A numerical modelling and experimental study of flow width dynamics on alluvial fans

Alluvial fans are dynamic landforms, the evolution of which is controlled by both external environmental forcing (climate, tectonics and base level change) and internal process-form feedbacks. The latter include changes in flow configuration (between sheetflow and channelized flow states), driven by aggradation and degradation, which may in turn promote changes in sediment transport capacity. Recent numerical modelling indicates that such feedbacks may lead to dramatic and persistent fan entrenchment in the absence of external forcing. However, the parameterization of flow width within such models is untested to date and is subject to considerable uncertainty. This paper presents results from an experimental study of flow width dynamics on an aggrading fan in which spatial and temporal patterns of fan inundation are monitored continuously using analysis of digital vertical photography. Observed flow widths are compared with results from a simple theoretical model developed for non-equilibrium (aggradational) conditions. Results demonstrate that the theoretical model is capable of capturing the first-order characteristics of width adjustment over the course of the experiment, and indicate that flow width is a function of fan aggradation rate. This illustrates that models of alluvial flow width derived for equilibrium conditions may have limited utility in non-equilibrium situations, despite their widespread use to date

Future C loss in mid-latitude mineral soils: climate change exceeds land use mitigation potential in France

Many studies have highlighted significant interactions between soil C reservoir dynamics and global climate and environmental change. However, in order to estimate the future soil organic carbon sequestration potential and related ecosystem services well, more spatially detailed predictions are needed. The present study made detailed predictions of future spatial evolution (at 250 m resolution) of topsoil SOC driven by climate change and land use change for France up to the year 2100 by taking interactions between climate, land use and soil type into account. We conclude that climate change will have a much bigger influence on future SOC losses in mid-latitude mineral soils than land use change dynamics. Hence, reducing CO2 emissions will be crucial to prevent further loss of carbon from our soils

Arable soil formation and erosion: a hillslope-based cosmogenic nuclide study in the United Kingdom

Arable soils are critical resources that support multiple ecosystem services. They are frequently threatened, however, by accelerated erosion. Subsequently, policy to ensure their long-term security is an urgent societal priority. Although their long-term security relies upon a balance between the rates of soil loss and formation, there have been few investigations of the formation rates of soils supporting arable agriculture. This paper addresses this knowledge gap by presenting the first isotopically constrained soil formation rates for an arable (Nottinghamshire, UK) and coniferous woodland hillslope (Shropshire, UK). Rates ranged from 0.026 to 0.096 mm yr−1 across the two sites. These rates fall within the range of previously published rates for soils in temperate climates and on sandstone lithologies but significantly differed from those measured in the only other UK-based study. We suggest this is due to the parent material at our sites being more susceptible to weathering. Furthermore, soil formation rates were found to be greatest for aeolian-derived sandstone when compared with fluvially derived lithology raising questions about the extent to which the petrographic composition of the parent material governs rates of soil formation. On the hillslope currently supporting arable agriculture, we utilized cosmogenically derived rates of soil formation and erosion in a first-order lifespan model and found, in a worst-case scenario, that the backslope A horizon could be eroded in 138 years with bedrock exposure occurring in 212 years under the current management regime. These findings represent the first quantitative estimate of cultivated soil lifespans in the UK

Bedrock geochemistry influences vegetation growth by regulating the regolith water holding capacity

Although low vegetation productivity has been observed in karst regions, whether and how bedrock geochemistry contributes to the low karstic vegetation productivity remain unclear. In this study, we address this knowledge gap by exploring the importance of bedrock geochemistry on vegetation productivity based on a critical zone investigation across a typical karst region in Southwest China. We show silicon and calcium concentrations in bedrock are strongly correlated with the regolith water loss rate (RWLR), while RWLR can predict vegetation productivity more effectively than previous models. Furthermore, the analysis based on 12 selected karst regions worldwide further suggest that lithological regulation has the potential to obscure and distort the influence of climate change. Our study implies that bedrock geochemistry could exert effects on vegetation growth in karst regions and highlights that the critical role of bedrock geochemistry for the karst region should not be ignored in the earth system mode

Rock crevices determine woody and herbaceous plant cover in the karst critical zone

The study of the critical zones (CZs) of the Earth link the composition and function of aboveground vegetation with the characteristics of the rock layers, providing a new way to study how the unique rock and soil conditions in karst regions affect the aboveground vegetation. Based on survey results of the rocks, soils and vegetation in the dolomite and limestone distribution areas in the karst area of central Guizhou, it was found that woody plant cover increases linearly with the number of cracks with a width of more than 1 mm, while the cover of herbaceous plants shows the opposite trend (p<0.01). The dolomite distribution area is characterized by undeveloped crevices, and the thickness of the soil layer is generally less than 20 cm, which is suitable for the distribution of herbaceous plants with shallow roots. Due to the development of crevices in the limestone distribution area, the soil is deeply distributed through the crevices for the deep roots of trees, which leads to a diversified species composition and a complicated structure in the aboveground vegetation. Based on moderate resolution imaging spectroradiometer (MODIS) remote sensing data from 2001 to 2010, the normalized differentiated vegetation index (NDVI) and annual net primary productivity (NPP) results for each phase of a 16-day interval further indicate that the NDVI of the limestone distribution area is significantly higher than that in the dolomite distribution area, but the average annual NPP is the opposite. The results of this paper indicate that in karst CZs, the lithology determines the structure and distribution of the soil, which further determines the cover of woody and herbaceous plants in the aboveground vegetation. Although the amount of soil in the limestone area may be less than that in the dolomite area, the developed crevice structure is more suitable for the growth of trees with deep roots, and the vegetation activity is strong. At present, the treatment of rocky desertification in karst regions needs to fully consider the rock-soilvegetation- air interactions in karst CZs and propose vegetation restoration measures suitable for different lithologies

An evaluation of the role of physical models in exploring form–process feedbacks in alluvial fans

Alluvial fans represent small-scale coupled transport/depositional systems that provide the opportunity to undertake detailed studies of non-equilibrium landform behaviour. Previous interpretation of fans in the field has tended to emphasise the importance of external controls on their evolution (e.g. climate and tectonics). However, recent theoretical models have shown that internal process-driven feedbacks may be equally important. Using a physical model, image analysis reveals temporal and spatial patterns in flow patterns over the building and fully-developed fan surface, both in the presence and absence of external forcing (change in water–sediment ratios). The pattern observed from the physical model is compared to patterns observed in both numerical model simulations and field fans in the Avoca Valley and Poerua in the Southern Alps, New Zealand. These comparisons led to a preliminary evaluation of the role of physical models in exploring form–process feedbacks in alluvial fans

An experimental investigation of autogenic behaviour during alluvial fan evolution

The findings from a series of laboratory experiments designed to examine the role of autogenic process-form interactions as controls on alluvial fan evolution are presented. The experimental approach follows the ‘similarity of processes’ concept and is therefore not scaled to a real world prototype. The experiments were conducted in the University of Exeter Sediment Research Facility using a 3 × 3 m fixed bed experimental table with controlled sediment and water supply fed to the fan apex, and evacuation of all sediment that passes the distal boundary. The evolution of the fan was recorded using high resolution time-lapse overhead photography to permit analysis of fan topography, flow patterns, channel migration and avulsion on the fan surface over the course of the experimental run. The qualitative observations from three experimental scenarios are discussed and evaluated in the context of current understanding of alluvial fan evolution. These demonstrate that in the absence of any change in the extrinsic variables there is an autogenic transition from sheetflow to channelised flow as the experimental fans evolve. This is consistent with theory that predicts this change in response to a decline in aggradation rate through time