PUBLICATIONS. Water Resources Research

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1 PUBLICATIONS Water Resources Research RESEARCH ARTICLE 1.12/215WR18136 Key Points: Beryllium-7, a fallout radionuclide, has potential for studying eventbased, soil redistribution Existing approaches to its use involve major constraints. A new approach is described The new approach is successfully validated at the catchment scale Validating a mass balance accounting approach to using 7 Be measurements to estimate event-based erosion rates over an extended period at the catchment scale Paolo Porto 1,2, Des E. Walling 2, Vanessa Cogliandro 3, and Giovanni Callegari 4 1 Dipartimento di Agraria - Universita degli Studi Mediterranea di Reggio Calabria, Feo di Vito, Reggio Calabria, Italy, 2 Geography, College of Life and Environmental Sciences, University of Exeter, Exeter, UK, 3 Dipartimento per la Innovazione nei Sistemi Biologici, Agroalimentari e Forestali Universita della Tuscia, Viterbo, Italy, 4 CNR Istituto per i Sistemi Agrari e Forestali per il Mediterraneo, Sezione Ecologia e Idrologia Forestale, Rende (Cs), Italy Correspondence to: P. Porto, paolo.porto@unirc.it Citation: Porto, P., D. E. Walling, V. Cogliandro, and G. Callegari (216), Validating a mass balance accounting approach to using 7 Be measurements to estimate event-based erosion rates over an extended period at the catchment scale, Water Resour. Res., 52, , doi:1.12/215wr Received 23 SEP 215 Accepted 8 JUN 216 Accepted article online 13 JUN 216 Published online 1 JUL 216 Abstract Use of the fallout radionuclides cesium-137 and excess lead-21 offers important advantages over traditional methods of quantifying erosion and soil redistribution rates. However, both radionuclides provide information on longer-term (i.e., 5 1 years) average rates of soil redistribution. Beryllium-7, with its half-life of 53 days, can provide a basis for documenting short-term soil redistribution and it has been successfully employed in several studies. However, the approach commonly used introduces several important constraints related to the timing and duration of the study period. A new approach proposed by the authors that overcomes these constraints has been successfully validated using an erosion plot experiment undertaken in southern Italy. Here, a further validation exercise undertaken in a small (1.38 ha) catchment is reported. The catchment was instrumented to measure event sediment yields and beryllium-7 measurements were employed to document the net soil loss for a series of 13 events that occurred between November 213 and June 215. In the absence of significant sediment storage within the catchment s ephemeral channel system and of a significant contribution from channel erosion to the measured sediment yield, the estimates of net soil loss for the individual events could be directly compared with the measured sediment yields to validate the former. The close agreement of the two sets of values is seen as successfully validating the use of beryllium-7 measurements and the new approach to obtain estimates of net soil loss for a sequence of individual events occurring over an extended period at the scale of a small catchment. VC 216. American Geophysical Union. All Rights Reserved. 1. Introduction Concern for problems of soil degradation and the sustainability of the soil resource [e.g., Montgomery, 27] as well as increasing recognition of the wide-ranging off-site impacts of soil erosion and the role of fine sediment in degrading aquatic ecosystems [e.g., Clark et al., 1985; Newcombe and MacDonald, 1991; Waters, 1995; Wood and Armitage, 1997; Kemp et al., 211] have created a growing need for information on rates of soil loss and soil redistribution. Sediment budgets are increasingly seen as a key tool for informing the design and implementation of sediment management and control programs in watersheds [Walling and Collins, 25; Gellis and Walling, 211] and information on rates of sediment mobilization and redistribution on the slopes of a watershed is a fundamental requirement for establishing a watershed sediment budget [Porto et al., 213]. This growing need for information on rates of soil loss and soil redistribution has directed attention to the development of novel approaches to documenting soil erosion and redistribution rates to replace or supplement more traditional approaches. The use of fallout radionuclides, and particularly cesium-137 ( 137 Cs) as a basis for documenting erosion and soil redistribution rates within fields or on watershed slopes has received considerable attention over the past ca. 3 years and the approach has been successfully used in many areas of the world [Ritchie and Ritchie, 25] and has been promoted by the International Atomic Energy Agency (IAEA) [see Zapata, 22; International Atomic Energy Agency (IAEA), 211]. Cesium-137 measurements provide an essentially unique means of obtaining spatially distributed and time-integrated information on soil redistribution rates [Walling and Quine, 1995]. Because the primary source of 137 Cs fallout in most areas of the world was the atmospheric testing of nuclear weapons in the late 195s and early 196s, 137 Cs measurements are primarily used to provide estimates of mean annual soil PORTO ET AL. VALIDATING A MASS BALANCE APPROACH TO USING 7 BE MEASUREMENTS 5285

2 1.12/215WR18136 redistribution rates for the period extending from the late 195s to the time of sampling. Excess lead-21 ( 21 Pb ex ) has also been used in a similar way to 137 Cs to obtain estimates of longer-term erosion and soil redistribution rates [Benmansour et al., 211; Matisoff and Whiting, 211; Porto and Walling, 212a,b; Walling and He, 1999; Walling, 22, 21]. Since this fallout radionuclide is of natural geogenic origin, its fallout can be seen as essentially continuous and its half-life of 22.3 years means that it can be used to obtain estimates of mean annual rates of soil redistribution extending back over ca. 1 years. One important limitation of 137 Cs and 21 Pb ex is their inability to provide estimates of soil redistribution rates over shorter timescales and particularly the event scale. Another fallout radionuclide, beryllium-7 [see Kaste et al., 22] with a half-life of 53 days has been seen as offering the potential to fulfil this need [Mabit et al., 28], but to date this potential has not been fully exploited [see Walling, 213]. Most existing attempts to use this radionuclide have followed the approach commonly used with 137 Cs. This involves comparison of the inventories sampled across the study area, after an erosion event, with a reference inventory obtained for an adjacent site with no evidence of erosion or deposition [e.g., Blake et al., 1999; Navas et al., 28; Schuller et al., 26, 21; Sepulveda et al., 28; Walling et al., 1999]. This approach introduces important constraints associated with ensuring that the inventories found at the reference site and the study area are the same prior to the event under investigation and that the inventories across the study area prior to the event are uniform and are not influenced by previous erosion events characterized by spatially variable soil redistribution, which would cause them to vary. These constraints effectively mean that use of the approach is restricted to events occurring after a long period with limited rainfall or after cultivation of the study area and to the first event in a sequence of events. This severely restricts its application. Attention has been directed to overcoming these constraints and Walling et al. [29] developed and successfully applied an approach that permits the period investigated to be extended through the wet season (i.e., for several months). However, the need for the first event to follow a long dry period or cultivation of the study area remains. Furthermore, the approach involves apportioning the total soil redistribution for the study period estimated for each sampling point on the basis of the contribution of each event or daily rainfall total to the total erosivity of the rainfall for the study period. This cannot take account of changes in sediment availability (i.e., exhaustion effects) through the study period. More recently, alternative approaches have been described and successfully applied by Porto and Walling [214] and Jha et al. [215]. In these a mass balance accounting procedure has been used to model changes in the inventory at each sampling point through time. This removes the need for comparison of the measured inventory with the reference inventory measured at the reference site. In essence, a reference inventory for an event is modeled for each sampling point based on the inventory measured after the previous event and the fallout received prior to and including the next event. Further details of the approach adopted by Porto and Walling [214] are provided below. Those authors successfully validated the approach using measurements undertaken on an erosion plot in southern Italy. They demonstrated close agreement between the soil loss associated with each event estimated using 7 Be measurements and that measured at the base of the plot. The study reported below reports a further attempt to validate the approach, at a larger scale and involving a small catchment located in southern Italy. 2. Using a 7 Be Mass Balance Accounting Approach to Document Soil Erosion Rates The basis for using 7 Be measurement to document erosion rates is similar to that for other fallout radionuclides [Mabit et al., 28], although attention focuses on individual events or short periods with several spells of heavy rainfall. Due to its short half-life, most of the 7 Be fallout is fixed within the upper ca. 1 cm of the soil and the inventory is soon reduced by decay. The radionuclide commonly exhibits a well-defined exponential depth distribution and if erosion or deposition occurs the deviation of the measured inventory A (Bq kg 21 ), relative to that of a local reference site A ref (Bq kg 21 ) can be used in combination with the depth distribution to estimate the depth or amount of soil involved. More specifically, the vertical distribution of 7 Be in the soil profile at a reference site can be represented by the following exponential function [see Walling et al., 1999] CðxÞ5CðÞ e 2x=h (1) where x 5 mass depth from soil surface (kg m 22 ); C(x) 5 the mass activity density or concentration (Bq kg 21 )of 7 Be at depth x; C() 5 the mass activity density or concentration (Bq kg 21 ) of the surface soil (at PORTO ET AL. VALIDATING A MASS BALANCE APPROACH TO USING 7 BE MEASUREMENTS 5286

3 1.12/215WR18136 x 5 ); h 5 relaxation depth (kg m 22 ) describing the profile shape and representing the mass depth at which C(x) reduces to 1/e C(). The mass of soil per unit area eroded from the sampling point, R (kg m 22 ), can therefore be calculated as R5h Ln A ref (2) A where size selective erosion occurs, the mobilized soil may be enriched in fines and therefore 7 Be relative to the parent soil. Failure to take this into account will result in overestimation of the mass of soil eroded. Following Walling et al. [211], a correction factor P can be incorporated into equation(2),where P represents the ratio of the 7 Be activity in mobilized sediment to that in the original soil. P is likely to be 1. R5 h P Ln A ref (3) A Because of difficulties associated with establishing the 7 Be activity of mobilized sediment and the surface soil (the latter will vary according to the depth sampled), P is usually estimated by comparing the particle size composition or the specific surface area of the mobilized sediment with that of the surface soil [see Walling et al., 211; Taylor et al., 213]. The close relationship between changes in the particle size composition or specific surface area of a soil or sediment sample and changes in its 7 Be activity has been emphasized by He and Walling [1996]. Where the inventory measured at a sampling point exceeds the reference inventory, deposition can be assumed to have occurred. Following Walling et al. [1999], the mass of sediment deposited per unit area, R (kg m 22 ) can be estimated by dividing the areal activity density in excess of A ref by the mean 7 Be mass activity density of the deposited sediment, C d (Bq kg 21 ): R 5 A2A ref C d (4) C d (Bq kg 21 ) is difficult to quantify empirically, However, if it is assumed to be equivalent to the mass activity density of the sediment mobilized from upslope eroding points, it can be calculated as ð P C e RdS C d 5 S ð RdS where C e (Bq kg 21 )indicatesthe 7 Be mass activity density of the sediment mobilized from the upslope contributing area S and P is the correction factor employed to take account of differences in 7 Be activity between the mobilized and deposited sediment, due to preferential deposition of coarser particles, which would cause the 7 Be activity of the deposited sediment to be less than that of the mobilized sediment. It represents the ratio of the 7 Be activity of deposited sediment to that of mobilized sediment, which is likely to be 1.AsinthecaseofP (see equation (3)), P is usually estimated by comparing the particle size composition or the specific surface area of the deposited sediment with that of mobilized sediment. As discussed above, the simple basis for using 7 Be measurements to estimate event or short-term soil redistribution amounts described above, although attractive in its simplicity, involves a number of requirements that severely restrict its application. The approach developed by Porto and Walling [214] provides a means of overcoming this limitation by sampling the sampling points at the beginning of the study period and resampling them after each major event or short period with several heavy storm events that causes erosion. The reference inventory required to represent A ref in equations (3) and (4) is determined for each sampling point independently. This is achieved by documenting the inventory at the sampling point shortly after the preceding event and using a mass balance accounting procedure with a daily timestep to calculate the inventory existing at the sampling point after the subsequent event, in the absence of erosion. This mass balance takes the form S (5) PORTO ET AL. VALIDATING A MASS BALANCE APPROACH TO USING 7 BE MEASUREMENTS 5287

4 1.12/215WR18136 AðtÞ5Aðt21Þ exp ð2kþ1fðtþ (6) where A(t-1), (Bq m 22 ) represents the 7 Be areal activity density existing at the end of the previous day, k, (day 21 ) represents the radioactive decay occurring during the intervening day, and F(t), (Bq m 22 ) represents the fallout input (flux) during the day under consideration. An estimate of A ref at each individual sampling point immediately after the event under consideration can therefore be obtained. This is compared with the measured value of A at the sampling point shortly after the event under consideration to estimate the erosion or deposition using equations (3) or (4). If there is a short delay in measuring A, the measured value can be corrected to represent the value immediately after the event required for comparison with A ref using the mass balance accounting procedure represented by equation (6) in reverse. This takes account of radioactive decay and further fallout inputs during the intervening period. Because direct measurements of fallout are unlikely to be available in many investigations, Porto and Walling [214] proposed that the values of FðtÞ for each day could be estimated using the same approach as that used by Walling et al. [29]. This assumes that 7 Be fallout is dominated by wet deposition, as demonstrated by researchers such as Wallbrink and Murray [1996], and reconstructs the fallout record using the record of daily rainfall and estimates of the 7 Be concentration in rainfall derived from measurements of the changing inventory at the reference site. If the reference inventory A ref is measured at an adjacent reference site at the end of each significant rainfall event or short period of heavy rainfall and the equivalent value is available for the preceding event, it is possible to estimate the mean 7 Be activity of the rainfall C m, (Bq l 21 ) for the intervening period using equation (7). Equation (7) calculates the value of C m required to produce the observed change in A ref between the two measurements of the reference inventory, while, taking account of radioactive decay i.e., C m 5 A ref ðt5tþ2a ref ðt5þ exp ð2k TÞ ð T I ðtþ exp ½2k ðt2tþšdt (7) where I(t), (lm 22 ), is the daily rainfall, and T is the number of days in the period considered. If the depth distribution of 7 Be at the reference site is established on each occasion that the reference site is sampled, this enables the values of h required by equation (3) to vary through the study period. The mass balance accounting approach developed by Porto and Walling [214] and described above makes it possible to use 7 Be measurements to estimate soil redistribution amounts for individual storm events or short periods with several heavy storm eventsl over extended periods (e.g., 1 2 years). This greatly extends the potential for using 7 Be measurement in soil erosion investigations. The approach was validated by Porto and Walling [214] using an erosion plot experiment in southern Italy. The 25m x 5m plot was maintained in a bare condition and the estimates of soil loss for 2 discrete events or closely spaced groups of events occurring over a ca. 18 month period provided by 7 Be measurements made after the events were compared with the direct measurements of soil loss obtained by collecting the runoff from the plot. There was close agreement between the measured soil loss for the individual events and the estimates of soil loss provided by the 7 Be measurements. The total measured soil loss for the 18 month period was 4.15 kg m 22 and a value of 4.25 kg m 22 was provided by the 7 Be measurements. Although the validation experiment described above provided convincing results, it could be seen as being limited in scope. It involved only a small area with a uniform slope which was maintained in a bare condition. Furthermore, it was not possible to validate estimates of deposition amounts provided by 7 Be measurements. All the 7 Be measurements were indicative of net soil loss from the sampling points. There was no evidence of deposition on the plot and the plot therefore only provided measurements of soil loss. Against this background, the authors have undertaken a second validation study. This was again located in southern Italy but involved a small catchment extending to 1.38 ha with complex topography more representative of the natural landscape. It is generally accepted that the use of 7 Be measurements to document soil redistribution is effectively limited to bare soils, since interception and storage of the 7 Be fallout by an overlying vegetation (e.g., crop) cover could compromise the basic assumptions of the approach. In this situation the inventory would comprise two components, namely, that associated with the 7 Be sequestered in the vegetation cover and that associated with the 7 Be fixed by the surface soil. Kaste et al. [211] have, for example, reported that ca. 5% of the 7 Be inventory was contained in the vegetation cover in a study undertaken is PORTO ET AL. VALIDATING A MASS BALANCE APPROACH TO USING 7 BE MEASUREMENTS 5288

5 1.12/215WR18136 N km Study area Stream gauge Coshocton wheel sampler 9 Contour (m) Sampling point 2m Figure 1. The location of the study area and study catchment. an arid environment in California, USA. The presence of a significant proportion of the 7 Be inventory in the vegetation cover would introduce problems in terms of only the second component being influenced by the soil redistribution which is being documented and the likelihood that the relative proportions of the inventory accounted for by the two components would be spatially variable. Furthermore, there are likely to be difficulties in establishing a representative reference inventory reflecting fallout receipt at the soil surface within a vegetated area. In the current study, an attempt has been made to explore the potential for extending the use 7 Be measurements to document soil redistribution into areas with a vegetation cover by undertaking the validation study in a watershed with a forest cover. In this case, the forest cover comprised Eucalyptus trees and was characterized by a lack of understorey vegetation and an essentially bare forest floor. Several assumptions were made regarding the influence of this vegetation cover on the use of 7 Be measurements to document soil redistribution in the catchment and these constitute part of the validation exercise. 3. The Study Area The study catchment (Figure 1), known as catchment W2 (1.38 ha), forms part of a soil erosion monitoring programme initiated by the National Research Council of Italy (CNR) in 1978 when three adjacent small catchments in Calabria (southern Italy) were instrumented by a Soil Conservation Project to establish the effects of afforestation on their hydrological response and sediment yield [Cinnirella et al., 1998; Porto et al., 29a]. The catchments are located near Crotone (35 m a.s.l., N, E) within the headwaters of the larger Crepacuore basin, which are incised into the Upper Pliocene and Quaternary sediments (clays, sandy clays and sands) underlying the local area. The climate of the study area is Mediterranean with a mean annual rainfall of ca. 67 mm for the period at Crotone (1 km distant). Most of the precipitation falls during the winter season that extends from October to March. The catchment has never been cultivated and historically it supported a rangeland vegetation cover. In 1968 it was planted with trees (Eucalyptus occidentalis) and these trees have been harvested twice (in 1978 and 199) with the tree cover being subsequently restored by natural regrowth. The harvested timber was extracted from the catchment manually in order to minimize soil disturbance. The tree cover is not uniform within the catchment and about 2% of its area, primarily along its northern margin, is characterized by PORTO ET AL. VALIDATING A MASS BALANCE APPROACH TO USING 7 BE MEASUREMENTS 5289

6 1.12/215WR18136 discontinuous trees and a sparse grass cover. Elsewhere, the tree cover is relatively uniform, although the canopy is not continuous. The soils under the eucalyptus trees are largely devoid of ground cover and much of the surface beneath the trees and sparse grass cover is therefore bare. The bare soils are exposed to rainsplash and surface runoff during heavy rainfall and significant sheet erosion occurs at such times. The area with discontinuous tree cover is the area of greatest soil loss [Porto et al., 25]. Based on existing records, the mean annual sediment yield from the catchment was calculated to be ca t yr 21 (23.8 t ha 21 yr 21 ). 4. Methods 4.1. Catchment Instrumentation Catchment W2 has been instrumented to measure rainfall, runoff and sediment yield since The water discharge at the catchment outlet is gauged using an H-flume structure [Porto et al., 21], equipped with a mechanical stage recorder. The sediment load passing the gauging structure is measured using a Coshocton wheel sampler installed immediately below the H-flume. This sampler diverts an aliquot of 1/2 of the flow, which is collected in a tank. After each storm event, representative samples are collected from the tank to determine the mean sediment concentration in the tank. The sediment concentrations in the tank samples are determined by oven drying at 15 C and the mean sediment concentration of the samples is calculated. The event sediment yield from the catchment is then calculated as the product of the mean sediment concentration and the total runoff volume for the event measured by the H-flume. The annual sediment yield is in turn calculated as the sum of the sediment loads for all storm events occurring during the year. Detailed records of sediment yield are available for the years and 26 until the present. The validation experiment reported here relates to a 2 month period starting in November 213 and continuing until June 215. Thirteen significant storm events that generated both runoff and soil loss were recorded during the study period. Because the study catchment is located ca. 25 km from the home University in Reggio Calabria, it was difficult to establish an intensive rainfall sampling program to document the 7 Be activity of the rainfall and calculate the daily fallout input. The lack of laboratory facilities to process rainfall samples to extract their 7 Be activity [see Wallbrink and Murray, 1996; Wilson et al., 23] and the lack of gamma detector capacity for measuring the extracted 7 Be or for direct assay of rainfall samples also precluded such a program. The procedure used by Walling et al. [29] and also employed by Porto and Walling [214], described above, was therefore used to estimate the daily fallout input. This was based on measuring the change in inventory associated with individual periods between the collection of cores from a reference plot and linking this to the record of daily rainfall, whilst taking account of radioactive decay, in order to estimate the daily fallout flux, as indicated by equation (7) The Validation Experiment The aim of the validation experiment was to use 7 Be measurements to document the soil redistribution and net soil loss associated with individual storm events, or composite events where several storms occurred during a short period, and to compare the resulting estimates of net soil loss with the measurements of sediment yield from the catchment. Direct comparison of the estimates of net soil loss from the study catchment with the measured sediment yields from the catchment provided the basis for the validation. Such direct comparison was possible because field observations during the study period had indicated that there was no significant storage of fine sediment in the ephemeral channel system of the study catchment and that channel erosion was not a significant sediment source within the catchment. The estimates of net soil loss from the slopes could therefore be directly compared with measured sediment yields. In addition, 137 Cs measurements had been used previously to derive an estimate of both the magnitude and the spatial distribution of the longer-term mean annual net soil loss from the catchment [Porto and Walling, 214] and comparison of the spatial patterns associated with both data sets were seen as providing further evidence to support the validation. The sampling points within the catchment used to collect soil samples for 7 Be analysis were selected to correspond with areas where the tree canopy was discontinuous and small clearings existed. The soil surface at these points was bare due to the lack of ground cover beneath the Eucalyptus trees. It was reasoned that the presence of the tree cover could effectively be ignored in using 7 Be measurements to estimate the soil redistribution amounts associated with the individual sampling points. There was PORTO ET AL. VALIDATING A MASS BALANCE APPROACH TO USING 7 BE MEASUREMENTS 529

7 1.12/215WR18136 limited interception of the rainfall by the tree cover at the sampling points and the fallout received could be assumed to be essentially the same as that at the reference site established adjacent to the study catchment in an area with minimal slope and free from trees. The reference plot was maintained in a bare condition by application of an herbicide. The soil redistribution rates documented at the sampling sites were assumed to be representative of those associated with the catchment surface more generally, since the surface conditions associated with the absence of ground cover were essentially identical to those where the forest canopy was more continuous. The possibility that soil redistribution rates associated with the latter areas might be lower than those associated with the sampling sites that were located in small clearings, due to the interception of rainfall by the forest canopy, was recognized. However, any difference was seen as likely to be limited, since the events generating storm runoff and sediment yield at the catchment outlet were all associated with high rainfall amounts, with a minimum of 25.6 mm, a maximum of mm and a mean of 85.7 mm, where interception losses would account for a relatively small proportion of the incident rainfall. Furthermore, the occurrence of surface wash across the slopes during these high magnitude storm events means that the amount of soil redistribution associated with a sampling point is influenced by runoff from a much wider area and is likely to be representative of that wider area. The number of sampling points that could be realistically deployed across the catchment was limited by practical constraints associated with the time available for sampling at a site distant from the home laboratory and the number of samples that could be subsequently analyzed. The short half-life of 7 Be (53 days) means that the 7 Be activity will be reduced below the detection limit relatively quickly (e.g., after ca 2 3 months) and it is therefore not possible to store samples for lengthy periods prior to analysis. The lengthy count times required by the gamma assay procedure (ca h) and the availability of only two gamma detectors limited sample throughput. The sampling strategy aimed to ensure that in most situations analysis of the samples collected after one storm event would have been completed prior to the occurrence of the next event. Continuity was important for the mass balance accounting. As a result, the number of sampling points was limited to 2. This precluded the establishment of detailed transects across different slopes. It was necessary to make the assumption that 2 sites, that were well distributed across the catchment and representative of the range of topography, would provide a representative sample of the soil redistribution rates associated with the catchment. This approach had been successfully employed by the authors in previous studies aimed at constructing the sediment budgets of small and medium-sized watersheds in the local region [see Porto et al., 29b, 211, 213]. The network of 2 sampling sites established within the study catchment is shown in Figure 1. The network aimed to provide a representative coverage of the catchment, with the exact location of the sampling points being constrained by the presence of openings in the tree canopy and, in part, by ease of access Soil Sampling and Laboratory Analysis Collection of soil samples from the catchment for 7 Be analysis involved two separate sampling programs for each major rainfall event that occurred during the study period. The first was designed to establish the magnitude of the soil redistribution produced by the individual events at each sampling point. This involved collecting samples shortly (e.g., 2 3 days) after the event from the 2 sampling points shown in Figure 1. Replicate (i.e., 2) bulk cores were collected from each sampling point using a 15.6 cm diameter core tube inserted into the soil to a depth of 2 cm. Care was taken to minimize disturbance of the surface of the sampling point during the sampling exercise and the shallow hole left by the coring device was infilled with soil in order to maintain the sampled area in as natural state as possible. The precise position of each sampling point was varied from event to event and progressively moved upslope, in order to avoid resampling an area already disturbed by previous sampling or that might be influenced by such disturbance. The sampling locations were located using markers and a GPS system, in order to facilitate resampling at the same locations. As indicated above, where tree cover existed in the vicinity of the sampling points, care was taken to ensure that cores were collected from points located in small clearings between the trees. Because of the constraints on the number of samples that could be analyzed discussed above, the 2 bulk cores collected after each event from each sampling site were combined and mixed and a single composite sample from each sampling point was analyzed for 7 Be content. When the network of sampling points was initially established, samples of surface soil ( 1 cm) were collected at each point to characterize their particle size composition. PORTO ET AL. VALIDATING A MASS BALANCE APPROACH TO USING 7 BE MEASUREMENTS 5291

8 1.12/215WR18136 Figure 2. Collection of sectioned cores from the reference plot. The photograph on the left shows the collection of the 2 cm deep cores from the reference plot using a plastic coring tube. The photo on the right shows the screw-driven core extrusion device fitted to the plastic core tube to permit extrusion of the core at 1 mm increments. The second coring program was undertaken essentially contemporaneously with the first and was directed to establishing both the magnitude and the depth distribution of the 7 Be inventory at an undisturbed reference plot with minimum slope. This reference plot was established ca. 3 m outside the boundary of the study catchment at the beginning of the experiment in October 213. A 4 m m plot was delimited, all vegetation was removed and its surface was cultivated and levelled by hand. By mixing the soil, the existing 7 Be activity in the surface soil was reduced to below the level of detection. The plot was subsequently maintained in a bare condition by application of herbicide. Both bulk cores for determination of the total inventory and depth incremental samples for determining the 7 Be depth distribution were collected from this plot. The former comprised two replicate bulk cores collected to a depth of 2 cm of using the 15.6 cm diameter corer used for sampling within the watershed. These were again combined to provide a single composite sample for analysis. The latter involved the collection of three 15.6 cm diameter cores to a depth of 2 cm using a custom-built coring device (see Figure 2). This permitted the short core tube containing the sample to be coupled with a screw-driven device for extruding the core (surface first) at fixed increments for slicing. The position of the sampling points within the reference plot was changed on each occasion that the plot was sampled, in order to ensure that the cores were collected from undisturbed points within the plot. The samples were transported to the Department of Agriculture at the University Mediterranea of Reggio Calabria for processing and analysis. In the case of the three cores collected from the reference site using the coring device shown in Figure 2 the device permitted the core to be extruded in 1 mm increments, starting from the surface. The matching individual 1mm depth increments extruded from the three cores collected after each event were bulked to provide a single composite sample for each 1 mm depth increment down to 2 cm. All samples, including the composite bulk samples and depth incremental samples from the reference plot, the composite bulk cores from the sampling points within the catchment and the samples of sediment collected from the storage tank at the catchment outlet were initially oven dried at 15 C, disaggregated, passed through a 2 mm sieve and homogenized prior to measurement of their 7 Be activity. A representative aliquot of the <2 mm fraction of the samples, comprising ca. 1 g of each bulk core sample and ca. 4 g for the composite core sections and the sediment samples from the tank, were then loaded into containers for 7 Be assay. Plastic pots (33 cm 3 )wereusedforthelarger(1g) samples and smaller Petri dishes (65 mm diameter, 15 mm high) for the smaller (4 g) samples. Beryllium-7 activities in the soil and sediment samples were measured by gamma-ray spectrometry at kev, using two Canberra p-type high-resolution low energy coaxial HPGe detectors (model GX42) coupled to amplifiers and a PC-based data collection system using the Canberra Genie 2 software. The detectors had a relative efficiency of 45% and were calibrated using the Canberra LABSOCS software (Laboratory Sourceless Calibration Software) which uses Monte Carlo procedures to establish the calibration efficiency for any type of geometry. The energy calibration was undertaken using a multigamma source and was further validated using several standard materials with known 137 Cs and 21 Pb activity. Count times varied from ca. 3, s, where a higher activity was found, to ca. 24, s where a lower activity PORTO ET AL. VALIDATING A MASS BALANCE APPROACH TO USING 7 BE MEASUREMENTS 5292

9 1.12/215WR18136 Table 1. The Inventories Measured at the Reference Site Shortly After the Rainfall Events (A ref ), the Measured Inventory (A) at the Sampling Points, the Rainfall Amounts (P e ) Associated With Each Event, the Rainfall Amounts (P t ) Associated With the Period Between the Commencement of the Study Period or the End of the Previous Event and the End of the Given Event, the 7 Be Concentration in the Rainfall During the Same Period Estimated From the Change in Inventory and the Rainfall Amount (C m ), the Shape Parameter (h ) Associated With the Depth Distribution Measured at the Reference Site (see Equation (1)), the Measured Sediment Yield Y i, and the Estimated Net Soil Loss R i (t) A ref a A a P e P t C m h Y i R i Code Event (Bq m 22 ) (Bq m 22 ) (mm) (mm) (Bq l 21 ) (kg m 22 ) (t) (t) EV /11/ EV-2 29/11 3/12/ EV-3 31/12/ EV-4 1 3/2/ EV-5 5 8/3/ EV-6 13/4/ EV-7 5 6/9/ EV-8 3 6/1/ EV-9 4 8/11/ EV /12/ EV /1/215 and /2/215 EV /3/ EV-13 17/3/ a These values relate to the inventories measured shortly after the event. Each value represents the mean inventory for the 2 points. was evident. This strategy provided results with an analytical precision of ca. 61% at the 95% level of confidence and a detection limit of ca. 1 Bq kg 21. For the composite samples associated with the cores collected within the watershed and from the reference plot, the measurement of mass activity density (Bq kg 21 ) was used to calculate the areal activity density or inventory (Bq m 22 ) associated with each sampling point at the time of sampling. In the case of the depth incremental samples collected from the reference site, the values of mass activity density obtained for each depth increment were used to establish the 7 Be depth distribution for the reference site at the time of sampling. In addition, the absolute grain size composition of the <2 mm fraction of selected sediment samples collected from the storage tank associated with the Coshocton Wheel and samples of surface soil collected from the sampling points within the catchment were determined using a laser granulometer, following pretreatment with hydrogen peroxide to remove the organic component and chemical dispersion with sodium hexametaphosphate. For several of the samples estimates of their specific surface area, assuming spherical particles, were also obtained. 5. Results 5.1. The Events Documented Information relating to the 13 major storm events causing sediment mobilization and measurable sediment yields from catchment W2 during the 2 month study period is presented in Table 1. The rainfall totals associated with both the event itself and the period extending from the previous sampling to the sampling undertaken after the event in question are listed for each event in Table 1. The latter value was required when estimating the 7 Be activity of the rainfall from the change in inventory at the reference site over that period. The record of daily rainfall totals for the study period is presented in Figure Measurements of Sediment Yield From the Experimental Catchment Measurements of sediment yield Y i (t) from the study catchment were obtained for each of the 13 events generating runoff and a measurable sediment yield that occurred during the study period extending from November 213 to June 215. The individual values of sediment yield, listed in Table 1, ranged over more than two orders of magnitude from.6 t to 14.3 t, with a mean of 3.6 t Be Inventories and Depth Distributions at the Reference Plot and Estimation of Fallout Inputs At the start of the experiment at the beginning of November 213, the reference site had been recently cultivated and the 7 Be inventories measured for the reference plot shortly before the first event in late November 213 were below the level of detection and were recorded as zero. The 7 Be inventories associated with PORTO ET AL. VALIDATING A MASS BALANCE APPROACH TO USING 7 BE MEASUREMENTS 5293

10 1.12/215WR /11/213 22/11/213 12/12/213 1/1/214 21/1/214 1/2/214 2/3/214 22/3/214 11/4/214 1/5/214 21/5/214 1/6/214 3/6/214 2/7/214 9/8/214 29/8/214 18/9/214 8/1/214 28/1/214 17/11/214 7/12/214 27/12/214 16/1/215 5/2/215 25/2/215 17/3/215 6/4/215 26/4/215 Event 3 Event 6 Event 7 Event 8 Event 1 Event 11 Event 13 Rainfall (mm) Event 4 Event 5 Event 9 Event 12 Event 1 Event 2 Figure 3. The record of daily rainfall totals for the study catchment for the period of the validation experiment. the bulk cores collected subsequently from the reference site, shortly after the individual events, are reported in Table 1. The 7 Be depth distributions associated with the sectioned cores collected from the reference plot are presented in Figure 4, with depth expressed as mass depth. All the profiles broadly conform to the shape expected for this radionuclide [see Wallbrink and Murray, 1996; Walling et al., 1999; Mabit et al., 28; Schuller et al., 21; Shi et al., 211; Walling, 213], with a maximum activity at the soil surface and an exponential decline with depth. Equation (1) was fitted to each profile to derive the values of h listed in Table 1. The fitted depth distributions are also depicted in Figure 4. For several 7 Be depth distributions there is some departure from the shape represented by the exponential function at depth. This in part reflects the low activities at depth which are close to the detection limit of the detectors used. However, the exponential functions are seen provide an acceptable fit to the 7 Be depth distribution in the surface layers (e.g., 3 kg m 22 ) which is the critical zone of the depth distribution when estimating soil loss. The maximum estimated soil loss for the events investigated was equivalent to a mean surface lowering of ca. 1 kg m 22 from the 2 sampling points. The values of h presented in Table 1 provide some evidence of a general trend related to catchment wetness, with low values associated with the dry conditions and low inventories at the beginning of the study period and values increasing as more rainfall occurs and the inventory increases. When the inventory subsequently declines due to lack of rainfall/fallout the h values also decline. This is similar to the pattern described by Porto and Walling [214] for a different area located in Southern Italy and by Walling et al. [29] for a study site in south-central Chile. The values of A ref, measured at the reference plot after each event are listed in Table 1 along with the estimates of C m for each selected period derived using equation (7). The range and variability of the C m values PORTO ET AL. VALIDATING A MASS BALANCE APPROACH TO USING 7 BE MEASUREMENTS 5294

11 1.12/215WR Be (Bq kg -1) 7Be (Bq kg-1) 7Be (Bq kg-1) 7Be (Bq kg-1) 7Be (Bq kg-1) /11/ /12/ /1/ /2/ /3/ Be (Bq kg -1) Be (Bq kg -1) Be kg -1) 7Be (Bq kg -1) (Bq /5/ /9/ /1/ /11/ Be (Bq kg-1) 7Be (Bq kg-1) 7Be (Bq kg-1) 7Be (Bq kg-1) /12/ /2/ /3/ /4/ Figure 4. Depth distributions of 7 Be activity in the surface soil of the reference plot shortly after the 13 rainfall events. The fitted lines represent the exponential depth distributions fitted to the measured depth distributions. is consistent with those reported by Wallbrink and Murray [1994] based on direct measurements of 7 Be activity in rainfall for individual rainfall events at a location in Canberra, Australia, which varied from.2 to 5.9 Bq l 21. The values of C m were used to reconstruct the record of daily 7 Be fallout required to implement the mass balance accounting procedure represented by equation (6) Be Inventories Within the Experimental Catchment The mean values of A(t) (Bq m 22 ) measured shortly after the individual events are listed in Table 1. These values represent the mean 7 Be inventory associated with the 2 composite samples collected from the 2 sampling points after each event. 213 was a dry year and a total of only 52 mm of rainfall was recorded between January and June 213. During the period from July to mid-november 213, prior to the event that occurred in late November, only 25 mm of rainfall were recorded. As a result, the 7 Be inventories within the catchment prior to the event in late November 213 were as expected very low and this was confirmed by measurements undertaken at several of the sampling points shortly before the event in late November. At this point in time the inventories were below the level of detection Converting 7 Be Measurements into Estimates of Soil Redistributions The estimates of soil redistribution associated with each of the 13 individual events or periods of heavy rainfall were obtained by applying equation (3) or (4) to the inventory values measured at each of the 2 sampling points shortly after the event. In this application, A represents the inventory for the point measured shortly after the event (i) being considered and A ref represents a point-specific reference inventory (i.e., the inventory that would have occurred on that sampling point at the end of the event, had no soil redistribution occurred during that event). A ref is estimated by taking the inventory A measured at the sampling point immediately after the previous event (i-1) and using equation (6) to establish the inventory that would have existed on that point in the absence of further soil redistribution. For several events there was a delay (e.g., 1 week) in measuring A and in this case the measured values were corrected to represent the value that PORTO ET AL. VALIDATING A MASS BALANCE APPROACH TO USING 7 BE MEASUREMENTS 5295

12 1.12/215WR would have occurred shortly after the event using the procedure outlined in 8 section 2. Where A < A ref, erosion is assumed to have occurred and equation (3) is applied to estimate the 6 amount of soil eroded. Where A > A ref deposition is assumed to have 4 2 SOIL SED Sediment (mean) occurred and equation (4) is applied to estimate the amount of deposition. In the absence of a point specific value Soil (mean) of h for use in equation (3), the approach employed by Porto and Walling [214] was adopted. In this, the Particle size (mm) value of h obtained for the 7 Be depth distribution documented at the reference Figure 5. Comparisons between the range of the grain size distributions of sediment (red) sampled at the watershed outlet with the range (black) of the grain site immediately after the event is used. In the case of the first event size distributions associated with surface soil within the catchment. The mean that occurred during the study period grain size distributions for the samples of surface soil and sediment collected from the study catchment are also shown. on 22 November 213, the reference inventory was assumed to be zero across the plot at the beginning of the study period, because of the long preceding dry period. This was confirmed by measurement of the inventory associated with several of the sampling points at this moment in time. These measurements showed that the inventory was below the limit of detection. Frequency (%) A comparison of the grain size distributions associated with some of the samples of sediment mobilized from the catchment and collected in the tank after each event with the grain size distributions of the surface soil collected from the 2 sampling points is provided by Figure 5. Figure 5 presents both the envelope curves for both sets of grain size distributions and the mean distributions for both soil and sediment. It indicates that the grain size distributions of the sediment samples are characterized by a greater spread than those of the soil samples, although the mean grain size distributions for soil and sediment are quite similar. The greater spread of the grain size distributions of the sediment was attributed in part to the difficulty of collecting a fully representative sample of the sediment stored in the collection tank at the outlet of the study catchment. It is to be expected that such a sample could overestimate the proportion of fines, and this could account for the finer nature of many of the sediment samples, relative to the soil samples. The mean grain size distributions shown in Figure 5 are quite similar, with the greatest differences between these two curves being of the order of 61%, which is seen as being the likely level of uncertainty associated with such curves, considering the sampling and laboratory analysis procedures. It is therefore suggested that Figure 5 provides no clear evidence that the sediment mobilized from the study catchment is consistently enriched in fines and the enrichment correction factor in equation (3) (P) was set to 1. Similarly, the lack of a clear differentiation between the grain size composition of sediment collected at the catchment outlet and that of the soil, suggested that the deposited sediment was unlikely to be substantially depleted in fines relative to the mobilized sediment and the correction factor P in equation (5) was also set to 1. These conclusions were further supported by the limited data available on the specific surface area of the sediment collected at the catchment outlet for individual events and the surface soil within the catchment. Analysis of five samples of each provided mean values of specific surface area of.424 m 2 g 21 for the soil samples and.465 m 2 g 21 for the sediment samples. Although these data suggest that the sediment samples might be characterized by an increased specific surface area relative to the surface soil, the difference was small. When estimating the net soil loss from the study catchment for the individual events based on the 7 Be measurements undertaken at the 2 sampling points shown in Figure 1, it was, as indicated above, assumed that these provided a representative sample of the soil redistribution rates within the catchment. The mean value, taking account of both positive (deposition) and negative (erosion) values, was seen as providing a meaningful estimate of the net soil loss associated with the event. Because field observations through the course of the study provided no evidence of significant storage of sediment in the ephemeral channel within the catchment or of channel erosion within that channel representing a significant additional source of sediment, the estimates of net soil PORTO ET AL. VALIDATING A MASS BALANCE APPROACH TO USING 7 BE MEASUREMENTS 5296

13 1.12/215WR18136 Estimated soil loss (t) loss from the catchment for the individual events provided by the 7 Be measurements were assumed to be directly comparable to the measured sediment yields for the events. The estimates of net soil loss for the individual events provided by the 7 Be measurements are listed in Table 1 and ranged from,.6 t to 13.6 t, with a mean of 4.2 t. The mean of 4.2 t is consistent with the mean of 3.6 t reported above for the measured sediment yields for the 13 events Measured sediment yield (t) Figure 6. A comparison of the estimates of net soil loss provided by the 7 Be measurements for the 13 storm events with the equivalent values of measured sediment yield from the study watershed Comparison of the Estimates of Net Soil Loss With the Measured Sediment Yields for Individual Events Figure 6 presents a plot of the estimates of net soil loss for the 13 events versus the measured sediment yields from the study watershed. 6. Discussion The results presented in Figure 6 indicate that the estimates of net soil loss from the experimental catchment for the 13 events generated using the 7 Be measurements are in close agreement with the measured sediment yields from the watershed for the same events. This agreement is further confirmed by the efficiency index [see Nash and Sutcliffe, 197] of.953 obtained for the comparison of the estimated and observed values Overall, Figure 6 suggests that there is no clear tendency for over- or under-estimation, although the results indicate that values of net soil loss estimated using the 7 Be measurements commonly overestimate the measured sediment yields, when the latter values are low (i.e., < 1 t). Summing the results for the 13 events during the study period, the value of net soil loss estimated using the 7 Be measurements was 53.9 t, whereas the measured sediment yield was as 47. t. The higher total associated with the estimate of net soil loss provided by the 7 Be measurements largely reflects the tendency for the 7 Be measurements to overestimate the low values of measured sediment yield (i.e., < 1t). Two possible reasons for this overestimation situation can be suggested viz, 1. Direct comparison of the estimates of net soil loss provided by the 7 Be measurements with the measured sediment yields assumes that there is good connectivity between the catchment surface, as represented by the 2 sampling points, and the catchment outlet. If temporary storage occurs within the catchment, this is likely to be greatest for low magnitude events characterized by less storm runoff and lower energy and therefore greater opportunity for deposition and storage. 2. As discussed above, there is no clear evidence of grain size effects associated with size-selective erosion and no correction factor was employed in equation (3). However, if such grain size effects do occur they are likely to be most marked for low magnitude events when detachment energy and transport capacity can be expected to be lowest and smaller particles may therefore be preferentially mobilized. This would result in overestimation of the net soil loss for low magnitude events. The grain size data currently available for the sediment were limited and it was not possible to explore this possibility further. A similar overestimation of low values of soil loss relative to the measured values was reported by Porto and Walling [214] in a study which applied the same mass balance accounting approach as used in the current study to a small erosion plot. This suggests that the second potential explanation outlined above may be more meaningful, since it could apply to both the erosion plot and the larger catchment. PORTO ET AL. VALIDATING A MASS BALANCE APPROACH TO USING 7 BE MEASUREMENTS 5297

14 1.12/215WR18136 Figure 7. The spatial distribution of soil redistribution within the study catchment as indicated by (a) 7 Be and (b) 137 Cs measurements. The values presented in Figure 7a relate to the total soil redistribution that occurred at each sampling point during the 2 month study period, whereas those presented in B relate to the mean annual soil redistribution for the longer time window (i.e., 5 years) associated with 137 Cs. When using the measured event sediment yields from the study catchment to validate the estimates of net soil loss provided by the 7 Be measurements, potential causes of divergence of the two sets of values such as those described above mean that it is unrealistic to expect perfect agreement. In this context, it is also important to consider the various sources of uncertainty associated with the two data sets. In the case of the measured event sediment yields, these include potential uncertainties associated with the discharge measurements provided by the H-flume, the sediment sampling equipment, sampling the collection tank and determining the sediment concentration of the samples. The overall uncertainty is judged to be of the order of 615%. Similarly, with the estimates of net soil loss derived from the 7 Be measurements uncertainty will be introduced by many components of the procedure used. These include the soil sampling and laboratory measurements of 7 Be activity for both the catchment and the reference site, the reconstruction of fallout inputs, and the numerical model used to estimate soil redistribution rates. Furthermore in this study, the final estimate of net soil loss was based on only 2 sampling points, which will clearly affect the precision of the final estimate. The uncertainty associated with the final estimate of net soil loss for each event is judged to be of the order of 625%. Detailed examination of the various sources of uncertainty and their propagation through the various calculations lies beyond the scope of this study, which was aimed at providing a preliminary validation of the results provided by the 7 Be measurements. The results obtained are, however, seen as providing confirmation of the general validity of those results and thus of the successful use of 7 Be measurements to document soil redistribution at the scale of a small catchment. A further test of the validity of the results provided by the 7 Be measurements involved generating a map showing the spatial pattern associated with the estimates of total soil redistribution within the catchment for all 13 events combined, based on the 2 sampling points. This map shown in Figure 7a was generated using a kriging interpolation procedure. The spatial pattern of soil redistribution shown is consistent with existing knowledge of areas characterized by higher rates of soil loss, which coincide with areas with a sparse tree cover. These are associated with the northern and eastern margins of the study catchment. Figure 7a can also be compared with an equivalent map of longer-term (ca. 5 years) mean annual soil redistribution rates produced for the study catchment based on 137 Cs measurements undertaken previously. This is presented in Figure 7b. In this case the map was based on a greater number of sampling points, which increases its spatial resolution and the time window of ca. 5 years means that the spatial pattern depicted is likely to reflect a degree of temporal smoothing when compared with the short period covered by the 7 Be measurements. Both these differences and the different units associated with the two maps limit direct comparison. However, the two maps are seen as being consistent in emphasizing that the areas of maximum soil loss within the catchment are located along the northern and eastern margins of the study catchment PORTO ET AL. VALIDATING A MASS BALANCE APPROACH TO USING 7 BE MEASUREMENTS 5298