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Topography is one of the key factors controlling soil erosion and redistribution of pedogenic material along slope. Land cover change can have an accelerating or retarding impact on topographically-controlled soil erosion rates, depending on the type and intensity of land use and management. In this study, we investigated the combined effect of hillslope gradient and land cover change on soil redistribution and rejuvenation in a subtropical region where Atlantic rain forest was converted to agricultural land. We used a two by two design, and evaluated the effect of hillslope gradient (steep vs. gentle) and land cover (forest vs. cropland) on the spatial pattern of soil weathering degree along slope. In four soil toposequences, soil weathering indices (Total Reserve in Bases, Chemical Index of Alteration, clay content, iron oxide content) and mineralogical assemblages were used to express the soil physico-chemical and mineralogical properties of the soil profiles. Our data showed that the spatial differentiation in chemical weathering degree along slope is strongly dependent on the hillslope gradient, with no to very limited differentiation in chemical weathering degree along the two gentle slopes and a clear differentiation along the steep slopes. Besides, there is an interaction effect between hillslope gradient and land cover. Forest conversion to cropland enhances erosion-driven soil redistribution with a marked effect on soil rejuvenation along steep slopes but no clear effect along gentle slopes. The comparative study based on four toposequences highlights that accelerated soil erosion after conversion of forests to cropland further enhanced lateral soil fluxes and redistribution of topsoil material along steep slopes, and led to soil rejuvenation and exposure of less weathered soil material at the eroding sites.

Although climate exerts a major control on mineral weathering and soil formation processes, the combined effect of vegetation and topography can influence the rate and extent of chemical weathering at the hillslope scale. In this paper, we examined spatial patterns in volumetric strain and soil weathering extent associated with topographic gradients and vegetation patterns. In a high Andean catchment, we selected 10 soil toposequences on andesitic flows: 5 under tussock grasses, 3 under cushion forming plants and 2 under native forest. Along each toposequence, one pit was excavated at the shoulder, backslope and toeslope resulting in 30 soil profiles. Depth‐weighted total soil porosity of the thirty soil profiles averaged 64±6%. The association between volumetric strain and soil organic C indicates that biotic agents can be effective in dilating the regolith during weathering. The young, postglacial volcanic soils were depleted in mono‐ and divalent cations, with total mass losses ranging between 793 and 1610 kg.m‐2. The accumulation of Al‐humus complexes in the soil matrix plays an essential role in chemical transformation of the non‐allophanic soils. Beyond the marginally significant topographic control on chemical weathering extent, our data show highly significant differences in chemical weathering extent between vegetation communities with total mass losses in forest soils being respectively 19% and 22% higher than in grasslands and cushion forming plants. The vegetation mosaic in alpine ecosystems might therefore provide essential clues to understand soil chemical weathering patterns caused by spatially varying soil particle and water residence times.

Images captured by unmanned aerial vehicles (UAVs) and processed by structure-from-motion (SfM) photogrammetry are increasingly used in geomorphology to obtain high-resolution topography data. Conventional georeferencing using ground control points (GCPs) provides reliable positioning, but the geometrical accuracy critically depends on the number and spatial layout of the GCPs. This limits the time and cost effectiveness. Direct georeferencing of the UAV images with differential GNSS, such as PPK (post-processing kinematic), may overcome these limitations by providing accurate and directly georeferenced surveys. To investigate the positional accuracy, repeatability and reproducibility of digital surface models (DSMs) generated by a UAV–PPK–SfM workflow, we carried out multiple flight missions with two different camera–UAV systems: a small-form low-cost micro-UAV equipped with a high field of view (FOV) action camera and a professional UAV equipped with a digital single lens reflex (DSLR) camera. Our analysis showed that the PPK solution provides the same accuracy (MAE: ca. 0.02 m, RMSE: ca. 0.03 m) as the GCP method for both UAV systems. Our study demonstrated that a UAV–PPK–SfM workflow can provide consistent, repeatable 4-D data with an accuracy of a few centimeters. However, a few flights showed vertical bias and this could be corrected using one single GCP. We further evaluated different methods to estimate DSM uncertainty and show that this has a large impact on centimeter-level topographical change detection. The DSM reconstruction and surface change detection based on a DSLR and action camera were reproducible: the main difference lies in the level of detail of the surface representations. The PPK–SfM workflow in the context of 4-D Earth surface monitoring should be considered an efficient tool to monitor geomorphic processes accurately and quickly at a very high spatial and temporal resolution.

Indurated volcanic deposits occur worldwide. In Ecuador, they are known as “cangahua”, a Quechua word for “hard, sterile soil”. About 15% of the cultivated area in the Highland region of Ecuador is underlain by cangahua. Improper cultivation practices lead to erosion and expo-sure of the hard cangahua material. The properties and mode(s) of for-mation of cangahua are not well understood. This thesis aims at shed-ding new light on the origin and processes responsible for the formation of cangahua. Three outcrops located in the Central and Northern Highland were stud-ied. These are comprised of tephra deposits, cangahua layers and soil horizons. Chemical and mineralogical analyses revealed that the parent materials of the cangahua layers were sourced from the closest volcanic centers. Most cangahuas probably relate to individual explosive erup-tions although some may have formed on tephra from different volca-noes. Grainsize analyses suggested that the parent materials correspond either to pyroclastic fall or flow deposits. Both geological (syn-deposition) and pedological (post-deposition) pro-cesses contributed to induration of cangahua’s parent material. As a re-sult, the weathering status of cangahua can vary significantly. Thus, cangahua classification should consider the stage at which induration occurs: geogenetic (syn-deposition induration via geological processes), pedogenetic (post-deposition induration via pedological processes) and pedo-geogenetic (syn-deposition induration via geological processes reinforced by post-deposition induration via pedological processes). Mineralogical analyses revealed that pedo-geogenetic cangahua layers dated at 23 and 32 cal ky BP were exposed to alternating wet-dry climat-ic conditions after and just before/during the Last Glacial Maximum in Ecuador., (AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 2019

Several Andean countries have planned to restore forest cover in degraded land to enhance the provision of multiple ecosystem services in response to international commitments such as the Bonn Challenge. Hydrological services, e.g. water supply, hydrological regulation and erosion mitigation, are particularly important to sustain the life of more than fifty million Andean people. While rapid and important forest cover changes have occurred during recent decades, critical information on the impact of forestation on hydrological services has not yet been synthesized in the context of Andean ecosystems. We define forestation as the establishment of forest by plantation or natural regeneration on areas that either had forest in the past or not. To help improve decision-making on forestation in the Andes, we reviewed the available literature concerning the impacts of forestation on water supply, hydrological regulation and mitigation of erosion and landslides. We also examined available data on the most relevant hydrological processes such as infiltration, evapotranspiration and runoff in forest stands. Hydrological services from native forests were also included as a reference state for comparing processes and services provided by forestation. Following systematic review protocols, we synthesized 155 studies using different methods, including meta-analyses and meta-regressions. Results show that forestation has had clear impacts on degraded soils, through reducing water erosion of soils and risk of moderate floods, increasing soil infiltration rate by 8 and topsoil organic matter (SOM). We found that 20 years of tree plantation was sufficient to recover infiltration rate and sediment yield close to the levels of native forests whereas SOM, soil water storage and surface runoff of native forests could not be recovered by forestation in the time scales examined. The benefits in terms of hydrological regulation are at the expense of a reduction in total water supply since

Although climate exerts a major control on mineral weathering and soil formation processes, the combined effect of vegetation and topography can influence the rate and extent of chemical weathering at the hillslope scale. In this paper, we examined spatial patterns in volumetric strain and soil weathering extent associated with topographic gradients and vegetation patterns. In a high Andean catchment, we selected 10 soil toposequences on andesitic flows: 5 under tussock grasses, 3 under cushion forming plants, and 2 under native forest. Along each toposequence, one pit was excavated at the shoulder, backslope, and toeslope resulting in 30 soil profiles. Depth‐weighted total soil porosity of the 30 soil profiles averaged 64 ± 6%. The association between volumetric strain and soil organic C indicates that biotic agents can be effective in dilating the regolith during weathering. The young, postglacial volcanic soils were depleted in mono‐divalent and divalent cations, with total mass losses ranging between 793 and 1610 kg/m2 . The accumulation of Al‐humus complexes in the soil matrix plays an essential role in chemical transformation of the nonallophanic soils. Beyond the marginally significant topographic control on chemical weathering extent, our data show highly significant differences in chemical weathering extent between vegetation communities with total mass losses in forest soils being, respectively, 19% and 22% higher than in grasslands and cushion‐forming plants. The vegetation mosaic in alpine ecosystems might therefore provide essential clues to underst

Mountains play an important role in the denudation of continents and transfer erosion and weathering products to lowlands and oceans. The rates at which erosion and weathering processes take place in mountain regions have a substantial impact on the morphology and biogeochemistry of downstream reaches and lowlands. The controlling factors of physical erosion and chemical weathering and the coupling between the two processes are not yet fully understood. In this study, we report physical erosion and chemical weathering rates for five Andean catchments located in the southern Ecuadorian Andes and investigate their mutual interaction. During a 4-year monitoring period, we sampled river water at biweekly intervals, and we analyzed water samples for major ions and suspended solids. We derived the total annual dissolved, suspended sediment, and ionic loads from the flow frequency curves and adjusted rating curves and used the dissolved and suspended sediment yields as proxies for chemical weathering and erosion rates. In the 4-year period of monitoring, chemical weathering exceeds physical erosion in the high Andean catchments. Whereas physical erosion rates do not exceed 30 t km−2 y−1 in the relict glaciated morphology, chemical weathering rates range between 22 and 59 t km−2 y−1. The variation in chemical weathering is primarily controlled by intrinsic differences in bedrock lithology. Land use has no discernible impact on the weathering rate but leads to a small increase in base cation concentrations because of fertilizer leaching in surface water.When extending our analysis with published data on dissolved and suspended sediment yields from the northern and central Andes, we observe that the river load composition strongly changes in the downstream direction, indicating large heterogeneity of weathering processes and rates within large Andean basins

Gully erosion is an important process of land degradation in mountainous regions, and is known to be one of the major sediment sources in eroded catchments. Recent studies have suggested that living and dead vegetation can be effective for ecosystem restoration, and large-scale restoration projects have been implemented in the tropical Andes in recent decades. However, few quantitative studies exist on the effectiveness of gully restoration to reduce sediment production and mobilization. In this study, sediment mobilization and transport was studied in five micro-catchments (< 1 km2) with different soil and water conservation treatments. The techniques that were used for soil and water conservation involve vegetation restoration on the hillslopes and check dams in active gully channels. To characterize the routing of sediment within the micro-catchments, we measured erosion and sediment deposition within the gully channels. Sediment yield was estimated from measurements of sediment accumulation in sediment traps that were constructed at the outlet of the micro-catchments. Flow barriers are shown to be very effective in stabilizing active gully systems in badlands through significant reduction (of>70%) of the amount of sediment exported from the micro-catchments. The construction of wooden barriers (or so-called check dams) in active gully channels enhances sediment deposition in the gully bed. The latter is strongly dependent on the rainfall intensity, as well as gully channel slope and vegetation cover. The experimental data suggest that there exists a threshold value of rainfall intensity (I30max) of about 23mmh−1, above which all sections of the gully system are actively contributing water and sediment to the river network. Also, forestation of active gully systems with rapidly growing exotic species such as Eucalyptus has a positive effect on the stabilization and restoration of the badlands, and effectively reduces the sediment export.

Ecuador is one of the first countries worldwide that set the environment as a national priority. The 2008 constitution established the concept of “Living Well” or “Buen Vivir” as a parameter of coexistence of the Ecuadorian society, and is based on equity and harmony with nature. The 2009–2013 [1] and 2013–2017 [2] National Plan for Good Living focused on conservation and sustainable management of natural heritage and biodiversity and mitigation and adaptation to climate change. These initiatives are internationally recognized for being at the forefront of socio-environmental rights [3,4]. One of the four principles of the “Plan Nacional para el Buen Vivir” was environmental sustainability, aiming to strengthen conservation, valorization and sustainable use of natural resources, ecosystem services and biodiversity [2]. The 2017–2021 National Development Plan [5] has been designed to further achieve environmental sustainability and territorial development.

The movement of sediment from mountainous uplands to continental margins, i.e. the sediment cascade, modifies the terrestrial surface and eventually drives the rate of topographic evolution. Understanding the pattern and rates of geomorphic mechanisms involved in the sediment cascade is of crucial importance to constrain biogeochemical cycles, long-term landscape evolution and potential feedbacks between tectonics, erosion and climate. Sediment cascading systems are still poorly constrained, as sediment budgets and connectivity can be highly variable in space and time due to the stochastic character of geomorphic processes. In an effort to improve our quantitative understanding of the sediment cascade in landslide-prone environments, this doctoral dissertation aims at constraining spatio-temporal sediment dynamics over annual to millennial time scales. We applied state-of-the-art geomorphic techniques to constrain sediment budgets and connectivity for a mountainous catchment located in the foothills of the Central Swiss Alps. At the annual time scale, we monitored topographic changes using 3D topographic reconstructions derived from aerial pictures acquired by a drone, and at the millennial time scale, we quantified catchment-scale denudation rates using in-situ produced cosmogenic radionuclides. By integrating sediment fluxes over different temporal scales, it becomes clear that geomorphic process rates in landslide-prone terrain are highly variable in space and time. Periods of sediment remobilisation by landsliding on the hillslopes do not necessarily coincide with sediment pulses in the river network. Only when hillslopes are geomorphically connected to the river channels, the sediment mobilised by landsliding on the slopes is effectively transferred to the channel network. Therefore, phases of low or high geomorphic activity in upland catchments are not necessarily indicative for the long-term sediment fluxes of mountainous catchments. Their spatio-temporal sed, (SC - Sciences) -- UCL, 2018

La capacidad hidroeléctrica en América del Sur ha aumentado en gran medida en la última década. En Perú, la energía hidroeléctrica representó aproximadamente el 50% de la producción total de energía en 2016. Las plantas hidroeléctricas en Perú comúnmente cuentan con un reservorio para regular el flujo en la temporada seca. Las altas tasas de erosión y transferencia de sedimentos en los Andes conducen a una acumulación rápida de sedimentos en los reservorios y a una disminución severa del almacenamiento. Sin embargo, hasta ahora se sabe poco sobre las evaluaciones cuantificadas de erosión y deposición y cómo los cambios climáticos influyen en estos procesos. Se estudia la cuenca del río Cañete, ya que juega un papel importante en el desarrollo social y económico de la región debido a su suministro de agua y energía a las zonas rurales y urbanas. La planta hidroeléctrica El Platanal, cuenta con el embalse Capillucas, ubicado en la cuenca media. En este trabajo, utilizamos un modelo hidrológico acoplado a un modelo de transporte de sedimentos para simular los cambios futuros en la descarga y la carga de sedimentos en el río. Cuatro escenarios fueron desarrollados y evaluados. Dos de ellos muestran un aumento de 8% y 15% respectivamente, de la tasa de precipitación anual, distribuida en la temporada de lluvias. Mientras que los otros evalúan el impacto de estos incrementos solo en los eventos extremos o picos. Este documento se centra en el impacto potencial del cambio climático en la dinámica de los sedimentos en los Andes peruanos, y evalúa los posibles cambios en la transferencia y movilización de sedimentos Así mismo, presenta nuevos datos sobre la sensibilidad de la infraestructura del agua y las instalaciones de energía hidroeléctrica a los cambios inducidos por el clima en la movilización de sedimentos en la red fluvial.

The hydropower capacity in South America has largely increased in the last decade. In Peru, hydropower is the largest energy resource accounting for about 60% of the total energy production in 2017. The hydropower plants in Peru commonly have a large impoundment facility, allowing to carry over the storage of river water from the wet season to supplement lower flows in the dry season. Erosion rates of catchments draining the steep escarpments of the Andean Cordillera, where hydropower plants are preferentially located, are known to range between 0.2 and 1.0 mm/yr, or roughly 520 to 2600 t/km2/yr. High rates of erosion and sediment transfer in the Andes lead to rapid accumulation of sediment in the hydropower reservoirs and a potentially severe decrease of the carryover storage. However, little is known to date concerning quantified assessments of erosion and deposition and how climatic changes influence these processes. This paper focuses on the potential impact of climate change on sediment dynamics in the Peruvian Andes, and evaluates the potential changes in sediment transfer and mobilization, and reservoir sedimentation. Within the Peruvian coastal mountain range, the catchment of the Cañete River is studied as it plays an important role in the social and economic development of the region due to its provision of water and energy to rural and urban areas. The 220MW El Platanal hydroelectric plant located on Canete’s river, involves the Capillucas reservoir dam that was constructed on the middle reach of the Cañete River. The lower part of the Cañete catchment is an arid desert, the middle subhumid part sustains subsistence agriculture, and the upper part is a treeless high-elevation puna landscape. Snow cover and glaciers are present at its headwaters located above 5000 masl. The retreat of glaciers due to climate change is expected to have an impact on water availability in Cañete’s headwaters, and on the production and mobilization of sediment within the river

Soils are a major reservoir for organic carbon storing more carbon (C) than vegetation and the atmosphere combined. Soil organic carbon (SOC) is regarded as a key soil property due to its effects on soil functions such as water retention or soil fertility. The removal of natural vegetation for agricultural purposes intensified soil redistribution processes and associated lateral SOC fluxes. There is now increasing acceptation that accelerated lateral SOC fluxes also alter the amount of carbon that is exchanged between soils and the atmosphere. Using a process-based, spatially-explicit modelling approach linking land cover change, soil redistribution, soil evolution and SOC cycling, this thesis highlights the strong temporal and spatial variability of the C budget at the catchment scale. Under soil erosion, SOC stock can reach a dynamic equilibrium at which the hillslope C export is compensated by the atmospheric replacement of the eroded C. The dynamic equilibrium follows a transient period of several decades in which the rate of C uptake from the atmosphere is steadily increasing. The SOC released from soils to the atmosphere quickly responded to anthropogenic land conversion while the erosion-induced C fluxes exhibited a very slow response time. Our model simulations suggest that, over the past millennium and for the environmental settings considered, the C sink could have almost offset the C losses related to land cover conversion. In addition, the long-term sustainability of the C sink and the establishment of a steady state regime are negatively affected by the impact of soil erosion on biomass productivity and soil resources. Using observations from a wide range of soil conditions, we showed that including biomass productivity evolution in the C budgeting improves the prediction of SOC losses and results in increased SOC losses and a reduced C sink intensity, relative to a scenario without soil erosion-productivity feedbacks. Finally, in contrast with environm, (SC - Sciences) -- UCL, 2018

Gully erosion is an important process of land degradation in mountainous regions, and is known to be one of the major sediment sources in eroded catchments. Recent studies have suggested that living and dead vegetation can be effective for ecosystem restoration, and large-scale restoration projects have been implemented in the tropical Andes in recent decades. However, few quantitative studies exist on the effectiveness of gully restoration to reduce sediment production and mobilization. In this study, sediment mobilization and transport was studied in five micro-catchments (< 1 km2) with different soil and water conservation treatments. The techniques that were used for soil and water conservation involve vegetation restoration on the hillslopes and check dams in active gully channels. To characterize the routing of sediment within the micro-catchments, we measured erosion and sediment deposition within the gully channels. Sediment yield was estimated from measurements of sediment accumulation in sediment traps that were constructed at the outlet of the micro-catchments. Flow barriers are shown to be very effective in stabilizing active gully systems in badlands through significant reduction (of>70%) of the amount of sediment exported from the micro-catchments. The construction of wooden barriers (or so-called check dams) in active gully channels enhances sediment deposition in the gully bed. The latter is strongly dependent on the rainfall intensity, as well as gully channel slope and vegetation cover. The experimental data suggest that there exists a threshold value of rainfall intensity (I30max) of about 23mmh−1, above which all sections of the gully system are actively contributing water and sediment to the river network. Also, forestation of active gully systems with rapidly growing exotic species such as Eucalyptus has a positive effect on the stabilization and restoration of the badlands, and effectively reduces the sediment export.

Mountains play an important role in the denudation of continents and transfer erosion and weathering products to lowlands and oceans. The rates at which erosion and weathering processes take place in mountain regions have a substantial impact on the morphology and biogeochemistry of downstream reaches and lowlands. The controlling factors of physical erosion and chemical weathering and the coupling between the two processes are not yet fully understood. In this study, we report physical erosion and chemical weathering rates for five Andean catchments located in the southern Ecuadorian Andes and investigate their mutual interaction. During a 4-year monitoring period, we sampled river water at biweekly intervals, and we analyzed water samples for major ions and suspended solids. We derived the total annual dissolved, suspended sediment, and ionic loads from the flow frequency curves and adjusted rating curves and used the dissolved and suspended sediment yields as proxies for chemical weathering and erosion rates. In the 4-year period of monitoring, chemical weathering exceeds physical erosion in the high Andean catchments. Whereas physical erosion rates do not exceed 30 t km−2 y−1 in the relict glaciated morphology, chemical weathering rates range between 22 and 59 t km−2 y−1 . The variation in chemical weathering is primarily controlled by intrinsic differences in bedrock lithology. Land use has no discernible impact on the weathering rate but leads to a small increase in base cation concentrations because of fertilizer leaching in surface water. When extending our analysis with published data on dissolved and suspended sediment yields from the northern and central Andes, we observe that the river load composition strongly changes in the downstream direction, indicating large heterogeneity of weathering processes and rates within large Andean basins.