Belgian Loess Belt Research Articles

Stability of organic matter in soils of the Belgian Loess Belt upon erosion and deposition

SummarySoil erosion has significant impacts on terrestrial carbon (C) dynamics. It removes C‐rich topsoil and deposits it in lower areas, which might result in its stabilization against microbial decay. Subsequently, C‐poor deeper horizons will be exposed, which also affects C stabilization. We analysed factors governing soil organic C (SOC) mineralization in topsoil (5–10 cm) and subsoil (75–100 and 160–200 cm) horizons from two contrasting sites (up‐slope compared with down‐slope) in the Belgian Loess Belt; we refer to these as eroding and depositional sites, respectively. Deposition of eroded soil material resulted in significantly increased SOC contents throughout the entire soil profile (2 m) and microbial biomass C in the topsoil. In a 28‐day incubation experiment we studied effects of O2 concentrations (0, 5 and 20%) and substrate (glucose) availability on C mineralization, soil microbial biomass and CaCl2‐extractable C. Carbon enrichment at the depositional site was accompanied by weak mineralization rates and small contents of water‐extractable organic C. Addition of glucose stimulated microbial growth and enhanced respiration, particularly in the subsoil of the depositional site. Availability of O2 showed the expected positive relationship with C mineralization in topsoils only. However, small O2 concentrations did not decrease C mineralization in subsoils, indicating that controls on C dynamics were different in top‐ and subsoils. We conclude that reduced C mineralization contributed to C accumulation as observed at depositional sites, probably because of poor availability of C in subsoil horizons. Limited availability of O2 in subsoils can be excluded as an important control of soil C accumulation. We hypothesize that the composition of the microbial community after burial of the organic‐rich material might play a decisive role.

Sensitivity of sediment dynamics to human actions and climatic events in the Belgian loess belt: a quantitative field study and modeling approach

Sensitivity of sediment dynamics to human actions and climatic events in the Belgian loess belt: a quantitative field study and modeling approach

Comparing landscape evolution models with quantitative field data at the millennial time scale in the Belgian loess belt

ABSTRACTThis study compares three landscape evolution models and their ability to correctly simulate measured 2500 year landscape evolution in two small catchments in the Belgian loess belt. WATEM LT and LAPSUS both model tillage and water erosion and deposition and have detachment‐limited descriptions for water erosion and deposition. Equations in LAPSUS are more mechanistic than those in WATEM LT. WATEM LTT resembles WATEM LT, but is a transport‐limited model. All three models are DEM‐based. Calibration and validation simulations were performed forward in time on (1D) transects for four spatial resolutions, and backward in time for (2D) catchments at 20 m resolution. For transects, model outputs were compared with discretized observations of transect shape. For catchments, outputs were compared with point observations of palaeo‐altitude, averaged over landscape element classes. For transects, the three models performed well, resulting in model efficiency factors of 0.92 to 0.99 for calibration and 0.62 to 0.96 for validation. However, for catchments, simulations showed that the transport‐limited WATEM LTT model could not realistically simulate long‐term landscape evolution. Performance of WATEM LT and LAPSUS at catchment scale was similar to that on transects, although LAPSUS has problems with backward calculation. Tests demonstrate that a transport‐limited approach cannot be used to model long‐term landscape evolution in the Belgian loess belt, which is in agreement with theoretical and empirical understanding of soil erosion processes in this environment. The difference in performance between transport‐limited and detachment‐limited models is clear only when the models are evaluated in a 2D catchment. The lack of such distinction when models were applied in a 1D transect highlights the importance of evaluating landscape evolution models in a 2D setting so that effects of flow convergence/divergence can be accounted for. Copyright © 2011 John Wiley & Sons, Ltd.

Mapping depth-to-clay using fitted multiple depth response curves of a proximal EMI sensor

As an alternative for the depth response approximations based on the theoretical Maxwell's equations, a procedure was proposed to fit depth response curves for different coil configurations. A 39 ha study area was selected in the Belgian loess belt, where loess material was situated on a Tertiary substrate. A survey with the DUALEM-21S electromagnetic induction instrument was carried out to map the depth-to-clay ( z clay). The depth response curves were fitted both for the vertical and perpendicular coil configurations using 85 depth observations of z clay. The resulting depth response curves R( z clay) were: R p, s ( z clay ) = 0 .8135 ⋅ e -1 .4131 ⋅ z clay s for the perpendicular coil configuration (with s as the intercoil spacing) and R v, s ( z clay ) = 0 .9802 ⋅ e -0 .8102 ⋅ z clay s for the vertical coil configuration. A set of 4 equations based on the developed depth response functions was used to model z clay at each of the 209 400 measurement points. These z clay predictions were validated using geo-electrical imaging. With two multi-electrode resistivity arrays, z clay was 1D-inverted at 95 locations along two transects, assuming a two-layered soil system. A coefficient of determination of 0.95, with a root mean-squared estimation error of 0.22 m, was found between the predicted and 1D-inverted depths. This procedure allowed the accurate 3D-reconstruction of the paleolandscape before the deposition of the loess. Flow lines were modelled on this paleosurface, revealing past or subsurface stream patterns not visible on the present relief.

Experimental rainfall–runoff data: Reconsidering the concept of infiltration capacity

► Combination of a variable rainfall intensity field infiltrometer and photography. ► Final infiltration rates and inundated area increase with rainfall intensity. ► Residues, sealing, microtopography interact with rainfall intensity and inundation. ► We argue for a dynamic rather than constant effective hydraulic conductivity ( K e ). Many infiltration models rely on an effective hydraulic conductivity parameter ( K e ) which is often determined in the field from rainfall simulation experiments on small plots. K e can be defined as the spatially averaged infiltration capacity when the soil is ‘field-saturated’ and steady state is reached. Then it equals the infiltration rate ( f ), provided ponding occurs. When a homogeneous surface is assumed, with negligible ponding depth, K e is constant and does not vary with rainfall intensity ( r ). We developed a drop infiltrometer that allows measuring K e on small plots under simulated rainfall intensities that vary between experiments. Infiltration experiments were conducted on a winter wheat field in the Belgian Loess Belt and various surface and soil properties were measured. Furthermore, photos were taken of the soil surface during the infiltration experiments for the determination of the inundated surface fraction. The results of the experiments show that K e is strongly dependent on rainfall intensity. In a statistical approach a dynamic K e could be estimated with a function of rainfall intensity, tillage treatment, percentage residue cover and bulk density. Observations indicate that microtopography, surface fraction covered by a sedimentary seal and macroporosity interact with rainfall intensity, surface ponding and infiltration. We propose that K e in physically based infiltration models should either be made dependent on dynamic state variables in a mechanistic way, such as ponding depth and water content or made dependent on rainfall intensity using an empirical relationship. With such adaptations, both surface runoff and erosion models might have more potential to deal with scale effects in runoff generation.

Catchment-scale carbon redistribution and delivery by water erosion in an intensively cultivated area

Large uncertainties still exist on both the rates of soil organic carbon (SOC) mobilization by soil erosion as well as the fate of the eroded C. Using a range of techniques we characterized soil C redistribution by water erosion in two small agricultural catchments in the Belgian Loess Belt. Results from rainfall simulations showed that C enrichment in eroded sediments was much higher than clay enrichment (average enrichment ratios of 2.47 and 1.36 on average respectively). Selectivity was also observed in depositional processes: sediments deposited within the catchments were depleted in C and finer fractions as compared to the source soil. Selectivity was much more pronounced in winter than in summer, which is related to the degree of aggregation of the transported/deposited sediment. The combined selectivity of erosion and depositional processes resulted in a significant C enrichment of the sediment exported from the catchments with the C enrichment ratio ( CER) between 1.2 and 3.0. Overall, catchment-scale C enrichment was higher than clay enrichment: enrichment ratios varied both with season and event intensity. The uncertainties on catchment-wide erosion and deposition rates precluded the closure of the sediment-C budgets. However, using a simple model we were able to show that both the magnitude and variation of observed catchment-scale CER was as expected from our understanding of C mobilization and deposition within the catchment. This suggests that most of the mobilized C was not mineralized during the erosion events but either re-deposited within the catchment or exported with the runoff. This was confirmed by the comparison of the elemental and isotopic signatures between exported sediments and source soil. In order to assess the role of erosion in the C cycle, understanding how C is mobilized and re-deposited selectively by erosion appears therefore as important as improving our understanding of the impact of erosion on C mineralization.

Dependence of effective hydraulic conductivity on rainfall intensity: loamy agricultural soils

AbstractEffective hydraulic conductivity (Ke) can be estimated with statistical models derived from datasets of field measured conductivities. Pedotransfer functions (PTFs) estimate constant Ke values, but suffer from large prediction errors because the functions usually do not account for soil structural heterogeneities. Rainfall‐runoff data have shown that the effective hydraulic conductivity, defined as final infiltration rate at steady state given ponding, is dependent on rainfall intensity. In this study a statistical approach to establish functions for rainfall intensity‐dependent Ke values is presented, including some specific functions for Western European loamy agricultural soils. Steady‐state rainfall experiments have been conducted with a drip‐type rainfall simulator at multiple intensities on small plots, installed on fields covered by four typical crops of the central part of the Belgian loess belt. A mixed linear model has been applied to temperature standardized and ln‐transformed values of apparent steady‐state infiltration rate and rainfall intensity data for the determination of significant explanatory variables and their parameters. Crop type was a necessary classification effect that accounts for much of the variation that could not be explained by continuous variables of soil and surface properties. However, soil surface bulk density, silt and sand content, tortuosity in surface microtopography, plant and residue cover (RC) were predictors that improved predictions combined with crop effect. Model efficiency (ME) values were between 0·81 and 0·97 for calibration and between 0·75 and 0·95 for validation. The application of the functions is limited to sloping, loamy agricultural soils in a Western European climate, during the spring and early summer growing phase, with not fully developed canopy cover and an RC of less than 20%. These limitations bound field conditions that are vulnerable to severe erosion events. A dynamic Ke could be applied in infiltration models, changing runoff and erosion response in hillslope models. Copyright © 2010 John Wiley & Sons, Ltd.

Limited influence of tillage management on organic matter fractions in the surface layer of silt soils under cereal - root crop rotations

Reduced tillage (RT) management may increase surface soil organic carbon (SOC) and nitrogen (N), particularly due to accumulation of labile organic matter (OM). We investigated the effect of RT compared with conventional tillage (CT) on the distribution of SOC and N over different soil fractions from 7 pairs of fields with cereal–root crop rotations, in the Belgian loess belt. Surface soil samples (0–100 mm) were physically fractionated according to a sequential sieving and density separation method into stable microaggregates, silt and clay, and free and occluded particulate OM fractions. RT management was previously found effective in increasing the organic C and organic N content of the surface soil (0–100 mm) at these 7 sites. Here, physical fractionation showed that the difference in amount of organic C and N in free particulate OM (fPOM), intra-microaggregate particulate OM (iPOM), and silt and clay associated OM between the RT and CT soils contributed 34, 29, and 37% of the increase in SOC and 35, 32, and 33% of the increase in N. The contribution of OC and N in iPOM and fPOM increased significantly on a relative basis under RT management. Only a modest increase in iPOM and slight enhancement of microaggregation was observed in RT compared with CT soils. We suggest that the repeated disturbance of soil by harvest of root crops and repeated use of cultivators and harrows may limit the accumulation of physically protected POM under RT management of these Western European cereal–root crop rotations. Instead, most of the accumulated OC and N in the surface horizons under RT management is present as free unprotected POM, which could be prone to rapid loss after (temporary) abandonment of RT management.

Soil erosion resistance effects on rill and gully initiation points and dimensions

AbstractRecent research has indicated the large spatial and temporal variation in soil erosion resistance against concentrated flow (SER). This study analyzes this variability in relation to rill and gully initiation locations on slopes and the downslope eroded volumes. The soil erodibility (Kc) and critical flow shear stress (τcr), were estimated from topsoil properties and correlated to eroded rill and gully volumes and their initiation points on slopes in the Belgian loess belt. Therefore, concentrated flow paths and topsoil properties were measured in their vicinity. The results show that rill and gully initiation points, and hence the lengths of concentrated flow paths, depend on τcr, which is controlled by soil surface conditions and can be predicted from saturated soil shear strength. Soil erosion control measures that increase soil shear strength (e.g. thalweg compaction), can therefore decrease rill and gully lengths. Once a rill or an ephemeral gully is initiated, its cross‐section was found to depend on Kc, which can be estimated from the soil water content, dry bulk density, and the dry density of roots and crop residues incorporated in the topsoil. 74% of the variation in the channel cross‐sectional area measured in the study area could be predicted from the combined effect of flow intensity and these three soil properties, whereas flow intensity alone could only account for 31% of the variation. Soil conservation measures affecting one of the soil properties that control Kc (e.g. double drilling of the thalweg, conservation tillage) can therefore decrease the cross‐sections of the concentrated flow paths. These findings also indicate that rill and gully initiation points are not only topographically controlled but also depend on the SER, which in turn determines the dimensions of these concentrated flow paths. Hence, knowledge of the variability in SER is indispensable. Copyright © 2009 John Wiley & Sons, Ltd.

The Neandertal lower right deciduous second molar from Trou de l'Abîme at Couvin, Belgium

A human lower right deciduous second molar was discovered in 1984 at the entrance of Trou de l'Abîme at Couvin (Belgium). In subsequent years the interpretation of this fossil remained difficult for various reasons: (1) the lack of taxonomically diagnostic elements which would support its attribution to either Homo ( sapiens) neanderthalensis or H. s. sapiens; (2) the absence of any reliable chronostratigraphic interpretation of the sedimentary sequence of the site; (3) the contradiction between archaeological interpretations, which attributed the lithic industry to a transitional facies between the Middle and Early Upper Palaeolithic, and the radiocarbon date of 46,820 ± 3,290 BP obtained from animal bone remains associated with the tooth and the flint tools. Thanks to recent progress regarding these three aspects, the tooth from Trou de l'Abîme may now be studied in detail. Analyses of the morphology and enamel thickness of the fossil yielded diagnostic characters consistent with an attribution to Neandertals. Re-examination of the lithic industry of Couvin shows that it corresponds to the late Middle Palaeolithic rather than a transitional facies. Furthermore, a new analysis of the site stratigraphy indicates that the unit situated above the archaeological layer in which the tooth was found is probably a palaeosol of brown soil type. Comparison with the regional cave sequences as well as with the reference sequence from the Belgian loess belt tends to show that the most recent palaeosol of this type is dated between 42,000 and 40,000 BP. This is consistent with both a recently obtained AMS result at 44,500 BP and the published conventional date.

A temporarily changing Holocene sediment budget for a loess-covered catchment (central Belgium)

This study presents a Holocene sediment budget for the Nethen catchment, a typical river catchment (55 km 2) in the Belgian loess belt. Soil erosion and hillslope sediment storage are quantified by extrapolating detailed data obtained from soil profile truncation studies in three representative zero-order sub catchments. Floodplain sediment storage is estimated by augerings along several transects across the main river and some of its tributaries. The sediment budget shows that ca. 38% of the soil eroded during the Holocene is redeposited as colluvium on hillslopes and in dry valley bottoms. Another 23% of the eroded sediment is now stored as alluvium in the floodplain. The remaining 39% or 21 × 10 6 Mg is exported from the catchment. Dating of both colluvial and alluvial sediment deposits reveals that sediment dynamics between the hillslopes, the dry valley bottoms and the floodplain behave highly non-linearly. Before ~ 500 BC, sediment delivery from the hillslopes to the river channels was near maximum. However, since the onset of significant agriculture in the Late Bronze Age–Early Iron Age, increased rates of soil erosion are only reflected in the colluvium, but not in the floodplain, resulting in very low hillslope sediment delivery ratios. From the Medieval period onwards, soil erosion increased even further, mainly as a result of a further increase in agricultural land use, but now also accelerated floodplain sedimentation took place due to an improved slope-channel coupling and the management of floodplains.

Holocene sediment budgets for upland catchments: The problem of soilscape model and data availability

Abstract This study presents a Holocene sediment budget for the upland catchment of the Speyerbach in the Palatinate Forest, southwestern Germany. The influences of both, data availability and the choice of an appropriate soilscape model on the sediment budget calculations are investigated. For budget calculations the spatial distribution of soils was derived from the soil map 1:50,000. Thickness values of soil truncation and colluvial burial were extracted from two soil data sets with varying information content. Data processing contained a disaggregation of the soil map with the help of land use data. In order to model the sediment budget a reference soil thickness (a so-called soilscape model), which represents the initial conditions is necessary. As upland soils are developed in periglacial solifluction sheets, the main solifluction sheet (MSS, “Hauptlage”) showing a constant thickness and being affected by soil forming processes was chosen. Determining the initial thickness of the MSS is a crucial point, as a comparison of reference values shows. The literature values are actual thicknesses and differ significantly between 30 and 100 cm among various studies. Therefore a statistical model using the local profile descriptions of the soil database for the Speyerbach catchment was set up. We found that the 95 percentile worked best to extract the depths of pristine soils. Hence, a pristine soil profile depth of 60 or 70 cm could be extracted for the Speyerbach catchment. Calculating the sediment budget for 60 cm returns a sediment delivery rate (SDR) of 64.8%. With a soilscape model of 70 cm a catchment SDR of 83.2% can be derived, which is 18.4% higher than the first one. The computed SDR is twice as high as the SDR determined by Houben et al. [Houben, P., Burggraaf, P., Hoffmann, T., Kleefeld, K., Zimmermann, A., Dikau, R., 2007. Reconstructing Holocene land-use change and sediment budgets in the Rhine system. PAGES News 15 (1), 17–18.] and Rommens et al. [Rommens, T., Verstraeten, G., Poesen, J., Govers, G., Van Rompaey, A.J.J., Peeters, I., Lang, A., 2005. Soil erosion and sediment deposition in the Belgian loess belt during the Holocene: establishing a sediment budget for a small agricultural catchment. The Holocene 15 (7), 1032–1043.]. This may reflect differences in physio-geographical settings and settlement history between a loess covered basin catchment and a deeply furrowed upland catchment. Since no datings or proxy data are available, the classification to individual settlement periods and furthermore an estimation of human impact is not possible yet. Future work will address the estimation of human impact by the analysis of historical data and datings of colluvial layers as well as a field validation of the computed sediment budgets.

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

AbstractSoil 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.

Establishing a Holocene sediment budget for the river Dijle

Abstract A Holocene sediment budget was constructed for the 758 km 2 Dijle catchment in the Belgian loess belt, in order to understand long-term sediment dynamics. Hillslope sediment redistribution was calculated using soil profile information from 809 soil augerings, which was extrapolated to the entire catchment using morphometric classes. As large parts of the forests within the catchment prove to have undergone little or no erosion since medieval times, a correction was applied for the presence of forests. Total Holocene erosion amounts 817 ± 66 Mt for the catchment, of which 327 ± 34 Mt was deposited as colluvium. This corresponds with a net Holocene soil erosion rate of 10.8 ± 0.8 × 10 3 Mg ha − 1 for the entire Dijle catchment. Alluvial deposits were studied through 187 augerings spread over 17 cross-valley transects. The total alluvial sediment deposition equals 352 ± 11 Mt or 42% of total eroded sediment mass. Results indicate that at the scale of a medium-sized catchment the colluvial sediment sink is as important as the alluvial sediment sink and should not be neglected. As a result the estimation of erosion through alluvial storage and sediment export would yield large errors. Dating of sediment units show an important increase in alluvial deposition from medieval times onwards, indicating the important influence of agricultural activities that developed from that period. Mean sediment export rates from the catchment for the last 1000–1200 years range between 0.8 and 1.3 Mg ha − 1 a − 1 and are consistent with present suspended sediment measurements in the Dijle. Erosion for agricultural land for this period is 9.2 ± 2.2 Mg ha − 1 a − 1 . Sediment budgets for the various tributary catchments provide an insight in the sources and sinks of sediment at different scales within the catchment.

A grassed waterway and earthen dams to control muddy floods from a cultivated catchment of the Belgian loess belt

Muddy floods, i.e. runoff from cultivated areas carrying large quantities of soil, are frequent and widespread in the European loess belt. They are mainly generated in dry zero-order valleys and are nowadays considered as the most likely process transferring material eroded from Cultivated hillslopes during the Holocene to the flood plain. The huge Costs Of Muddy flood damages justify the urgent installation of control measures. In the framework of the 'Soil Erosion Decree' of the Belgian Flemish region, a 12 ha-grassed waterway and three earthen dams have been installed between 2002-2004 in the thalweg of a 300-ha cultivated dry valley in the Belgian loess belt. The measures served their purpose by preventing any Muddy flood in the downstream village, despite the Occurrence of several extreme rainfall events (with a maximum return period of 150 years). The catchment has been intensively monitored from 2005-2007 and 39 runoff events were recorded in that period. Peak discharge (per ha) was reduced by 69% between the upstream and the downstream extremities of the grassed waterway (GWW). Furthermore, runoff was buffered for 5-12 h behind the dams, and the lag time at the outlet of the catchment was thereby increased by 75%. Reinfiltration was also observed within the waterway, runoff coefficients decreasing by a mean of 50% between both extremities of the GWW. Sediment discharge was also reduced by 93% between the GWW's inflow and the Outlet. Before the installation of the control measures, specific sediment yield (SSY) of the catchment reached 3.5 t ha(-1) yr(-1) and an ephemeral gully was observed nearly each year in the catchment. Since the control measures have been installed, no (ephemeral) gully has developed and the SSY of the catchment dropped to a mean of 0.5 t ha(-1) yr(-1). Hence, sediment transfer from the cultivated dry valley to the alluvial plain should dramatically decrease. Total cost of the control measures that are built fora 20 year-period is very low (126 (sic) ha(-1)) compared to the mean damage cost associated with muddy floods in the study area (54 (sic) ha(-1) yr(-1)). Similar measures should therefore be installed to protect other flooded villages of the Belgian loess belt and comparable environments. (C) 2008 Elsevier B.V. All rights reserved.

Fingerprinting of soil organic matter as a proxy for assessing climate and vegetation changes in last interglacial palaeosols (Veldwezelt, Belgium)

Soil characteristics in palaeosols are an important source of information on past climate and vegetation. Fingerprinting of soil organic matter (SOM) by pyrolysis-GC/MS is assessed as a proxy for palaeo-reconstruction in the complex of humic layers on top of the Rocourt pedosequence in the Veldwezelt-Hezerwater outcrop (Belgian loess belt). The fingerprints of the extractable SOM of different soil units are related to total organic carbon content, δ 13C and grain-size analysis. Combined results indicate that the lower unit of the humic complex reflects a stable soil surface, allowing SOM build-up, intensive microbial activity and high decomposition. Higher in the profile, decomposition and microbial activity decrease. This is supported by a shift in the isotopic signal, an increased U ratio and evidence of wildfires. Although the chemical composition of the extracted SOM differed greatly from recent SOM, fingerprinting yielded detailed new information on SOM degree of decomposition and microbial contribution, allowing the reconstruction of palaeo-environmental conditions during pedogenesis.

Seasonal evolution of runoff generation on agricultural land in the Belgian loess belt and implications for muddy flood triggering

AbstractMuddy floods due to agricultural runoff are a widespread and frequent phenomenon in the European loess belt, and particularly in central Belgium. These floods are triggered when high quantities of runoff are generated on cropland and cause severe erosion. Three soil surface characteristics are relevant to determine the runoff potential of cultivated soils: soil cover by crops and residues, soil surface crusting and roughness. These characteristics have been observed on 65 cultivated fields throughout 2005. A heavy rainfall event representative for events triggering muddy floods in the region (60 mm h−1 during 30 minutes) has been simulated using a 0·5 m2 simulator on fields with the 17 most observed combinations of soil surface characteristics in central Belgium. Runoff is not observed in the case of (ploughed) bare uncrusted soils, nor in the case of soils covered by crops showing a transitional crust and a moderate roughness (1–2 cm). In the cases where runoff has been observed, mean runoff coefficients ranged from 13% (wheat in July) to 58% (sugar beet or maize in May and June). Grassed buffer strips (GBSs) and grassed waterways (GWWs) show a higher runoff coefficient (62% for GBSs and 73% for GWWs) than most cultivated soils (13–58%). Furthermore, it is demonstrated that small plot measurements can be used to estimate runoff generation at the field scale. A classification of runoff generation risk based on the surveys of soil surface characteristics has been applied to common crops of central Belgium. February as well as the period between May and September are the most critical for runoff at the field scale. However, it appears from monitoring of a 16 ha catchment that the highest runoff volumes and peak discharges are recorded between May and August after heavy rainfall, explaining why 85% of muddy floods are recorded during this period in central Belgium. Copyright © 2007 John Wiley & Sons, Ltd.

Rainfall‐induced consolidation and sealing effects on soil erodibility during concentrated runoff for loess‐derived topsoils

AbstractFlume experiments simulating concentrated runoff were carried out on remolded silt loam soil samples (0·36 × 0·09 × 0·09 m3) to measure the effect of rainfall‐induced soil consolidation and soil surface sealing on soil erosion by concentrated flow for loess‐derived soils and to establish a relationship between soil erodibility and soil bulk density. Soil consolidation and sealing were simulated by successive simulated rainfall events (0–600 mm of cumulative rainfall) alternated by periods of drying. Soil detachment measurements were repeated for four different soil moisture contents (0·04, 0·14, 0·20 and 0·31 g g−1). Whereas no effect of soil consolidation and sealing is observed for critical flow shear stress (τcr), soil erodibility (Kc) decreases exponentially with increasing cumulative rainfall depth. The erosion‐reducing effect of soil consolidation and sealing decreases with a decreasing soil moisture content prior to erosion due to slaking effects occurring during rapid wetting of the dry topsoil. After about 100 mm of rainfall, Kc attains its minimum value for all moisture conditions, corresponding to a reduction of about 70% compared with the initial Kc value for the moist soil samples and only a 10% reduction for the driest soil samples. The relationship estimating relative Kc values from soil moisture content and cumulative rainfall depth predicts Kc values measured on a gradually consolidating cropland field in the Belgian Loess Belt reasonably well (MEF = 0·54). Kc is also shown to decrease linearly with increasing soil bulk density for all moisture treatments, suggesting that the compaction of thalwegs where concentrated flow erosion often occurs might be an alternative soil erosion control measure in addition to grassed waterways and double drilling. Copyright © 2007 John Wiley & Sons, Ltd.

Seasonal variations in soil erosion resistance during concentrated flow for a loess-derived soil under two contrasting tillage practices

Soil erodibilty during concentrated flow ( K c) and critical flow shear stress ( τ cr), both reflecting the soil's resistance to erosion by concentrated runoff, are important input parameters in many physically-based soil erosion models. Field data on the spatial and temporal variability of these parameters is limited but crucial for accurate prediction of soil loss by rill or gully erosion. In this study, the temporal variations in K c and τ cr for a winter wheat field on a silt loam soil under three different tillage practices (conventional ploughing, CP; shallow non-inversion tillage, ST; deep non-inversion tillage, DT) in the Belgian Loess Belt were monitored during one growing season. Undisturbed topsoil samples (0.003 m 3) were taken every three weeks and subjected to five different flow shear stresses ( τ = 4–45 Pa) in a laboratory flume to simulate soil detachment by concentrated flow. To explain the observed variation, relevant soil and environmental parameters were measured at the time of sampling. Results indicated that after two years of conservation tillage, K c(CP) > K c(DT) > K c(ST). K c values can be up to 10 times smaller for ST compared to CP but differences strongly vary over time, with an increasing difference with decreasing soil moisture content. The beneficial effects of no-tillage are not reflected in τ cr. K c values vary from 0.006 to 0.05 sm −1 for CP and from 0.0008 to 0.01 sm −1 for ST over time. Temporal variations in K c can be mainly explained by variations in soil moisture content but consolidation effects, root growth, residue decomposition and the presence of microbiotic soil crusts as well play a role. τ cr values increase with increasing soil shear strength but K c seems more appropriate to represent the temporal variability in soil erosion resistance during concentrated flow. The large intra-seasonal variations in K c, which are shown to be at least equally important as differences between different soil types reported in literature, demonstrate the importance of incorporating temporal variability in soil erosion resistance when modelling soil erosion by concentrated flow.

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.