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Catena 77 (2009)

Establishing a Holocene sediment budget for the river Dijle

Bastiaan Notebaert ⁎, Gert Verstraeten, Tom Rommens, Bart Vanmontfort,Gerard Govers, Jean Poesen

K.U. Leuven, Department of Earth and Environmental Sciences, Celestijnenlaan 200E 3001, Leuven, Belgium

Received 8 November 2007; received in revised form 1 February 2008; accepted 4 February 2008

Abstract

A Holocene sediment budget was constructed for the 758 km2 Dijle catchment in the Belgian loess belt, in order to understand long-termsediment dynamics. Hillslope sediment redistribution was calculated using soil profile information from 809 soil augerings, which wasextrapolated to the entire catchment using morphometric classes. As large parts of the forests within the catchment prove to have undergone littleor no erosion since medieval times, a correction was applied for the presence of forests. Total Holocene erosion amounts 817±66 Mt for thecatchment, of which 327±34 Mt was deposited as colluvium. This corresponds with a net Holocene soil erosion rate of 10.8±0.8×103 Mg ha−1

for the entire Dijle catchment. Alluvial deposits were studied through 187 augerings spread over 17 cross-valley transects. The total alluvialsediment deposition equals 352±11 Mt or 42% of total eroded sediment mass. Results indicate that at the scale of a medium-sized catchment thecolluvial sediment sink is as important as the alluvial sediment sink and should not be neglected. As a result the estimation of erosion throughalluvial storage and sediment export would yield large errors. Dating of sediment units show an important increase in alluvial deposition frommedieval times onwards, indicating the important influence of agricultural activities that developed from that period. Mean sediment export ratesfrom 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 sedimentmeasurements 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 tributarycatchments provide an insight in the sources and sinks of sediment at different scales within the catchment.© 2008 Elsevier B.V. All rights reserved.

Keywords: Soil erosion; Alluvial sediment storage; Sediment budget; Human impact; Holocene; Sediment delivery

1. Introduction

Historical soil erosion and sediment deposition have reshapedthe landscape, especially in those regions where anthropogenicland cover change has been important. These changes can bevery clear, as is demonstrated by traces of ancient gully systems,infilled valleys and cultivation steps (e.g. Bork, 1989; Larue,2002; Stankoviansky, 2003; Vanwalleghem et al., 2003; Larue,2005). However in most cases, slope morphology has changed

⁎ Corresponding author. Fax: +32 16 32 29 80.E-mail addresses: Bastiaan.notebaert@ees.kuleuven.be (B. Notebaert),

Gert.verstraeten@ees.kuleuven.be (G. Verstraeten), rommens.tom@gmail.com(T. Rommens), Bart.Vanmontfort@ees.kuleuven.be (B. Vanmontfort),Gerard.govers@ees.kuleuven.be (G. Govers), Jean.poesen@ees.kuleuven.be(J. Poesen).

0341-8162/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.catena.2008.02.001

only moderately, with slope profile truncation on steep slopes andsediment deposition at footslopes and in alluvial environments(e.g. Bork, 1983; Trimble, 1999; Knox, 2006; Rommens et al.,2007). A thorough understanding of the present landscape there-fore implies the study of past processes. Much information can begained from the study of contemporary processes and their asso-ciated rates. The alluvial and colluvial deposits are also importantas an archive of past environments. They can be used to constructtemporal frameworks for historical erosion, and in this way theygive an indication of the importance of past erosion processes(e.g. Lang, 2003). Using these data, the influence of land use andclimate change on the fluvial systems can be studied (Lang andBork, 2006).

An important tool to study the sediment dynamics in catch-ments and the controlling factors are sediment budgets. Suchbudgets are the accounting of sources, sinks and pathways of

Fig. 1. The study area with indication of the main cities (1: Leuven; 2: Wavre;3: Ottignies; 4: Court-St-Etienne; 5: Genappes) and the main rivers (a: Dijle;b: Thyle; c: Orne; d: Train; e: Laan; f: Nethen; g: IJsse).

151B. Notebaert et al. / Catena 77 (2009) 150–163

sediment in a unit region for a time unit (Slaymaker, 2003; Reidand Dunne, 2003). Temporarily changing sediment budgets canindicate how land use changes within the catchment impact sedi-ment transport at various scales (e.g. Trimble, 1999). According toFoulds and Macklin (2006), sediment budgeting is a necessarytool to understand the role of land use change on catchmentstability, as it identifies reach-scale zones of sediment transfer andstorage. Several studies have constructed catchment-wide sedi-ment budgets for relative short periods ranging from days to a fewdecades (e.g. Trimble, 1983; Walling and Quine, 1993; Beach,1994; Page et al., 1994; Trimble, 1999; Fryirs and Brierley, 2001;Walling et al., 2002, 2006). However, few studies have concen-trated on long-term sediment budgets (e.g. spanning the entireHolocene), as this is more difficult, mainly because of the largeruncertainties and the lack of precise dates. Most long-termsediment budget studies therefore focus on relatively smallcatchments (b100 km2) for which sufficiently detailed data can begathered relatively easily (e.g. Clemens and Stahr, 1994; Rom-mens et al., 2005; Houben, 2006; Verstraeten et al., in press). Forlarger river catchments, often only one sediment budget compo-nent, for instance floodplain storage, is assessed (e.g. Houben,2006; Hoffmann et al., 2007). In some case studies, a late-Holocene sediment budget has been reconstructed using acombined field and modelling approach. For the 380 km2 largeGeul catchment, de Moor and Verstraeten (2008) reconstructed asediment budget for the last 1000 years by estimating the alluvialstorage from field data and the hillslope erosion and sedimentdeposition masses using a spatially distributed model approach.For medium-sized catchments, which provide the link betweenprocesses operating at hillslopes and fluvial processes in largedrainage basins, balanced sediment budgets spanning the entireHolocene are still rare (Macaire et al., 2002).

The objectives of this study are therefore threefold. Firstly, weaim to construct a Holocene sediment budget for the medium-sized Dijle catchment (758 km2) in the Belgian Loess Belt. Forthis river catchment, information on historic erosion and depo-sition amounts is available for smaller spatial units (Rommenset al., 2005, 2006). Secondly, the possibility of upscaling of localinformation to larger spatial units, in order to arrive at a Holocenesediment budget for the entire catchment. In this way an attemptwas made to identify the main sediment sources and sinks.Thirdly, in addition to the quantitative approach, a timeframe fordepositional processes is established and will be compared withexisting information from local sediment archives.

2. Study area

This study focuses on the 758 km2 catchment of the river Dijleupstream the city of Leuven, Belgium (Fig. 1). The topographyconsists of an undulating plateau in which several rivers areincised. The slopes are in general less than 5%, but slopes steeperthan 50% occur regularly along the axes of themajor streams. Theheight of the plateau varies between 165 m a.s.l. in the south to80 m a.s.l. in the north of the catchment, while the floodplain issituated around 25 m a.s.l. around Leuven.

The southern part of the drainage network consists of threeimportant rivers (Dijle, Thyle and Orne) with more or less the

same dimensions. These three rivers join near Court-Saint-Etienne to form the main River Dijle (Fig. 1). The width of thefloodplain downstream from this confluence varies between200 and 1800 m, whereas in the upstream parts the floodplainwidth generally does not exceed 150 m. More to the north,several other tributaries join the main branch, of which theLaan, IJse, Nethen and Train are the most important.

The plateau mainly consists of Tertiary sands and clayscovered with Pleistocene loess. Locally, this loess cover can beabsent, resulting in sandy outcrops. In the southern part of theDijle catchment the rivers have locally cut through the sandyTertiary deposits, resulting in small outcrops of more resistantPalaeozoic rocks. Soils in the region are mainly loess-derivedluvisols according to the FAO (1998).

Current land use is rather diverse: the plateau areas are mainlycovered with cropland, but several large forest stands occur,especially in the northeastern (Meerdaal forest) and northwestern(Zonien forest) part of the catchment (Fig. 2). The floodplains areused for grassland and forests. Large built up areas cover both thefloodplain and the nearby slopes and plateaux between Court-St-Etienne and Bas-Wavre, and some important residential areaswere built around forested plateau sites.

Palynological information (Mullenders and Gullentops, 1957;Mullenders et al., 1966; De Smedt, 1973) taken from sedimentcores in the Dijle floodplain near Leuven shows that the catch-ment wasmainly forested during the first half of the Holocene andthat the first agricultural crops occurred in the Subboreal phase(Mullenders et al., 1966) The oldest known settlement datesfrom around 5000 BP (Diriken, 1989) and numerous finds fromother historical periods (Bronze Age, Iron Age, Roman Period

Fig. 3. Location of augering sites. Sites with dating results of alluvial deposits:1: Korbeek-Dijle; 2: St-Joris-Weert. Sites with augering data for slope pro-cesses: 3: Loonbeek; 4: Billande; 5: Ottenburg; 6: Nodebais; 7: Hamme-Mille;8: Beauvechain.

Fig. 2. Spatial distribution of forests within the Dijle catchment.

152 B. Notebaert et al. / Catena 77 (2009) 150–163

and Medieval Period) can be found throughout the catchment(Van Hove et al., 2005; Peeters, 2007; Vanwalleghem et al., inpress).

Although anthropogenic disturbances become more impor-tant from the Subatlantic period onwards (Mullenders et al.,1966; De Smedt, 1973), it is only in the Roman period thatthe human influence on the landscape becomes significant. Theforest was extensively cleared to provide agricultural land.Continuity between the Roman and medieval land occupation isnot very clear, but it appears that agricultural land remainedwidespread. However, some large areas remained forested fromat least the 14th century until present, such as the Meerdaalforest in the northeastern part of the catchment (Vanwalleghemet al., 2006). In these forests pre-medieval landforms like man-made closed depressions (Vanwalleghem et al., 2007) andgullies dated by 14C and OSL dating to the Bronze Age andRoman Period (Vanwalleghem et al., 2003, 2005, 2006, inpress) were preserved. Comparable gullies were found in theZonien forest (Arnould-De Bontridder and Paulis, 1966) andwere observed within this study in several other forests in theDijle catchment. This indicates that these forests were not proneto severe erosion for at least the last few centuries. The Zonienforest is reported to have been forested since the 12th century(Langohr, 2001).

3. Materials and methods

In order to reconstruct a Holocene sediment budget, all com-ponents of the sediment budget were quantified. A differentapproachwas followed to quantify themass of sediment deposited

within the floodplain, and the mass of soil eroded and sedimentredeposited on the hillslopes.

3.1. Fluvial deposition

In total 17 cross-sections across the floodplain wereestablished to characterize and quantify the fluvial sedimentstorage within the catchment (Fig. 3) and 187 hand augeringswere made along these cross-sections. A detailed description foreach 5 cm soil depth was made for each augering. The inter-pretation of the cross-sections and the identification of sedi-mentological units were based on fluvial architecture concepts,for which the sediment texture was assessed in the field (e.g.Miall, 1985; Houben, 2007). In order to quantify the totalvolume and mass of sediment stored within the entire catchmentusing information derived from these cross-sections, a flood-plain polygon was digitized using soil maps, topographic maps,field observations and, where available, detailed LIDAR DEM's.The floodplain was consequently divided in homogenous zones,for which it is assumed that the thickness of the Holocene alluvialdeposits is effectively constant, and for each zone a representativecross-section was selected. The sediment mass for each zone wascalculated using:

Mzone ¼ Azoned Mcs ð1Þwith:

Mzone the mass of mineral sediment (t) deposited in the alluvialzone;

Azone the area (m2) of the alluvial zone;

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Mcs the average mineral sediment mass per unit surfacearea (Mg m−2) for the cross-section representative forthe alluvial zone.

Values for Mcs were calculated as follows:

Mcs ¼Xj

i¼1

Munit i ð2Þ

with:

j number of alluvial units present in the cross-section;Munit i the average mineral sediment mass per unit surface

area (Mg m−2) for alluvial unit i in the cross-sectionrepresentative for the alluvial zone.

Values forMunit i are calculated with a correction factor for theoccurrence of high percentages of organic matter (Verstraeten andPoesen, 2001):

Munit i ¼ dunit i � 11

DBDMSþ kOM

1�kOMð Þ�DBDOM

ð3Þ

with:

dunit i average thickness of alluvial unit i in the consideredcross-section (m);

%OM percentage organic matter;DBDOM the dry bulk density of the organic matter (Mg m−3);DBDMS dry bulk density of the clastic component (Mg m−3).

The sum of the sediment masses of the different fluvial unitswithin a zone, gives the total mass of deposited sedimentswithin the alluvial zone. For the Nethen subcatchment, we madeuse of the data reported by Rommens et al. (2006).

In order to derivemore information on the chronology of fluvialsediment deposition, AMS-radiocarbon dating was performed on8 samples. All samples were chosen at the transition from onefluvial unit to another, corresponding to changes in organic mate-rial content. Additional radiocarbon data for the Nethen floodplainwere obtained from Rommens et al. (2006). Calibration of radio-carbon dating was performed with Oxcal 4.0 (https://c14.arch.ox.ac.uk/oxcal, build number 22; Bronk Ramsey, 2008) using theIntcal 04 curve (Reimer et al., 2004).

The precision of the calculations can be estimated usingGaussian error propagation. Errors of average sedimentologicalunit thickness (augering depth), DBD and the area of the alluvialplain can be considered independent (e.g. Rommens et al., 2006).The error on individual sedimentary units in coring observations is5 cm, while the thickness of these units ranges from 1 m to morethan 3 m, resulting in an relative error of 5% or less. Therefore anerror of 5% was assumed on average depths of the differentsedimentological units when these depths are calculated based onprofiles. Where insufficient augerings where undertaken to make adetailed subdivision into different sedimentological units, theaverage depth is taken to be the arithmetic mean of the individualaugerings. Values and errors for %OM, DBDOM and DBDMS arebased on Rommens et al. (2006). The error associated with the

digitizing of the alluvial plain was estimated for each polygonindividually, based on the expected error in the width of thepolygon. The error in the polygon width mainly depends on theability to define of the alluvial plain (e.g. steep or gentle valleyslopes). It was assumed that all cross-sections are representative forthe given floodplain zones, thus no error was used for the extra-polation of the cross-section data to the entire floodplain zones.

3.2. Hillslope erosion and deposition

The loess in which most soils are developed typicallycontained 10–13% CaCO3 upon deposition (Goossens, 1987),but gradually decalcification of the upper loess layers occurred.This allowed the mobilization of clay and the development of atypical luvisol with an eluviation horizon (E) and a clay accu-mulation horizon or argic horizon (Bt). Later on these typicalluvisol soil profiles were altered by soil erosion and sedimentdeposition. Under the assumption that the depth of the differentsoil horizons is constant within the study area, i.e. that decal-cification and soil profile development occurred independentlyof the slope gradient and slope aspect, observed soil profiles canbe compared with the theoretical local undisturbed luvisol pro-file to estimate the depth of erosion. For a detailed description ofthe methodology we refer to Rommens et al. (2005).

Data from 633 augerings at four sites (Beauvechain, Nodebais,Hamme-Mille, Ottenburg; see Fig. 3) were derived from existingdatasets (Vanmontfort et al., 2004; Rommens et al., 2005). Inaddition, 176 hand augerings were made at two other study sites,namely Loonbeek and Bilande (Fig. 3). For each augering thesedimentological record was described in detail and (wherepossible) the different soil horizons were determined. Apparentlyuneroded soil profiles on plateau top sites were used to determinethe local (undisturbed) reference soil profiles. All datasets werefiltered to exclude soil augerings in man-made closed depressions(Gillijns et al., 2005; Vanwalleghem et al., 2007; Rommens et al.,2007). Finally the depth of the decalcification front was used todetermine the local erosion depth for each augering.

In order to extrapolate the augering data to the whole Dijlecatchment, morphometric classes were used. This ‘AveragePer Unit method’ (APU) was also successfully used by otherstudies (Lewis and Lepele, 1982; Bork, 1983; Macaire et al., 2002;Rommens et al., 2005). Mapping of the morphometric units foreach augering site was performed by digitizing the contour lineswith a height interval of 2.5 m from topographic maps at a scale of1:10,000. These contour lines were used to create a raster DTMwith a pixel resolution of 20 m (hereafter called ‘DTM1’), usingArcGIS™. This raster was in turn used to define the morphometricclasses. Colluvial (dry) valleys were defined as a buffer aroundpixels with an upslope area of 4 ha or more draining towards thatpixel. However, the width of this buffer depends again on theupslope area, as downstream these colluvial valleys tend to becomewider. All other pixels were classified according to the local slopegradient in 4 classes (0%–3%, 3%–5%, 5%–8%, N8%). Theseclasses were chosen arbitrarily, but in accordance with Rommenset al. (2005) to make comparison possible.

Average erosion and deposition depths of the morphometricunits were calculated for each corresponding soil augering dataset.

Fig. 4. A typical cross-section with indication of the various alluvial sedimentary units in the main valley of the River Dijle near Korbeek-Dijle (see also Fig. 3).Datings are uncalibrated ages BP (see also Table 3).

154 B. Notebaert et al. / Catena 77 (2009) 150–163

A limited number of augerings (in total 26) were reallocated toanother unit, as they were erroneously classified as being in- oroutside the colluvial valleys due to small shifts of the thalweg on theDTM. The average erosion and deposition depths for eachmorpho-metric unit at the different sites were weighted using the localdistribution of the morphometric units, in order to calculate a totalaverage value for each unit. Using this weighting, the influence ofunevenly distributed augering densities on the average value wasavoided.

For the entire Dijle catchment a second DTM (hereafter called‘DTM2’) was available based on contour lines of the 1:50,000topographical map of the Belgian National Geographic Institute(NGI). For a description of the DTM creation we refer to Van

Table 1Overview of soil augering data in alluvial valleys

Location Subcatchment Floodplain width (m) Drainage a

Korbeek-Dijle Main valley 1010 748.3Sint-Joris-Weert Main valley 1400 642.4Pecrot Main valley 1270 442.5Archennes Main valley 780 353.5Thy Upper-Dijle 205 40.7Loupoigne 1 Upper-Dijle 107 14.9Loupoigne 2 Upper-Dijle 101 8.8Terlanen Laan 390 135.1Couture-Saint-Germain Laan 165 20.4Bonlez Train 150 39.4Blanmont Orne 130 34.6Chastres Orne 70 30.5Cortil 2 Orne 80 12.5Cortil 1 Orne 55 3.6Bois Quinaux Orne 30 5.3Nil-St-Vincent-St-Martin Nil 100 29.1Tourinnes-St-Lambert Nil 70 23.9

Rompaey et al. (2001). This DTM was used to define the samemorphometric classes as described above, complemented with thealluvial plains. As the topography is somewhat smoothed on thisDTM2 compared to the DTM1, the slope values had to bemultiplied by a factor of 1.368 before defining the slope-units.This factor was calculated by matching the slope-histograms. TheDTM2 was not used initially for the average erosion and de-position calculations per unit area as described above, because theposition of the geomorphic units is rather poorly defined, resultingin a very large number ofwrongly classified soil augerings,mainlyaround the dry valleys. On the other hand, the general pattern iswell represented. Next, the morphometric map derived fromDTM2 was combined with the calculated average erosion and

rea (km2) Average thickness of alluvialsediment deposits (m)

Average mineral sediment massper unit surface (Mg m−2)

5.26 5.7±0.53.5 4.1±0.34.5 5.6±0.46.2 6.5±0.55.1 5.7±0.57.2 7.9±0.67.4 7.9±0.64.3 6.0±0.77.2 8.8±0.17.1 7.9±0.71.4 1.8±0.22.8 3.7±0.44.6 5.5±0.34.5 5.3±0.52.0 2.6±0.35.0 5.3±0.74.0 4.1±0.5

Fig. 5. Alluvial sediment storage per unit of downstream valley length within thecatchment (Mg m−1).

155B. Notebaert et al. / Catena 77 (2009) 150–163

(colluvial) deposition rates to obtain total eroded and colluvialsediment masses for the entire Dijle catchment.

When extrapolating soil erosion and sediment depositiondepths derived from the different augering sites to the wholecatchment, it is assumed that the erosion and deposition historyfor these sites is representative of the whole catchment. However,the augering data were all collected at agricultural sites (croplandor pasture), while large parts of the catchment are at presentcovered by forests. Most of these forests occur on historical mapsof the end of the 18th century (Carte de Cabinet des Pays-BasAutrichiens et de la Principauté deLiège, count de Ferraris, 1777),and for several forests there are strong indications that they wereforested since the Middle Ages onwards (see above). This meansthat total erosion and hillslope sediment deposition depths will beoverestimated, if we assume that these areas experienced the samerates of erosion and deposition as those that were continuouslyused for cropland. According to Langohr and Sanders (1985),there is no indication of substantial colluvial deposits within theZonien forest.

To include the occurrence of these historical forests within thesediment budget, a correction factor for these areas was applied.This correction factor was based on the dating of two colluvialdeposits in the Nethen subcatchment that show that about 60%to 85% of the colluviation depth took place during and afterthe Medieval period (Rommens et al., 2005; Rommens, 2006;Rommens et al., 2007). As demonstrated above, it can be expectedthat most of these forests were not prone to soil erosion since theMiddle Ages, but erosion phases before this period cannot beexcluded. Extrapolating the dating results of colluvial deposits, itcan be assumed that total erosion under present-day forests is, as amaximum, 25% of total erosion on current agricultural land.Therefore, the estimated erosion and colluvial deposition depthswithin the forests were reduced by 75% compared to agriculturalland. For the colluvial valleys within forests smaller than 30 ha, nocorrection was applied, as these forests are typically trapping thesediment originating in the surrounding agricultural land. Thespatial extent of forest within the Dijle catchment was digitizedfrom topographical maps (1972–1992) and aerial photographs(1994–2000) (Fig. 2).

Finally, sediment volumes were converted to sediment massesusing a soil and sediment dry bulk density of 1.53 Mg m−3 (seeRommens et al., 2005). The errors estimated for the slope resultsare based on Gaussian error propagation (see above). It wasassumed that there was no error on the morphometric mapping.

4. Results

4.1. Fluvial deposits

Alluvial deposits in the main floodplain of the Dijle show atypical pattern with three major units (Fig. 4). The oldest depo-sitional unit consists of a peat layer or highly organic clasticsediments containing ca 15% organic matter. This unit is overlainby a second unit, which contains mainly clastic material, albeitmixed with few peaty and organic layers, and often containingplant remains. The upper parts of unit 2 are often less organic andcontain almost no peat and plant remains, but frequently there is at

the top a small but well developed peat layer. The third and upperdepositional unit contains no recognizable plant remains or peat,except for the presence of contemporary roots. This layer is oftenthe thickest layer.

Alluvial deposits in some of the main tributaries (e.g. Laan,Orne and parts of the Upper-Dijle) generally consist of the sameunits, although they are less distinguishable, which confirmsdata for the Nethen (Rommens et al., 2006). In other maintributaries (e.g. Train, parts of the Upper-Dijle) this image isdisturbed by the presence of calciferous deposits includingtravertine (own data; Geurts, 1976). Furthermore, in the smallertributaries there is a large lateral variation in the presence of peatand organic deposits in units 2 and 3.

There is a large variation in average thickness of the alluvialdeposits between the different augering sites, and as a conse-quence also in the average mineral sediment mass per unitsurface area (Mg m−2) (Table 1). The largest deposited massesper unit area of floodplain can be found in the upper valleys ofthe Dijle and Laan. The lowest values are found in the LowerOrne valley (Blanmont cross-section). The general trend showsthat the thickness of alluvial deposits and deposition per unitarea within the floodplain is decreasing downstream. Averagesediment storage in mass per m valley length is represented inFig. 5, to give an insight in the geographic distribution.

Combining the augering data with the digitized floodplain,yields a total alluvial deposition of 352±11 Mt (megaton or

Table 3AMS-radiocarbon dating results

Sample ID Lab code Location Depth (m) Stratigraphic position Material 14C age (BP) 1 Sigma calibratedcalendar age (BC–AD)

2 Sigma calibratedcalendar age (BC–AD)

SJW12-265 Beta-226188 St-Joris-Weert 2.65 Top of the peat layerat the top of unit 2

Organic Cresidue

710±40 1263 AD–1299 AD 1224 AD–1314 AD1370 AD–1380 AD 1357 AD–1389 AD

SJW20-340 Beta-226189 St-Joris-Weert 3.40 Top of unit 1 Wood 5770±40 4686 BC–4582 BC 4718 BC–45244570 BC–4557 BC

SJW22-445 Beta-226190 St-Joris-Weert 4.45 Top of unit 1 Wood 5500±40 4442 BC–4424 BC 4450 BC–4318 BC4371 BC–4327 BC 4296 BC–4263 BC4283 BC–4271 BC

KOR_0163 UtC 14827 Korbeek 2.90 Top of unit 1 Organic Cresidue

2473±35 753 BC–686 BC 765 BC–483 BC668 BC–611 BC 467 BC–415 BC597 BC–523 BC

KOR_0164 UtC 14828 Korbeek 4.60 Bottom of unit 1 Plant 6646±48 5623 BC–5543 BC 5639 BC–5488 BCKOR_04C UtC 14830 Korbeek 3.30 Peat layer at the

top of unit 2Charcoal 1280±36 679 AD–724 AD 658 AD–783 AD

739 AD–771 AD 789 AD–813 AD844 AD–857 AD

OH_03340 UtC 14831 Korbeek 3.40 10 cm below top ofpeat layer (unit 1);valley edge

Organic Cresidue

973±36 1019 AD–1049 AD 996 AD–1006 AD1086 AD–1123 AD 1012 AD–1157 AD1138 AD–1151 AD

OH_07455 UtC 14832 Korbeek 4.55 10 cm below topof peat layer (unit 1)

Organic Cresidue

6508±49 5527 BC–5466 BC 5606 BC–5595 BC5441 BC–5423 BC 5560 BC–5367 BC5406 BC–5383 BC

Table 4Average erosion (E, m) and deposition (D, m) depths of the different morphometric units for the used augering datasets (n: number of augerings used for the calculationis given in parenthesis)

Nodebais Hamme-Mille Beauvechain Ottenburg Loonbeek Bilande Average

Number of augerings 185 87 187 174 79 97 809Slope 0–3% E (n) 0.34 (39) 0.36 (7) 0.39 (65) 0.77 (72) 0.69 (11) 0.27 (1) 0.46 (195)

D (n) 0.10 (46) 0.14 (8) 0.07 (65) 0.16 (113) 0 (11) 0 (4) 0.10 (247)Slope 3–5% E (n) 0.91 (20) 0.78 (5) 0.45 (53) 1.20 (13) 1.88 (2) 0.91 (11) 0.81 (104)

D (n) 0.13 (22) 0.18 (9) 0.11 (54) 0.27 (35) 0 (7) 0 (14) 0.13 (141)Slope 5–8% E (n) 1.22 (22) 0.97 (15) 0.91 (28) 0.35 (1) 1.50 (11) 1.31 (27) 1.10 (104)

D (n) 0.05 (23) 0.17 (22) 0.09 (29) 0.24 (14) 0.47 (16) 0.03 (31) 0.12 (135)Slope N8% E (n) 1.58 (25) 1.28 (15) – – 1.93 (17) 1.66 (30) 1.56 (87)

D (n) 0.9 (39) 0.17 (25) – 0.32 (7) 0.27 (36) 0.30 (41) 0.24 (148)Thalweg E (n) 0.75 (18) 1.17 (6) 0.85 (28) – 2.46 (4) 1.55 (2) 0.41 (58)

D (n) 1.72 (55) 2.27 (23) 1.33 (39) 2.01 (5) 2.89 (9) 3.88 (7) 2.38 (138)

Table 2Overview of the floodplain area and the alluvial sediment deposition masses within the different subcatchments

Catchment Area (km2) Floodplainarea (km2)

Number of studiedcross-sections

Floodplaindeposits (Mt)

Fraction offloodplaindeposits

Upper tributaries 263 15.0 10 78.7±6.5 22.5%Upper-Dijle 79 3.9 3 26.8±2.1 7.6%Thyle 71 4.6 0 24.7±5.8 7.1%Orne 113 6.5 7 27.2±2.0 7.8%

Lower tributaries 404 22.3 3 145.1±6.1 41.1%Train 77 3.7 1 28.5±2.4 8.1%Nethen 54 2.8 12 13.8±2.6 3.7%Laan 139 9.5 2 66.8±4.2 19.0%IJse 75 4.0 0 20.5±2.3 5.8%Small tributaries 58 2.4 0 15.5±1.2 4.4%

Main valley 94 22.5 4 127.8±5.7 36.4%Total catchment 758 59.9 17 351.6±10.6 100%

156 B. Notebaert et al. / Catena 77 (2009) 150–163

Table 5Areas of the different morphometric units within the study area (km2) and the percentage of forest within each hillslope unit

Slope 0–3% Slope 3–5% Slope 5–8% Slope 8%+ Thalweg Alluvium

Area on morphometric map (km2) 196 135 120 184 65 58Of which is forested 12.8% 13.3% 17.5% 37.0% 16.9% –

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106 Mg) clastic material. About 22% is deposited in uppertributaries (south of Court-St-Etienne), 41% in the lowertributaries (north of Court-St-Etienne) and 36% in the mainvalley (Table 2).

AMS-dating results for the main floodplain are representedin Table 3 (see also Fig. 4). For the top of the first sedimen-tological unit several dates are available, suggesting that the endof the peat development varies within the same cross-section.Two dates are available for the (top of the) peat layer at the topof the second unit, indicating that the top of this unit dates fromthe Middle Ages.

4.2. Hillslope erosion and deposition

Erosion and deposition rates for the different augering sites arerepresented in Table 4. Erosion depths could be calculated foronly 547 of the 809 augering, as for the other augerings noinformation about the depth of the decalcification front waspresent, mainly because the loess layer was completely decal-cified, or because no loess layer was present. The morphometricmap (summarized in Table 5) was combined with the averageerosion depths (Table 4) to calculate the erosion and depositionvolumes for the entire catchment (Table 6, Fig. 6). Incorporatingthe correction factor for the forests in the calculation, the totalerosion amounts to 533×106 m3 or 817±66 Mt sediment, whilethe total colluvial deposition amounts to 214×106 m3, which isequivalent to 327±34 Mt sediment. This means that 490±75 Mthas moved from the hillslopes towards the fluvial system. Thetotal Holocene soil erosion for the entire catchments is 10.8×103±0.8×103 Mg ha−1. If the correction factor for forests is notapplied, the rates are much higher, i.e. 656×106 m3 (1004 Mt)erosion, 250×106 m3 (383 Mt) deposition and a hillslope sedi-ment export of 621 Mt.

Table 6Erosion and sediment deposition masses (Mt) for the different morphometric units

Slope 0–3

Morphometric map without correction for forests Erosion (Mt) 139Erosion (%) 14%Deposition (Mt) 31Deposition (%) 8%

Morphometric map with forests Erosion (Mt) 125Erosion (%) 15%Deposition (Mt) 28Deposition (%) 9%

Nethen catchment Erosion (Mt) 14.6Erosion (%) 33%Deposition (Mt) 3.3Deposition (%) 13%

Both the cases with and without forests considered within the calculations are repre

4.3. The sediment budget of the Dijle catchment

With the given calculations a sediment budget for the entireDijle catchment was calculated (Fig. 6, Table 7). Sediment exportout of the catchment amounts to 138±75 Mt. In order to compareour results with those of a previous estimate of the sediment budgetfor the Nethen (Rommens et al., 2006), we extracted data for thissubcatchment from the morphometric map and combined it withthe erosion and deposition depths. The results of this comparisonare given in Table 6, using themorphometricmaps and the averageerosion and deposition rates reported in Table 4. Similar infor-mation for the other tributaries provides more insight in the geo-graphic distribution of erosion and sedimentation (Table 7).

5. Discussion

5.1. Hillslope sediment redistribution

A major weakness of the calculations for slope and plateaupositions is the assumption that the original soil profile is ho-mogenous throughout the different study sites. However, 184augerings of undisturbed soil profiles in theMeerdaal forest showan average depth of 2.48 m for the decalcification front, with astandard deviation of 0.63 m (Vanwalleghem, pers. com.). Thesame database contains 215 cores where no decalcification frontwas observed. The upper and lower limits of the Bt horizon showless variation, respectively 0.13 m (n=233) and 0.24 m (n=216).This shows that these borders would be better indicators for thesoil profile truncation calculations. However, in practice the upperBt border is often truncated under agricultural land, while theidentification of the lower Bt border is difficult or even impossiblefor large parts of the augering datasets. Only soil textural datacould present a solution for a reliable delineation of the Bt.

% Slope 3–5% Slope 5–8% Slope 8%+ Thalweg Total

170 202 452 41 1004±7617% 20% 45% 4%26 23 67 236 383±407% 6% 17% 62%154 175 326 36 817±6619% 21% 40% 4%24 20 49 208 327±347% 6% 15% 64%10.4 8.2 9.8 3.0 46±422% 17% 22% 7%1.6 0.9 1.5 17.3 25±26% 4% 6% 68%

sented.

Fig. 6. Sediment budget of the Dijle catchment.

158 B. Notebaert et al. / Catena 77 (2009) 150–163

With the applied method, the calculations for the most severelyeroded sites are probably somewhat underestimated: for augeringswhere the calcareous loess occurs at the surface, the erosion depthwas set equal to the reference profile depth of the decalcified layer.For areas where the calcareous loess is outcropping, it is mostprobable that also a part of this calcareous layer was eroded.

Between and within augering sites, there is a large variationin erosion and deposition depths for the different morphometricunits (Table 4). When plotting the local erosion depths and slopefor the different augerings, within the different subcatchments,it is evident that there is no linear relation between both (Fig. 7).This shows that slope gradient alone is not enough to explainthe local variation in erosion depths. Although Rommens et al.(2005) showed the applicability of the APU method for theerosion and deposition calculations, further research is neededfor a better extrapolation of total historic soil erosion depths,including factors other than slope.

It is clear that these methodological limitations result in a largeuncertainty on the calculated total hillslope erosion volumes.However, it remains difficult to quantify this uncertainty. It wasattempted to establish this largely by applying a statistical analysiswhereby a large number of augering data were used. It is expectedthat in this way, individual augerings— or entire augering sites—with somewhat deviating results, will have a limited influence onthe overall average erosion and deposition amounts. The errorassociated with the eroded soil mass equals 8%, while the errorassociated with the deposited sediment mass is ±10%. These

Table 7Sediment budget for the Dijle catchment and the different subcatchments

Catchment Surface (km2) Erosion (Mt) Colluvialdeposition (Mt)

Nethen 54 46.0 24.5Thyle 71 77.4 30.1IJse 75 80.0 28.7Train 77 91.1 35.3Upper-Dijle 79 91.7 36.9Orne 113 111.7 57.8Laan 139 161.3 58.5Entire Dijle catchment 758 817.0 327.4

errors are rather low as a large datasetwas used.However, it shouldbe stressed that this error only includes extrapolation errors, underthe assumption that soil erosion and sediment deposition is relatedto topography.

5.2. Alluvial storage

Dating results show large variations in the age of the top of thefirst (oldest) sedimentological unit (Table 3, Fig. 4). Moreover,two samples from the top of unit 1 with an absolute heightdifference of about 1 m and a horizontal distance of about 30 m,yield dates within the same time period (BETA226189 andBETA226190). Although we have insufficient data too, this suggeststhat the assumption of a uniform vertical aggradation of thefloodplain is not correct. It is, however, clear that the major peatdevelopment of the first unit probably stopped around 6500 BP(5527 cal BC–5383 cal BC) for some parts of the valley, andcontinued for much longer at the edges of the valley. The secondunit indicates a transition towards a phase with more sedimentinput in the valley. But on top of it often a peat layer is present,indicating a new period with reduced sediment input in thevalleys. Dating results show that this peat layer formed some-where during the Medieval period, yet there are insufficient datesavailable to better constrain the exact timing of this peat forma-tion. The general pattern indicates that the majority of the flood-plain deposition took place after the early Medieval period(Fig. 8). This suggests that the important agricultural activities

Hillslopeexport (Mt)

HSDR (%) Alluvialdeposits (Mt)

Catchmentexport (Mt)

SDR (%)

21.5 46.8 13.0 8.6 18.647.3 61.1 24.7 22.6 29.251.3 64.2 20.5 30.8 38.555.7 61.2 28.5 27.3 29.954.8 59.8 26.8 28.0 30.553.9 48.2 27.2 26.7 23.9

102.9 63.8 66.8 36.1 22.4489.6 59.9 351.6 138.0 16.9

Fig. 7. Relation between slope gradient and hillslope erosion (gross soil profile truncation) for all augerings where the decalcification front is present.

159B. Notebaert et al. / Catena 77 (2009) 150–163

which developed from this period onwards are responsible for thelarge sediment input.

This general pattern fits well into the timeframe of sedimentdeposition developed for the Nethen valley (Rommens et al.,2006), although the recent sediment deposition rates seem to besomewhat lower for the Nethen. Dating results from colluvialdeposits within theNethen catchment (Rommens, 2006; Rommenset al., 2006, 2007) show a start of colluviation around 7000–6000BP (5900–4800 cal BC)with increasing amounts from2500–3000BP (1300–500 cal BC) onwards. From theRoman period onwards,colluviation was already significant. At first sight this pattern is notevident in the alluvial deposits and there seems to be a time lagbetween colluvial and alluvial depositions (see also Rommenset al., 2006; Verstraeten et al., in press). More precise dating ofalluvial deposits, and more specific dating of the upper parts of thesecond unit is necessary to confirm this time lag. It would also

Fig. 8. Average accumulated sediment in the floodplain through time: 1) Sint-Joris-W2006); 3) Shaded area: combination of dates for Korbeek, Dijle main floodplain (th

provide a better understanding of the history and mechanisms ofsediment transport within the catchment.

5.3. Sediment budget

In the calculation of the sediment budget, we took into accountthat parts of the catchments were most likely forested for longtime periods. If no correction was applied, total eroded andcolluvial sediment masses, as well as the sediment export, wouldbe larger. Indeed, sediment export would then increase by 95% to270 Mt, whereas total erosion would equal 1004 Mt, whichcorresponds to a 23% increase. However, the relative importanceof the various components of the sediment budget changes onlyslightly (Table 7).

Direct comparison between the sediment budget estimated inthis study with that of the Nethen subcatchment estimated by

eert, Dijle main floodplain (this study); 2) Nethen floodplain (Rommens et al.,is study).

160 B. Notebaert et al. / Catena 77 (2009) 150–163

Rommens et al. (2006) and Verstraeten et al. (in press) is notstraightforward. First of all, they did not take into accountthe presence of forests. This is not realistic, especially for theNethen catchment, as the large Meerdaal forest makes up 25%of the catchment. Secondly, in the present study, we made use ofa more extensive augering dataset which resulted in averageerosion and colluvial sediment deposition volumes for the entireDijle catchment, which differ slightly from the average volumesused in earlier studies. And finally, a more detailed mapping ofthe colluvial valleys was carried out in this study, whereby thewidth of the colluvial deposits varies with upslope drainage areaas can be evidenced in the field. Therefore, it can be expectedthat the calculated sediment budget for the Nethen subcatch-ment yields more realistic results than former calculations.

The sediment budgets that were established in this paperincorporate hillslope erosion, hillslope deposition and net flood-plain deposition. It does not mean, however, that this sedimentbudget can be considered as a fluvial sediment budget. Indeed, weestimated total volumes of erosion and sediment deposition onhillslopes, irrespective of the process that caused the redistributionof soil. For instance, tillage operations can be held responsible forthe majority of the current-day soil redistribution volumes withinfields (e.g. Govers et al., 1996; Van Oost et al., 2005). From theobserved soil profile truncations and colluvial profiles, it cannot beestimated which fraction was eroded or deposited by tillage or bywater erosion processes. Van Oost et al. (2005) show that there isan important shift in the relative contribution of tillage and watererosion processes to soil redistribution in recent decades, wherebytillage erosion is nowadays more important than water erosion. Inthe longer term it is clear that water erosion is the dominantprocess, and thus, the majority of the erosion and sediment depo-sition volumes on hillslopes calculated in our sediment budget doreflect water erosion processes. Another important soil degrada-tion process is soil loss due to crop harvesting (Poesen et al., 2001;Ruysschaert et al., in press), which has become important in thisstudy area since the late 19th century. Part of the observed soilprofile truncation is caused by this process, and thus it is inherentlyincorporated in the sediment budget. There is, however, nodetailed time frame available to estimate the importance of tillageerosion and soil losses due to crop harvesting on a decade scale forthis catchment, and thus it is very difficult to estimate the relativecontribution of water erosion in the sediment budget.

On the other hand, some fluvial erosion and deposition pro-cesses are not incorporated in the sediment budget, namely riverchannel erosion and deposition. Changes in channel width anddepth would possibly have an influence on total sediment export.Available data give insufficient information about historicalchanges in channel dimensions, but even if these changes wouldbe relatively important, they would still represent a negligible neterosion amount compared to the hillslope erosion amounts. Forinstance, an average channel widening of 5 m combined with anaverage deepening of the channel by 1 m over the 39 km longriver stretch downstream Court-St-Etienne, would represent a netcontribution of 0.28 Mt sediment to the river channel, which isvery low compared to the 482 Mt entering the fluvial network byhillslope processes. On the other hand, the gross channel erosionand deposition amounts can have a much stronger influence on

the relative importance of the various sediment budget compo-nents (see e.g. Trimble, 1997, 1999). There is insufficient infor-mation to estimate these values over the long term. Recent data ona 1010 m long stretch show that deposition caused by channeldisplacement equals roughly 10.3Mgm−1, or 7.1×103 m3 for theentire stretch, for a 30 year period (1969–2002) (Notebaert et al.,submitted for publication). Extrapolating this value over the mainstretch over the last 10,000 years implies a total deposition of134 Mt. Total channel erosion is the same if we assume thatchannel dimensions remained constant (no net deepening orwidening). These values are similar to the total estimated sedi-ment export and represent one third of the total floodplaindeposition amount. However, this value does not necessarilycorrespondwith the total mass of bed and pointbar depositswithinthe catchment. Indeed, augering data show that these depositsoften cover small parts of the alluvial deposits, suggesting that theriver is mostly eroding and reworking these old bed deposits,instead of eroding floodplain deposits. There are no data availableto validate the estimates, nor are there indications that the valuesfor the studied stretch are representative for other stretches or timeperiods, and thus their use within the budget is not justified.

5.4. Implications of the sediment budget

The construction of a long-term sediment budget makes itpossible to calculate sediment delivery ratios (SDR) for longertime periods. The SDR of the catchment is defined as the ratiobetween the sediment export and the gross erosion within thecatchment, while the hillslope sediment delivery ratio (HSDR) isdefined as the ratio between the sediment exported fromhillslopes towards the fluvial system and the gross erosionwithin the catchment. Although recently it was argued that ‘theconcept of SDR is a fallacy’ (Parsons et al., 2006), we believe thatSDR remains an important tool for understanding the sedimentdynamics within a catchment, as it represents an essential part ofthe sediment budget which is closely connected with all theprocesses occurring in the sediment cycle (erosion, transport anddeposition at different scales). For the Dijle catchment, the SDRequals 17%. The SDR values for the different subcatchmentshow also large variations, which cannot only be attributed totheir differences in size (Table 7). It is clear that differences incatchment environment play an important role. If the correctionfor forests is not incorporated in the overall budget, the SDRincreases to 27%. This indicates that the assumptions anddefinitions that are used for the different terms of a sedimentbudget have important implications for this sediment budget andin particular the SDR. Therefore it is only possible to comparebudgets or delivery ratios when all calculations were made in thesame way. As was demonstrated above, the budget for the Dijlecatchment is not a pure water erosion budget. Therefore it is notpossible to make a comparison with sediment budgets whichwere, for instance, the erosion component was estimated using awater erosion modelling approach (e.g. Trimble, 1999; de Moorand Verstraeten, 2008).

The sediment budget clearly shows that at the scale of theDijle catchment, colluvial deposition (both on slopes and in dryvalleys) is as a sink as important as the floodplain. This implies

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that the calculation of erosion amounts from floodplain storageamounts or from sediment export rates (e.g. Hoffmann et al.,2007) is not justified. This will systematically lead to a majorunderestimation of erosion amounts. The HSDR of the differentsubcatchments show important variations. From this it is clearthat extrapolating alluvial storage and export data to derivecolluvial deposition amounts can result in large errors.

For the non-forested area (557 km2) in the Dijle catchment,the total erosion mass was estimated at 754 Mt, which cor-responds to a mean erosion rate of 1.4±0.1 Mg ha−1 yr−1 forthe entire Holocene. If we assume that about 75±15% of soilerosion took place during the last 1000–1200 years (see above),the average erosion rate on agricultural land for this periodequals 9.2±2.2 Mg ha−1 yr−1. Other studies that applied similarmethodologies in the Belgian loess areas report erosion ratesranging from 2.8 to 17.6 Mg ha−1 yr−1 for the last 1000 years,whereas present-day rates of water erosion processes equal 2.6and 16.7 Mg ha−1 yr−1 (Verstraeten et al., 2006). Likewise,historical sedimentation rates calculated with different butcomparable methods (like colluvial infilling of closed depres-sions) also yield comparable rates (Verstraeten et al., 2006). Nolong-term independent data on sediment export are available tovalidate the sediment budget estimated in this study. However, itis possible to compare the estimated sediment export rate withpresent-day rates of fluvial sediment transport by the RiverDijle. From the radiocarbon dates it can be concluded thatroughly 60% of alluvial, and 75% of colluvial sediment depo-sition took place in the last 1000–1200 years. If we apply thesame ratio to the sediment export, sediment export for the last1000 years ranges between 0.8 and 1.3 Mg ha−1 yr−1. Thesedata are in correspondence with contemporary suspended sedi-ment measurements for the Dijle near Leuven, which equals0.9Mg ha−1 yr−1 (Verstraeten et al., 2006). The above-mentionedvalues are those that were estimated by applying a correctionfactor for forests. If we were to establish a sediment budgetwithout considering the role of forests, average sediment exportfor the last 1000 years would equal 1.6 to 2.4Mg ha−1 yr−1. Suchhigh values are very unlikely for a time period of 1100 years. Itwas only during an exceptionally wet winter period that sedimentexport values of 2.1 Mg ha−1 yr−1 were measured (Steegen,2001). This stresses again the need to take land use into accountwhen extrapolating erosion rates observed on agricultural land toentire catchments.

6. Conclusion

AHolocene sediment budget was established for the 758 km2

large Dijle catchment, situated in the Belgian loess belt. Theresulting historical erosion volumes for agricultural land areconsistent with the findings of other studies within the Belgianloess belt.

The erosion and deposition calculations for the Nethen sub-catchment are more realistic than those reported by previousstudies (Rommens et al., 2006; Verstraeten et al., in press) asmore augering data are used for the estimation of hillslopeprocesses. Also morphometric mapping of colluvial dry valleyswas improved, and the presence of historical forests was taken

into account. The incorporation of historical land use into thebudget results in large differences, showing that land use hasmajor implications on the sediment dynamics. Augering dataderived from agricultural land can thus not simply be extra-polated across the entire catchment. It is clear that erosioncannot only be explained by the local slope, and future researchshould concentrate on the refinement of the extrapolation of theaugering datasets by testing the inclusion of other explanatoryvariables.

Based on sedimentary properties, three sedimentologicalunits can be recognised in the alluvial deposits throughout thecatchment. Preliminary dating results show that the end of thepeat development of unit 1 is rather variable (see Table 3), whilethe peat layer at the top of the second unit yields medieval dates.It is clear that the major part of alluvial sediment deposition tookplace after the early medieval period and can be attributed to theimportant agricultural activities that developed from this periodon. These results are in accordance with the dating results fromboth alluvial and colluvial deposits in the Nethen subcatchment(Rommens et al., 2005, 2006, 2007). More detailed dating ofalluvial and colluvial deposits is needed to see whether there is atime lag between both.

The resulting sediment budget gives a detailed insight into the(historical) sediment sources and sinks within the catchment andtheir geographical distribution. Total erosion amounts to 817±66 Mt, hillslope deposition 327±33 Mt (40%) and alluvial de-position 352±11Mt (43%). This results in a HSDR of 60% and aSDR of 17%. Both the HSDR and SDR are variable within thedifferent subcatchments, reflecting the differences in sedimentdynamics. The presence of high amounts of colluvium and thevariable (H)SDRs show that alluvial storage and export cannot beused as a proxy for erosion. The calculated sediment export ratefalls well within the range of contemporary suspended sedimentyield measurements, indicating that the applied methodologyyields a realistic sediment budget.

Acknowledgements

This research is part of a project funded by the Fund forScientific Research — Flanders (research project G.0583.06).Their support is gratefully acknowledged. The authors would alsolike to thank Jeroen Monsieur, Bjorn Dieu and the several Msc.students in physical geography for their assistance during fieldwork. We also thank Dr. Tom Vanwalleghem for providing soilaugering data on the Meerdael forest, and Dr. Dirk Goossens forthe discussion on Belgian loess soils.

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