Reconstructing ancient topography through erosion modelling

One of the main aims of geomorphology is to understand how geomorphic processes change topography over long time scales. Over the last decades several landscape evolution models have been developed in order to study this question. However, evaluation of such models has often been very limited due to the lack of necessary field data. In this study we present a topography based hillslope erosion and deposition model that is based on the WATEM/SEDEM model structure and works on a millennial time scale. Soil erosion, transport and deposition are calculated using slope and unit contributing area. The topography is iteratively rejuvenated by taking into account modelled erosion and deposition rates, thereby simulating topographic development backwards in time. A first attempt has been made to spatially evaluate the model, using detailed estimates for historical soil erosion and sediment deposition volumes, obtained from an augering campaign in a small catchment in the Belgian Loess Belt. The results show that the model can simulate realistic soil redistribution patterns. However, further research is necessary in order to deal with artificial flaws that cause routing problems and significantly influence results. Common problems and issues related to this type of backward modelling are also discussed.     

Joint interpretation of hydrological and geophysical data: electrical resistivity tomography results from a process hydrological research site in the Black Forest Mountains,Germany

The use of electrical resistivity tomography (ERT; non‐intrusive geophysical technique) was assessed to identify the hydrogeological conditions at a surface water/groundwater test site in the southern Black Forest, Germany. A total of 111 ERT transects were measured, which adopted electrode spacings from 0·5 to 5 m as well as using either Wenner or dipole‐dipole electrode arrays. The resulting two‐dimensional (2D) electrical resistivity distributions are related to the structure and water content of the subsurface. The images were interpreted with respect to previous classical hillslope hydrological investigations within the same research basin using both tracer methods and groundwater level observations. A raster‐grid survey provided a quasi 3D resistivity pattern of the floodplain. Strong structural heterogeneity of the subsurface could be demonstrated, and (non)connectivities between surface and subsurface bodies were mapped. Through the spatial identification of likely flow pathways and source areas of runoff, the deep groundwater within the steeper valley slope seems to be much more connected to runoff generation processes within the valley floodplain than commonly credited in such environmental circumstances. Further, there appears to be no direct link between subsurface water‐bodies adjacent to the stream channel. Deep groundwater sources are also able to contribute towards streamflow from exfiltration at the edge of the floodplain as well as through the saturated areas overlying the floodplain itself. Such exfiltrated water then moves towards the stream as channelized surface flow. These findings support previous tracer investigations which showed that groundwater largely dominates the storm hydrograph of the stream, but the source areas of this component were unclear without geophysical measurements. The work highlighted the importance of using information from previous, complementary hydrochemical and hydrometric research campaigns to better interpret the ERT measurements. On the other hand, the ERT can provide a better spatial understanding of existing hydrochemical and hydrometric data. Copyright © 2009 John Wiley & Sons, Ltd.     

Evaluation of a dynamic multi‐class sediment transport model in a catchment under soil‐conservation agriculture

Soil erosion models are essential tools for the successful implementation of effective and adapted soil conservation measures on agricultural land. Therefore, models are needed that predict sediment delivery and quality, give a good spatial representation of erosion and deposition and allow us to account for various soil conservation measures. Here, we evaluate how well a modified version of the spatially distributed multi‐class sediment transport model (MCST) simulates the effectiveness of control measures for different event sizes. We use 8 year runoff and sediment delivery data from two small agricultural watersheds (0·7 and 3·7 ha) under optimized soil conservation. The modified MCST model successfully simulates surface runoff and sediment delivery from both watersheds; one of which was dominated by sheet and the other was partly affected by rill erosion. Moreover, first results of modelling enrichment of clay in sediment delivery are promising, showing the potential of MCST to model sediment enrichment and nutrient transport. In general, our results and those of an earlier modelling exercise in the Belgian Loess Belt indicate the potential of the MCST model to evaluate soil erosion and deposition under different agricultural land uses. As the model explicitly takes into account the dominant effects of soil‐conservation agriculture, it should be successfully applicable for soil‐conservation planning/evaluation in other environments. Copyright © 2008 John Wiley & Sons, Ltd.     

Model based analysis of lateral and vertical soil carbon fluxes induced by soil redistribution processes in a small agricultural catchment

Soil redistribution on arable land significantly affects lateral and vertical soil carbon (C) fluxes (caused by C formation and mineralization) and soil organic carbon (SOC) stocks. Whether this serves as a (C) sink or source to the atmosphere is a controversial issue. In this study, the SPEROS‐C model was modified to analyse erosion induced lateral and vertical soil C fluxes and their effects upon SOC stocks in a small agricultural catchment (4·2 ha). The model was applied for the period between 1950 and 2007 covering 30 years of conventional tillage (1950–1979) followed by 28 years of conservation tillage (1980–2007). In general, modelled and measured SOC stocks are in good agreement for three observed soil layers. The overall balance (1950–2007) of erosion induced lateral and vertical C fluxes results in a C loss of ?4·4 g C m^–2 a^–1 at our test site. Land management has a significant impact on the erosion induced C fluxes, leading to a predominance of lateral C export under conventional and of vertical C exchange between soil and atmosphere under conservation agriculture. Overall, the application of the soil conservation practices, with enhanced C inputs by cover crops and decreased erosion, significantly reduced the modelled erosion induced C loss of the test site. Increasing C inputs alone, without a reduction of erosion rates, did not result in a reduction of erosion induced C losses. Moreover, our results show that the potential erosion induced C loss is very sensitive to the representation of erosion rates (long‐term steady state versus event driven). A first estimate suggests that C losses are very sensitive to magnitude and frequency of erosion events. If long‐term averages are dominated by large magnitude events modelled erosion induced C losses in the catchment were significantly reduced. Copyright © 2011 John Wiley & Sons, Ltd.     

Towards constraining the magnitude of global agricultural sediment and soil organic carbon fluxes

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 in 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.     

Segregation of soilscapes using statistics in remote sensing

The use of remotely sensed data in the form of vertical aerial photographs has been in practice since several decades. Launching of Landsat satellites and their capability together multispectral scanner data afforded soil scientists enhanced capability for mapping soils. Technological advances in computer processing of Landsat MSS data coupled with ancillary information provided added advantages. Still difficulties exist in identifying the interference to vegetation and in segregation of narrowly defined soilscapes. Utility of statistical data, obtainable from computer analysis, and an aid in understanding the reflectance characteristics of various types of vegetation and soilscapes has been discussed in this paper. The final output obtained by computer processing compared with existing soil maps of the area registered more future prospects for utilisation of statistics in segregation of soilscapes and vegetation.     

Flow velocity profiles in the Lower Scheldt estuary

Recent acoustic Doppler current profiler (ADCP)-measurements in the Scheldt estuary near Antwerp, Belgium, revealed anomalous, i.e. anti-clockwise circulations in a left bend during the major part of the flood period; these circulations were established shortly after the turn of the tide. During ebb, anti-clockwise circulations persisted, as predicted by classical theory. These data were analysed with a 3D and a 1DV-model. The 3D simulations reveal that the anomalous circulations are found when salinity is included in the computations—without salinity “normal” circulations were found. From analytical and 1DV simulations, it is concluded that a longitudinal salinity gradient ${\partial S} \mathord{\left/ {\vphantom {{\partial S} {\partial x}}} \right. \kern-0em} {\partial x}$ may induce a near-bed maximum in flow velocity reversing the direction of the secondary currents. The 1DV-model was then used to assess the contribution of various processes one by one. It was found that because of a reduction in vertical mixing, the vertical velocity profile is not at equilibrium during the first phase of accelerating tide, further enhancing the effects of ${\partial S} \mathord{\left/ {\vphantom {{\partial S} {\partial x}}} \right. \kern-0em} {\partial x}$ . A small vertical salinity gradient ${\partial S} \mathord{\left/ {\vphantom {{\partial S} {\partial z}}} \right. \kern-0em} {\partial z}$ appeared to have a very large effect as the crosscurrents of the secondary circulations induced by ${\partial S} \mathord{\left/ {\vphantom {{\partial S} {\partial x}}} \right. \kern-0em} {\partial x}$ became an order of larger magnitude. However, at the site under consideration, the effects of transverse salinity gradients, generated by differential advection in the river bend, were dominant: adverse directions of the secondary circulations were found even when the vertical velocity profile became more regular with a more or less logarithmic shape, i.e. when the effects of ${\partial S} \mathord{\left/ {\vphantom {{\partial S} {\partial x}}} \right. \kern-0em} {\partial x}$ and ${\partial S} \mathord{\left/ {\vphantom {{\partial S} {\partial z}}} \right. \kern-0em} {\partial z}$ did not play a dominant role anymore. It is argued that data on the secondary velocity structure, which can be measured easily owing to today’s developments in ADCP equipment, may serve as an indicator for the accuracy at which the salinity field is computed with 3D numerical models. Moreover, the large effect of the salinity structure on the velocity field must have a large impact on the morphological development of estuaries, which should therefore be accounted for in morphological modelling studies.