabled us to obtain lavaka mobilization rates ranging between 18 ± 3 and 311 ± 82 t ha-1 yr-1 for the six study areas, with an average of 108 ± 26 t ha-1 yr-1. This does not only show that the Malagasy highlands are currently rapidly eroding by lavaka, but also that lavaka erosion is spatially variable, requiring the assessment of a large area in order to obtain a meaningful estimate of the average erosion rate.With this study we demonstrate that medium-resolution global DEMs can be used to accurately estimate the volumes of gullies exceeding 800 m² in size, where the proposed breakpoint-method can be applied without requiring the availability of a higher resolution DEM. This might aid geomorphologists to quantify sediment mobilisation rates by highly variable processes such as gully erosion or landsliding at the regional scale, as illustrated by our first assessment of regional lavaka mobilization rates in the central highlands of Madagascar.

Over the past few decades, developments in remote sensing have resulted in an ever-growing availability of topographic information on a global scale. A recent development is TanDEM-X (TerraSAR-X add-on for digital elevation measurements), an interferometric synthetic aperture radar (SAR) mission of the Deutsche Zentrum für Luft- und Raumfahrt, providing near-global coverage and 12 m resolution digital elevation models (DEMs). Moreover, ongoing developments in uncrewed aerial vehicle (UAV) technology have enabled acquisitions of topographic information at a sub-meter resolution. Although UAV products are generally preferred for volume assessments of geomorphic features, their acquisition remains a time-consuming task and is spatially constrained. However, some applications in geomorphology, such as the estimation of regional or national erosion quantities of specific landforms, require data over large areas. TanDEM-X data can be applied at such scales, but this raises the question of how much accuracy is lost because of the lower spatial resolution. Here, we evaluated the performance of the 12 m TanDEM-X DEM to (i) estimate gully volume, (ii) establish an area–volume relationship, and (iii) determine mobilization rates through comparison with a higher-resolution (0.2 m) UAV structure-from-motion (SfM) DEM and a lower resolution (30 m) Copernicus DEM. We did this for six study areas in the Lake Alaotra region (central Madagascar), where lavaka (gullies) are omnipresent and surface area changes over the period 1949–2010s are available for 699 lavaka. Copernicus-derived lavaka volume estimates were systematically too low, indicating that the Copernicus DEM is too coarse to accurately estimate volumes of geomorphic features at the lavaka scale (100–105 m2). Lavaka volumes obtained from TanDEM-X were similar to UAV-SfM volumes for the largest features, whereas the volumes of smaller features were generally underestimated. To deal with this bias we introduce a breakpoint analysis to eliminate volume reconstructions that suffer from processing errors as evidenced by significant fractions of negative volumes. This elimination allowed the establishment of an area–volume relationship for the TanDEM-X data with fitted coefficients within the 95 % confidence interval of the UAV-SfM relationship. Our calibrated area–volume relationship enabled us to obtain large-scale lavaka mobilization rates ranging between 18 ± 3 and 311 ± 82 tha-1yr-1 for the six different study areas, with an average of 108 ± 26 tha-1yr-1 for the full dataset. These results indicate that current lavaka mobilization rates are 2 orders of magnitude higher than long-term erosion rates. With this study we demonstrate that the global TanDEM-X 12 m DEM can be used to accurately estimate volumes of gully-shaped features at the lavaka scale (100–105 m2), where the proposed breakpoint method can be applied without requiring the availability of a higher-resolution DEM. Furthermore, we use this information to make a first assessment of regional lavaka erosion rates in the central highlands of Madagascar.

During the Anthropocene, the impact of humans on Earth surface processes has increased exponentially, often surpassing the importance of natural drivers. Also in mountainous areas, landslide mobilization rates are exacerbated by human disturbances of the landscape such as deforestation, road constructions, and mining processes. However, investigating these interactions remains difficult in many regions due to a lack of sufficiently long observation periods, preferably over a large area, so that the presence of extreme landslide events (triggered by rainfall or earthquakes) does not induce an observation bias. Here, we investigate landslide mobilization rates in the densely populated North Tanganyika-Kivu Rift Region (NTK Rift), a prominent landslide hotspot in Africa. We use ca. 2,400 panchromatic aerial photographs from 1958 in combination with recent satellite imagery to assess the long-term landslide mobilization rates over a large area of ca. 21,000 km2.By estimating the volume of the deep-seated and shallow rapidly-moving landslides using empirical volume-area scaling relationships, we estimate that the average landslide mobilization rate in the NTK Rift is ca. 31 m3 km-2 year-1 in actively incising, rejuvenating landscapes and ca. 12 m3 km-2 year-1 in relict landscapes. The mobilization rates in the NTK Rift are dominated by the largest landslides. For instance, the 15 largest deep-seated landslides account for 50% of the total rate. Overall, we observe mobilization rates in the NTK Rift that are somewhat lower than what a global model predicts. These relatively low rates could be explained by four factors: (i) the absence of major landslide-triggering earthquakes during our 60-year observation period, (ii) the exclusion of earthflows from our analysis due to a lack of information on the depth and velocity of these instances, (iii) the relatively large size of our study area which reduces biases linked to extreme rainfall, (iv) the fact that the NTK Rift is a mountain range in an extension area, which differs from orogenic mountainous areas, where most landslide mobilization rates are reported; and (v) uncertainties on the global landslide mobilization rate model.In rejuvenated landscapes, roughly 5% of the sediment mobilization by rapidly-moving landslides is linked to human activity, while in relict landscapes this figure rises to 18%, notably due to mining and road construction. The role of human activity is limited as compared to the recent occurrence of some large landslides, which seem linked to natural causes and dominate the overall mobilization rates. Moreover, the limited role of human activity must be balanced with the fact that the NTK Rift, although highly populated, remains relatively untouched by major road infrastructure constructions. While previous studies have found that deforestation has a large impact on the landside risk (i.e. the incidence of landslide fatalities), its impact on the observed mobilization rates appears to be much less important. The landslides associated with deforestation are commonly shallow debris avalanches with a limited size and rather high mobility.Overall, our results significantly contribute to a better understanding of landslide mobilization and its controlling factors, especially by proving much-needed long-term observations for a currently under-researched type of environment.

Lavaka (gullies) are often considered as the prime indication of a currently ongoing human-induced environmental crisis in Madagascar's highlands. Yet, lavaka are known to have existed long before human arrival and account for the majority of the long-term sediment input into the highland rivers and floodplains. The role of anthropogenic disturbances in their formation therefore remains highly debated and it is unclear whether lavaka erosion has recently increased. Here,we address these questions by evaluating the dynamics of lavaka in the Lake Alaotra region (central Madagascar). An overall birth to stabilization ratio of 6.1 indicates a rapid lavaka population growth over the period 1949–2010s. Using data on lavaka development we calculated a mean lavaka population age of 410 ± 40 years and estimate that the disequilibrium started at 870 ± 430 cal. BP. Floodplain sedimentation starts to increase around 1000 cal. BP and peaks over the last 400 years, thereby independently confirming this time frame of increased lavaka activity. Lavaka population dynamics modelling shows that a strong increase in environmental pressure over the last centuries is needed to attain current disequilibrium levels. A general drying of the climate since 950 cal. BP in combination with the introduction of cattle and growing human presence around 1000 cal. BP will likely have triggered the increase in lavaka erosion. However, the recent acceleration cannot be explained by climatic changes alone and seems to be linked to increased anthropogenic pressure on the environment. As such, we offer a fresh and quantitatively supported perspective on lavaka dynamics and human impact in central Madagascar, where our methodology can be used in other locations where similar questions on geomorphic equilibrium need to be answered. ispartof: Science Of The Total Environment vol:806 issue:Pt 2 ispartof: location:Netherlands status: Published online

Effective disaster risk reduction is often hampered by a general scarcity of reliable data collected on disastrous events, particularly in the Global South. Novel approaches are therefore necessary to alleviate this constraint, particularly with regard to reducing extensive risks. A geo-observer network, consisting of 21 reporters, was established in the Rwenzori region (Uganda) in February 2017 to collect data on eight different disasters using smartphone technology. Within the first 15 months of operation, a total of 319 disaster reports were submitted. A large majority of the reported disasters were reached by the geo-observers within 2 days after their occurrence. The analysis of reporting activity shows a large divergence, with one third of the most active geo-observers accounting for nearly 75% of all reports. By using an existing landslide susceptibility map as a proxy of expected landslide prevalence, this reporting divergence is demonstrated to be at least partially driven by a difference in disaster occurrences. This is confirmed by the results of a survey held among the geo-observers. Survey results also showed that the participants are more driven by non-pecuniary benefits rather than financial compensation. The data collected during the first 15 months of operation indicates that extensive risks in the region are underestimated and demonstrates the added value of participatory sensing to compensate for the current lack of well-functioning official data collection mechanisms. This pilot project is a proof of concept for participatory sensing to collect high quality data even in remote contexts where smartphone technology is not generally adopted. It can serve as a precedent or example for other regions where extensive risks are poorly understood but pose significant threat to the population.

Over the past decades, developments in remote sensing have resulted in an ever growing availability of topographic information on a global scale. A recent development is TanDEM-X, an interferometric SAR mission of the Deutsche Zentrum für Luft- und Raumfahrt providing near-global coverage and high resolution DEMs. Moreover, ongoing developments in unmanned aerial vehicle (UAV) technology has enabled acquisitions of topographic information at a sub-meter resolution. Although UAV products are generally preferred for volume assessments of geomorphic features, their acquisition remains time-consuming and is spatially constrained. However, some applications in geomorphology such as the estimation of regional or national erosion quantities of specific landforms, require data over large areas. TanDEM-X data can be applied at such scales, but this raises the question of how much accuracy is lost because of the lower spatial resolution. Here, we evaluated the performance of the 12 m TanDEM-X DEM to i) estimate gully volumes, ii) establish an area-volume (A-V) relationship, and iii) determine mobilization rates, through comparison with a high resolution (0.2 m) UAV-SfM DEM and lower resolution (30 m) SRTM DEM. We did this for six study areas in the Lake Alaotra region (central Madagascar) where lavaka (gullies) are omnipresent and lavaka surface area changes over the period 1949–2010s are available for 699 lavaka. SRTM derived lavaka volume estimates were systematically too low, indicating that the SRTM DEM is too coarse to accurately estimate volumes of geomorphic features at the lavaka-scale (100–100 000 m2). Lavaka volumes obtained from TanDEM-X were similar to UAV-SfM volumes for the largest features, whereas the volumes of smaller features were generally underestimated. To deal with this bias we introduce a breakpoint analysis to eliminate volume reconstructions that suffer from processing errors as evidenced by significant fractions of negative volumes. This elimination allowed the establishment of an area-volume relationship for the TanDEM-X data that is within the 95 % confidence interval of the UAV-SfM A-V relationship. Our calibrated area-volume relationship enabled us to obtain large-scale lavaka mobilization rates ranging between 18 ± 6 and 289 ± 125 ton ha−1 yr−1 with an average of 102 ± 41 ton ha−1 yr−1. These results indicate that current lavaka mobilization rates are two orders of magnitude higher than long-term erosion rates. With this study we demonstrate that the global TanDEM-X 12 m DEM can be used to accurately estimate volumes of gully-shaped features at the lavaka-scale (100–100 000 m2), where the proposed breakpoint-method can be applied without requiring the availability of a higher resolution DEM. Furthermore, we use this information to make a first assessment of regional lavaka erosion rates in the central highlands of Madagascar.

Schistosomiasis is a tropical parasitic disease affecting more than 200 million people worldwide, predominantly in Africa. The World Health Organization recently highlighted the importance of targeted control of the freshwater snails acting as intermediate hosts for the parasites causing schistosomiasis. However, because of a shortage of trained experts and resources, detailed information on spatiotemporal snail distributions, which is needed for targeted control measures, is often missing. We explore the potential of citizen science to build these much-needed datasets through fine-grained, frequent snail sampling. We trained a network of 25 citizen scientists to weekly report on snail host abundances in 77 predefined water contact sites in and around Lake Albert (western Uganda). Snail abundance, together with marked GPS locations, water chemistry parameters, and photographs of the identified snails are recorded and submitted using the freely available mobile phone application KoBoToolbox. Trained researchers then engage in remote, semi-automatic validation of the submissions, after which there is an opportunity to provide targeted feedback to the citizen scientists. Five months after the operationalisation of the network, a total of 570 reports were submitted and personalized feedback was given, resulting in lasting improvements in subsequent reporting and snail genus identification. The preliminary results show the possibility of citizen science to independently obtain reliable data on the presence of schistosome snail hosts. We therefore argue that citizen-driven monitoring on a high spatiotemporal resolution could help to generate the much-needed data to support local targeted snail control measures in remote and/or resource-limited environments.

Effective disaster risk reduction (DRR) presupposes awareness among key stakeholders on the causal factors that exacerbate disaster risks as well as a feeling of ownership over proposed DRR measures. Yet, the prevailing top-down communication of risk and the expert-centered knowledge have a limited impact in bringing significant positive change. Serious games respond to the need for a community-based DRR approach as they encourage a collective recognition of societal issues and co-learning at the different levels of the DRR governance system. However, there is still a gap in understanding how serious games facilitate co-creation of knowledge. In this article, we introduce DisCoord, a serious game first implemented in the landslide- and flood-prone Rwenzori mountains in South-West Uganda. Designed as a public pedagogy tool, Discoord bridges diverse views and sets of knowledge of DRR stakeholders separated by spatial and socio-cultural domains. Secondly, through a qualitative analysis of the dialogues, narratives and observation notes during the actual game sessions and debriefings, we explored the factors and processes that influence knowledge co-creation. Three different knowledge actors are involved in the co-creation process, namely the game designers, game facilitators and players. These actors have diverse pre-game views, which are expressed through the game rules, arguments, game strategies, and game outcomes, and are confronted within the public space provided by the game. We find that crises experienced during the game, real-life based arguments provided by the players and own interpretations by the players are key factors in the co-creation process. This study leads us to conclude that games like DisCoord are useful as a public pedagogy intervention as they bring different forms of knowledge together in a public space and facilitate co-learning. This paper also contends that countering a top-down approach of risk communication using a public pedagogy approach requires an openness towards the unpredictable, de-centered DRR, and plural co-learning outcomes.

Deforestation is associated with a decrease in slope stability through the alteration of hydrological and geotechnical conditions. As such, deforestation increases landslide activity over short, decadal timescales. However, over longer timescales (0.1–10 Myr) the location and timing of landsliding is controlled by the interaction between uplift and fluvial incision. Yet, the interaction between (human-induced) deforestation and landscape evolution has hitherto not been explicitly considered. We address this issue in the North Tanganyika–Kivu rift region (East African Rift). In recent decades, the regional population has grown exponentially, and the associated expansion of cultivated and urban land has resulted in widespread deforestation. In the past 11 Myr, active continental rifting and tectonic processes have forged two parallel mountainous rift shoulders that are continuously rejuvenated (i.e., actively incised) through knickpoint retreat, enforcing topographic steepening. In order to link deforestation and rejuvenation to landslide erosion, we compiled an inventory of nearly 8000 recent shallow landslides in © Google Earth imagery from 2000–2019. To accurately calculate landslide erosion rates, we developed a new methodology to remediate inventory biases linked to the spatial and temporal inconsistency of this satellite imagery. Moreover, to account for the impact of rock strength on both landslide occurrence and knickpoint retreat, we limit our analysis to rock types with threshold angles of 24–28∘. Rejuvenated landscapes were defined as the areas draining towards Lake Kivu or Lake Tanganyika and downstream of retreating knickpoints. We find that shallow landslide erosion rates in these rejuvenated landscapes are roughly 40 % higher than in the surrounding relict landscapes. In contrast, we find that slope exerts a stronger control on landslide erosion in relict landscapes. These two results are reconciled by the observation that landslide erosion generally increases with slope gradient and that the relief is on average steeper in rejuvenated landscapes. The weaker effect of slope steepness on landslide erosion rates in the rejuvenated landscapes could be the result of three factors: the absence of earthquake-induced landslide events in our landslide inventory, a thinner regolith mantle, and a drier climate. More frequent extreme rainfall events in the relict landscapes, and the presence of a thicker regolith, may explain a stronger landslide response to deforestation compared to rejuvenated landscapes. Overall, deforestation initiates a landslide peak that lasts approximately 15 years and increases landslide erosion by a factor 2 to 8. Eventually, landslide erosion in deforested land falls back to a level similar to that observed under forest conditions, most likely due to the depletion of the most unstable regolith. Landslides are not only more abundant in rejuvenated landscapes but are also smaller in size, which may again be a consequence of a thinner regolith mantle and/or seismic activity that fractures the bedrock and reduces the minimal critical area for slope failure. With this paper, we highlight the importance of considering the geomorphological context when studying the impact of recent land use changes on landslide activity.

Accurate and complete inventory of natural hazard occurrence and their level of impact is a key first step to risk assessment, but it remains a challenge, especially for high frequency low impact events that rarely makes it to the news media. This challenge is even greater in rural areas of developing countries such as Uganda, where limited IT facilities prevent dissemination of information through social media. Here we report on a citizen-science initiative to monitor small-scale disasters (landslides and floods) occurring in the Rwenzori Mountains. A network of citizen (geo-)observers was established in February 2017 to collect temporally explicit geo-referenced information on eight different hazards and their impact using smartphone technology. Since then, over 500 hazard occurrences have been reported. However, such dataset needs to be assessed for its accuracy and potential biases before being used for scientific analysis. In this study, we evaluate the accuracy and completeness of the geo-observer-based disaster reports. First, systematic errors are reduced by peer reviewing the reports and implementing automatic tests to assess potential errors in detection and biases. Then, we compare the geo-observer-based records with two independent inventories collected through systematic field mapping and satellite imagery mapping, focusing on landslide and flood events for the period between May 2019 and May 2020. Results show over 95% of the geo-observer reports validated in the field were correctly identified and recorded less than 5 days after the occurrence (60% true positives, 1% false positives and 39% false negatives). For the satellite imagery mapping, 29% were true positives, 43% false positives and 28% false negatives. Geo-observers provide near real time disaster information on the location and level of impact, something difficult to achieve with systematic field and satellite imagery mapping. Depending on the topography of the area and the weather conditions, it can take several days to weeks before a cloud-free satellite image of a place can be obtained. The false negatives in the Geo-observer data are due to the tendency to report mainly occurrences along roads and rural foot paths since such occurrences are easily seen and accessed. Isolated small and inaccessible landslides are often not seen or reported to the Geo-observers. While satellite imagery mapping provides an opportunity to record disaster occurrences even in extremely inaccessible places, small landslides are often missed while shallow ones can easily be confused with freshly cleared vegetation for crop planting. Citizen science-based disaster reporting therefore not only provide the spatial occurrence of disasters but also the temporal and weather-related information, necessary for disaster risk analysis.

On the nexus of humans and their environment, landslide risk is in essence dynamic. In mountainous areas over the world, the need for agricultural land incites people to settle on steeper (more landslide-prone) terrain at the expense of ecosystems. At the same time, the degradation of ecosystems, for example through deforestation, leads to a considerable increase in landslide hazard. Although the link between deforestation and landslide hazard/risk has been widely recognized, it remains poorly quantified. This is especially the case in the Global South where historical land cover and landslide records are scarce. In this study, we investigate 58 years of forest cover changes, population dynamics, and landslide risk in the Kivu Rift. This mountainous region presents similar geomorphic and climatic conditions across three countries: Burundi, the eastern part of the Democratic Republic of the Congo (DRC), and Rwanda. First, we use contemporary landslide and deforestation data (2000-2016) to explicitly quantify the interactions between these two processes. Second, we reconstruct the annual forest cover changes between 1958 and 2016 by means of a cellular automaton of which the output converges to four forest cover products (1958, 1988, 2001, 2016). We derive the 1958 forest data from an inventory of nearly 2,400 panchromatic aerial photographs, available at the Royal Museum for Central Africa. The forest data for 1988, 2001, and 2016 are readily available and derived from satellite imagery. Next, we estimate the yearly historical landslide hazard dynamics by applying the contemporary deforestation-landslide relationship to the historical forest cover changes. Finally, an approximation of the landslide risk (expected fatalities per 100,000 inhabitants), is calculated for four epochs (1975, 1990, 2000, 2015) and derived from the product of the corresponding hazard map and population density grids.During our entire period of observation, the landslide risk is higher in the DRC than in Rwanda and Burundi. While the risk in Rwanda and Burundi displays a slightly decreasing trend, the risk seems more volatile in the DRC. Here, the initial risk in 1975 is high due to the concentration of a small population along the steep northwestern coast of Lake Kivu. In the following 15 years, the risk in the DRC decreases sharply, only to soar again in the nineties. This sudden increase in risk can be linked to two factors: demographic changes and environmental degradation. During the nineties, the location of the Congolese people shifted towards steeper terrain. This shift is explained by the relocation of hundreds of thousands of Rwandan refugees and internally displaced people following the First and Second Congo War, but also by the economic opportunities provided by the booming, often informal, mining industry. Deforestation has also contributed to the higher landslide risk in the DRC, as large parts of the primary forest have been cut to satisfy the land and fuelwood demand of the fast-growing population.With our analysis, we demonstrate that a landslide risk assessment is more than the reflection of the current environmental conditions. The legacy of environmental and societal dynamics resonates in contemporary landslide risk.

Madagascar is an island characterized by a sharp vegetation gradients in landscape and the extent to which the central highlands were once covered by forest is still under debate. Stable carbon isotope ratios have been an important tracer to understand vegetation shifts in a landscape over time because forest plants (following the C3 photosynthetic pathway) and grasses (following the C4 pathway for the majority of tropical and subtropical species) show a different degree of isotope fractionation. The main aim of this study is to provide insight into the past vegetation history of the grasslands in the Lake Alaotra region (central Madagascar). This region is under high anthropogenic pressure as it is the most important rice-producing region in Madagascar. While much of the Lake Aloatra catchment consists of grassland-covered hills, with a high density of lavaka (large erosional features), pristine forests are located just east of the lake and these form the most western part of a larger rainforest-covered region extending up to the east coast of the island. Soil profiles were sampled along the hillslope gradient at both forested and grassland sites, whereby carbon stocks were quantified and δ13C values of soil organic carbon (OC) were measured to find evidence for past forest vegetation in the grassland sites. The soil organic carbon content of grassland soil profile was extremely low, from 0.4 to 1.8% in the top layer and rapidly decreasing to 0.2 % below 100 cm depth. The current vegetation predominantly consists of C4 grasses (δ13C ~-13 ‰), yet soil δ13C-OC ranges between -25.9 and -16.6‰, and most profiles show a decrease in δ13C-OC with depth, in contrast to observations in the (C3-dominated) forest profiles, which show a typical enrichment in 13C with depth. δ13C values in grassland and forest profiles converge to similar values (within 2.1 ±1.8 ‰) at depths below ~80cm, suggesting that the grasslands in the Lake Alaotra region have developed on formerly forested soils. Moreover, the soil OC stock of grasslands was ~55.6% lower than along the forested hillslopes for the upper 30 cm layer. Our results are consistent with the hypothesis that a vegetation change has occurred in the lake Alaotra region, and offer a promising avenue to expand this approach on a wider scale to help understand the vegetation cover change in the central highland of Madagascar.

Effective disaster risk reduction (DRR) presupposes awareness among key stakeholders on the causal factors that exacerbate disaster risks as well as a feeling of ownership over proposed DRR measures. Yet, the prevailing top-down communication of risk and the expert-centered knowledge have a limited impact in bringing significant positive change. Serious games respond to the need for a community-based DRR approach as they encourage a collective recognition of societal issues and co-learning at the different levels of the DRR governance system. However, there is still a gap in understanding how serious games facilitate co-creation of knowledge. In this article, we first introduce a serious game, called DisCoord, as a public pedagogy tool that bridges diverse views and sets of knowledge of DRR stakeholders separated by spatial and socio-cultural domains. Second, through a qualitative method of analysis of the 10 game sessions in Uganda, we examine the factors and processes that influence knowledge co-creation. The game actors – game designers, game facilitators and players – primarily steer and influence the co-creation process. These actors have diverse pre-game views, which are expressed through the game rules, arguments, game strategies, and game outcomes, and are confronted within the public space provided by the game. We find that crises experienced during the game, real-life based arguments provided by the players and own interpretations by the players are key factors in the co-creation process. This study leads us to conclude that games like DisCoord are useful as public pedagogy intervention as they bring different forms of knowledge together in a public space and facilitate co-learning. This paper also contends that countering a top-down approach of risk communication using a public pedagogy approach requires an openness towards the unpredictable, de-centered DRR, and plural co-learning outcomes.

To understand the driving processes for changes in African ecosystems and related problems such as soil degradation, it is crucial to gain insight in the relative importance of human disturbance and climate change. Madagascar is known for its particularly high erosion rates in the central highlands, yet the role of human disturbance versus natural processes are not well understood and is a topic of ongoing debate. Recent studies have challenged the traditional view that the currently observed intense erosion processes and sediment fluxes in Madagascar are mainly driven by recent large-scale deforestation. However, at present almost no quantitative data is available to couple vegetation dynamics and sediment fluxes over time in Madagascar. This study aims to provide more insight in landscape changes (vegetation changes, sediment mobilization and deposition) in central Madagascar, and in the specific role of man and climate. The study focuses on the 1800 km² catchment of Lake Alaotra, located ca 200 km northeast of Antananarivo. Lake Alaotra is formed in a graben system in the highlands of Madagascar, and is the largest freshwater lake of the country (400 km²). A pollen record from the lake was used to reconstruct regional vegetation changes. Radiocarbon dates of extracted pollen provide a detailed chronostratigraphic framework. Augerings and radiocarbon dates from floodplains and marshes in the catchment were used to reconstruct the sediment deposition history. The pollen record and charcoal data shows the vegetation changes over the last 3000 years. The main observed shift in vegetation is a transition from a woodland/grassland mosaic towards an open grassland, starting ca 1850 years ago, which coincides with the onset of human activities. Data on floodplain sedimentation show an increase in accumulation rates in the last 600 years, from ca 1 mm yr-1 to ca 30 mm yr-1, which can be linked to increased hillslope erosion rates during that time period. The sedimentation wave, however, does not reach Lake Alaotra nor the surrounding marshes as floodplains act as a buffer. Overall, this study provides a spatial and temporal integrated reconstruction of vegetation changes in the Lake Alaotra catchment and the link with sediment mobilization and deposition, thereby providing a better understanding of environmental changes in central Madagascar and its driving forces.

Different erosion processes deliver large amounts of terrestrial soil organic carbon (SOC) to rivers. Mounting evidence indicates that a significant fraction of this SOC, which displays a wide range of ages, is rapidly decomposed after entering the river system. The mechanisms explaining this rapid decomposition of previously stable SOC still remain unclear. In this study, we investigated the relative importance of two mechanisms that possibly control SOC decomposition rates in aquatic systems: (i) in the river water SOC is exposed to the aquatic microbial community which is able to metabolize SOC much more quickly than the soil microbial community and (ii) SOC decomposition in rivers is facilitated due to the hydrodynamic disturbance of suspended sediment particles. We performed different series of short-term (168 h) incubations quantifying the rates of SOC decomposition in an aquatic system under controlled conditions. Organic carbon decomposition was measured continuously through monitoring dissolved O2 (DO) concentration using a fiber-optic sensor (FireStingO2, PyroScience). Under both shaking and standing conditions, we found a significant difference in decomposition rate between SOC with aquatic microbial organisms added (SOC + AMO) and without aquatic microbial organisms (SOC − AMO). The presence of an aquatic microbial community enhanced the SOC decomposition process by 70 %–128 % depending on the soil type and shaking–standing conditions. While some recent studies suggested that aquatic respiration rates may have been substantially underestimated by performing measurement under stationary conditions, our results indicate that the effect of hydrodynamic disturbance is relatively minor, under the temperature conditions, for the soil type, and for the suspended matter concentration range used in our experiments. We propose a simple conceptual model explaining these contrasting results.

The Malagasy highlands are scattered with large inverse teardrop-shaped gullies called lavaka, which are by many considered as the prime indication of a currently ongoing human-induced environmental crisis. Yet, these gullies are known to have existed long before human arrival on the island, resulting in the highly debated role of anthropogenic disturbances on their formation. Here, we assess the dynamics of 700 lavaka in the lake Alaotra region from 1949 to the 2010s by using historical aerial pictures and present day satellite imagery. An overall birth to stabilization ratio of 6.1 indicates a currently rapid growing lavaka population. Observed growth-, birth- and stabilization rates allowed us to calculate a mean lavaka population age of 410 ± 40 years, and estimate that the current crisis started at 943 ± 430 cal. yr BP. This timeframe corresponds well with the “subsistence-shift”, where people move from hunting and foraging to farming and herding practices upon the introduction of cattle in the region. These findings were integrated into a novel, temporally explicit lavaka population model - building upon the observed lavaka growth-, birth- and stabilization rates and lavaka size distributions - where different environmental pressure scenarios were tested. Modelling outcomes show that the currently observed lavaka crisis largely results from a rapid increase in environmental pressure over the last centuries, likely caused by the combined effects of deforestation and overgrazing related to human population growth and the introduction of cattle. With this study we show the potential of an integrated data-modeling approach, where demographic concepts are applied to geomorphological features, allowing to link their evolution with past anthropogenically driven environmental changes.

Deforestation increases landslide activity over short, contemporary timescales. However, over longer timescales the location and timing of landsliding is controlled by the interaction between uplift and fluvial incision. Yet, the interaction between (human-induced) deforestation and landscape evolution has hitherto not been explicitly considered. We address this issue in the North Tanganyika-Kivu Rift region (East African Rift). In recent decades, the regional population has grown exponentially and the associated expansion of cultivated and urban land has resulted in widespread deforestation. On a much longer time scale, tectonic uplift has forged two parallel mountainous Rift shoulders that are continuously rejuvenated through knickpoint retreat, enforcing topographic steepening. In order to link deforestation and rejuvenation to landslide erosion, we compiled an inventory of nearly 8,000 recent shallow landslides in Google Earth© imagery from 2000–2019. To accurately calculate landslide erosion rates, we developed a new methodology to remediate inventory biases linked to the spatial and temporal inconsistency of this satellite imagery. We find that erosion rates in rejuvenated landscapes are roughly 40 % higher than in the surrounding relict landscapes, upstream of retreating knickpoints and outside of the Rift shoulders. This difference is due to the generally steeper relief in rejuvenated landscapes which more than compensates for the fact that rejuvenated slopes, when compared to similarly angled slopes in relict zones, often display a somewhat lower landslide erosion rate. These lower rates in the rejuvenated landscapes could be the result of a drier climate, the omission of earthquake-induced landslide events in our landslide inventory, and potentially a smaller regolith stock. More frequent extreme rainfall events in the relict zones, and possibly the presence of a thicker regolith, cause a stronger landslide response to deforestation compared to rejuvenated landscapes. Overall, deforestation initiates a landslide peak that lasts approximately 15 years and increases landslide erosion by a factor 2 to 8. Eventually, landslide erosion in deforested land falls back to a level similar to that observed under forest conditions, most likely due to the depletion of the most unstable regolith. Landslides are not only more abundant in rejuvenated landscapes but are also smaller in size, which may be a consequence of the seismic activity that fractures the bedrock and reduces the minimal critical area for slope failure. With this paper, we highlight the importance of considering the geomorphological context when studying the impact of recent land use changes on landslide activity.

© 2018 Springer-Verlag GmbH Germany, part of Springer Nature Over the past decades, the global exposure of persons and assets to natural hazards, like landslides, has increased faster than that their vulnerability has decreased. Therefore, the Sendai Framework for Disaster Risk Reduction (DRR) made a call in 2015 for more investment in DRR. Recent studies exposed the lack of scientific evaluation data for selecting DRR measures. The objective of this research is to develop a participatory methodology for identifying appropriate DRR measures based on stakeholder groups’ perceptions. More specifically, we propose a social multi-criteria evaluation which combines a two-phased participatory multi-criteria analysis with an institutional analysis for its design and a discourse analysis for interpreting its outcomes. This method is then tested for a case study focusing on landslides in the Rwenzori Mountains region (Uganda) to illustrate both its relevance and practical challenges. The results from the case study highlight the perceived importance of the acceptability of measures above cost, feasibility and effectiveness criteria, by district and village stakeholder groups. Frequently recommended DRR actions, such as disaster risk zonation or relocation, are shown to be perceived negatively by local stakeholders despite being put forward in national policies. This study contributes to the DRR literature as it proposes a new method to support DRR-related decision-making. It also contributes to the understanding of landslide risk reduction in the Rwenzori Mountains region as it presents the first attempt to identify appropriate landslide risk reduction measures for the region. ispartof: Landslides vol:16 issue:9 pages:1793-1807 status: Published online

The international agenda for disaster risk reduction, through the Hyogo Framework for Action and the Sendai Framework for disaster risk reduction, promotes decentralised platforms as an indispensable strategy to achieve effective and efficient disaster risk management. Based on empirical data from the Rwenzori Mountains region, we question the implications of this type of network governance for disaster risk management. We embed our observations in an analytical framework that combines literature on network governance with insights from politics of disaster, notably scale and blame theories. In this study, we investigate the implications for disaster risk reduction through the analysis of three processes of scale structuration observed in contemporary West Uganda: (i) incomplete decentralisation, (ii) blame dissolution, and (iii) scale jumping. We argue that decentralised platforms in Uganda co-produce unequal risk, as they are used as spatial tactics to centralise power for the ruling party and enable blame dissolution and scale jumping. From our analysis we draw broader conclusions on drivers and implications of the implementation of disaster network governance in countries that are primarily governed hierarchically and that endorse the international frameworks of disaster risk reduction.

Citizen science (CS) is promoted as a useful practice for the achievement of the Sustainable Development Goals (SDGs). In this contribution we explore how CS aligns to the SDGs overarching pledge to ‘Leave no one behind’. We propose a framework to evaluate exclusionary processes in CS. We interlink three dimensions of CS inspired by existing CS typologies with five factors underpinning exclusionary processes. With this, we are able to situate existing literature on various exclusionary effects in CS within a structured framework. We hope this contribution sparks a discussion and inspires practitioners’ reflections on a more inclusive practice in CS.

Both landscape rejuvenation through tectonic uplift and human-induced deforestation are known to increase landslide (LS) activity. Yet, the interaction between deforestation and landscape evolution has hitherto not been explicitly considered. Here, we investigate how shallow LS frequency is impacted by deforestation and landscape rejuvenation through knickpoint retreat in the Kivu Rift (East African Rift) while accounting for rock strength and slope steepness. In the past 12 Ma, the Kivu Rift has been characterized by tectonic uplift which gave rise to knickpoints in the river profiles enforcing topographic steepening. On a much shorter timescale, the rapidly growing population in the Rift has gradually expanded its cultivated and urban land leading to widespread deforestation.We compiled an inventory of almost 8000 shallow LSs using Google Earth imagery. To quantify LS frequency, we developed a new method that accounts for the temporal and spatial inconsistency of satellite imagery coverage. To characterize long-term landscape evolution, we identified (i) 672 non-stationary knickpoints in the Rift and (ii) quantified the impact of lithology on slope threshold angles (TA). We identified two homogenous lithological groups: one group of younger/weaker lithologies (540 Ma, TA=27.9 +/- 0.3°). Further analysis focused on the latter group since it covers 85% of the study area and contained more than 95% of the observed LSs.The overall shallow LS frequency in the rejuvenated landscapes inside the rift is 0.039 LS/km2/yr versus 0.010 LS/km2/yr in the relict landscapes outside the rift. Generally, LS frequency on recently deforested slopes increased by 200 to 800% in comparison to forested land. There is no notable difference in LS frequency on equally steep non-forested slopes (i.e. slopes deforested at least several decades ago) inside and outside of the rift. However, forest slopes of similar steepness are 2-3 times more sensitive to landsliding within the rift. We propose two mechanisms that might explain the higher frequency of landsliding on similar topographies within the rift: (i) the active undercutting by rivers may lead to slope destabilization without significantly increasing the average slope gradient as extracted from the SRTM DEM and (ii) tectonic uplift may induce rock and regolith fracturing, leading to weaker, more LS-prone slopes. The fact that we did not observe differences in LS frequency on hillslopes that were deforested long ago suggests that on such slopes a new equilibrium is established whereby these aforementioned mechanisms are no longer important.In conclusion, one of the key factors why the rejuvenated landscape inside the rift is more sensitive to landsliding is the higher prevalence of threshold slopes due to active incision. However, the impact of rejuvenation cannot be understood by considering only its effects on overall topography. Deforestation dramatically increases LS frequency in both relict and rejuvenated landscapes, in the first decades after forest cover removal.

Rivers receive large amounts of terrestrial soil organic carbon (SOC) and transport them from land to the ocean. Mounting evidence indicates that a large fraction of the eroded SOC, which is often very old, is quickly decomposed upon entering the river and never reaches the ocean. The mechanisms explaining this rapid decomposition of previously stable SOC remain unclear. In this study, we investigated the relative importance of two mechanisms possibly explaining this rapid SOC decomposition: (i) in the river water SOC is exposed to a different microbial community which is able to metabolise SOC much more quickly than the soil microbial community and (ii) SOC decomposition in rivers is facilitated due to the hydrodynamic disturbance of sediment. We performed different series of short-term (168h) incubations quantifying the rates of SOC decomposition in an aquatic system under controlled conditions. Organic carbon decomposition was measured continuously through monitoring dissolved O2 concentration using a fiber-optic meter (FirestingO2, PyroScience). In the control treatment, bottles of 320 ml of river water sampled from Dijle river (Leuven, Belgium) were used, without headspace, under dark conditions in a temperature-controlled room (20℃). In a second treatment, soil material was added to river water filtered at 0.2 um to remove aquatic micro-organisms (MO) (SOC-MO treatment). The effect of the presence of an aquatic microbial community on SOC decomposition was simulated by adding an inoculum of unfiltered river water to a bottle containing the same soil material (SOC+MO treatment). Secondly, we investigated the effect of water motion on respiration rates by simulating the hydrodynamic disturbance of soil particles using a swing system to keep particles suspended in the water. All treatments described above were conducted under both standing- and shaking conditions. Each experiment was repeated six times and two types of soil were tested: one from arable land (sandy loam, 2.4%OC), and the other from a temperate forest site (sandy loam, 5.0%OC). Our result show that SOC indeed further mineralized in a riverine environment. Under both shaking and standing conditions, we found a significant difference between SOC-MO and SOC+MO treatments (paired t-tests, p0.05). While some recent studies suggested that aquatic respiration rates may have been substantially underestimated by performing measurement under stationary conditions, our results indicate that this effect is relatively minor, at least under the temperature conditions and for the suspended matter concentration range (500 mg/L arable land soil; 200 mg/L forest soil) used in our experiments.

Madagascar shows very high erosion rates and, in some regions, the landscape is dotted by major gullies called “lavaka”. Lavaka are also very frequent in the surroundings of Lake Alaotra, a large, shallow lake in the Malagasy highlands. A central question with respect to landscape evolution in Madagascar is to what extent human impact has triggered environmental change in terms of vegetation, erosion rates (lavaka formation) and sediment dynamics. Sedimentary archives in lakes such as lake Alaotra can be of great help to resolve this question provided that we understand how sediment and carbon are mobilised and transported through the landscape. In this study, we traced pathways of sediment and carbon fluxes through this eroding landscape, from the eroded hillslopes over various sediment deposition zones (floodplains, reservoirs, marshes..) to Lake Alaotra. Detailed profiles taken along convex hillslope transects (grasslands and primary forest), in the marsh peat, floodplains and lake were analyzed for carbon and nitrogen content, texture, and stable carbon isotope ratios (δ13C). Along the grassland hillslopes, soil OC content is extremely low, from 0.4 to 1.8% in the top layer and rapidly decreasing to 13C ~-13 ‰), yet soil δ13C ranges between -24 and -18‰, and most profiles show a decrease in δ13C with the depth – in contrast to observations in most C3-dominated systems. Contrary to our expectations, Lake Alaotra was found not to be a major sink of hillslope-derived sediments and/or carbon. Sediment cores from different parts of the lake have high OC contents (5 to 18%) and contain only minor amounts of sand, the dominant grain size class on the hillslopes. The high OC content of the lake sediments, in combination with data on C/N ratios and δ13C indicate that the OC in the lake sediments is mainly derived from the surrounding marshes and in situ primary production rather than from terrestrial C eroded from the catchment. Floodplains are likely a key sink for soil-derived sediments: similar to hillslope soils, sediment profiles in the floodplains show a low %OC and relatively high δ13C values ranging between -21 and -14‰.We conclude that most of the detritic sediments and carbon mobilised on the hillslopes through erosion do not reach lake Alaotra, even though erosion rates in the landscape are extremely high. Studying sedimentary profiles in the lake may provide information on environmental change (e.g. through changes in carbon contents and/or characteristics) but is insufficient to understand the entire sediment and carbon cascades in the Malagasy landscape.

Regional landslide inventories are often prepared by several different experts, using a variety of data sources. This can result in a combination of polygon and point landslide data, characterized by different meanings, uncertainties and levels of reliability. The propagation of uncertainties due to such heterogeneous data is a relevant issue in statistical landslide susceptibility zonation at supra-local scale. In the inhabited highlands of the Rwenzori Mountains, we compare different approaches and mapping units to provide a robust methodology for susceptibility mapping using a combination of landslide point and polygon data. First, the effect of the uncertainty related to a point representation of landslides is assessed comparing slope unit-based and pixel-based analyses, using digital elevation models with different resolutions. Secondly, with regard to landslide polygon inventories, we compare the use of thresholds versus a presence/absence of the depletion centroid or a randomly selected point in the landslide polygon in order to identify slope units with landslides. Based on these results, we prepare regional slope unit-based susceptibility maps using a logistic regression model calibrated with the landslide polygon inventory and validated with the point inventory. Although pixel-based mapping remains the most common approach for statistical landslide susceptibility zonation, our analysis clearly favours the use of slope units as a powerful tool to prepare regional susceptibility maps and, in particular, to exploit heterogeneous information in a consistent way.

Landslides affect millions of people worldwide, but theoretical and empirical studies on the impact of landslides remain scarce, especially in Sub-Saharan Africa. This study proposes and applies a method to estimate the direct impact of landslides on household income and to investigate the presence of specific risk sharing and mitigation strategies towards landslides in a tropical and rural environment. An original cross-sectional household survey is used in combination with geographical data to acquire detailed information on livelihoods and on hazards in the Rwenzori mountains, Uganda. Ordinary least squares regressions and probit estimations with village fixed effects are used to estimate the impact of landslides and the presence of mitigation strategies. Geographical information at household level allows to disentangle the direct impact from the indirect effects of landslides. We show that the income of affected households is substantially reduced during the first years after a landslide has occurred. We find that members of recently affected households participate more in wage-employment or in self-employed activities, presumably to address income losses following a landslide. Yet, we see that these jobs do not provide sufficient revenue to compensate for the loss of income from agriculture. Given that landslides cause localized shocks, finding a significant direct impact in our study indicates that no adequate risk sharing mechanisms are in place in the Rwenzori sub-region. These insights are used to derive policy recommendations for alleviating the impact of landslides in the region. By quantifying the direct impact of landslides on household income in an agricultural context in Africa this study draws the attention towards a problem that has been broadly underestimated so far and provides a sound scientific base for disaster risk reduction in the region. Both the methodology and the findings of this research are applicable to other tropical regions with high landslide densities. publisher: Elsevier articletitle: The direct impact of landslides on household income in tropical regions: A case study from the Rwenzori Mountains in Uganda journaltitle: Science of The Total Environment articlelink: http://dx.doi.org/10.1016/j.scitotenv.2016.01.171 content_type: article copyright: Copyright © 2016 Elsevier B.V. All rights reserved. ispartof: Science of the Total Environment vol:550 pages:1032-1043 ispartof: location:Netherlands status: published