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TECHNICAL REPORT
Landscape and Watershed Processes

Factors Controlling Sediment and Phosphorus Export from Two Belgian Agricultural Catchments

^a Lab. for Experimental Geomorphology, K.U. Leuven, Redingenstraat 16, 3000 Leuven, Belgium
^b Lab. for Soil Fertility and Soil Biology, K.U. Leuven, Kardinaal Mercierlaan 92, 3001 Heverlee, Belgium

* Corresponding author (gerard.govers{at}geo.kuleuven.ac.be)

Received for publication June 19, 2001.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Sediment and total phosphorus (TP) export vary through space and time. This study was conducted to determine the factors controlling sediment and TP export in two agricultural catchments situated in the Belgian Loess Belt. At the outlet of these catchments runoff discharge was continuously measured and suspended sediment samples were taken during rainfall events. Within the catchments vegetation type and cover, soil surface parameters, erosion features, sediment pathways, and rainfall characteristics were monitored. Total P content and sediment characteristics such as clay, organic carbon, and suspended sediment concentration were correlated. Total sediment and TP export differ significantly between the monitored catchments. Much of the difference is due to the occurrence of an extreme event in one catchment and the morphology and spatial organization of land use in the catchments. In one catchment, the direct connection between erosive areas and the catchment outlet by means of a road system contributed to a high sediment delivery ratio (SDR) at the outlet. In the other catchment, the presence of a wide valley in the center of the catchment caused sediment deposition. Vegetation also had an effect on sediment production and deposition. Thus, many factors control sediment and TP export from small agricultural catchments; some of these factors are related to the physical catchment characterisics such as morphology and landscape structure and are (semi)permanent, while others, such as vegetation cover and land use, are time dependent.

Abbreviations: OrgC, organic carbon content • Q, runoff discharge • SDR, sediment delivery ratio • SSC, suspended sediment concentration • TP, total phosphorus

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
ALTHOUGH phosphorus is a natural component in the soil, farmers add phosphorus to the soil by using fertilizers in order to increase crop yield (Hofman and Verloo, 1989). As a result, the topsoil of many fields in Belgium has a phosphorus content that is higher than necessary to ensure a normal crop production. In a recent study, Vanongeval et al. (1996) found that almost 60% of the samples taken in the loess region of Belgium had a phosphorus content above crop needs. Phosphorus is one of the main elements in eutrophication problems because it is often the limiting element in surface waters (Vlaamse Milieu Maatschappij, 1998). Phosphorus is brought into the drainage network from both point sources and diffuse sources (Svendsen et al., 1995). Phosphorus losses from point sources can easily be detected and a remediation procedure can be conducted. Phosphorus losses from diffuse sources, such as agriculture, are more difficult to measure and to correct.

The transport of sediment-associated nutrients such as phosphorus from the soil to the river network is complex because it is influenced by many processes such as soil erosion, sediment transport, and deposition within the catchment (e.g., Gburek et al., 2000), and because parameters like rainfall and soil surface characteristics are variable in time and space. All these factors will result in variable phosphorus losses. Nevertheless, in order to reduce nonpoint phosphorus pollution from agricultural land, information on fluxes of pollutants, their origins, and transfer mechanisms is required (Dorioz and Ferhi, 1993).

This study concentrated on the factors controlling the phosphorus export from two small agricultural catchments in central Belgium where considerable water erosion occurs (Steegen et al., 1998). We address two questions: (i) What is the relationship between sediment characteristics and phosphorus export? (ii) Are there differences in phosphorus export between two comparable catchments? If so, what are the controlling factors?

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The agricultural catchments, locally called Kinderveld and Ganspoel, are situated in the Belgian Loess Belt near Leuven. The drainage area of the Kinderveld catchment is 250 ha, whereas the Ganspoel catchment is 117 ha (Fig. 1). Both catchments have a rolling topography and their slope distribution is plotted in Fig. 2. The main crops in the catchments are wheat (Triticum aestivum L.), maize (Zea mays L.), sugar beet (Beta vulgaris L.), potato (Solanum tuberosum L.), and chicory (Cichorium intybus L.), with woodland and pastures found on steeper slopes and in some of the thalwegs (Table 1). The soils within the catchment are mainly loess-derived Alfisols, but in some places sandy outcrops occur. A dispersed topsoil sample typically contains 7 to 14% clay, 75 to 80% silt, and 9 to 17% sand (Beuselinck et al., 2000). The TP content of the topsoil in both catchments is very similar, with an average value of 675 mg kg^-1 for the Ganspoel catchment (n = 65) and 668 mg kg^-1 for the Kinderveld catchment (n = 10) (randomly sampled over both catchments). These loess-derived soils are highly susceptible to water erosion (Govers, 1991; Poesen and Govers, 1990). After one or a series of rainfall events, rills and gullies often develop on the cultivated fields and interrill erosion due to sheet-wash also occurs. The sediment that is produced during these storm periods is partly deposited within the catchment (Beuselinck et al., 2000), but a considerable part is transported to the outlet of the catchment and into the river system.

View larger version (61K): [in this window] [in a new window]   Fig. 1. Contour maps of the Kinderveld and Ganspoel catchments indicating the road system, drainage area, and brooklet in which the San Dimas flumes were installed (contour interval = 5 m). The thalwegs as mentioned in Table 6 are also indicated (TW1–4).

View larger version (29K): [in this window] [in a new window]   Fig. 2. Histogram of slope distributions in the Kinderveld and Ganspoel catchments.

The suspended sediment concentration (SSC) was determined by oven-drying the samples at 105°C for 24 h. The grain-size distribution of the dispersed samples was measured using a laser diffractometer (Coulter LS-100; Coulter Corporation, Miami, FL). The grain-size distributions were converted to sieve-pipette data using the equations proposed by Beuselinck et al. (1998).

About 25% of the oven-dried samples were selected for phosphorus and organic carbon analysis. Samples were chosen so that the variations in sediment concentration within an event were well covered. The Walkley and Black method was used to determine the organic carbon content by titration (Walkley and Black, 1934). Total phosphorus (TP) and mineral phosphorus content of each sample was determined using the method of Walker and Adams (described by Olsen and Sommers [1982] and modified by Takken and Verstraten [1996]). This method uses H₂SO₄ to extract phosphorus from both an ignited and unignited soil sample. No distinction was thus made between soluble and particulate phosphorus. Sharpley et al. (1981) found that even with a short contact time between suspended sediment and runoff water, sorption of phosphorus occurs. Because the suspended sediment concentrations in the studied catchments are larger than those measured in their study, it may be assumed that most of the phosphorus is transported with the sediments during runoff events. The organic P content was calculated as the difference between TP (ignited) and mineral P (unignited) content. The relationship between TP, sediment characteristics, and runoff discharge were analyzed using regression analysis as implemented in the SAS package (SAS Institute, 1985). In order to test whether there was a statistically significant difference between the two catchments, a dummy variable was used (Wonnacott and Wonnacott, 1977).

Monthly field surveys were made within the catchment and included measurements of crop type, surface roughness (by measuring height differences in microtopography), vegetation cover percentage and degree of crusting (by comparing the field situation with photographs [Govers, unpublished data, 1986]), and erosion and sediment deposition features. Furthermore, at the end of one or a series of rainfall events, volumetric data on gully and rill erosion and the extent and thickness of sediment deposits were collected. In total, three such feature surveys were conducted in the Kinderveld catchment and two in the Ganspoel catchment. The location of ephemeral gullies was mapped using a global positioning system (GPS) and gully volumes were obtained by measuring their width and depth at several places along a longitudinal profile. In areas where rill erosion occurred, rill width and depth were measured along transects perpendicular to the flow direction. The location of the transect as well as the area represented by the transect were mapped using GPS. Sediment deposition areas were located and mapped using GPS and the average thickness of the deposited sediment was derived from several sediment thickness measurements (about 1 measurement per m²). Sediment thickness was measured by making a small vertical cut through the deposit with a small trowel. Recent sediment deposits could easily be distinguished from older ones by differences in color, and/or because they were deposited on crops or freshly tilled fields. Using these data, a sediment delivery ratio (SDR; i.e., the ratio between sediment delivered at the catchment outlet and total erosion in the basin) could be calculated for major erosion events (Walling, 1983).

Most of the runoff events that occurred during the observation period were adequately sampled. However, for an extreme event in Kinderveld on 20 May 1997, only data on water level variations and two runoff samples taken manually to determine the sediment concentration were available due to equipment failure when the measurement flume was overtopped. The calculation of total runoff discharge and sediment export for this event was explained in more detail by Steegen et al. (2000). During the event of 6 June 1998 in the Ganspoel catchment, the flume at the outlet was obstructed by grasses. Consequently, runoff discharge was overestimated. Moreover, the inflow of the automatic sampler was also obstructed by grasses and no reliable samples were taken. The total sediment export was then calculated as the difference between all erosion features found in the catchment (rills, gullies, and interrill erosion) and the sediment volumes that were deposited on the fields. For all these calculations, interrill erosion was assumed to be 10% of total rill and gully erosion volumes (Ludwig et al., 1992).

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Measured TP of the samples taken in the Kinderveld catchment ranged between 598 and 2168 mg kg^-1. In the Ganspoel catchment values were between 583 and 1892 mg kg^-1. The mineral P content of the samples was on average 73% of the TP content, with a minimum of 61% and a maximum of 85%.

A linear, positive correlation was found between TP and organic carbon in the sediment, with similar relationships for both catchments (Fig. 3, Table 2). A log-linear function was observed between TP and SSC in the outflow in both catchments, with no significant difference between them (Table 2). There was a strong relationship between TP and clay in suspended sediment in the Kinderveld catchment, but not in the Ganspoel catchment (Table 2). A fourth parameter, runoff discharge, was not well correlated to TP (Table 2).

View larger version (15K): [in this window] [in a new window]   Fig. 3. Relationship between total phosphorus (TP) of the suspended sediments and organic carbon content (OrgC).

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

The log-linear relationship between SSC and TP and/or particulate P was also reported by other authors (e.g., Schreiber and Rausch, 1979; Probst, 1985; Sharpley and Smith, 1990, 1991; Sharpley et al., 1991; Garbrecht and Sharpley, 1993; Hodun and Burt, 1997). The value of this negative exponent ranged between -0.1991 (this study) and -0.4 (Sharpley and Smith, 1990). These relationships are plotted in Fig. 4 in the range for which they were developed. This figure shows that TP and particulate P contents vary strongly between catchments. However, despite differences in soils and land use, as well as catchment hydrology and size, the relative rate at which P contents decrease with increasing SSC is more or less constant. Considering the strong linkage between P contents and sediment characteristics as described above, the relative variation of organic carbon content with increasing SSC should also be rather similar in all these catchments.

View larger version (23K): [in this window] [in a new window]   Fig. 4. Relationship between total phosphorus and particulate phosphorus (P) and suspended sediment concentration (SSC) from this study and from studies reported in the literature.

Sediment and Phosphorus Budgets for the Two Catchments
The catchments of Kinderveld and Ganspoel, which are situated within the same physiographic region and have similar land use, differ in TP and sediment export by a factor four to five. This difference may be explained by the spatial variability of the erosion processes. Wendt et al. (1986) and Nearing et al. (1999) have described differences in plot response during the same rainfall event. Although this can be an important factor at the plot scale, we believe that it was not the most important factor for our catchments because such small-scale variations may be expected to average out over larger areas. Differences in management practices or phosphorus application could possibly also be responsible for the observed differences. However, although the land is owned by different farmers, these reasons can be excluded as farmers in this area all apply the same agricultural practices.

Differences in topography may also affect the overall soil erosion risk in the catchment (Fig. 2). Simulations using a steady-state two-dimensional erosion–deposition model (WATEM) have shown that the water erosion risk is on average clearly higher in the Ganspoel catchment (11.4 Mg ha^-1) compared with the Kinderveld catchment (7.6 Mg ha^-1), which is due to the fact that in the Ganspoel catchment some very steep slopes are under cultivation (Van Oost et al., 2000). Therefore, this factor certainly does not explain the lower sediment yield observed in the Ganspoel catchment.

The differences in TP and sediment export between the two catchments must therefore be attributed to other factors. Extreme rainfall events and the spatial organization of land use in both catchments may have been the most important controls. The TP export from the Kinderveld catchment was highly influenced by the occurrence of an extreme event on 20 May 1997 with a recurrence interval of 10 yr (16 mm in 15 min; Demarée, 1985). This event accounted for almost 52% of the TP production over the whole observation period. Many authors have already pointed to the importance of storm periods in TP export (Dorioz and Ferhi, 1993; Frere et al., 1977; Pionke et al., 1996; Smith et al., 1993). As the SSC were very high during this event, its relative importance in the total sediment production (60%; 2325 Mg) was higher than in the TP production. Without this event, the sediment production in the Kinderveld catchment decreases to 1580 Mg, with a corresponding export rate of 2.5 Mg ha^-1 yr^-1. Total P export equals 2.4 kg ha^-1 yr^-1 without this extreme event. In the Ganspoel catchment, the rainfall event on 6 June 1998 yielded more than half of the sediment and TP production, but this event had only a recurrence interval of 2 yr and was thus much less extreme than that of the Kinderveld catchment (12.4 mm in 49 min; Demarée, 1985). The occurrence of important rainfall events is the major factor explaining the huge differences in sediment and TP export between different summer periods (Table 4).

Even after eliminating the extreme event in the Kinderveld catchment, the export rates for sediment and phosphorus are still twice as high as those in the Ganspoel catchment. Another factor that could explain differences in sediment and phosphorus export rates between the two catchments is the spatial organization of land use in the catchment. High suspended sediment export rates from catchments occur when there is a good connection between the erosion zones and the catchment outlet. Several factors, such as the presence of a road system, the development of rills and gullies, and the vegetation cover in the catchment, can influence this connectivity. In both catchments, the overall road density is comparable (37 m ha^-1 for the Kinderveld catchment and 32 m ha^-1 for the Ganspoel catchment). However, if only those roads are included that serve as runoff collectors (i.e., where water running off from adjacent fields is concentrated and subsequently evacuated to the catchment outlet), the density is much higher in the Kinderveld catchment: 24 m ha^-1, compared with 7 m ha^-1 for the Ganspoel catchment. Many subcatchments of the Kinderveld catchment deliver water and sediment directly to nearby roads or sunken lanes, and because the hydraulic roughness of these paved roads is low, the sediment produced on the parcels can then easily be transported to the catchment outlet. Moreover, different parts of the Kinderveld catchment come together in a sunken lane that delivers its water and sediment directly to the permanent brooklet where the measurement station was installed (Fig. 1). In the Ganspoel catchment, the measurement station is placed in a brooklet that is situated at the side of a rather flat and wide valley floor (Fig. 1). In this catchment the sunken lanes are not connected with the main erosion sources of the catchment. In addition, rills and gullies that develop on the steep valley slopes deposit the eroded sediment at the point where they enter the wide valley bottom (Fig. 5). As a result, the measured SDRs are much higher in the Kinderveld compared with those in the Ganspoel catchment (Table 3). If sediment deposition in the wide valley bottom of the Ganspoel catchment had not occurred, the SDR for the Ganspoel catchments would be much higher, resulting in a higher annual sediment and TP export (Table 3).

View larger version (41K): [in this window] [in a new window]   Fig. 5. Valley-floor sediment deposition downslope of eroded areas in the Ganspoel catchment after the event of 6 June 1998 (as measured by a global positioning system [GPS]). The map shows the lowest area of the catchment south of the brooklet (contour interval = 5 m).

View larger version (30K): [in this window] [in a new window]   Fig. 6. Histogram of vegetation cover distributions in the Kinderveld catchment during the winter periods of 1996–1997 and 1997–1998 (for each year measured in the month of February).

The reduced vegetation cover in the winter period resulted in higher SSC for similar discharges, as the soil surfaces were more susceptible to water erosion and sediment delivery was higher due to the absence of vegetation-induced sediment deposition (Steegen et al., 2000). However, total sediment and TP export in the winter period was much lower than in the summer period. In the Kinderveld catchment only 11% of the total sediment production and 16% of the TP export occurred during the winter period. In the Ganspoel catchment, the contribution of the winter period was even lower at 4% of the sediment export and 6% of the TP export. The reason for this different contribution is that during the winter rainfall intensities were much lower than during the summer, resulting in low runoff discharges and thus in a low erosion capacity of the overland flow.

The slightly higher contribution of the winter period in the TP export compared with the sediment export was due to the characteristics of the exported sediment. Although, for a given discharge, SSC is higher during the winter period than during the summer period, the highest SSC occurred during the summer period when the runoff discharges were highest (Steegen et al., 2000). Since higher SSC resulted in lower TP (Table 2), sediment export is relatively more important than TP export in summer periods compared with winter periods.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
We found that the sediment and TP export from small agricultural catchments can only be understood if the spatial organization of sediment-producing and deposition areas within the catchments is included in the analysis. Information on sediment-producing and depostion areas cannot be obtained from measurements at the catchment outlet only. These data have to be complemented by detailed field observations made within the catchment. This also implies that a direct extrapolation of the sediment and TP export measured at the outlet of an experimental catchment to a larger surface area may lead to considerable errors. These errors are partly due to the effect of extreme rain events and may to some extent be lessened by extending the measuring period.

Sediment and TP delivery to the catchment outlet were also strongly dependent on catchment morphology, the spatial organization of land use, as well as the connectivity between sediment-producing areas and the catchment outlet. The effect of these factors may be evaluated by the application of a spatially distributed erosion–sediment deposition model (e.g., Van Oost et al., 2000). If data are available, such a model may, after calibration, be applied to larger areas in order to obtain more appropriate estimates of sediment and TP export on a regional scale.

	ACKNOWLEDGMENTS

 
This research is part of a project supported by the Fund for Scientific Research Flanders (G0215-96), by the Katholieke Universiteit Leuven (OT 95/15), and by the European Union (FAIR CT95-0458). Their support is gratefully acknowledged. Furthermore, we want to thank M. Hens and P. L'Hoëst for their help with the phosphorus analysis.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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