Description: Reliable quantitative data on the extent and rates of soil erosion are needed to understand the global significance of soil-erosion induced carbon exchange and to underpin the development of science-based mitigation strategies, but large uncertainties remain. Existing estimates of agricultural soil and soil organic carbon (SOC) erosion are very divergent and span two orders of magnitude. The main objective of this study was to test the assumptions underlying existing assessments and to reduce the uncertainty associated with global estimates of agricultural soil and SOC erosion. We parameterized a simplified erosion model driven by coarse global databases using an empirical database that covers the conterminous USA. The good agreement between our model results and empirical estimates indicate that the approach presented here captures the essence of agricultural erosion at the scales of continents and that it may be used to predict the significance of erosion for the global carbon cycle and its impact on soil functions. We obtained a global soil erosion rate of 10.5 Mg ha -1 y -1 for cropland and 1.7 Mg ha -1 y -1 for pastures. This corresponds to SOC erosion rates of 193 kg C ha -1 y -1 for cropland and 40.4 kg C ha -1 y -1 for eroding pastures and results into a global flux of 20.5 (±10.3) Pg y -1 of soil and 403.5 (±201.8) Tg C y -1 . Although it is difficult to accurately assess the uncertainty associated with our estimates of global agricultural erosion, mainly due to the lack of model testing in (sub-)tropical regions, our estimates are significantly lower than former assessments based on the extrapolation of plot experiments or global application of erosion models. Our approach has the potential to quantify the rate and spatial signature of the erosion-induced disturbance at continental and global scales: by linking our model with a global soil profile database, we estimated soil profile modifications induced by agriculture. This showed that erosion-induced changes in topsoil SOC content are significant at a global scale (an average SOC loss of 22% in 50 years) and agricultural soils should therefore be considered as dynamic systems that can change rapidly. Copyright © 2011 John Wiley & Sons, Ltd.

Description: Morphological features of braided rivers (bars, channels and pools) experience major changes in area, shape and spatial distribution as a response to i) the pulsation of discharge during a flood and ii) the bed evolution induced by floods. In this paper, at-a-station relationships between water level and planform configuration are investigated on the Tagliamento River, a large gravel-bed braided river in northeast Italy, over a 2-year study period comprising three bankfull events and several small-to-medium floods. The analysis was performed on two 1-km long reaches, characterised by different riparian vegetation cover. Ground-based images with an hourly temporal resolution were acquired using software-controlled, digital cameras. Bars, channels, pools and vegetated patches were manually digitised on more than 100 rectified images. Sequences of constant-level images spanning the study period were used to quantify the impact of floods on the stability of at-a-station relationships and on the turnover rate of water bodies. The analysis shows that wetted area increases almost linearly with water level in both reaches. The average number of branches per cross section peaks at intermediate flow levels, increasing from 2 at low flow up to 6–7. The number of branches displays the largest fluctuations over time, with significant changes produced also by moderate floods. Turnover rates are high in both reaches, with more than 30% of wetted areas at low flow converting into bare gravel in less than 2 months. Vegetation colonisation is found to limit the mobility of the low flow channels over time by concentrating the flow in fewer, deeper anabranches. The number of channels per cross section is 30 – 40% less in the vegetated reach and the proportion of low flow water bodies in the same position after 12 months increases from 3% to 14%. Copyright © 2011 John Wiley & Sons, Ltd.

Description: The flooding susceptibility of alluvial fans in the Southern Apennines has long been neglected. To partly address this oversight, we focus on the region of Campania which contains highly urbanised piedmont areas particularly vulnerable to flooding. Our findings are based on stratigraphic analysis of the fans and morphometric analysis of the basin-fan systems. Using geomorphological analysis we recognised active alluvial fans while stratigraphic analysis together with statistical analysis of the morphometric variables was used to classify the fans in terms of the transport process involved. The results indicate that in the examined geological context, the best discrimination between debris flow ( Df ) and water flood ( Wf ) processes is achieved by means of two related variables, one for the basin ( feeder channel inclination, Cg) and one for the fan ( fan length, Fl ). The probability that an unclassified fan belongs to group Wf is computed by applying a logistic function in which a p value exceeding 0.5 indicates that a basin/fan system belongs to group Wf . This important result led to the classification of the entire basin/fan system data. As regards process intensity, debris flow-dominated fans are susceptible to the occurrence of flows with high viscosity and hence subject to more severe events than water flood-dominated fans. Bearing this in mind, the data gathered in this study allow us to detect where alluvial fan flooding might occur and give information on the different degrees of susceptibility at a regional scale. Regrettably, urban development in recent decades has failed to take the presence of such alluvial fans into account due to the long recurrence time (50-100 years) between floods. This paper outlines the distribution of such susceptibility scenarios throughout the region, thereby constituting an initial step to implementing alluvial fan flooding control and mitigation. Copyright © 2011 John Wiley & Sons, Ltd.

Description: The relative importance of tectonics, climate, base level and source lithology as primary factors on alluvial-fan evolution, fan morphology and sedimentary style remain in question. This study examines the role of catchment lithology on development and evolution of alluvial megafans (〉30 Km in length), along the flanks of the Kohrud Mountain range, NE Esfahan, central Iran. These fans toe out at axial basin river and playa-fringe sediments towards the centre of basin and tectonics, climatic change and base-level fluctuations, were consistent for their development. They formed in a tectonically active basin, under arid to semiarid climate and a long term (Plio-Pleistocene to Recent) change from wetter to drier conditions. The key differences between two of these fans, Soh and Zefreh fans, along the west and south flanks of this mountain range, is that their catchments are underlain by dissimilar bedrock types. The source-area lithologies of the Soh and Zefreh fans are in sedimentary and igneous terrains respectively and these fans developed their geometry mainly in response to different weathering intensities of their catchment bedrock lithologies. Fan surface mapping (based on 1/50000 topographic maps, satellite images, and fieldwork), revels that the geomorphic evolution of these fans differs in that the relatively large-scale incision and through trenching of the Soh fan is absent in the Zefreh fan. Whereas the limited sediment supply of the Soh fan has resulted in a deep incised channel, the Zefreh fan has remained aggradational with little or no trenching into proximal to medial fan surface due to its catchment bedrock geology, composed mainly by physically weathered volcaniclastic lithology and characterized by high sediment supply for delivery during episodic flash floods. Sediment supply, which is mainly a function of climate and source lithology, is a dominant driver behind the development of fan sequences in alluvial megafans. Copyright © 2011 John Wiley & Sons, Ltd.

Description: Soil detachment in concentrated flow is due to the dislodging of soil particles from the soil matrix by surface runoff. Both aggregate stability and shear strength of the topsoil reflect the erosion resistance of soil to concentrated runoff, which are important input parameters in predicting soil detachment models. This study was conducted to develop a formulae to predict soil detachment rate in concentrated flow by using the aggregate stability index ( As ), root density ( R d ) and saturated soil strength ( σ s ) in the subtropical Ultisols region of China. The detachment rates of undisturbed topsoil samples collected from eight cultivated soil plots were measured in a 3.8 m long, 0.2 m wide hydraulic flume under five different flow shear stresses were applied ( τ  = 4.54, 9.38, 15.01, 17.49 and 22.54 Pa). The results indicated that the stability index ( As ) was well related with soil detachment rate, particularly for the results obtained with high flow shear stress (22.54 Pa), and the stability index ( As ) has a good linear relationship with concentrated flow erodibility factors ( K c ). There was a positive linear relationship between saturated soil strength ( σ s ) and critical flow shear stress ( τ c ) for different soils. A significant negative exponential relationship between erodibility factors ( K c ) and root density ( R d ) was detected. This study yielded two prediction equations that allowed a comparison of their efficiency in assessing soil detachment rate in concentrated flow. The equation including the root density ( R d ) may have a better correlation coefficients ( R 2  = 0.95). It was concluded that the formulae based on the stability index ( As ), saturated soil strength ( σ s ) and root density ( R d ) has the potential to improve methodology for assessing soil detachment rate in concentrated flow for the subtropical Chinese Ultisols. Copyright © 2011 John Wiley & Sons, Ltd.

Description: UK peatlands are affected by severe gully erosion with consequent impacts on ecosystem services from these areas. Incision into the peat can damage the vegetation and hydrology and lead to increases in carbon loss and sediment transfer downstream. Gullies represent then a conduit for and a hotspot of carbon loss but the relatively high water tables of gullies have meant that they have been identified as areas with a high restoration potential because of easily restored peat-forming conditions. This study uses a series of gully sites, subject to different restoration interventions, to investigate differences in carbon pathways (DOC, CO 2 ) and hydrology between restoration strategies and gully position. The results show that the position within the gully (interfluve, gully side, or gully floor) does not significantly affect water quality but that it plays a significant role in CO 2 exchange. Gully floors are areas of high photosynthesis and ecosystem respiration, though net ecosystem exchange is not significantly different across the gully. Whilst gully position plays a role in the cycling of some carbon species, this study highlights the importance of vegetation as a key control on carbon cycling. Copyright © 2011 John Wiley & Sons, Ltd.

Description: Progressive rock-fall failures in natural rock slopes are common in many environments, but often elude detailed quantitative documentation and analysis. Here we present high-resolution photography, video, and laser scanning data that document spatial and temporal patterns of a 1.2-year-long sequence of at least fourteen rock falls from the Rhombus Wall, a sheeted granitic cliff in Yosemite Valley, California. The rock-fall sequence began on 26 August 2009 with a small failure at the tip of an overhanging rock slab. Several hours later, a series of five rock falls totaling 740 m 3 progressed upward along a sheeting joint behind the overhanging slab. Over the next three weeks, audible cracking occurred on the Rhombus Wall, suggesting crack propagation, while visual monitoring revealed opening of a sheeting joint adjacent to the previous failure surface. On 14 September 2009 a 110 m 3 slab detached along this sheeting joint. Additional rock falls between 30 August and 20 November 2010, totaling 187 m 3 , radiated outward from the initial failure area along cliff (sub)parallel sheeting joints. We suggest that these progressive failures might have been related to stress redistributions driving fracture propagation along sheeting joints behind the cliff face. Mechanical analyses indicate that tensile stresses should occur perpendicular to the cliff face and open sheeting joints, and that sheeting joints should propagate parallel to a cliff face from areas of stress concentrations. The analyses also account for how a sheeting joint can propagate to lengths many times greater than its depth behind a cliff face. We posit that as a region of failure spreads across a cliff face, stress concentrations associated with previous failures will likely tend to spread with it, inciting further crack propagation and rock falls. Copyright © 2011 John Wiley & Sons, Ltd.

Description: Quaternary period palaeoenvironmental and palaeoclimatic reconstructions are based on a wide and diverse array of proxy data sets, some of which are geomorphological in nature. In drylands, where organic proxies may be limited, the use of landforms is particularly important, but challenging. The capacity to establish the age of depositional forms, particularly through the use of luminescence dating, has advanced the use of landforms in dryland palaeo-research, though interpretation of these ‘geoproxy’ records can be complex, especially at the nexus of palaeoclimate and palaeonvironmental interpretations of past conditions. In this paper the use of aeolian and lacustrine forms in Quaternary research is considered, focusing on the relationships between dynamics, form and climate, and on the essential linkage between process research and palaeoenvironmental research. It is concluded that landform analysis is a critical part of dryland palaeoenvironmental/climate reconstruction, contributing a different set of data compared to other data sources, in terms of the elements of past conditions that are revealed. Five principles are identified to improve the use of geoproxy records in Quaternary research: 1) greater use of geomorphic process studies by Quaternary scientists, to better inform palaeolandform interpretation; 2) further development of the use of chronometric data, especially in terms of interpreting large data; 3) interpret landform records in location-specific contexts, not in general terms; 4) capitalise of the complexity of spatially-extensive landform records, which may offer better representations of real Quaternary environmental complexity than ‘at a point’ proxies; 5) establish ways of integrating spatially-extensive geoproxy records with other palaeoenvironmental records. These challenges are major, but not insurmountable, and should represent goals for geomorphologists, chronologists and quaternary scientists alike. Copyright © 2011 John Wiley & Sons, Ltd.

Description: Channel fills are common elements of Holocene river systems and older fluvial sequences, but surprisingly little is known about formation and their sedimentary build-up. Abandoned channels result from channel shifting processes at various scales, including meander cutoff and channel-belt avulsion. Channel-fill sequences are of importance as containers of palaeoenvironmental proxy-records, can be used to reconstruct palaeochannel dynamics and derive palaeoflood records, and contain materials that allow dating the abandonment. Integrated knowledge on the dynamic nature (geometrical and physical insights) of channel abandonment and resultant sedimentary recording is a necessity for comparing and collating records from series of abandoned channel fills. This paper intends to make channel-fill sedimentological sequences more useful recorders of channel abandonment processes and palaeofloods, for which improved understanding is needed of the internal build-up of channel fills. We review oxbow lake infilling along meandering rivers, and supplement this with highly detailed descriptions of two selected field examples of channel fills from the apex-region of the Netherlands’ Rhine delta. From these examples it becomes clear that regional setting and type of abandonment result in different channel-fill end-members; oxbow cutoffs generally produce thick laminated clayey fills as the channel entrance is plugged rapidly, avulsion-abandoned channels are filled with coarse (proximal) deposits as a result of a maintained open river connection. Field examples of channel fills are integrated with knowledge on channel abandonment dynamics in meander cutoff and bifurcating river situations, including insights from recent numerical modeling. We propose a sedimentary-architecture descriptive scheme that distinguishes elements from two stages of channel-fill development; (i) the abandonment stage with initial proximal fill, and (ii) the subsequent fully abandoned palaeochannel that collects distal fill. Copyright © 2011 John Wiley & Sons, Ltd.

Description: There has been limited success in determining critical thresholds of ground cover or soil characteristics that relate to significant changes in runoff or sediment production at the microscale (〈1 m 2 ), particularly in semi-arid systems where management of ground cover is critical. Despite this lack of quantified thresholds, there is an increasing research focus on the two-phase mosaic of vegetation patches and inter-patches in semi-arid systems. In order to quantify ground cover and soil related thresholds for runoff and sediment production, we used a data mining technique known as conditional inference tree analysis to determine statistically significant values of a range of measured variables that predicted average runoff, peak runoff, sediment concentration and sediment production at the microscale. On Chromic Luvisols across a range of vegetation states in semi-arid south-eastern Australia, large changes in runoff and sediment production were related to a hierarchy of different variables and thresholds, but the percentage of bare soil played a primary role in predicting runoff and sediment production in most instances. The identified thresholds match well with previous thresholds found in semi-arid and temperate regions (including the approximate values of 30%, 50% and 70% total ground cover). The analysis presented here identified the critical role of soil surface roughness, particularly where total ground cover is sparse. The analysis also provided evidence that a two-phase mosaic of patches and inter-patches identified via rapid visual assessment could be further delineated into distinct groups of hydrological response, or a multi-phase rather than a two-phase system. The approach used here may aid in assessing scale-dependent responses and address data non-linearity in studies of semi-arid hydrology. Copyright © 2011 John Wiley & Sons, Ltd.