Lake sedimentological and ecological response to hyperthermals : Boltysh impact crater, Ukraine

Acknowledgements Initial drilling of the Boltysh meteorite crater was funded by Natural Environment Research Council (NERC) grant NE/D005043/1. The authors are extremely grateful to the valuable scientific contributions of S. Kelley and I. Gilmour. The constructive and critical reviews by M. Schuster and an anonymous reviewer greatly helped to improve this manuscript.Peer reviewedPostprin

The geomorphology of coarse clastic surfaces in arid environments

This study explores the linkages between slope form and slope process in arid environments. In doing so, questions of the development of slopes in arid environments are examined. The age of many arid environment surfaces, combined with the sporadic nature of formative events, means that long-term surface and slope development remains an elusive question in geomorphology. Deserts have inspired many of the most enduring theories of landscape evolution and continue to provide a test-bed for new and emerging ideas in geomorphology. The clast-mantled surface of the northeast Jordan Badia presents an ideal opportunity to study the links between surface character and slope processes in arid environments. The northeast Badia also provides an opportunity to explore theories of slope development and the behaviour of earth surface systems. The nature of the clast covered ground surface has been assessed using a new digital aerial photography and image analysis technique. A field study of surface processes has been used to explore links between surface form and slope process. Additionally, a computer based simulation of long-term modification of the spatial distribution of surface clast has been undertaken. Given the subtle variation in earth surface form between disparate locations, a new semi-quantitative method of locating sample sites has been developed. The characterization of surface form has identified statistically significant relationships between ground surface character and two-dimensional slope form. Systematic variations in ground surface configuration, both within and between basalt flows, are found to be indicative of the action of slope processes. The first study of ground surface hydrology in the north eastern Badia has been undertaken. The results from a series of rain-storm simulation experiments show subtle but significant links between the action of surface processes and variations in ground surface form. The controls on surface process are diverse and vary in significance with position in the landscape. A combination of ground surface characterization and process studies has identified several interesting geomorphological phenomena The surfaces exhibit systematic variations in structure and organization. Homeostatic links between form and process are clearly apparent, which suggests that surface form influences and is influenced by process action via a process of positive feedbacks. Given the sporadic and infrequent recurrence of formative events in arid environments, a modelling approach has been developed to understand the long-term, spatial dynamics of the ground surface. The model has been used to simulate structure in the surface clast arrangement and the sensitivity of surface organization to physically constrained variations in model parameters. The model also allows the surfaces to be considered as self-organizing earth surface systems. The model results provide new insights into the process-form linkages in operation on clast-mantled arid surfaces. The model results provide new ways of examining and understanding the dynamics of clast mantled arid surfaces and have implications for the application of self-organization in geomorphology

Understanding Structure and Function in Semiarid Ecosystems: Implications for Terrestrial Carbon Dynamics in Drylands

This study advances understanding of how the changes in ecosystem structure and function associated with woody shrub encroachment in semi-arid grasslands alter ecosystem carbon (C) dynamics. In terms of both magnitude and dynamism, dryland ecosystems represent a major component of the global C cycle. Woody shrub encroachment is a widespread phenomenon globally, which is known to substantially alter ecosystem structure and function, with resultant impacts on C dynamics. A series of focal sites were studied at the Sevilleta National Wildlife Refuge in central New Mexico, USA. A space-for-time analogue was used to identify how landscape structure and function change at four stages over a grassland to shrubland transition. The research had three key threads: 1. Soil-associated carbon: Stocks of organic and inorganic C in the near-surface soil, and the redistribution of these C stocks by erosion during high-intensity rainfall events were quantified using hillslope-scale monitoring plots. Coarse (>2 mm) clasts were found to account for a substantial proportion of the organic and inorganic C in these calcareous soils, and the erosional effluxes of both inorganic and organic C increased substantially across the vegetation ecotone. Eroded sediment was found to be significantly enriched in organic C relative to the contributing soil with systematic changes in OC enrichment across the vegetation transition. The OC enrichment dynamics observed were inconsistent with existing understanding (derived largely from reductionist, laboratory-based experiments) that OC enrichment is largely insignificant in the erosional redistribution of C. 2. Plant biomass: Cutting-edge proximal remote sensing approaches, using a remotely piloted lightweight multirotor drone combined with structure-from-motion (SfM) photogrammetry were developed and used to quantify biomass carbon stocks at the focal field sites. In such spatially heterogeneous and temporally dynamic ecosystems existing measurement techniques (e.g. on-the-ground observations or satellite- or aircraft-based remote sensing) struggle to capture the complexity of fine-grained vegetation structure, which is crucial for accurately estimating biomass. The data products available from the novel SfM approach developed for this research quantified plants just 15 mm high, achieving a fidelity nearly two orders of magnitude finer than previous implementations of the method. The approach developed here will revolutionise the study of biomass dynamics in short-sward ecogeomorphic systems. 3. Ecohydrological modelling: Understanding the effects of water-mediated degradation processes on ecosystem carbon dynamics over greater than observable spatio-temporal scales is complicated by significant scale-dependencies and thus requires detailed mechanistic understanding. A process-based, spatially-explicit ecohydrological modelling approach (MAHLERAN - Model for Assessing Hillslope to Landscape Erosion, Runoff and Nutrients) was therefore comprehensively evaluated against a large assemblage of rainfall runoff events. This evaluation highlighted both areas of strength in the current model structure, and also areas of weakness for further development. The research has improved understanding of ecosystem degradation processes in semi-arid rangelands, and demonstrates that woody shrub encroachment may lead to a long-term reduction in ecosystem C storage, which is contrary to the widely promulgated view that woody shrub encroachment increases C storage in terrestrial ecosystems.NERC Doctoral Training Grant (NE/K500902/1)NSF Long Term Ecological Research Program at the Sevilleta National Wildlife Refuge (DEB-1232294

Modelling runoff generation and connectivity for semi-arid hillslopes and small catchments

The processes relating to runoff generation in a semi-arid environment at the hillslope scale are poorly understood. This research considers the amount and origin of water reaching the channel during a storm event using a combination of field experiments and computer simulation techniques. From the field experiments, it was found that the key controls on runoff generation at the point scale are the surface cover of rock fragments, vegetation cover, slope gradient and surface roughness. The effect of land management was found to be greater than geology. The simulation modelling work investigated the controls on runoff generation at the hillslope and small catchment scales. It was found that the storm characteristics are far more important than surface properties in determining the amount of discharge from a slope. The temporal fragmentation of the rainfall was found to control the distribution of flow path lengths and hence the amount of discharge leaving a slope. The key surface controls on the form of the discharge hydrograph are slope length, slope gradient and the hydrological properties at the base of the slope. The origin of runoff was investigated using autonomous software agents able to trace the flow of water through a catchment. This technique is able to give a unique picture of the origin of runoff within a catchment. It was shown that the spatial pattern of the origin of runoff is complex and varies significantly between catchments. This research has shown that there are two key themes in determining the amount of runoff reaching the channel network: the interplay between the distribution of flow path lengths generated during a storm and the distributions of flow lengths to the channel as a function of the landscape. The second theme relates to the importance of the spatial structure of hydrological areas within the landscape

Structures and processes of the initial ecosystem development phase in an artificial water catchment (Final report CRC/TR 38)

Objective of the Transregional Collaborative Research Centre (CRC/TR) 38 was the study of structures and processes of the initial ecosystem development. It was assumed that the initial phase is characterized by less structured and therefore less heterogeneous ecosystems. Thus, analysis of young ecosystems in their initial stages should provide better insights into ecosystem functioning. Following this basic concept, the idea of the CRC/TR 38 was to analyze the establishment of new structures and processes which lead to a growing structuring and in consequence to a growing complexity and heterogeneity in an artificially created watershed. Further, with the help of this step-by-step development of the ecosystem it was aimed to learn from occurring feedbacks, which appear between old and newly emerging structures and patterns in order to better understand also the behavior of more mature systems. Special emphasis was placed on the spatial and temporal dynamics of both evolving structures and related processes and their interactions. In summary, the CRC/TR 38 was able to identify a number of structures and processes that are considered to be relevant and specific for young systems.Ziel des Sonderforschungsbereichs/Transregio (SFB/TRR) 38 war die Untersuchung von Strukturen und Prozessen der initialen Ökosystemgenese. Es wurde angenommen, dass die initiale Entwicklungsphase durch geringere Strukturierung und damit durch eine geringere Heterogenität der Ökosysteme gekennzeichnet ist. Entsprechend sollte die Untersuchung von Ökosystemen in ihrer initialen Entwicklungsphase verbesserte Erkenntnisse zur Funktion von Ökosystemen bieten. Diesem Konzept folgend untersuchte der SFB/TRR 38 in einem künstlichen Wassereinzugsgebiet die Entwicklung von neuen Strukturen und Prozessen, die zu einer zunehmenden Strukturierung und damit zu einer zunehmenden Heterogenität führte. Weiterhin war beabsichtigt, mit Hilfe der schrittweisen Entwicklung des Ökosystems auftretende Rückkopplungsprozesse zwischen alten und sich neu etablierenden Strukturen und Mustern zu erkennen und damit auch das Verständnis der Funktionsweise gereifter Ökosysteme zu verbessern. Besonderes Augenmerk wurde auf die zeitliche und räumliche Dynamik sowohl der sich entwickelnden Strukturen als auch der damit verbundenen Prozesse und ihrer Interaktionen gelegt. Zusammenfassend kann gesagt werden, dass der SFB/TRR 38 in der Lage war, zahlreiche Strukturen und Prozesse zu identifizieren, die als relevant und spezifisch für junge Ökosysteme betrachtet werden können

Semantic array programming in data-poor environments: assessing the interactions of shallow landslides and soil erosion

This research was conducted with the main objective to better integrate and quantify the role of water-induced shallow landslides within soil erosion processes, with a particular focus on data-poor conditions. To fulfil the objectives, catchment-scale studies on soil erosion by water and shallow landslides were conducted. A semi-quantitative method that combines heuristic, deterministic and probabilistic approaches is here proposed for a robust catchment-scale assessment of landslide susceptibility when available data are scarce. A set of different susceptibility-zonation maps was aggregated exploiting a modelling ensemble. Each susceptibility zonation has been obtained by applying heterogeneous statistical techniques such as logistic regression (LR), relative distance similarity (RDS), artificial neural network (ANN), and two different landslide-susceptibility techniques based on the infinite slope stability model. The good performance of the ensemble model, when compared with the single techniques, make this method suitable to be applied in data-poor areas where the lack of proper calibration and validation data can affect the application of physically based or conceptual models. A new modelling architecture to support the integrated assessment of soil erosion, by incorporating rainfall induced shallow landslides processes in data-poor conditions, was developed and tested in the study area. This proposed methodology is based on the geospatial semantic array programming paradigm. The integrated data-transformation model relies on a modular architecture, where the information flow among modules is constrained by semantic checks. By analysing modelling results within the study catchment, each year, on average, mass movements are responsible for a mean increase in the total soil erosion rate between 22 and 26% over the pre-failure estimate. The post-failure soil erosion rate in areas where landslides occurred is, on average, around 3.5 times the pre-failure value. These results confirm the importance to integrate landslide contribution into soil erosion modelling. Because the estimation of the changes in soil erosion from landslide activity is largely dependent on the quality of available datasets, this methodology broadens the possibility of a quantitative assessment of these effects in data-poor regions

Identifying runoff processes on the plot and catchment scale

International audienceRainfall-runoff models that adequately represent the real hydrological processes and that do not have to be calibrated, are needed in hydrology. Such a model would require information about the runoff processes occurring in a catchment and their spatial distribution. Therefore, the aim of this article is (1) to develop a methodology that allows the delineation of dominant runoff processes (DRP) in the field and with a GIS, and (2) to illustrate how such a map can be used in rainfall-runoff modelling. Soil properties were assessed of 44 soil profiles in two Swiss catchments. On some profiles, sprinkling experiments were performed and soil-water levels measured. With these data, the dominant runoff processes (DRP) were determined using the Scherrer and Naef (2003) process decision scheme. At the same time, a simplified method was developed to make it possible to determine the DRP only on the basis of maps of the soil, topography and geology. In 67% of the soil profiles, the two methods indicated the same processes; in 24% with minor deviations. By transforming the simplified method into a set of rules that could be introduced into a GIS, the distributions of the different DRPs in two catchments could be delineated automatically so that maps of the dominant runoff processes could be produced. These maps agreed well with manually derived maps and field observations. Flood-runoff volumes could be quite accurately predicted on the basis of the rainfall measured and information on the water retention capacity contained in the DRP map. This illustrates the potential of the DRP maps for defining the infiltration parameters used in rainfall-runoff models

Modeling soil erosion and reservoir sedimentation at hillslope and catchment scale in semi-arid Burkina Faso

Soil erosion is a major factor of land degradation in Sub-Saharan Africa. The loss of nutrient-rich topsoil from hillslopes causes severe agricultural problems for an extremely vulnerable agricultural society that depends on soil quality as a fundamental base for its livelihood. The removal of soil in source areas leads to sediment accumulation in sink areas such as dammed reservoirs. Especially the siltation of small reservoirs is seen as a serious environmental threat in Burkina Faso, where more than a thousand dams have been built to store unevenly distributed rainwater for the dry season. These dams are in danger of losing their function as essential water reservoirs for domestic use, irrigation and stock watering in the near future. This study presents an integrative, scale-dependent approach to assess on-site and off-site impacts of soil erosion by quantifying the magnitude and intensity of soil loss/deposition at hillslope and catchment scales and by considering the spatial dimension of these processes in a complex landscape system in southwestern Burkina Faso. At the hillslope scale, the spatial variability of soils is analyzed by soil profile investigations along a catena and subsequently considered in soil erosion simulations by the physically-based WEPP model. WEPP model predictions indicate that although average soil loss rates simulated for the entire hillslope are comparatively low, they can be forty times higher at particular hillslope positions. These spatial differences, even in the relatively flat terrain of Burkina Faso, are also confirmed by Cs-137 measurements with averaged soil loss rates of less than 5 t/ha/yr and maximum erosion rates of more than 50 t/ha/yr in erosion hotspots. The identification of such hazard zones can be used to target site-specific land management options. WEPP model simulations show that the application of stone lines, minimum tillage, contour farming and residue management could reduce soil loss by up to 95 %, 70 %, 55 % and 45 % at these erosion-prone hillslope positions. At the catchment scale, sedimentation rates of three reservoirs are analyzed by bathymetric surveys, sediment core sampling and sediment yield calculations using the soil erosion and sediment delivery model WaTEM/SEDEM. For the model, a digital elevation models is generated and land-cover maps are derived from remote sensing images. A comparison between the initial and actual reservoir bed morphology shows that the reservoirs have lost approximately 10-15 % of their original storage capacity and more than 60 % of their inactive storage volume in the last 15 to 20 years. During that period, a sedimentation layer of 0.3 m to 0.5 m thickness has accumulated on the reservoir bed. This was verified by stratigraphical changes and downcore variations in sediment properties and Cs-137 concentrations. Predictions by WaTEM/SEDEM show similar magnitudes of siltation with specific sediment yield rates of 0.5 t/ha/yr to 3.4 t/ha/yr. These results indicate that the half-life of the dams might be reached in about 25 years assuming constant siltation rates under current conditions. In order to identify the sediment source areas and the potential soil-erosion risk zones leading to these high siltation rates, a spatially-explicit soil erosion and deposition hazard maps generated by WaTEM/SEDEM can be used. These hazard maps present a powerful tool to support policy makers in their decisions on which landscapes are primarily at risk and where action plans for sustainable soil and water conservation should be implemented.Modellierung von Bodenerosion und Sedimentation von Stauseen auf Hang- und Wassereinzugsgebietsebene im semi-ariden Burkina Faso Die Bodenerosion hat einen wesentlichen Einfluss auf die Landdegradierung semi-arider Gebiete in Afrika südlich der Sahara. Der Abtrag von humusreichem Oberboden am Hang verursacht schwerwiegende landwirtschaftliche Probleme, insbesondere für eine fragile Ackerbaugesellschaft, die von einer guten Bodenqualität zur Sicherung ihrer Existenzgrundlage abhängig ist. Der Abtrag von Bodensedimenten aus Quellgebieten hat gleichzeitig die Akkumulation von Sedimenten in Zielgebieten wie beispielsweise eingedämmten Rückhaltebecken zur Folge. Insbesondere die Verschlämmung von Kleinstauseen stellt für das Land Burkina Faso, in dem über tausend Staudämme gebaut worden sind, um das Regenwasser der uneinheitlich verteilten Niederschläge für die Trockenzeit stauen zu können, ein zunehmendes Umweltproblem dar. In naher Zukunft drohen diese Staudämme ihre Funktion als unverzichtbare Wasserspeicher für Haushalt, Bewässerungsfeldbau und Viehzucht zu verlieren. Um die Auswirkungen von Bodenerosion sowohl on-site als auch off-site zu bewerten, verfolgt die vorliegende Arbeit einen integrativen, skalenabhängigen Ansatz, bei dem einerseits das Ausmaß und die Intensität von Bodenerosion und deren Ablagerung auf Hang- und Einzugsgebietsebene quantifiziert werden und bei der andererseits die räumliche Dimension dieser Prozesse in einem komplexen Landschaftssystem im Südwesten Burkina Fasos berücksichtigt wird. Auf der Hangebene wird die räumliche Variabilität von Boden anhand von Bodenprofilen entlang einer Catena untersucht und in die Erosionsmodellierung mittels des physikalisch-basierten WEPP-Model miteinbezogen. Die Ergebnisse der Modellierung verdeutlichen, dass, obwohl die simulierten Abtragsraten für den Gesamthang als vergleichsweise gering einzuschätzen sind, diese bis zu vierzigfach erhöht an einzelnen Hangbereichen auftreten können. Auch Cs-137 Messungen bestätigen mit durchschnittlichen Abtragsraten von weniger als 5 t/ha/yr für den Gesamthang und maximalen Abtragsraten von mehr als 50 t/ha/yr in gefährdeten Zonen diese hohen, räumlichen und für die verhältnismäßig flache Landschaft Burkina Fasos auffälligen Differenzen. Die Identifikation dieser Gefährdungszonen kann jedoch einer standortspezifischen Anpassung von Landnutzungsmethoden dienen. Die Simulationsergebnisse des WEPP-Models zeigen, dass durch die Anlegung von Steinwällen, weniger intensiven Bodenbearbeitungsmethoden, Konturpflügen und Mulchsaat der Bodenabtrag an diesen besonders erosionsgefährdeten Hangbereichen um bis zu 95 %, 70 %, 55 % und 45 % reduziert werden könnte. Auf Wassereinzugsgebietsebene wird die Sedimentationsrate von drei Kleinstaudämmen durch bathymetrische Methoden, Sedimentbohrungen und die Berechnung der Sedimentfracht anhand des Bodenerosions- und Sedimentaustragsmodelles WaTEM/SEDEM ermittelt. Für die Modellierung wird ein digitales Höhenmodel erstellt, Landbedeckungskarten werden wiederum von Fernerkundungsbildern abgeleitet. Ein Vergleich zwischen der ursprünglichen und aktuellen Stauseemorphologie zeigt, dass die Staubecken in den letzten 15 bis 20 Jahren zwischen 10-15% ihrer gesamten Speicherkapazität und mehr als 60% ihres inaktiven Speichervolums eingebüßt haben. In diesem Zeitraum hat sich auch eine Sedimentschicht von 0.3 m bis 0.5 m Mächtigkeit auf dem Stauseeboden abgelagert, was mittels stratigraphischer Analysen und Veränderungen sedimentspezifischer Eigenschaften sowie Cs-137 Konzentrationen im Bohrkern belegt wird. Modellberechnungen mit WaTEM/SEDEM bestätigen einen ähnlich hohen Verschlämmungsgrad mit spezifischen Sedimentfrachtraten von 0.5 t/ha/yr bis 3.4 t/ha/yr. Die Ergebnisse verdeutlichen, dass - setzt man gleichbleibende Sedimentationsraten unter den heutigen Bedingungen voraus - die Halbzeit der prognostizierten Lebensdauer der Staudämme in etwa 25 Jahren erreicht sein wird. Um Sedimentbereitstellungsgebiete und potentielle Erosionsrisikobereiche, die zu diesem hohen Sedimenteintrag in Stauseen führen, zu identifizieren, dienen räumlich-detaillierte Bodenabtrags- und Akkumulationsgefahrenkarten, die mit WaTEM/SEDEM erstellt werden. Diese Gefahrenkarten können schließlich von Entscheidungsträgern als sinnvolle Planungsgrundlage genutzt werden, um festzulegen, welche Landschaftsbereiche vorrangig als Risikogebiete ausgewiesen werden sollten, um dort nachhaltige Boden- und Wasserschutzmaßnahmen zu implementieren