The use of caesium-137 measurements to establish a sediment budget for the Start catchment, Devon, UK

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1 Hydrological Sciences-Journal-des Sciences Hydrologiques, 42(3) June The use of caesium-137 measurements to establish a sediment budget for the Start catchment, Devon, UK PHILIP N. OWENS, DESMOND E. WALLING, QINGPING HE, & JO SHANAHAN Department of Geography, University of Exeter, Exeter EX4 4RJ, UK IAN D. L. FOSTER Division of Geography, Coventry University, Coventry CV1 5FB, UK Abstract Caesium-137 ( l37 Cs) measurements have been used to investigate the delivery of sediment from the hillslopes to a lake which marks the downstream limit of the small agricultural Start basin in Devon, UK. Total 137 Cs inventories and 137 Cs depth distributions in sediment cores were used to estimate that the eroded sediment stored within the fields and on the flood plain of the main river was equivalent to sediment yields of ca. 21 and 30 t km" 2 year 1, respectively. Based on published information on sediment accumulation in the lake, the minerogenic sediment yield from the basin was estimated to be ca. 29 t km" 2 year 1. The erosion rate on the hillslopes in the basin, calculated as the sum of the sediment yield to the lake and the two storage components, is ca. 80 t km' 2 year'. Of the soil eroded from the slopes more than 60% is stored at intermediate locations and the sediment delivery to the lake is less than 40%. The calculated soil erosion rate for the Start basin is consistent with an estimate of the erosion rate for the basin of the nearby Old Mill Reservoir of ca. 63 t knr 2 year 4. Utilisation d'une technique de mesure par le césium-137 pour l'établissement du bilan sédimentaire du bassin versant "Start", dans le Devon (Royaume-Uni) Résumé Un processus de mesure utilisant le caesium-137 ( 137 Cs) a été employé afin d'étudier l'apport de sédiments provenant des versants du petit bassin versant agricole "Start" (Devon, Royaume-Uni) à un lac constituant sa limite aval. Des inventaires de 137 Cs et l'étude de sa distribution en fonction de la profondeur dans des carottes de sédiments ont permis d'estimer que l'apport de sédiments provenant de l'érosion retenus dans les champs et dans la plaine d'inondation de la rivière principale étaient respectivement égaux à 21 et 30 t knr 2 an" 1. Selon les informations publiées au sujet du stockage de sédiments dans le lac, l'apport de sédiments minéraux provenant du bassin a pu être évalué à 29 t knr 2 an" 1. Le taux d'érosion sur les versants du bassin, somme de l'apport des sédiments vers le lac et des deux composantes de stockage est donc de 80 t knr 2 an" 1. Plus de 60% du sol érodé est retenu sur des sites intermédiaires et l'apport de sédiments au lac est inférieur à 40%. Le taux d'érosion du sol calculé pour le bassin "Start" est comparable à celui d'un bassin voisin "Old Mill Reservoir" (Réservoir du Vieux Moulin) qui est de 63 tkm" 2 an" 1. INTRODUCTION Most existing sediment yield studies have been primarily concerned with calculating the output of sediment from the contributing drainage basin. Sediment yields are Open for discussion until 1 December 1997

2 406 Philip N. Owens et al. commonly estimated either by monitoring the flux of sediment passing a specific location in a river channel or by calculating the volume of sediment trapped over a known period of time in a lake or reservoir. Such studies have provided a wealth of information on the nature and magnitude of the sediment fluxes from drainage basins of different sizes and characteristics (Walling & Webb, 1996). Furthermore, investigations of temporal patterns of sediment yield have enabled the effects of intrinsic and extrinsic forcing variables to be examined, thus providing a means of predicting likely future changes in sediment transport associated with climatic and land use changes (Wolman, 1967; Foster, 1995; Walling, 1995). Increasingly, however, there has been a realization that such an approach, based on documenting sediment fluxes at the basin outlet, provides only a partial picture of sediment transfers in drainage basins. Many recent studies (Walling, 1983; Roberts & Church, 1986; Phillips, 1991) have shown that storage of mobilized sediment at intermediate locations within the basin, such as upslope of field boundaries, within river buffer zones, on the channel bed and on flood plains, can often be of a similar magnitude, or even greater, than documented yields. Downstream estimates of sediment yield will frequently underestimate upland rates of erosion. This situation is further complicated by the dynamic nature of sediment storage and remobilization (Trimble, 1995). Thus, under certain circumstances, the reworking of sediment from storage locations could increase sediment yields, while upland erosion rates remain constant or may even be declining (Trimble, 1983). Storage effects, therefore, can complicate the interpretation of sediment yields and may mask the true nature of sediment transfers in the upstream drainage basin. Sediment budget and routing studies provide an alternative and, by definition, more rigorous approach to the examination of catchment sediment dynamics (Dietrich & Dunne, 1978; Swanson et al, 1982; Phillips, 1991; Slaymaker, 1993; Walling & Quine, 1993). Sediment budgets quantify the mobilization, transport and storage of sediment in drainage basins or smaller units, while sediment routing studies focus on the overall movement of sediment through a series of landscape units (Swanson et al, 1982). At its most rigorous, a sediment budget requires the recognition and quantification of sediment sources, transport processes and storage elements and the identification of linkages between these, and this will require information on the magnitude and frequency characteristics of individual transport processes (Dietrich et al, 1982). Less rigorous sediment budget studies attempt to establish possible sediment sources, storage sites and transport rates in a drainage basin (Campo & Desloges, 1994). Although the sediment budget approach provides a useful framework for the examination of catchment sediment dynamics, it is often difficult to obtain the information necessary to establish a sediment budget at the catchment scale. Traditional approaches to documenting soil erosion and sediment deposition tend to suffer from a number of operational problems and temporal and spatial sampling constraints. The use of caesium-137 ( 137 Cs) measurements has been identified as an alternative approach to obtaining the necessary data (Walling & Bradley, 1990; Loughran et al, 1992). The advantage of using the 137 Cs technique is that it can be used to trace sediment movement within the catchment over a common time period (Ritchie & McHenry,

3 The use ofcaesium-137 measurements to establish a sediment budget ). This paper reports a study which uses 137 Cs measurements to quantify the delivery of sediment from the hillslopes to a lake basin which marks the downstream limit of a small (12.8 km 2 ) agricultural catchment in Devon, UK. This study uses 137 Cs measurements to identify sediment sources and estimate rates of soil erosion and within-field sediment storage, to provide information on rates and patterns of flood plain sedimentation, and to examine lake sedimentation within a small drainage basin. This information is then used along with published data on lake sediment-based sediment yields (O'Sullivan et al, 1991) to construct a tentative sediment budget for this catchment. The results are subsequently compared with historical sediment yield data reconstructed for the catchment of the nearby Old Mill Reservoir (Foster & Walling, 1994) in order to place the study basin in a regional context. THE CAESIUM-137 TECHNIQUE Caesium-137 measurements have been used extensively over the last two decades for tracing soil and sediment redistribution within the landscape and for dating lacustrine and other sediment (Ritchie & McHenry, 1990). The advantages of the 137 Cs technique for such studies is that a wealth of retrospective information can be obtained on medium-term (ca. last 40 years) rates and patterns of soil and sediment redistribution, on the basis of limited field work (i.e. the collection of soil and sediment samples as opposed to long-term monitoring). Although the 137 Cs technique is now well documented, the following section provides a brief review of the salient features of this radionuclide and of its potential geomorphological applications. Fallout 137 Cs is an artificial radionuclide with a half life of ca years, most of which was produced as a result of above-ground thermonuclear weapons testing in 160 cr 140- m Q. 120 o 100 o c o 80- o Q. CD T3 15 c < I JLEMjnSLB Year Fig. 1 Temporal variation of total annual bomb-derived 137 Cs fallout to the northern hemisphere. Fallout from the Chernobyl accident in 1986 is also illustrated (based on Cambray et al., 1989).

4 408 Philip N, Owens et al. the 1950s and 1960s. Radiocaesium was ejected into the stratosphere where it circulated globally. Deposition on the land surface was first recorded in the early 1950s and was primarily associated with precipitation. Fallout was greater in the northern hemisphere than in the southern hemisphere, because more atmospheric testing took place in the former, and was greatest in lower latitudes. Within latitudinal bands there is often a good relationship between the amount of 137 Cs fallout and precipitation. The temporal pattern of fallout is well documented in many regions of the world (Fig. 1), such that after a peak in fallout in 1963, levels have declined and, apart from the Chernobyl accident in 1986, negligible amounts of fallout have been recorded since the mid-1980s. In most environments, fallout 137 Cs reaching the land surface was rapidly and tightly sorbed to the fine fraction of surficial soils and sediments. Although over time 137 Cs may migrate slowly down the profile, in most cases the majority of the 137 Cs is retained in the top few centimetres of the soil or sediment profile (Owens et al., 1996). It is the known temporal pattern of 137 Cs fallout to the Earth's surface and its strong association with surficial sediment that enable 137 Cs measurements to be used effectively for dating and tracing purposes. Fallout 137 Cs has been widely used to date recent sediments from lacustrine, flood plain, wetland and other environments (Pennington et al., 1976; DeLaune et al, 1978) on the basis that the fallout input has a clearly defined temporal pattern (Fig. 1). The vertical distribution of 137 Cs in a sediment profile can, therefore, be related to the known record of fallout in that region. Thus, the deepest occurrence of 137 Cs in the profile can be approximately equated with the onset of 137 Cs fallout in the early 1950s, while peaks in activity concentration can be equated with peaks in fallout in 1959 and In areas that received radiocaesium fallout due to the Chernobyl incident in 1986, an additional marker horizon may exist which can also be employed for dating purposes. In water bodies that receive a considerable amount of sediment due to erosion from the contributing catchment, the shape of the 137 Cs profile will also reflect this input of 137 Cs and a more rigorous approach is required to interpret the 137 Cs depth profile (Walling & He, 1992; He et al., 1996). In the absence of strong variations in precipitation, over a relatively small area (i.e. a field or small basin) the total 137 Cs fallout can generally be assumed to be spatially uniform. The cumulative atmospheric deposition flux, including radioactive decay to the present, can be determined by measuring the total 137 Cs activity per unit surface area for a level, stable undisturbed site and this is usually referred to as the reference inventory (Sutherland, 1991; Owens & Walling, 1996). Sites that are experiencing soil erosion will have 137 Cs inventories lower than the reference inventory. Sites where deposition is occurring will be characterized by inventories greater than the reference inventory and deeper profile distributions, reflecting the addition of soil-associated 137 Cs to the surface of the pre-existing soil/sediment profile (Walling & Quine, 1990). Rates of soil erosion and deposition can be estimated by comparing the 137 Cs inventory at a particular sampling point with the local reference inventory. By using a suitable numerical model for either uncultivated or cultivated soil (Walling & Quine, 1990; Quine, 1995) it is possible to convert values of 137 Cs inventory to estimates of the mean erosion or deposition

5 The use of caesium-137 measurements to establish a sediment budget 409 rate occurring at the sampling point since the onset of,37 Cs fallout. A similar approach can be used to estimate rates and patterns of overbank sedimentation on flood plains (Walling & He, 1993). Caesium-137 measurements can also be used to identify the dominant sources of transported (i.e. fluvial suspended) sediment and recently deposited (i.e. flood plain and lacustrine) sediment by using the fingerprinting approach (Walling et al, 1993). In undisturbed soils most 137 Cs fallout accumulates in the surface horizons, giving high activity concentrations. In soils that have been cultivated, surface l37 Cs activity concentrations are considerably reduced as a result of the mixing of soil and 137 Cs within the plough layer. As most soil erosion processes remove soil from or close to the top of the profile, particles mobilized from uncultivated and cultivated fields will have different 137 Cs activity concentrations. Furthermore, the 137 Cs activity concentration of channel bank (and gully) source materials tends to be significantly lower than that associated with surface materials. This is because of the near-vertical surface geometry of channel banks and because, in most cases, much of the material delivered to the channel from bank erosion is likely to originate from below the depth to which 137 Cs fallout penetrates. Thus, by comparing the 137 Cs activity concentrations of mobilized or deposited sediment with typical concentrations for cultivated soils, uncultivated soils and channel bank material, and by correcting for differences in the particle size composition of source materials and sediment, it is possible to make quantitative statements regarding the likely source type and areas (He & Owens, 1995; Walling & Woodward, 1995; Wallbrink et al, 1996). STUDY AREA AND METHODOLOGY Study area The Start catchment is a small (12.8 km 2 ) agricultural basin located in the South Hams region of Devon, UK. (Fig. 2). It drains into Slapton Ley, a small (0.77 km 2 ) eutrophic coastal lake system which developed as a result of the shoreward movement of coastal sediment as post-glacial sea levels rose. The Ley is divided into two wetland areas: the Higher Ley (which is now largely a reed swamp); and the Lower Ley (which is an open water lake with a discontinuous reed fringe). The two water bodies are linked by an open water channel which carries water from the Higher Ley to the Lower Ley via Slapton Bridge. The outflow of the lake-catchment system is at the southern end of the Lower Ley. The Start catchment is the larger of the two catchments which drain into the Lower Ley, and drains about 30% of the total catchment area (46 km 2 ) of Slapton Ley. The catchment is composed of flat ridges which are dissected by steep valleys. The underlying geology is of Lower Devonian age with the coarser Meadfoot Beds overlying finer Dartmouth Slates. The soils are dominantly freely drained acid brown earths (Denbigh 1 Series), characterized by a clay loam texture with a high content of silt-sized particles (30-40%) and fragments of weathered slate, although typical brown podzolic soils of the Manod Series are found on the valley slopes and

6 410 Philip N. Owens et al. c r~"^ \ HALWELL/YN \ 1 Gara S^-^ 2 SiaptonWood S ^ 3 Start f 4 Stokeley Barton ' / i / 1 / \^~ l\r^< /\ 1 \ M \ \ U-~ J ( ( ^ J y Q3 Minor Drainage Basins m Open water / Wetland s~~- River -~ Drainage divide V. SLAPTON 0\ v «Higher Ley 0 km 1 \ 47/ START T BAY Jlower Ley 4 r<7 JTORCROSS Outflow Sluice Shingle and sand Rivers Coniferous trees Non coniferous trees Field boundaries Roads Marsh km Fig. 2 Location ((a) and (b)) and characteristics (c) of the catchments draining to Slapton Ley, and of the study area (d): (b) shows the approximate location of the Slapton Ley (i) and Old Mill Reservoir (ii) catchments; (d) shows the approximate location of the uncultivated (i) and cultivated (ii) fields examined in this paper. bottoms. Land use of the catchment above Deer Bridge is dominated by permanent and temporary pasture, with arable land accounting for about 30% of the area. The study area experiences a maritime climate. Mean annual precipitation at the nearby Slapton village for the period 1961 to 1993 was 1049 mm (although precipitation for the study area was probably slightly higher) and the mean annual runoff for the Start catchment is ca. 535 mm (Burt & Heathwaite, 1996).

7 The use of' caesium-137 measurements to establish a sediment budget 411 Methodology Most fieldwork was undertaken in 1991 and 1992, except in the case of a cultivated field from which samples were collected in Soil and flood plain sediment cores were collected using metal core tubes driven into the ground using a percussion hammer and removed with a portable winch and frame. Soil cores were collected to depths in excess of 40 cm using a metal core tube of 7.0 cm internal diameter. Most soil cores were bulked but some were sectioned into 1.0 or 2.0 cm increments in order to determine the depth distribution of 137 Cs. Long (0.8 m) cores of flood plain sediment were collected at three locations using an 11.2 cm internal diameter core tube. These cores were sectioned into increments of various thickness. Other (bulk) flood plain sediment cores were collected to depths of at least 60 cm using a metal tube of 7.0 cm internal diameter. A sub-sample of the bottom ca. 2 cm of sediment in each core was removed and analysed separately in order to ensure that the core contained all of the 137 Cs present in the profile. All samples were air-dried and ground prior to analysis by gamma spectrometry. Sediment cores were collected from the Lower Ley using a mini-mackereth corer equipped with 5.2 cm internal diameter plastic core tubes. The apparatus was deployed from a small inflatable boat. Upon return to the laboratory, the sediment was carefully extruded from the core tubes, cut into 2 cm increments, freeze-dried and prepared for gamma spectrometry. Radiocaesium measurements were undertaken using an HPGe co-axial y-ray detector coupled to a multi-channel analyser. The dried samples were gently ground with a mortar and pestle and passed through a 2 mm sieve before being placed in suitable containers for y-spectrometry. For the bulk samples, Marinelli beakers were used, while for the sectioned cores plastic pots, of similar diameter to the detector end cap, were used. Counting times were typically in the range to s. The detectors were calibrated with Standard Reference Materials and radionuclide standards. The absolute particle size composition of one of the flood plain cores was determined with a Malvern Mastersizer after removal of organic matter with hydrogen peroxide and chemical (sodium hexametaphosphate) and ultrasonic dispersion. RESULTS AND DISCUSSION Sediment sources, soil erosion and within-field storage Foster et al. (1996) used mineral magnetic measurements to demonstrate that the dominant source of sediment transported and deposited in the Start catchment since the 1970s is derived from the erosion of topsoil from agricultural fields, and that contributions from subsoil sources have been negligible. They also suggested that before the 1970s there was a greater contribution from subsoil sources, probably derived from channel bank erosion, since there is no evidence of gully erosion in the catchment.

8 412 Philip N, Owens et al. Although no suspended sediment samples have been collected from the study basin for use in sediment source fingerprinting (He & Owens, 1995; Walling & Woodward, 1995), the main contemporary source of sediment transported in the study basin can be inferred by comparing the 137 Cs content of the most recently deposited sediment on the flood plain surface with that of potential source materials (Table 1). Even allowing for the possible enrichment or depletion of 137 Cs in sediment deposited on the flood plain surface due to particle size effects (He & Walling, 1996), it is evident from Table 1 that the average B7 Cs content of the most recently deposited sediment reflects more closely the content of topsoil rather than subsoil sources. Furthermore, at present there is no visual evidence of major areas of channel bank erosion, although trampling of the bank by cattle may introduce sediment to the channel locally at certain times. Table Cs content of representative samples of topsoil (top 1 cm) from uncultivated and cultivated fields, channel bank material and of surface samples of flood plain sediment. Sample Uncultivated topsoil Cultivated topsoil Channel bank Flood plain sediment 137 Cs content (mbq g" 1 ) The range for bank sediment is for banks between 0.3 and 1.0 m in height and is based on sampling and theoretical considerations. The flood plain sediment has been corrected for the mass of organic material produced in situ on the flood plain surface (ca. 30% of the mass of the sediment samples) but has not been corrected for differences in the particle size composition of sediment and potential sources materials. As described above, the topography of the study area is characterized by ridges which are dissected by valleys. Fields located on the ridges are generally flat and located away from channels and are, therefore, unlikely to contribute a significant amount of eroded sediment to the main channel network. However, the fields on the valley sides are typically very steep and located close to the channels, and are likely to be characterized by both high rates of soil loss and potentially high sediment delivery to the channel network. Furthermore, most of the fields in the study catchment are characterized by the presence of lateral hedge boundaries, which at the base of steep valley slopes are known to impede the movement of sediment from the field and consequently are often associated with zones of net sediment storage immediately upslope. In order to estimate rates of soil loss from the hillslopes and the sediment delivery to the channel, attention was directed towards the steep valley slopes of the main Start stream and sediment cores were collected from a pasture and a cultivated field in this valley (the pasture field is described in detail in Foster et al. (1996)). Although only two fields were examined, they are representative of the steep valley slopes in this catchment. In the cultivated field (1.5 ha), 24 bulk sediment cores were collected in 1996 (see Fig. 2 for location). The field is characterized by a ca. 3 m wide zone of sediment storage which lies upslope of a hedge boundary at the base of the field. Multiple sediment cores collected in 1992 from areas of flat,

9 The use ofcaesium-137 measurements to establish a sediment budget 413 non-eroding, undisturbed pasture in the Start catchment were used to calculate a local reference inventory of 261 mbq cm 2 which, taking account of radioactive decay, is equivalent to a value of ca. 240 mbq cm 2 in Caesium-137 inventories for each of the bulk cores were converted to estimates of net soil redistribution rate at the individual sampling points using a numerical mass-balance model (Owens, 1994) and these are presented in Fig. 3. Of the 24 cores collected, eight have 137 Cs inventories less than the reference inventory indicating net soil loss, while 16 have higher 137 Cs inventories, indicating net soil accumulation. Although areally this field is dominated by net sediment accumulation, six of the cores were collected within the very narrow sediment storage zone at the base of the slope which only occupies about 3 % of the sampled m Fig Cs~based estimates of soil redistribution for an area within a steep cultivated field in the Start catchment (see Fig. 2 for location).

10 414 Philip N. Owens et al. area. The average rate of soil loss for the area of the field experiencing erosion (42%) was 19 t ha 4 year 1 while the average rate of sediment deposition for the area of the field experiencing accumulation (minus the zone at the base of the slope) (55%) was 5.3 t ha" 1 year" 1. The average sediment accumulation rate in the storage zone (3% of field) was > 100 t ha" 1 year" 1. For the field as a whole, the average rate of soil erosion was ca. 8 t ha" 1 year" 1, of which the equivalent of ca. 6 t ha" 1 year" 1 of sediment was deposited within the field {ca. 50% of this was focused in the storage zone at the base of the slope). Thus, in this example, sediment redistribution within the field appears to be dominant and only ca. 25% (2 t ha" 1 year" 1 ) of the soil mobilized by erosion was exported from the field towards the channel network. The values presented above provide a useful complement to data presented in Foster et al. (1996) for sediment accumulation in a storage zone upslope of a hedge boundary in a steep pasture field on the opposite side of the valley (Fig. 2). Although no attempt was made to examine soil redistribution within this field in detail, the 137 Cs inventories of three cores collected from this storage zone (average 410 mbq cm" 2 ) were used to estimate that approximately 60 t of sediment had been deposited upslope of the hedge boundary since Foster et al. (1996) estimated that this storage was equivalent to a mean yield from the entire field of 0.16tha"' year" 1. As with the cultivated field above, these results suggest that a significant amount of the soil mobilized by erosion processes is stored within the field. In the case of the pasture field described by Foster et al. (1996), the estimate of sediment storage was based on only a small zone at the base of the slope that occupied ca. 1 % of the area of the field, and the total amount of sediment stored in the field as a whole could be considerably greater than the equivalent yield of 0.16 t ha" 1 year 1. In the cultivated field (Fig. 3), sediment deposition in the storage zone upslope of the hedge boundary only accounted for ca. 50% of the total sediment storage in the field. However, it is becoming increasingly recognized that for cultivated fields tillage operations may result in enhanced soil redistribution. Quine et al. (1993) estimated that tillage displacement was responsible for 34% of the soil erosion and 65% of the soil aggradation in fields near Xifeng, China. A significant proportion of the within-field storage evidenced by the cultivated field could therefore represent soil mobilized by tillage. Flood plain storage The flood plain of the Start stream has a history of inundation during winter storms that has resulted in flooding of local roads and the development of a wetland environment. In turn, this flooding has resulted in the accumulation of overbank deposits. Fig. 4 maps the present day and historical extent of flooding and sedimentation as represented by the area of wetland depicted on a series of Ordnance Survey maps since Fig. 4 demonstrates that whereas wetland was found only downstream of Deer Bridge in 1889, it currently extends over 1 km upstream. Historical records also document a significant and sustained increase in water level in the Ley since 1920, which may have been partly responsible for the expansion of the area of

11 The use of caesium-137 measurements to establish a sediment budget 415 u c 3 c o O U) O" CD E, ci o <n O (LUO) mcteq

12 416 Philip N. Owens et al. wetland after this time. However, there is evidence for increased soil erosion in the post-war period which coincided with a phase of agricultural intensification and, in particular, an increase in stocking densities (Heathwaite & Burt, 1993). This could also account for the rapid expansion of the wetland area and, thus, the area inundated by flooding between 1955 and 1965 (Fig. 4). In order to determine the magnitude and extent of sediment storage on the flood plain of the main Start stream, sediment cores were collected from the locations shown on Fig. 4 using a nested sampling strategy. Initially, a series of bulked cores were collected upstream of Deer Bridge at intervals of between 200 and 300 m, in order to provide information on the extent of sedimentation. One area was subsequently selected for more detailed examination and, within this area of ca. 40 m x 100 m, three sectioned cores and 24 bulk cores were collected to provide detailed information on the depth distribution of 137 Cs and the spatial pattern of overbank sedimentation within a relatively small area. The aim was to scale-up this information to the entire area of the flood plain. Figure 4 presents the total inventories for the bulked cores and the 137 Cs depth profiles and inventories for the three sectioned cores. The total inventories for all of these cores range between 321 and 795 mbq cm" 2, with the lowest values occurring at the upstream and downstream limits of the sampled area. The variation in the magnitude of the inventories partly reflects the sampling location relative to the channel and the flood plain topography, which influence sedimentation rates and the 137 Cs content of the deposited sediment (Walling & He, 1993). In all cases, 137 Cs inventory values are considerably greater than the local reference inventory for the time the cores were collected. The excess I37 Cs (the total inventory minus the reference inventory) reflects the 137 Cs associated with suspended sediment deposited on the flood plain during overbank flows. In the three sectioned cores 137 Cs occurs to depths in excess of 30 cm (Fig. 4). The depth at which 137 Cs reaches a maximum activity in the flood plain sediment core provides a chronological marker which can be equated with the peak of atmospheric fallout in 1963 (Fig. 1). Figure 4 also provides information on the clay (to which most 137 Cs is sorbed) content of the sediment in core c, and this can be seen to be approximately constant with depth. There is no evidence of changes in the clay content of the sediment which might otherwise complicate any interpretation of the location of the peak in 137 Cs concentration. However, in most soils and sediments a net downward migration of 137 Cs associated with internal processes such as bioturbation, leaching, translocation etc. can occur, and this should be taken into account in order to avoid overestimating the sedimentation rate. Based on data presented by Owens et al. (1996), the 137 Cs migration rate is assumed to be 0.10 cm year" 1 or 0.04 g cm' 2 year" 1. The average deposition rates between 1963 and 1991 (the time the cores were collected) for cores a, b and c, respectively, are ca. 0.94, 0.77 and 1.26 cm year" 1 (ca. 0.17, 0.16 and 0.54 g cm" 2 year" 1 ). Figure 5 shows the spatial variation of the 137 Cs inventories and surface concentrations associated with the samples collected from the area of intensively sampled flood plain illustrated in Fig. 4. In all cases, the total 137 Cs inventories of the bulk cores are greater than the local reference inventory, confirming the evidence

13 The use of caesium-137 measurements to establish a sediment budget All

14 418 Philip N. Owens et al. provided by the three sectioned cores described above that there has been a considerable amount of sediment deposition within this area during periods of overbank flow. The values for the 137 Cs inventory and the 137 Cs content of the surface sediment at each sampling location have been used to estimate the sedimentation rate at each sampling location on the flood plain using the procedure described in Walling & He (1993). The estimated sedimentation rates presented in Fig. 5 represent mean rates since the start of 137 Cs atmospheric fallout in 1954 (Fig. 1). Sedimentation rates range between <0.10 and >0.60 g cm 2 year 1, and the highest rates occur at the edge of the channel and generally decrease with distance from the channel. The mean sedimentation rate for the sampled area is 0.31 g cm" 2 year 1 or 31 t ha 4 year" 1. The total area of sedimentation above Deer Bridge extends to 11 ha (Fig. 4), and therefore the average mass of suspended sediment stored on the flood plain since 1954 has been ca. 340 t year" 1. These estimates of sedimentation rate agree well with information presented by Owens (1990), where the volume of recent sediment deposited during overbank events was estimated by simple surveying techniques. Seven cross-sections of the valley between Deer Bridge and the upstream limit of the flood plain were used to calculate an average sedimentation rate of between 0.80 and 2.6 cm year" 1 (the range represents the uncertainty associated with the onset of sedimentation) which is similar to that estimated above based on the 137 Cs depth distributions for three flood plain cores. Owens (1990) estimated a total storage of sediment on the flood plain of between 341 and 1136 t year" 1, which is equivalent to a sediment input from the upstream catchment area of between 30 and 95 t km" 2 year" 1 (Foster et al., 1996). Here, we use the lower value as this is more consistent with the flood plain 137 Cs data described above. Deer Bridge i37cs (mbq g- 1 ) Core location and 137 Cs inventory (mbq cm" 2 ) km Fig Cs inventories for three sediment cores and l37 Cs depth distribution for core a from Slapton Lower Ley.

15 The use of caesium-137 measurements to establish a sediment budget 419 Lacustrine sedimentation In order to investigate the accumulation of sediment derived from the Start catchment in the Lower Ley, three cores of the lake-bottom sediment were collected from the bay into which the Start stream flows. Fig. 6 illustrates the total 137 Cs inventories of these cores and the depth distribution of 137 Cs in core a. In core a there is an increase in the 137 Cs content of sediment towards the sediment-water interface and the majority of 137 Cs is contained in the top 18 cm. The 137 Cs inventories of the three cores are similar and are considerably lower than the local reference inventory (261 mbq cm 2 ). However, the l37 Cs inventories of the lake sediment cores cannot be directly compared with the local reference inventory because not all 137 Cs input to the lake surface from the atmosphere will be deposited on the lake bed and because 137 Cs may not be deposited uniformly over the lake bed due to sediment focusing. However, both the 137 Cs depth profile for core a and the low 137 Cs inventories for the three cores indicate that net sediment accumulation on the lake bed since the onset of 137 Cs fallout in the 1950s has been limited. As well as the low delivery of sediment from the Start catchment due to intermediate storage within fields and on the flood plain, the lack of sediment accumulation in the Lower Ley partly reflects the behaviour of sediment in the lake. The trap efficiency of the lake has been estimated to be 76% and therefore not particularly high and, because of the very shallow nature of the water body (mean depth 1.55 m), wind generated currents are likely to mix and resuspend the bottom sediment (and associated I37 Cs), which may be subsequently refocused or lost from the lake at the outflow. Although in this case it is not possible to use the 137 Cs measurements of the sediment to provide an estimate of the sediment yield to the lake from the Start catchment, the total sediment flux to the Lower Ley has been estimated by Foster et al. (1996) by recalculating data presented in O'Sullivan et al. (1991). Foster et al. (1996) calculated that the average specific sediment yield to the Lower Ley for the period 1977 to 1987 from the two catchments draining into the Lower Ley (Start and Stokeley Barton; Fig. 2) was 29 t km" 2 year 4. A SEDIMENT BUDGET It is possible to use the information presented above to construct a tentative sediment budget for the Start catchment. The sediment yield to the lake is calculated from published data, whereas 137 Cs measurements are used to estimate the amount of sediment stored on the flood plain and within the steep fields in the main valleys. The amount of soil lost from the fields is assumed to equal the sum of these three components. At this stage, no estimate has been made of within-channel sediment storage (Sutherland, 1990). As 137 Cs measurements have been used to investigate most of the components, the time period over which rates of sediment redistribution have been estimated are similar (i.e. mid 1950s to the early or mid 1990s). Also, the data are likely to be temporally representative as they represent medium-term averages for a ca. 40 year period. The only exception is the sediment yield to the

16 420 Philip N. Owens et al. lake, which is an average value for the period 1977 to The average sediment yield to the Lower Ley from the contributing area for the period 1977 to 1987 was ca. 29 t km" 2 year 1. This value is approximately equivalent to the estimate of the amount of sediment stored on the flood plain, which is equivalent to a yield from the upstream catchment of ca. 30 t km" 2 year" 1. The amount of sediment stored within fields, particularly upslope of hedge boundaries, is more difficult to estimate. Here, the storage of sediment within fields is defined as that soil which has accumulated in the zone immediately upslope of hedge boundaries, as in the absence of these artificial boundaries this sediment would be exported towards the channel network. Upslope of this storage zone, soil is redistributed within the field (i.e. from areas of net erosion to areas of net accumulation) but in the absence of the hedge boundaries this material would not be exported from the field. In the cultivated field as a whole, sediment deposition was equivalent to ca. 6 t ha" 1 year" 1 (600 t km" 2 year" 1 ), although much of this could be ascribed to soil redistribution by tillage, and the storage of sediment in the zone of accumulation upslope of the hedge boundary (i.e. that sediment stored due to the effect of the hedge boundary) was estimated to be equivalent to ca. 3 t ha" 1 year" 1 (300 t km" 2 year" 1 ). For the pasture field, the storage of soil immediately upslope of the hedge boundary was estimated to be equivalent to a yield from the entire contributing area of 0.16 t ha 1 year 1 (16 t km" 2 year" 1 ). As discussed earlier, the majority of the catchment is composed of flat ridges which occupy about 70% of the catchment, and in these areas there is little visual evidence for significant sediment storage upslope of hedge boundaries, whereas there is considerable sediment storage upslope of hedge boundaries in fields on the steep valley slopes. The majority of the land use in the steep valleys is pasture (ca. 60%) and cultivated land occupies no more than 20% of the land at any one time (the remainder is woodland). Therefore, for the catchment as a whole, the areaweighted value for within-field storage has been estimated at ca. 21 t km" 2 year" 1. This value is, however, sensitive to the assumptions upon which it is based, particularly the area of the catchment occupied by steep valleys and the percentages of land use. Furthermore, the land use values for the valley slopes are for the present day and are not average values since the 1950s (the onset of 137 Cs fallout), and this may result in an underestimation of the mean field storage since this time as Heathwaite & Burt (1993) have documented a steady decrease in the amount of cultivated land in the catchments of Slapton Ley since the late 1930s. Figure 7 provides a schematic representation of the sediment budget for the Start catchment. If it is assumed that the average amount of sediment lost from fields due to soil erosion processes is the sum of the yield to the lake and the amount of sediment stored upslope of hedge boundaries and on the flood plain, this value is ca. 80 t km" 2 year" 1. It is estimated that ca. 26% and ca. 38% of the material mobilized by erosion processes is stored within fields and on the flood plain, respectively, and that the sediment delivery ratio associated with the sediment yield to the lake is ca. 36%. These figures represent revised values based on additional data and therefore differ slightly from those presented in Foster et al. (1996), although the conclusions and implications of both studies remain the same. The sediment delivery ratio for the Start catchment is low when compared to values cited for some catchments

17 The use oj'caesium-}'37 measurements to establish a sediment budget Field storage > 26% ' ' > Floodplain storage 38% Sediment yield to lake 36% Fig. 7 Sediment budget for the Start catchment. (Sutherland, 1990; Walling & Quine, 1993), but it is comparable to that estimated for other basins where sediment storage has been identified as an important component of the sediment budget (Trimble, 1983; Roberts & Church, 1986; Phillips, 1991). The sediment budget estimated for the Start catchment is consistent with data for the catchment of the nearby Old Mill Reservoir (Fig. 2), which has a similar land use, lithology and climate. Foster & Walling (1994) used the sediment deposited in the reservoir to estimate that the average sediment yield to the reservoir for the period 1954 to 1992 was ca. 63 t km 2 year 1, and that the yield for the last 15 years was ca. 90 t km" 2 year" 1. In the Old Mill catchment there is only a very limited amount of flood plain and the amount of sediment stored upslope of hedge boundaries is also likely to be less than in the Start catchment because of differences in field size between the two basins. CONCLUSION Caesium-137 measurements have been used along with published sediment yield data for the Lower Ley to quantify the delivery of sediment from the hillslopes to the lake and consequently establish a sediment budget for the Start catchment. Of the soil eroded from fields, more than 60% is stored immediately upslope of artificial hedge boundaries at the base of steep slopes in the main valleys and also on the flood plain of the Start stream. The sediment delivery to the lake is estimated to be less than 40%. The dominance of sediment storage at intermediate locations in the sediment budget of this catchment has important implications for the interpretation of the sediment dynamics (and also sediment-associated contaminants) of this catchment (Foster et al., 1996), which might otherwise have been misinterpreted if only the sediment yield data at the catchment outlet had been considered. There is clearly a need for more studies of this type to be performed in order to understand the response of sediment mobilization, transport and deposition processes at the catchment scale to intrinsic and extrinsic forcing variables, and the use of 137 Cs measurements offers considerable promise in this context.

18 422 Philip N. Owens et al. Acknowledgements Most of the work presented in this paper was undertaken while P. N. Owens and J. Shanahan were in receipt of UK NERC postgraduate studentships and Q. He was in receipt of a University of Exeter postgraduate scholarship and an ORS award. The authors would like to thank S. Bradley and T. Quine for assistance with preliminary fieldwork, and H. Jones, T. Bacon (Exeter University) and the Cartographic Section, Division of Geography, Coventry University, for producing the diagrams. K. Chell and local landowners are also gratefully acknowledged for allowing access to Slapton Ley and the study sites. MEKERENCES Burt, T. P. & Heathwaite, A. L. (1996) The hydrology of the Slapton catchments. Field Studies 8, Cambray, R. S., Playford, K., Lewis. G. N. J. & Carpenter, R. C. (1989) Radioactive fallout in air and rain: results to the end of UKAEA Report AERE-R HMSO, London. Campo, S. H. & Desloges, J. R. (1994) Sediment yield conditioned bv glaciation in a rural agricultural basin of southern Ontario, Canada. Phys. Geog. 15, DeLaune, R. D., Patrick, Jr., W. H., & Buresh, R. J. (1978) Sedimentation rates determined by '"Cs dating in a rapidly accreting saltmarsh. Nature (London) 27S, Dietrich, W. E. & Dunne. T. (1978) Sediment budget for a small catchment in moutainous terrain. Z. Geomorphol. Suppl. Bd 29, Dietrich, W. E., Dunne, T., Humphrey, N. F. & Reid, L. E. (1982) Construction of sediment budgets for drainage basins. In: Sediment Budgets and Routing in Forested Drainage Basins ed. by F. J. Swanson, R. J. Janda. T. Dunne & D. N. Swanston, USDA'Forest Service General Technical Report PNW-141, Portland, USA. Foster, I. D. L. (1995) Lake and reservoir bottom sediment as a source of soil erosion and sediment transport data in the OK. In: Sediment and Water Quality in River Catchments ed. by I. D. L. Foster, A. M. Gurnell & B. W. Webb, Wiley, Chichester, UK. Foster, I. D. L. & Walling, D. E. (1994) Using reservoir deposits to reconstruct changing sediment yields and sources in the catchment of the Old Mill Reservoir, South Devon, UK, over the past 50 years. Hydrol. Sri. J. 39, Foster, I. D. L., Owens, P. N. & Walling, D. E. (1996) Sediment yields and delivery in the catchments of Slapton Lower Ley, South Devon, UK. Field Studies 8, He, Q. & Owens, P. (1995) Determination of suspended sediment provenance using caesium-137, unsupported lead-210 and radium-226: a numerical mixing model approach. In: Sediment and Water Quality in River Catchments, ed. by I. D. L. Foster, A. M. Gurnell & B. W. Webb, Wiley, Chichester, UK. He, Q. & Walling, D. E. (1996) Interpreting particle size effects in the adsorption of l37 Cs and unsupported 2 "'Pb by mineral soils and sediments. J. Environ. Radioactivity 30, He, Q., Walling, D. E. & Owens, P. N. (1996) Interpreting the l3; Cs profiles observed in several small lakes and reservoirs in southern England. Chem. Geol. 129, Heathwaite, A. L. & Burt, T. P. (1993) The evidence for past and present erosion in the Slapton catchment, southwest Devon. In: Past and Present Soil Erosion ed. by M. Bell & J. Boardman Oxbow Monograph 22, Oxford, UK. Loughran, R. J., Campbell, B. L., Shelley, D. J. & Elliott, G. L. (1992) Developing a sediment budget for a small drainage basin in Australia. Hydrol. Process. 6, O'Sullivan, P. E.. Heathwaite, A. L., Appleby, P. G., Brookfield. D.. Crick, M. W., Moscrop, C, Mulder, T. P., Vernon, N. J. & Wilmhurst, J. M. (1991) Palaeolimnology of Slapton Ley. Devon. Rydrobiologia 214, Owens, P. N. (1990) Valley sedimentation at Slapton, South Devon, and its implications for the estimation of lake sediment-based erosion rates. In: Soil Erosion on Agricultural Land. ed. bv J. Boardman, I. D. L. Foster & J. A. Dearing Wiley, Chichester, UK. Owens, P. N. (1994) Towards improved interpretation of caesium-137 measurements in soil erosion studies. PhD Thesis, University of Exeter, UK. Owens, P. N. & Walling, D. E. (1996) Spatial variability of caesium-137 inventories at reference sites: an example from two contrasting sites in England and Zimbabwe. Appl. Radiât. Isot. 47, Owens, P. N.. Walling, D. E. & He, Q. (1996) The behaviour of bomb-derived caesium-137 fallout in catchment soils. J. Environ. Radioactivity 32, Pennington, W., Cambray, R. S., Eakins, J. D. & Harkness, D. D. (1976) Radionuclide dating of the recent sediments of Blenham Tarn. Freshwater Biol. 6, Phillips, J. D. (1991) Fluvial sediment budgets in the North Carolina Piedmont. Geomorphology 4, Quine, T. A. (1995) Estimation of erosion rates from caesium-137 data: the calibration question. In: Sediment and Water Quality in River Catchments ed. bv I. D. L. Foster, A. M. Gurnell & B. W. Webb Wiley, Chichester, UK. Quine, T. A., Walling, D. E. & Zhang, X. (1993) The role of tillage in soil redistribution within terraced fields on the

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