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Journal of Environmental Quality 31:870-879 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Landscape and Watershed Processes

Regional Scale Variability in Sediment and Nutrient Delivery from Small Agricultural Watersheds

Gert Verstraeten*,a,b and Jean Poesena

a Laboratory for Experimental Geomorphology, Katholieke Universiteit Leuven, Redingenstraat 16, B-3000 Leuven, Belgium
b Postdoctoral Fellow of the Fund for Scientific Research—Flanders

* Corresponding author (gert.verstraeten{at}geo.kuleuven.ac.be)

Received for publication May 29, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Although many studies have pointed out the various controlling factors of sediment and nutrient delivery on a plot or watershed scale, little is known on the spatial variability of sediment and nutrient delivery on a regional scale. This study was conducted to reveal regional variations in sediment-associated nutrient delivery in central Belgium. Sediment deposited in 13 small retention ponds was sampled and analyzed for total phosphorus (TP), K, Mg, and Ca content. The TP content of the sediment deposits varied from 510 to 2001 mg P per kg sediment. Nutrients are predominantly fixed on the very fine sediment fraction (<16 µm), which is the reason why the nutrient trap efficiency of the ponds is only a fraction of the sediment trap efficiency. Average nutrient trap efficiency of the studied ponds varies between 4 and 31%, whereas sediment trap efficiency varies between 10 and 72%. For watersheds ranging from 7 to 4873 ha, sediment yield ranged between 1.2 and 20.6 Mg ha-1 yr-1, whereas TP export varied from 1.8 to 39.7 kg ha-1 yr-1. The observed spatial variability in nutrient losses is primarily attributed to regional variations in erosion and sediment yield values and to a far lesser degree to the spatial variations in fertilizer application. Redistribution of manure in the framework of an agricultural policy may increase the rate of nutrient delivery by ways of erosion and sediment transport.

Abbreviations: dBD, dry bulk density • ER, enrichment ratio • NC, nutrient content • NE, nutrient export • NTE, nutrient trap efficiency • SSY, area-specific sediment yield • STE, sediment trap efficiency • TP, total phosphorus


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
NUTRIENTS APPLIED in agriculture are often fixed to soil particles (Clark et al., 1985; Sibbesen, 1995). Soil erosion and sediment delivery processes are therefore not only responsible for high sediment loads in rivers, but also for high concentrations of nutrients found in rivers and riverbed sediments. This can lead to severe eutrophication, certainly in the case of phosphates, with major effects on the aquatic ecosystem (Clark et al., 1985). Therefore, control of sediment loss can lead to a reduction in nutrient loading to surface waters via runoff. However, before appropriate measures can be taken, it is necessary to define those areas that significantly contribute to nutrients in the drainage system. This needs to be done at the watershed scale, addressing the sources and pathways of sediment and sediment-associated nutrients within a watershed and the question of which fields are contributing to nutrient export (Steegen et al., 2001; Sharpley et al., 2000). Is is also necessary to study the problem at the regional scale by comparing the export of sediment and sediment-associated nutrients between individual watersheds, and discover which watershed and/or region is a major supplier to the sediments and nutrients found in larger river systems.

Numerous studies have attempted to identify and to quantify the various factors controlling sediment and nutrient delivery at the field or watershed scale but these studies are mostly limited to small study areas. At a larger scale, other processes or factors may explain regional variations in erosion, sediment delivery, and transfer of sediment-associated nutrients to river systems. However, this is rarely studied, mainly due to a lack of reliable data at regional scales. Data on sediment-associated nutrient delivery at the watershed scale are mainly gathered by analyzing the nutrient content of suspended sediment taken at regular time intervals at gauging stations (Garbrecht and Sharpley, 1993; Gburek et al., 2000; Steegen et al., 2001). Such techniques can provide information on the temporal dynamics of sediment and sediment-associated nutrient export. However, this requires not only the use and maintenance of expensive monitoring equipment, but also time for collecting and analyzing the suspended sediment samples. Therefore, these methods are most often limited to a selected number of study areas. These problems can be overcome by studying sediment deposits in small retention ponds. This technique not only reveals the spatial variation of sediment-associated nutrient export, but it also cost much less.

Throughout the world, several million ponds are constructed for irrigation, water supply, or flood control (Verstraeten and Poesen, 2000). In many of these ponds, sediment deposition can be observed. The availability of such ponds makes them ideal for the study of sediment delivery on a regional scale. For the Loess Belt of central Belgium, Verstraeten and Poesen (2001a) studied the sediment export from 26 small agricultural watersheds (7–5000 ha) through the use of sediment deposits in small flood retention ponds (0.02 to 1 ha). In this study, sediment sampled in 13 flood retention ponds in central Belgium was analyzed for nutrient content (TP, Ca, K, Mg), with particular attention to TP, in order to assess the sediment-associated nutrient export from the agricultural watersheds. The sediment-associated nutrient export value for each watershed draining to a retention pond can be calculated by:

[1]
where NE represents the sediment-associated nutrient export (kg ha-1 yr-1), SV the measured volumetric sediment accumulation rate (m3 yr-1), dBDi the dry bulk density of sediment sample i (Mg m-3), NCi the sediment-associated nutrient content of sample i (mg kg-1), n the number of samples, A the watershed area (ha), and NTE the nutrient trap efficiency of the pond (%). The nutrient trap efficiency of a pond, reservoir, or lake refers to the proportion (%) of the inflowing nutrients that is trapped and deposited in this pond, reservoir, or lake. Since most of the studied nutrients in runoff from agricultural land are bound to sediment particles (Sharpley et al., 2000) it can be expected that nutrient trap efficiency (NTE) is related to the sediment trap efficiency (STE). However, since most nutrients are fixed to the finest particles (Holtan et al., 1988; Ongley, 1982; Sibbesen, 1995; Steegen et al., 2001) and the efficiency of a pond for trapping these finer particles is normally lower than that of the bulk sediment, NTE is likely to be smaller than STE. This was, for instance, observed for a reservoir receiving inflow from a farmed watershed in Missouri (Rausch and Schreiber, 1981) where the STE over a 3-yr period equaled 85%, but the TP trap efficiency for this period was 77%. For the 1-ha detention pond in the Goodwin Creek watershed (Mississippi), Cooper and Knight (1990) measured a STE of 77% and a TP trap efficiency of 72% over a 5-yr period. For a selected runoff event, STE of a retention pond in Ohio was 88.6%, whereas the TP trap efficiency was only 56.1% and for the fine particulate fraction (<2 µm) only 26.6% (Bhaduri et al., 1995). It is clear that the difference between STE and NTE may vary considerably depending on the ability of the pond or reservoir to trap the fine particles and on the relative presence of these fine particles in runoff. This implies that when Eq. [1] is used to assess the nutrient export from watersheds, an accurate assessment of NTE needs to be made. Given the large difficulties that may arise when predicting STE for small ponds for a series of runoff events (Verstraeten and Poesen, 2000), this is not readily done. In this study, we will therefore present a methodology to predict the NTE of small ponds using a trap efficiency model (STEP, Verstraeten and Poesen, 2001b).

The overall objectives of this study are therefore to (i) assess the regional scale delivery of sediment-associated nutrient delivery through small pond sediment deposits, (ii) predict the NTE of small ponds, and (iii) discuss the implications of regional scale variability in nutrient delivery for central Belgium.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Study Area
In central Belgium, more than 100 retention ponds have been built for preventing small-scale flooding of housing properties and road infrastructure over the past few decades (Verstraeten and Poesen, 1999). Their sizes range from 50 m3 to more than 5 million m3 corresponding to watersheds ranging from 25 to 50000 ha. Central Belgium is part of the large European Loess Belt, extending from northern France to the Ukraine. The soils in the study area are loess-derived Alfisols. Regional differences in soil texture, however, exist. The eastern and southern part of the study area is dominated by silt loam soils with silt (2–63 µm) contents of around 80% and clay (<2 µm) contents of 10 to 20%, while in the western part the sand (>63 µm) contents may be as high as 40 to 50%. Much of the area is used for intensive agriculture; the most important crops are winter wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), sugar beet (Beta vulgaris L.), potatoes (Solanum tuberosum L.), maize (Zea mays L.), and chicory (Cichorium intybus L.). Regional differences in crops and crop rotation schemes exist. The study area is located at the transition from the northern coastal plains (elevation 0–5 m) and the southern lowland plateau (up to 200 m). This transition zone is incised by several rivers running south to north, creating a rolling topography. In the eastern part of the study area, where the loess deposits mainly cover Tertiary sands, this incision is very clear. In the western part, however, Aeolian deposits cover alternating Tertiary clays and sands. In this part, wider valley bottoms between more isolated hills and ridges can be observed. Mean annual precipitation in central Belgium varies between 700 and 800 mm and is well distributed throughout the year, although rain erosivity has a summer maximum (June to August; Bollinne, 1982).

The loess-derived soils are very susceptible to water erosion processes (i.e., inter-rill, rill, and ephemeral gully erosion). Mean intensities of water erosion on cultivated sloping land in central Belgium ranges between a few to more than 10 Mg ha-1 yr-1, but it can be more than 100 Mg ha-1 yr-1 for individual field parcels on steep slopes during extreme rainfall events (Bollinne, 1982; Govers and Poesen, 1988; Govers, 1991; Poesen and Govers, 1994; Vandaele and Poesen, 1995; Poesen et al., 1996; Steegen et al., 2000). Within the watersheds, significant sediment deposition occurs (e.g., Beuselinck et al., 2000; Steegen et al., 2000, 2001) and only a part of the produced sediment is effectively transported to the river system. Mean annual area-specific sediment yield (SSY) values for watersheds ranging between 7 and 4873 ha range between 0.4 and 20.6 Mg ha-1 yr-1 whereby SSY decreases with an increase in watershed area by a power function (Verstraeten and Poesen, 2001a). Regional variations in SSY were explained by regional variations in landscape morphology and morphometry.

Retention Pond Descriptions and Surveys
Thirteen small flood retention ponds in central Belgium were selected for studying the sediment-associated nutrient export. Information on these ponds and the watersheds that drain into them is given in Table 1. Some of these ponds are dry for most of the year, except during and shortly after runoff events. The bottom surface of these ponds is often covered with grasses, herbs, and twigs. Other ponds, however, remain flooded during the whole year, thereby prohibiting the growth of vegetation on the pond bottom. For one series of ponds, the sediment surface of each pond was measured every year with an automatic theodolite. Using the SURFER software (Golden Software, 1999), the sediment surface was constructed by linear interpolation between the measured points. By comparing two successive surveys in SURFER, the volume of sediment that was deposited in the period between the two surveys was calculated. This volume was then used to calculate the annual sediment deposition rate (SV in Eq. [1], Table 1). For another series of ponds, data on SV were provided by the local authorities, who regularly (every 3 to 5 yr) had to dredge the ponds after they had filled completely with sediment deposits. The dredged sediment volume (m3) was divided by the number of years between two successive dredging operations to become SV. For some ponds, both methods have been used to calculate SV (Table 1).


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  		Table 1. Characteristics of the 13 selected flood retention ponds in central Belgium and their corresponding watersheds.

 
Sediment Sample Collection
Sediment samples were taken in the selected ponds and analyzed for dry sediment bulk density (dBD), particle-size distribution, and nutrient content (NC). Sample locations were chosen according to a regularly spaced grid in order to incorporate the total spatial variability in these sediment characteristics. The coring density varied between 0.0014 and 0.09 ha core-1 (median = 0.025), which is relatively high compared with the majority of lake sediment studies (Foster et al., 1990). Submerged sediment samples, or samples of highly saturated sediments, were taken with a Beeker sampler (Set 04.20.SA; Eijkelkamp Agrisearch Equipment, Giesbeek, the Netherlands). This is a piston corer with clear perspex tube (internal diameter = 57 mm) of different lengths (600, 1000, and 1500 mm). At the bottom of the piston, an inflatable valve assures no sediment losses when raising the piston corer to the surface. In the dry parts of the retention ponds, the sediment deposits were sampled using a thin-walled metal corer (internal diameter = 37.8 mm, length = 207 mm). Within the retention pond of Ciplet, multiple cores were taken to identify sedimentary events. These cores had a mean depth of 9 to 16 cm including the whole sediment deposit sequence in the pond. The sediment deposits could easily be distinguished from the bed of the ponds by a distinct difference in color or texture (e.g., a sand bed under a silt loam deposit), or because the bed of the pond was made of concrete.

Sample Analysis
The sediment was removed from the corers in the laboratory and all the samples were weighed after being oven-dried (24 h at 105°C). Dry sediment bulk density (dBD) was calculated by dividing the dry sediment mass by the volume it originally occupied in the sediment deposits. Sediment particle-size distribution was obtained using a laser diffractometer (Coulter [Miami, FL] LS-100) and converted to sieve-pipette values by using the equations of Beuselinck et al. (1998). Seven particle-size classes were measured: <2, 2 to 4, 4 to 8, 8 to 16, 16 to 32, 32 to 63, and >63 µm. In this paper, a distinction is made between clay (<2 µm), fine silt (2–16 µm), coarse silt (16–63 µm) and sand (>63 µm). In total, 64 of the oven-dried samples were selected for nutrient and organic matter analysis. The selection of these samples was done such that (i) they are spatially distributed within each pond and (ii) the mean dBD of these samples corresponds to the overall mean dBD of the sediment deposits in each pond based on all the samples. This is necessary to ensure that the nutrient export values calculated with Eq. [1] are accurate. The nutrient content (TP, Mg, Ca, and K) of the sampled sediment deposits was determined by a multi-acid digestion (HNO3 and HCl) of the sediment samples according to DIN (Deutsche Industrienorm) 38414/S7 followed by inductively coupled plasma (ICP) spectrometry. The detailed cores taken in the retention pond of Ciplet were subdivided in 10 to 14 subsamples with a varying thickness of 0.1 to 2 cm according to clear variations in color (Fig. 1) . Both sediment particle-size distribution and nutrient content were determined for each subsample.



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  		Fig. 1. Sediment core taken in the Ciplet retention pond showing four sedimentary events (total length = 14.9 cm).

 

   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Nutrient Content of Sediment Samples
The nutrient content of the sediment samples varies between the selected retention ponds (Table 2). Even within a single pond, large variability in nutrient content could be observed. The TP content of a single sediment sample, for instance, varies from 456 mg kg-1 as the lowest measured value in the Hammeveld pond, to 2327 mg kg-1 as the highest measured value in the Broenbeek pond. Mean TP content for each retention pond varied from 510 to 2001 mg kg-1. Similar observations could be made for K, Mg, and Ca. Additionally, a weak but significant correlation exists between the nutrients in each pond (correlation coefficients between K, Mg, and TP ranged between 0.27 and 0.63, which are all significant at the 99% confidence level), except for the relation between Ca and K, Mg, and P (correlation coefficients ranged between -0.16 and 0.16 with p values > 0.05). Sediments in the Holsbeek retention pond, for instance, have very high values of TP and K compared with other ponds but rather low values for Ca. This suggests that differences in the availability of sediment-associated nutrients (in particular Ca) and their delivery exist between the selected watersheds.


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  		Table 2. Nutrient content of sediment deposits in 13 flood retention ponds in central Belgium.

 
The measured values are comparable with those found in other studies. De Boer (1994), for instance, measured the nutrient content of lake sediments from agricultural watersheds in the Canadian Prairies for which the P content ranged from 250 to 1750 mg kg-1 and that of Mg from 2500 to 10000 mg kg-1. The TP content of the sediment flowing into a small agricultural retention reservoir in Missouri was 450 mg kg-1 (Schreiber and Rausch, 1979). For two small agricultural watersheds (117 and 250 ha) in central Belgium, Steegen et al. (2001) measured TP content of the suspended sediment, which varied between 583 and 2168 mg kg-1.

For suspended sediments, it was concluded from previous studies (e.g., Ongley, 1982; Sibbesen, 1995) that nutrients, and in particular phosphorus, are primarily fixed to clay-sized particles. However, when the total nutrient content for each sediment sample was considered, no significant relationship with sediment texture was found in this study (Fig. 2) . Even if this relation was analyzed for each retention pond separately, no significant improvement could be observed. This can be explained by the observation that the sampled sediment deposits were a mixture of many sediment deposition events with different magnitudes. This becomes clear if the detailed sediment samples taken in the Ciplet retention pond are considered. Figure 3 shows the variation in sediment texture and sediment-associated nutrient content with depth for a sample taken in the Ciplet retention pond. The correlation between nutrient content and sediment texture is much better, with Pearson correlation coefficients ranging between 0.65 and 0.98 (all significant at a 99% confidence level), except for the relation between Ca and the sand fraction (r = -0.31, p = 0.15). In these cores, four sedimentary events could be identified. At the bottom of each event layer, light colored sediments were found that primarily consist of coarse silt and fine sands, while at the top of each event layer, very fine silt and clay is deposited as a dark colored sediment. This marked difference can be explained by the selectivity of the sediment deposition process. During inflow, first the coarsest sediment particles will settle down. When inflow has ceased and the flow currents in the pond are negligible, fine particles will settle on top of the previously deposited coarse particles. The nutrient content of the sediment deposits shows the same contrast: the bottom of each sediment layer is very poor in nutrients compared with the top sediments. The enrichment of nutrients and carbon in the top sediments explains why their color is darker. Furthermore, it can be observed that, for individual events within one pond, there is a good correlation between all nutrients. The relationship between nutrient content and sediment texture for the profiles sampled in more detail is therefore very good compared with such a relationship using bulk samples covering the total sediment depth (Fig. 4) . The rather low variation in nutrient content with sediment texture for the bulk samples is explained by the mixing of sediment layers having high and low nutrient content, obscuring the correlation.



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  		Fig. 2. Relation between total phosphorus (TP) and the fine sediment fraction (<16 µm) for all analyzed sediment samples taken in 13 retention ponds.

 


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  		Fig. 3. Variation of sediment texture and sediment-associated nutrient content with depth for one of the sediment cores taken in the Ciplet retention pond (see also Fig. 1).

 


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  		Fig. 4. Relationship between total phosphorus (TP) content and sediment texture both for detailed samples taken in the Ciplet retention pond and for bulk samples covering the total sediment depth.

 
These results indicate that the use of retention pond sediments provides good quality data only for the study of bulk nutrient content, not for detailed analysis of relationships between sediments and nutrients, unless sedimentary units can be distinguished in core profiles.

Nutrient Trap Efficiency
Before the sediment-associated nutrient export can be calculated with Eq. [1], a representative value of the nutrient trap efficiency (NTE) needs to be assessed. Measurements of NTE require time- and resource-consuming sampling of both the in- and outflow of a retention pond. This would completely compensate the advantage when using sediment deposits in ponds to assess area-specific sediment yield or nutrient export. Therefore, NTE needs to be modeled. Many models used to predict the sediment trap efficiency of reservoirs (e.g., Brown, 1943; Churchill, 1948; Brune, 1953; Heinemann, 1981) do not provide accurate predictions for small-sized ponds (Verstraeten and Poesen, 2000) nor can they be applied for nutrients bound to sediment particles, as these models do not make a separate prediction for various particle-size classes. This is necessary, however, as sediment deposition can be highly selective for small ponds. For the sediment deposits in a small retention pond in southern Germany, Weigand et al. (1998) estimated the enrichment ratios (ER) for particulate P, compared with the P content of the soils, at 0.3 to 1.4 with a mean value smaller than 1. This means that the sediment deposits are depleted in particulate P compared with the origin of these sediments, the topsoil. The runoff sediments sampled before the runoff entered the pond, however, did have a mean ER of 1.8, which means that the selectivity of the sediment deposition process in the pond is even higher than what can be concluded from the ER of the sediment deposits alone. The low sediment trap efficiency for the fine sediment fraction, on which most of the nutrients are bounded, also causes the NTE to be lower than the overall STE.

The STEP model (Verstraeten and Poesen, 2001b) is a physically based model, based on the principles of sediment deposition, that predicts the sediment trap efficiency for as many particle-size classes as there are input data available. It also allows the calculation of ER values for the deposited sediment for each particle size class. This ER is the ratio of the fraction of a particle-size class in the deposited sediment to the fraction of this particle-size class in the sediment that flows into the pond. It is assumed that the ER of the sediment-associated nutrients equals the ER for the fraction < 16 µm because (i) the results from the measurements in the Ciplet retention pond (Fig. 3) clearly show that the fraction < 16 µm is rich in nutrients and (ii) simulation with the STEP model shows for most ponds that the ER is lower than 1 for this size fraction and higher than 1 for the fraction > 16 µm. The NTE can then be calculated by:

[2]

Figure 5 shows the results of simulations with the STEP model for the Hammeveld retention pond (2000 m3) for the period 1934–1966. A large variability in both the STE and the NTE can be observed. Simulations for the period 1934–1966 were made since detailed rainfall data (10-min interval) were available. For the period in which sediment deposition is studied, no detailed rainfall data were available and the corresponding NTE could therefore not be calculated with STEP. However, it is assumed that the mean simulated NTE provides a good estimate of the NTE for the period under study (Verstraeten and Poesen, 2001b). The mean NTE for the whole simulation period equals only 28% compared with 68% for the STE. Table 3 shows mean long-term simulated NTE values for the other studied ponds. For most ponds the NTE is very low compared with the STE. Most ponds are relatively small and are not very efficient in trapping the fine sediment fraction. This strongly contrasts with the findings for larger reservoirs where NTE is very close to STE.



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  		Fig. 5. Annual variation of sediment (STE) and sediment-associated nutrient trap efficiency (NTE) for the Hammeveld retention pond as simulated with the STEP model.

 

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  		Table 3. Simulated long-term annual sediment (STE) and particulate nutrient trap efficiency (NTE) for the 13 studied flood retention ponds in central Belgium.

 
Sediment-Associated Nutrient Export
Table 4 presents the sediment-associated nutrient export values for each watershed as calculated by Eq. [1]. Total sediment-associated P export for the watersheds calculated with pond sediments ranged from 1.8 to 39.7 kg ha-1 yr-1 TP. This range (a factor of 20) is much larger than the range in mean nutrient content of the sediment samples (a factor of 4) taken in each retention pond (Table 2). This higher variability is attributed to a larger range in sediment yield values compared with the range in nutrient content. For the Hannut and Hammeveld retention ponds, for instance, the mean nutrient contents of the sediment deposits are similar, but the large difference in sediment yield values for these watersheds results in a large difference in nutrient losses for these watersheds. These findings confirm again that soil erosion and sediment transport are very important processes and pathways that control the amounts of nutrients that are delivered to river systems (e.g., Garbrecht and Sharpley, 1993; Sharpley et al., 2000). The values on TP export are also comparable with the limited results that were obtained in other studies on the nutrient delivery in the Loess Belt of central Belgium. Steegen et al. (2001) measured a TP export from two small agricultural watersheds (117 and 250 ha) of 1.3 and 4.9 kg ha-1 yr-1 TP.


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  		Table 4. Sediment yield and sediment-associated nutrient export for 13 watersheds in central Belgium using retention pond sediments.

 
On the other hand, it can be observed that the NC values of the sediment samples taken in the Steenbeek and St.-Jansbeek retention ponds are at least as high as the NC of the other pond sediments, while that of the Broenbeek is even more than twice as high. Given that in these ponds the sediment deposits are much coarser with a median particle diameter of around 60 to 70 µm compared with only 30 µm in most other ponds, it may be concluded that the fine sediment fraction (<16 µm) in these three retention ponds has a higher NC compared with the corresponding fraction in the other ponds. If it is assumed that the nutrients are associated with the fine sediment fraction (<16 µm), the NC of this fine fraction (NC<16µm) equals:

[3]

For the Steenbeek, St.-Jansbeek, and Broenbeek pond sediments this NC<16µm would then be 10500, 11300, and 22200 mg kg-1, respectively. The value of NC<16µm for the other ponds ranges between 3800 mg kg-1 for the Wolvengracht pond to 6900 mg kg-1 for the Holsbeek pond. It can be argued that this difference will also exist for the NC of the topsoil in these respective watersheds, which is confirmed by an analysis of the chemical soil fertility of the Belgian arable land based on the analysis of 75000 soil samples (Vanongeval et al., 1996). Their study showed that the TP content of the topsoil is significantly higher in the watersheds draining to the Steenbeek, St.-Jansbeek, and Broenbeek retention ponds (±1000–1500 mg kg-1), which are all located in the western part of central Belgium, compared with the other watersheds in the central and eastern part of central Belgium (±400–600 mg kg-1). These regional variations in NC of the topsoil are most probably related to regional variations in fertilizer application rates (P2O5) (Fig. 6a) . For the municipality of Langemark-Poelkapelle, in which the watersheds of the St.-Jansbeek and Broenbeek are located, the average fertilizer application rate (P2O5) on cultivated land was 148 kg ha-1 P2O5 in 1999, while this was only 71 kg ha-1 P2O5 for the municipality of Bertem, in which the Hammeveld watershed is located (Vlaamse Landmaatschapij, 2001). These figures reflect the use of locally produced organic and chemical fertilizers. The regional differences in total fertilizer application as shown on Fig. 6a is even more pronounced if only the organic fertilizers (manure) from local livestock are considered (Fig. 6b). In Langemark-Poelkapelle (Broenbeek and St.-Jansbeek watersheds) manure production was 142 kg ha-1 yr-1 (96% of total fertilizer use) and only 31 kg ha-1 yr-1 (44% of total fertilizer production) for Bertem (Hammeveld watershed) (Vlaamse Landmaatschapij, 2001). In the western part of Belgium, significantly more cattle, pigs, and poultry are held than in the eastern part, leading to more fertilizer production. In Langemark-Poelkapelle, for instance, the density of cattle, pigs, and poultry is 1.93, 13.23, and 73.51 animals ha-1, respectively, while this only amounts to 0.67, 0.8, and 0.11 animals ha-1 for Bertem (data from the 1997 agricultural census; National Institute for Statistics, 1997). Due to new legislation that became active in the 1990s, the fertilizer surplus needs to be redistributed to regions where less manure is produced. This agricultural policy is meant to reduce the risk of phosphorus and nitrogen saturation in overfertilized regions to further prevent nutrient leaching to the ground water. The differences in TP in the topsoil, which are the result of tens of years of fertilizer application, may therefore not reflect the actual orthophosphate application if the transport of orthophosphates is taken into account (Fig. 6c). Total P2O5 application rates on cultivated land are then more or less equal in all regions of central Belgium: for example, 120 kg ha-1 P2O5 in Langemark-Poelkapelle and 121 kg ha-1 P2O5 in Bertem (Vlaamse Landmaatschapij, 2001). The regional variations in TP in the topsoil and the related sediments are thus a second important factor that explains the TP export from agricultural watersheds. Since most watersheds with important fertilizer application are less prone to erosion, this factor is of less importance in understanding the nutrient delivery to river systems, compared with water erosion processes. The actual redistribution of manure from regions with small water erosion rates to regions with larger erosion rates, however, may have an adverse effect on the nutrient delivery to river systems.



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  		Fig. 6. Spatial distribution of P2O5 production and application on agricultural land within Flanders. (a) The total P2O5 application rate without taking into account P2O5 redistribution, (b) the local animal P2O5 production, and (c) the actual P2O5 application rate taking into account P2O5 redistribution. Units of P2O5 application and production rates are expressed in kg P2O5 ha-1 agricultural land (includes cropland and grassland) (Vlaamse Landmaatschappij, 2001). The location of the studied retention ponds is indicated with a *.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
In this study, a methodology to assess the regional variability in sediment and nutrient delivery through sediment deposits in small ponds is discussed. Sediment deposits in small flood retention ponds in central Belgium were analyzed for their nutrient content in order to calculate the nutrient delivery from agricultural watersheds to the river drainage system. The nutrient trap efficiency of the pond was a very important parameter to be assessed. Simulations with the STEP model showed that nutrient trap efficiency for small ponds is much smaller compared with the sediment trap efficiency, as the ponds are not capable of trapping the fine sediment fraction. The results indicate that a substantial reduction in delivery of nutrients to the river system can best be achieved by (i) controlling soil losses by water erosion and sediment delivery rates, (ii) constructing retention ponds having a high trap efficiency for the fine sediment fraction, and (iii) reducing the input of nutrients on the fields through fertilizers. Redistribution of manure to areas more prone to soil erosion in the framework of an agricultural policy, which aims to reduce nutrient leaching to the ground water, may increase the rate of nutrient delivery to the rivers in central Belgium via ways of overland flow paths, erosion, and sediment transport.


   ACKNOWLEDGMENTS
 
The authors wish to thank Martine Breynaert, Ingrid De Bruyne, Ivan Geeraerts, Anton Van Rompaey, and An Steegen for assisting in surveying the ponds and for sampling and analyzing sediment samples for dry bulk density and particle-size distribution; Nina Desmet for assisting with the analysis of the sediment-associated nutrient content; Prof. Dr. M. Geypens for providing logistical support for the analysis of the nutrient content; Prof. Dr. E. Van Hecke and Hilde Vandenhoeck for providing data on the agricultural census; and the Vlaamse Landmaatschappij –afdeling Mestbank, Brussels for providing data on fertilizer use.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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Home page
 Progress in Physical GeographyHome page
Yihe Lu, Bojie Fu, Liding Chen, Guohua Liu, and Wei Wei
Nutrient transport associated with water erosion: progress and prospect
Progress in Physical Geography, December 1, 2007; 31(6): 607 - 620.
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