Gerard Govers

The European Soil Erosion Model (EUROSEM): a dynamic approach for predicting sediment transport from fields and small catchments

The European Soil Erosion Model (EUROSEM) is a dynamic distributed model, able to simulate sediment transport, erosion and deposition over the land surface by rill and interill processes in single storms for both individual fields and small catchments. Model output includes total runoff, total soil loss, the storm hydrograph and storm sediment graph. Compared with other erosion models, EUROSEM has explicit simulation of interill and rill flow; plant cover effects on interception and rainfall energy; rock fragment (stoniness) effects on infiltration, flow velocity and splash erosion; and changes in the shape and size of rill channels as a result of erosion and deposition. The transport capacity of runoff is modelled using relationships based on over 500 experimental observations of shallow surface flows. EUROSEM can be applied to smooth slope planes without rills, rilled surfaces and surfaces with furrows. Examples are given of model output and of the unique capabilities of dynamic erosion modelling in general.

Evaluating the effects of changes in landscape structure on soil erosion by water and tillage

Landscape structure, or the spatial organization of different land units, has an impact on erosion and sedimentation on agricultural land. However, current erosion models emphasize the temporal, and less the spatial, variability of relevant parameters so that the effects of changes in landscape structure have hitherto not been studied in detail. Therefore, a spatially distributed water and tillage erosion model that allows the incorporation of landscape structure is presented. The model is applied to three study sites in the Belgian Loam Belt where significant changes in landscape structure occurred over the last fifty years. Erosion rates were shown to change by up to 28% however, with decreases as well as increases occurring. These could be explained by the interaction of changes in land use with changes in the position of field boundaries. Thus, landscape structure is an important control when the effect of environmental change on erosion risk is to be assessed.

Grain-size analysis by laser diffractometry: comparison with the sieve-pipette method

Abstract Results of a traditional sedimentation technique for grain-size analysis (the sieve-pipette method) were compared with those of a laser diffraction grain-size analyser, the Coulter LS-100. Milled quartz samples and silty soil samples were used to investigate the effect of particle shape. The reproducibility of the Coulter LS-100 was better than that of the sieve-pipette method, except for the sand fraction. The agreement between the two methods is strongest for the milled quartz samples. The Coulter LS-100 underestimated the clay content of the silty soil samples and generally overestimated the clay content of the milled quartz samples. This indicates the importance of particle morphology. The milled quartz grains are very angular and somewhat elongated whereas the clay minerals in the clay fraction of the silty soil samples usually have a platy shape. Reduced major axis relationships are calculated which may be used to convert Coulter LS-100 results to those of the sieve-pipette method. Correlation is strongest for percentiles size data. The Coulter LS-100 underestimates the clay, but correctly estimates the sand fraction. This results in a weak relationship for the total silt fraction as measured by the two techniques. Therefore, an alternative method is proposed for estimating this fraction. A comparison of our data with published data and relationships suggests that the calibration relationships are quite robust as long as the clay mineralogy of the samples is similar.

Impact of vegetation on flow routing and sedimentation patterns: three-dimensional modeling for a tidal marsh

[1] A three-dimensional hydrodynamic and sediment transport model was used to study the relative impact of (1) vegetation, (2) micro-topography, and (3) water level fluctuations on the spatial flow and sedimentation patterns in a tidal marsh landscape during single inundation events. The model incorporates three-dimensional (3-D) effects of vegetation on the flow (drag and turbulence). After extensive calibration and validation against field data, the model showed that the 3-D vegetation structure is determinant for the flow and sedimentation patterns. As long as the water level is below the top of the vegetation, differences in flow resistance between vegetated and unvegetated areas result in faster flow routing over unvegetated areas, so that vegetated areas are flooded from unvegetated areas, with flow directions more or less perpendicular to the vegetation edge. At the vegetation edge, flow velocities are reduced and sediments are rapidly trapped. In contrast, in between vegetated areas, flow velocities are enhanced, resulting in reduced sedimentation or erosion. As the water level overtops the vegetation, the flow paths described above change to more large-scale sheet flow crossing both vegetated and unvegetated areas. As a result, sedimentation patterns are then spatially more homogeneous. Our results suggest that the presence of a vegetation cover is the key factor controlling the long-term geomorphic development of tidal marsh landforms, leading to the formation of (1) unvegetated tidal channels and (2) vegetated platforms with a levee-basin topography in between these channels.

Patterns of rock fragment cover generated by tillage erosion

Abstract Intensively cultivated areas in the upper part of the Guadalentin catchment (southeast Spain) show a systematic spatial pattern of surface rock fragment cover (Rc). The objective of this paper is to quantify and to explain this spatial rock fragment cover pattern. Therefore, a map of an intensively cultivated area of 5 km2 was digitised, and for each pixel total topographic curvature was calculated. Next, rock fragment cover was determined photographically at 35 sites with a range of total slope curvatures. A linear relation between total curvature and rock fragment cover was found, except for narrow concavities. It was hypothesised that this pattern can be explained by a significant net downslope movement of rock fragments and fine earth by tillage. The displacement distances of rock fragments by tillage with a duckfoot chisel were measured by monitoring the displacement of tracers (painted rock fragments and aluminium cubes) on 5 sites having different slopes. The rare of tillage erosion for one tillage pass with a duckfoot chisel, expressed by the diffusion constant (k), equals 282 kg/m for up and downslope tillage and only 139 kg/m for contour tillage. Nomograms indicate that mean denudation rates in almond groves due to tillage erosion (3 to 5 tillage passes per year) can easily amount to 1.5–2.6 mm/year for contour tillage and up to 3.6–5.9 mm/year for up- and downslope tillage for a field, 50 m long and having a slope of 20%. These figures are at least one order of magnitude larger than reported denudation rates caused by water erosion in similar environments. Hence tillage erosion contributes significantly to land degradation. The downslope soil flux induced by tillage not only causes considerable denudation on topographic convexities (hill tops and spurs) and upper field boundaries but also an important sediment accumulation in topographic concavities (hollows and valley bottoms) and at lower field boundaries. Kinetic sieving (i.e. the upward migration of rock fragments) by the tines of the duckfoot chisel also concentrates the largest rock fragments in the topsoil in such a way that a rock fragment mulch develops in narrow valleys and at the foot of the slopes. These results clearly indicate that tillage erosion is the main process responsible for the observed rock fragment cover pattern in the study area. Since the study area is representative for many parts of southern Spain where almond groves have expanded since 1970, the results have a wider application. They show to what extent intensive tillage of steep slopes has contributed to the increase in soil degradation, to changes in hillslope morphology (i.e. strong denudation of convexities, development of lynchets and rapid infilling of narrow valley bottoms) and to the development of rock fragment cover patterns which control the spatial variability of the hydrological and water erosion response within such landscapes.

Geomorphic threshold conditions for ephemeral gully incision

Abstract Ephemeral gully erosion in cultivated land is an important source of sediment that is frequently being overlooked and not accounted for in soil erosion studies. Furthermore, little is known about the factors controlling ephemeral gully erosion. In this paper the available information on the initiation and location of (ephemeral) gullies is summarised, focusing on the relationship between the upslope drainage area ( A ) and the critical slope gradient ( S cr ) for ephemeral gully initiation. By plotting the non-linear relationship between critical slope gradient (measured immediately upstream of the incision head) and drainage area (at the incision head) for gullied sites it was possible to draw a straight line on log-log paper through the lower-most points for each of the available datasets, representing a critical slope-area relationship for incision. Consequently, below this line no incision occurs. This line or critical relation can also be written as a power function between critical slope and area. Although many factors vary between the different datasets, the exponent of the drainage area relationship (−0.4) showed very little variation. The observed critical slope-area relationship can be related to a simple model of channel initiation by overland flow. Furthermore, this relationship can be used to identify potentially unstable sites where control measures should be undertaken.

Comparison of routing algorithms for digital elevation models and their implications for predicting ephemeral gullies

ABSTRACT Six routing algorithms, describing how flow (and water borne material) will be routed over Digital Elevation Models, are described and compared. The performance of these algorithms is determined based on both the calculation of the contributing area and the prediction of ephemeral gullies. Three groups of routing algorithms could be identified. Both from a statistical and a spatial viewpoint these groups produce significantly different results, with a major distinction between single flow and multiple flow algorithms. Single flow algorithms cannot accommodate divergent flow and are very sensitive to small errors. Therefore, they are not acceptable for hillslopes. The flux decomposition algorithm, proposed here, seems to be preferable to other multiple flow algorithms as it is mathematically straightforward, needs only up to two neighbours and yields more realistic results for drainage lines. The implications of the routing algorithms on the prediction of ephemeral gullies seem to be somewhat coun...

Landscape-scale modeling of carbon cycling under the impact of soil redistribution: The role of tillage erosion

Despite its global significance, soil-atmosphere carbon (C) exchange under the impact of soil redistribution remains an unquantified component of the global C budget. Here we use radionuclide and soil organic carbon (SOC) data for two agricultural fields in Europe to undertake a spatial analysis of sediment and SOC fate during erosion and deposition in agricultural uplands. C fluxes induced by soil redistribution are quantified by incorporating C dynamics in a spatially distributed model including both water- and tillage-induced soil redistribution (SPEROS-C). The SOC patterns predicted by SPEROS- C are in good agreement with field observations and show that in upland areas, tillage erosion and deposition exerts a large influence on SOC redistribution and soil profile evolution at a timescale of a few decades. The formation of new SOC at eroding sites and the burial of eroded SOC below plough depth provide an important mechanism for C sequestration on sloping arable land in the order of 3–10 g C m 2 yr 1 . Any attempt to manage agricultural land to maximize sequestration must fully account for erosion, burial and fate of eroded and buried SOC across the landscape and must also account for the correlation between tillage and erosion.

Modelling long-term tidal marsh growth under changing tidal conditions and suspended sediment concentrations, Scheldt estuary, Belgium

Abstract Existing numerical models simulating the vertical growth of tidal marshes have only, to a very limited degree, been validated using observed data. In this study, we describe a refined zero-dimensional time-stepping model, which is based on the mass balance approach of Krone [in: Coastal Sediments ’87, 1987, pp. 316–323], Allen [Mar. Geol. 95 (1990) 77–96] and French [Earth Surf. Process. Landforms 18 (1993) 63–81]. The model is applied and evaluated, using field data on suspended sediment and tidal regime as input and the historical growth of a specific minerogenic tidal marsh in the Scheldt estuary (Belgium) as independent data for model testing. First, the historical rise of the marsh surface during the past 55 years is reconstructed based on land use and vegetation cover changes, which are dated using aerial photographs and which are recognised in sediment cores. After marsh formation, the marsh surface builds up very quickly and asymptotically to an equilibrium level relative to the tidal frame. Second, temporal variations in suspended sediment concentration (SSC) were measured above the actual marsh surface during a 1-year period. These measurements show that the SSC, in the water that floods the marsh surface at the beginning of an inundation, increases linearly with maximum inundation height. The application of existing models, which assume a constant incoming SSC, leads to an underestimation of the observed historical growth and to biased predictions under scenarios of future sea-level rise. However, after incorporation of the relationship between SSC and inundation height, the observed vertical growth is successfully simulated. This leads to the conclusion that not only the decrease in tidal inundation, but also the decrease in SSC with decreasing marsh inundation height, is of great importance to fully explain and successfully simulate the long-term vertical morphodynamics of tidal marshes.

Rill erosion on arable land in Central Belgium: Rates, controls and predictability

Abstract A survey was set up in Central Belgium in order to assess rill erosion rates on arable land. Mean rill erosion rate for all fields in the survey during a three year period was 0.36 kg/m2. Statistical analysis of the data allowed to calculate mean estimates for the slope and length exponent of 1.45 and 0.75 respectively. These results agree well with other data. Mean values should be used with caution as a rather important variation in slope and length exponents was found which could not be explained. Apart from topography, vegetation cover is by far the most important control on between-field variations in rill erosion rate. Other significant factors are soil texture, aggregate stability and, for silty soils, the state of the surface layer. Generally, the available information allows only a limited explanation of the observed variation in rill erosion rates. Although it is believed that further improvement is certainly possible, a considerable margin of error on individual predictions has to be accepted, not only because the process mechanics of rill erosion is not very well known, but also because the controlling factors cannot all be measured within the framework of an extensive survey. It may therefore be useful to express predictions in probabilistic terms.