The impact of depression removal on catchment geomorphology,soil erosion and landscape evolution

The current generation of landscape evolution models use a digital elevation model for landscape representation. These programs also contain a hydrological model that defines overland flow with the drainage network routed to an outlet. One of the issues with landscape evolution modelling is the hydrological correctness of the digital elevation model used for the simulations. Despite the wide use and increased quality of digital elevation models, data pits and depressions in the elevation data are a common feature and their removal will remain a necessary step for many data sets. This study examines whether a digital elevation model can be hydrologically correct (i.e. all depressions removed so that all water can run downslope) before use in a landscape evolution model and what effect depression removal has on long‐term geomorphology and hydrology. The impact on sediment transport rates is also examined. The study was conducted using a field catchment and a proposed landform for a post‐mining landscape. The results show that there is little difference in catchment geomorphology and hydrology for the non‐depression removed and depression removed data sets. The non‐depression removed and depression removed digital elevation models were also evaluated as input to a landscape evolution model for a 50 000 year simulation period. The results show that after 1000 years there is little difference between the data sets, although sediment transport rates did vary considerably early on in the simulation. Copyright © 2007 John Wiley & Sons, Ltd.     

Linking landslides,hillslope erosion,and landscape evolution

This Virtual Issue highlights 10 recent innovative, unconventional, or otherwise significant contributions to Earth Surface Processes and Landforms that help advance the state‐of‐the‐art in research on linkages between landslides, hillslope erosion, and landscape evolution. The selected studies address this feedback within a temporal spectrum that ranges from the event to the millennial scale, thus underscoring the importance of detailed field observations, high‐resolution digital topographic data, and geochronological methods for increasing our capability of quantifying landslide processes and hillslope erosion. Copyright © 2009 John Wiley & Sons, Ltd.     

Process–form relationships in Southern Italian badlands: erosion rates and implications for landform evolution

Characteristic badlands are incised into Plio‐Pleistocene clays in Basilicata, southern Italy, creating steep, scarp slopes with knife‐edge ridges (calanchi) and small dome‐shaped forms (biancane). Erosion pin data for the period 1997–2003 give mean annual erosion rates for dome‐shaped biancane in the range 9–19 mm a^?1, while rates for the calanchi scarps are lower, at 7–10 mm a^?1. The erosion pin data also show a non‐linear relationship with slope angle. Maximum erosion rates coincide with a slope angle of 35°, within an envelope defined by combining the theoretical effects of both rainsplash and surface weathering. Monitoring of surface changes and erosion rates for two 0·5 m² cleared swathes on biancane forms reveals a complex relationship between weathering and erosion. Characteristic forms can develop from large blocks of intact clay bedrock over a time period of less than 30 a. The implications of the measured erosion rates for the landform association of mountain front/pediment/domed inselberg are explored. Copyright © 2005 John Wiley & Sons, Ltd.     

Climate‐driven decrease in erosion in extant Mediterranean badlands

Badland areas provide some of the highest erosion rates globally. Most studies of erosion have insufficient lengths of record to interrogate the impacts of decadal‐scale changes in precipitation on rates of badland erosion in regions such as the Mediterranean, which are known to be sensitive to land degradation and desertification. Erosion measurements, derived from field monitoring using erosion pins, in southern Italy during the period 1974–2004 are used to explore the impacts of changing precipitation patterns on badland erosion. Erosion on badland inter‐rill areas is strongly correlated with cumulative rainfall over each monitoring period. Annual precipitation has a substantial dynamic range, but both annual and winter (December, January, February) rainfall amounts in southern Italy show a steady decrease over the period 1970–2000. The persistence of positive values of the winter North Atlantic Oscillation index in the period 1980–2000 is correlated with a reduction in the winter rainfall amounts. Future climate scenarios show a reduction in annual rainfall across the western and central Mediterranean which is likely to result in a further reduction in erosion rates in existing badlands. Copyright © 2010 John Wiley & Sons, Ltd.     

Hillslope erosion measurement—a simple approach to a complex process

The measurement of hillslope erosion can be a difficult, costly and time‐consuming activity. Many techniques are available, ranging from using environmental tracers, to LiDAR. Erosion measurements using erosion pins are assessed and compared with regional scale erosion data, hillslope data obtained using ¹³⁷Cs and erosion modelling results. The pins produced erosion rates which are within the range determined using ¹³⁷Cs and model data but above that of regional denudation rates. Our findings demonstrate that inexpensive erosion pins can provide reliable data on hillslope erosion. © 2015 Commonwealth of Australia. Hydrological Processes © 2015 John Wiley & Sons Ltd.     

Modelling of sediment dynamics in a laboratory‐scale experimental catchment

The variability of hillslope form and function is examined experimentally using a simple model catchment in which most landscape development parameters are either known or controlled. It is demonstrated that there is considerable variability in sediment output from similar catchments, subjected to the same hydrological processes, and for which the initial hillslope profiles are the same. The results demonstrate that, in the case of catchments with a linear initial hillslope profile, the sediment output is initially high but reduces through time, whereas for a concave initial profile the sediment output was smaller and relatively constant. Concave hillslope profiles also displayed reduced sediment output when compared with linear slopes with the same overall slope. Using this experimental model catchment data, the SIBERIA landscape evolution model was tested for its ability to predict temporal sediment transport. When calibrated for the rainfall and erodible material, SIBERIA is able to simulate mean temporal sediment output for the experimental catchment over a range of hillslope profiles and rainfall intensities. SIBERIA is also able to match the hillslope profile of the experimental catchments. The results of the study provide confidence in the ability of SIBERIA to predict temporal sediment output. The experimental and modelling data also demonstrate that, even with all geomorphic and hydrological variables being known and/or controlled, there is still a need for long‐term stream gauging to obtain reliable assessments of field catchment hydrology and sediment transport. Copyright © 2005 John Wiley & Sons, Ltd.     

Hillslope and point based soil erosion – an evaluation of a Landscape Evolution Model

Excessive soil erosion and deposition is recognised as a significant land degradation issue. Quantifying soil erosion and deposition is a non-trivial task. One of these methods has been the mathematical modelling of soil erosion and deposition patterns and the processes that drive them. Here we examine the capability of a landscape evolution model to predict both soil erosion rate and pattern of erosion and deposition. This numerical model (SIBERIA) uses a Digital Elevation Model (DEM) to represent the landscape and calculates erosion and deposition at each grid point in the DEM. To assess field soil redistribution rates (SRR) and patterns the distribution of the environmental tracer ¹³⁷Cs has been analysed. Net hill slope SRR predicted by SIBERIA (a soil loss rate of 1.7 to 4.3 t ha^-1 yr^-1) were found to be in good agreement with ¹³⁷Cs based estimates (2.1 – 3.4 t ha^-1 yr^-1) providing confidence in the predictive ability of the model at the hillslope scale. However some differences in predicted erosion/deposition patterns were noted due to historical changes in landscape form (i.e. the addition of a contour bank) and possible causes discussed, as is the finding that soil erosion rates are an order of magnitude higher than likely soil production rates. The finding that SIBERIA can approximate independently quantified erosion and deposition patterns and rates is encouraging, providing confidence in the employment of DEM based models to quantify hillslope erosion rates and demonstrating the potential to upscale for the prediction of whole catchment erosion and deposition. The findings of this study suggest that LEMs can be a reliable alternative to complex and time consuming methods such as that using environmental tracers for the determination of erosion rates. The model and approach demonstrates a new approach to assessing soil erosion that can be employed elsewhere. © 2018 John Wiley & Sons, Ltd.     

Modelling centennial sediment waves in an eroding landscape – catchment complexity

Sediment flux dynamics in fluvial systems have often been related to changes in external drivers of topography, climate or land cover. It is well known that these dynamics are non‐linear. Recently, model simulations of fluvial activity and landscape evolution have suggested that self‐organization in landscapes can also cause internal complexity in the sedimentary record. In this contribution one particular case of self‐organization is explored in the Sabinal field study area, Spain, where several dynamic zones of sedimentation and incision are observed along the current river bed. Whether these zones can be caused by internal complexity was tested with landscape evolution model (LEM) LAPSUS (Landscape Process Modelling at Multi‐dimensions and Scales). During various 500 year simulations, zones of sedimentation appear to move upstream and downstream in eroding river channels (‘waves’). These waves are visualized and characterized for a range of model settings under constant external forcing, and the self‐organizing process behind their occurrence is analysed. Results indicate that this process is not necessarily related to simplifications in the model and is more generic than the process of bed‐armouring that has recently been recognized as a cause for complexity in LEM simulations. We conclude that autogenic sediment waves are the result of the spatial propagation in time of feedbacks in local transport limited (deposition) and detachment limited (erosion) conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

Slope–channel coupling between pipes,gullies and tributary channels in the Mocatán catchment badlands,Southeast Spain

The dispersive nature of the highly sodic silts of the Triassic‐rich unit of the Góchar formation plays a fundamental role in the erosion of the Mocatán catchment badlands in Almería, where a rejuvenating pipe and incised channel network is rapidly evacuating slope materials. Referring to concepts of medium‐ and long‐term landscape evolution, and incorporating contemporary thoughts on the role of connectivity in system evolution from the geomorphological literature, this paper attempts to develop a conceptual model of the way geologic, topographic, material property and ecological factors combine to explain the complex geomorphological evolution of the site. An electronic distance measurement (EDM) survey was undertaken using a Leica TC3100, to produce a detailed topographic map. This database was supplemented by geomorphological, geological and ecological data derived from ground survey and remote sensing, and further morphometric analysis undertaken. Preferred orientations of channel segments, and the topographic distribution of pipe‐roof‐collapse features and outfalls in relation to known stratigraphic controls, suggests that, once coupled to the slope‐base channel, pipe networks develop in a systematic, sequential way. A wave of incision moving up the main channel reconnects channels with slopes, and the resulting increased hydraulic gradients on sideslopes encourage extensive deep pipe development for the first time in these materials. Once major pipe development is possible, three‐dimensional pipe networks enlarge and then collapse to form an extensive, partially coupled steep‐sided gully network. From this perspective, the coupling of the pipe to a rejuvenating channel is a significant intrinsic threshold event and the main reason that badlands have developed locally in these dispersive materials. It is concluded that erosion in this landscape will only be suppressed after a considerable period of slope‐base stability, which could allow a gradual loss of sodium from the surface by leaching or organic exchanges. Both theoretical and management implications are explored. Copyright © 2007 John Wiley & Sons, Ltd.     

Wind erosion of Mancos Shale badland ridges by sudden drops in pressure

One process of erosion of Mancos Shale badlands near Hanksville, Utah, appears to be caused by nearly instantaneous drops in air pressure accompanying gusts of wind. A series of sharp-crested bedrock ridges trend nearly perpendicular to the strong, gusty southwesterly winds that precede cold fronts passing through the area. The Bernoulli effect, resulting from the explosive onset of wind gusts in which the wind over the ridges can accelerate from 7 to 14 m s⁻¹, can cause nearly instantaneous pressure drops of 1·27 mmHg. This provides a unit lifting force of 0·01697 N. Since the average gravitational force acting on a unit area of the crust is only 0·00883 N, this force is sufficient to lift the crust, exposing the underlying weathered shale chips to further wind erosion. Soils susceptible to this type of erosion consist of polygonally cracked surface crust averaging 1·2 cm thick overlying a porous subsoil of silt-sized shale chips. The arid environment permits complete soil drying between weather fronts, greatly reducing the cohesion that would occur if the soil were moist. The pressure drops, and the erosion caused by them, were observed on the lee side of bedrock ridges about 10 m high, within 1 m of the ridge crest. Landforms resulting from this process are micro-cirque forms located near the ridge crests. Continued development of micro-cirques eventually forms cliffs on the lee sides of the ridges. © 1997 by John Wiley & Sons, Ltd.     

Landscapes on the edge: examining the role of climatic interactions in shaping coastal watersheds using a coastal–terrestrial landscape evolution model

Incised coastal gullies (ICGs) are dynamic features found at the terrestrial‐coastal interface. Their geomorphic evolution is driven by the interactions between processes of fluvial knickpoint migration and coastal cliff erosion. Under scenarios of future climate change the frequency and magnitude of the climatological drivers of both terrestrial (fluvial and hillslope) and coastal (cliff erosion) processes are likely to change, with an adjunct impact on these types of coastal features. Here we explore the response of an incised coastal gully to changes in both terrestrial and coastal climate in order to elucidate the key process interactions which drive ICG evolution. We modify an extant landscape evolution model, CHILD, to incorporate processes of soft‐cliff erosion. This modified version, termed the Coastal‐Terrestrial‐CHILD (CT‐CHILD) model, is then employed to explore the interactions between changing terrestrial and coastal driving forces on the future evolution of an ICG found on the south‐west Isle of Wight, UK. It was found that the magnitude and frequency of storm events will play a key role in determining the future trajectory of ICGs, highlighting a need to understand the role of event sequencing in future projections of landscape evolution. Furthermore, synergistic (positive) and antagonistic (negative) interactions were identified between coastal and terrestrial parameters, such as wave height intensity and precipitation duration, which act to modulate the impact of changes in any one parameter. Of note was the role played by wave height intensity in driving coastal erosion, which was found to play a more important role than sea‐level rise in determining rates of coastal erosion. This highlights the need for a greater focus on wave height in studies of soft‐cliff erosion. Copyright © 2014 John Wiley & Sons, Ltd.     

Did tillage erosion play a role in millennial scale landscape development?

Landscape evolution models (LEMs) quantitatively simulate processes of sedimentation and erosion on millennial timescales. An important aspect of human impact on erosion is sediment redistribution due to agriculture, referred to herein as tillage erosion. In this study we aim to analyse the potential contribution of tillage erosion to landscape development using LEM LAPSUS. The model is calibrated separately for a water erosion process (i) without tillage and (ii) with tillage. The model is applied to the ~250 km² Torrealvilla case study catchment, SE Spain. We were able to simulate alternating sequences of incision and aggradation, that are important on longer (millennial) timescales. Generally, model results show that tillage erosion adds to deposition in the lower floodplain area, but neither water erosion alone nor water with tillage erosion together could exactly reproduce the observed amounts of erosion and sedimentation for the case study area. In addition, scale effects are apparent. On hillslopes, tillage may contribute importantly to erosion and may fill local depressions. If assessed on the catchment scale, sediments from tillage erosion eventually reach the lower floodplain area where they contribute to deposition. However, water erosion was observed in the model simulations to be the most important process on the catchment scale. This is the first time that tillage erosion has been explicitly included in a landscape evolution model at a millennial timescale and large catchment scale. Copyright © 2012 John Wiley & Sons, Ltd.     

Hillslope erosion in a grassland environment: Calibration and evaluation of the SIBERIA landscape evolution model

Field measurement and modelling of soil erosion provides insights into landscape systems as well as the potential for enhanced landscape management. There are a number of field and numerical methods by which soil erosion and deposition can be quantified. Here we examine the capability of the SIBERIA landscape evolution model to quantify short-term erosion and deposition on a well-managed cattle grazing landscape on the east coast of Australia. The model is calibrated by two methods (1) a geomorphological approach using a site digital elevation model (DEM) and soil data and (2) a laboratory-scale flume. The two calibration processes resulted in similar model input parameters and estimated erosion rates of 3.1 t ha⁻¹ year⁻¹ and 4.4 t ha⁻¹ year⁻¹, respectively. These were found to closely match erosion rates estimated using the environmental tracer ¹³⁷Cs (2.7–4.8 t ha⁻¹ year⁻¹). However, erosion and deposition estimated at individual points along the hillslope was not well correlated with ¹³⁷Cs at the same position due to the temporal averaging of the model and microtopography. Sensitivity analysis showed the model was more sensitive to parameterisation than sub-DEM-scale topography. This places confidence in the model's ability to estimate erosion and deposition across an entire hillslope and catchment on decadal time scales. We also highlight the robustness and flexibility of the calibration methods.     

A 13‐year record of erosion on badland sites in the Karoo,South Africa

Land degradation in South Africa has been of concern for more than 100 years with both climate change and inappropriate land management (overgrazing) being proposed as primary drivers. However, there are few quantitative studies of degradation and, in particular, few of erosion by water. Badlands, taken here to be the landform which results from extreme erosion, have been notably neglected. We report on 13 consecutive years of erosion pin measurements of badland erosion on 10 study sites in the Sneeuberg uplands of the eastern Karoo in South Africa. The study sites are on Holocene colluvium which mantles footslopes. They have been subject to overgrazing for at least 100 years, c. 1850–1950. Currently they are lightly grazed by sheep. The area receives about 500 mm rainfall per year. The sites are remote, with only informal, farmer‐operated, daily raingauges nearby. The nearest sub‐daily raingauge is c. 55 km distant. Also we report on an analysis of the erosion pin data which focuses on establishing the origins and context of the badlands, including the relationship between study sites and adjacent valley‐bottom gully systems; compare erosion rates on our study sites with rates determined by erosion pins on other badland sites; and discuss the implications of these erosion rates for landscape development and off‐site impacts. Net erosion rates on the study sites are relatively high compared with global badland rates and range from 3.1 to 8.5 mm yr^‐1 which may be extrapolated to 53 to 145 t ha yr^‐1 (using a measured bulk density of 1.7 g cm^‐3). However, comparisons with badland sites elsewhere are difficult because of different measuring methodologies, lithologies, climate and dominant processes. Erosion rates on the study sites are strongly influenced by rainfall amounts and, in particular, by daily rainfall events which exceed ~10 mm: this is the threshold intensity at which runoff has been observed to commence on badlands. Of significance, but of lesser influence, is weathering, mainly by wetting and drying: this prepares bare surfaces for erosion. However, questions remain regarding the role of site characteristics, and of processes at each site, in determining between‐site differences in erosion rate. Crude extrapolation of current rates of erosion, in conjunction with depths of incision into the badlands, suggests that badland development started around 200 years ago, probably as a response to the introduction of European‐style stock farming which resulted in overgrazing. We assume, but cannot quantify, the additional influence of periods of drought and burning in the erosional history of the area. Intermittent connection of these badlands to valley‐bottom gullies and therefore to small farm dams and ultimately to large water storage reservoirs increases their impact on local water resources. Copyright © 2015 John Wiley & Sons, Ltd.