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Humans are now the most important geomorphic agent on the planet and accelerated erosion in agricultural landscapes results in high magnitude lateral organic carbon (OC) fluxes and significant perturbation of the land–ocean carbon flux. Nevertheless, the net effect of these lateral carbon fluxes on the C cycle is poorly constrained and there is no consensus as to whether they drive a net source or net sink of atmospheric CO 2 . Here, we test the hypothesis that, under sustained erosional forcing, soil carbon stocks on hillslopes reach a new equilibrium state in which all carbon exported with erosion is replaced; and, therefore, erosion results in a net sink for atmospheric CO 2 at the scale of eroding hillslopes. The evidence from our study site, in the Loess Plateau of China, is consistent with this hypothesis. Despite net export of OC equivalent to ca. 10% NPP, we found that all of the eroded OC was replaced and, therefore, that the sink strength was equal to the C export rate. This sets the upper limit of the erosion-induced sink term at the scale of whole watershed. The fate of the exported carbon in reservoirs, floodplains, riverbeds and the ocean ultimately controls the watershed-scale sink strength. Nevertheless, the full replacement observed here suggests that erosion does not induce a C source, irrespective of the fate of the exported carbon, at least for high-input agricultural systems. Finally, we propose that assessment of the C cycle perturbation associated with erosion-induced lateral C fluxes must be made an integral part of accounting mechanisms for climate change mitigation strategies that are based on land use change and C sequestration in terrestrial environments. [ABSTRACT FROM AUTHOR]

Abstract: We present results from a study of post-glacial alluvial fan evolution in New Zealand, in which a series of >83,000 numerical simulations of fan formation were conducted, within a Monte Carlo framework, in order to evaluate the potential for reconstructing past environmental conditions by matching simulated and observed fan morphologies. Our results indicate that simulations based on a wide range of model boundary conditions and internal process parameterisations are able to reproduce the observed fan characteristics. Such equifinal behaviour is a product of an imposed limit on fan progradation and associated autogenic feedbacks, which drive fan entrenchment even in the absence of external changes in water or sediment supply. We find little evidence that the majority of preserved fan terraces that formed post-entrenchment did so during periods of higher than average discharge or sediment supply to the fan apex, or that evidence for significant changes in sediment supply associated with major climate shifts are likely to be preserved in the current fan morphology. These results illustrate the difficulties that are inherent in the interpretation of alluvial landforms as records of past environmental conditions. They also highlight the need for more rigorous quantification of uncertainties in field and model-based interpretation of landscape responses to environmental change. [Copyright &y& Elsevier]

Abstract: Reduced-complexity models have considerable potential as tools for elucidating river behaviour over periods of 100–104 years and, consequently, for addressing fundamental questions concerning the scale-dependent nature of explanation in geomorphology. This paper proposes a simple subdivision of reduced-complexity models of river behaviour into two categories that mirror methodological developments in fluvial geomorphology over the past 50 years. First, high-resolution cellular approaches that are implemented within a framework that resolves process-form feedbacks at small time and space scales. Second, models that incorporate section-averaged representations of channel geometry and processes, and that are typically underpinned by regime theory and equilibrium concepts. Examples of both model types are presented here, in the form of a cellular representation of stream braiding and a combined lattice-network model of alluvial fan evolution. Simulations conducted using these models demonstrate how small-scale process-form interactions determine the emergence of larger-scale channel and fan morphology and, in so doing, regulate system response to external forcing. In this sense, both models demonstrate that internal feedbacks play a critical role in controlling river responses to environmental change over historic and Holocene timescales. However, both classes of model are characterised by uncertainty in their parameterisation of geomorphic processes, such that internal feedbacks and thresholds for channel response to external forcing may vary substantially between competing models. Methods of refining both approaches are considered, and hybrid models based on lattice-network structures and mechanistic representations of channel process-form interactions are identified as a means of addressing the shortcomings of existing strategies. [Copyright &y& Elsevier]