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Abstract: Soil erosion and associated off-site sedimentation are threatening the sustainable use of the Three Gorges Dam. To initiate management intervention to reduce sediment yields, there is an increasing need for reliable information on soil erosion in the Three Gorges Reservoir Region (TGRR). The purpose of this study is to use 137Cs tracing methods to construct a sediment budget for a small agricultural catchment in the TGRR. Cores were taken from a pond and from paddy fields, for 137Cs measurements. The results show that the average sedimentation rate in the pond since 1963 is 1.50 g cm-2 yr-1 and the corresponding amount of sediment deposited is 1,553 t. The surface erosion rate for the sloping cultivated lands and the sedimentation rate in the paddy fields were estimated to be 3,770 t km-2 yr-1 and 2,600 t km-2 yr-1, respectively. Based on the estimated erosion and deposition rates, and the area of each unit, the post 1970 sediment budget for the catchment has been constructed. A sediment delivery ratio of 0.5 has been estimated for the past 42 years. The data indicate that the sloping cultivated lands are the primary sediment source areas, and that the paddy fields are deposition zones. The typical land use pattern (with the upper parts characterized by sloping cultivated land and the lower parts by paddy fields) plays an important role in reducing sediment yield from agricultural catchments in the TGRR. A 137Cs profile for the sediment deposited in a pond is shown

to provide an effective means of estimating the land surface erosion rate in the upstream catchment. Keywords: Pond deposition; Soil erosion rate; Sediment budget; 137Cs; Sediment delivery ratio; Three Gorges Reservoir Region

Introduction

Soil erosion and sedimentation have become worldwide environmental problems, due to their impact on land degradation, soil fertility and water contamination (Lal 2001; Namr and Mrabet 2004). Furthermore, severe erosion may threaten the safe operation of water conservancy projects (IAEA 1998). The Three Gorges Reservoir Region (TGRR) is an area of serious soil erosion in the upper reaches of the Yangtze River and the evaluation and quantification of soil erosion in the area have attracted increasing attention during the past several decades, especially after the construction of the Three Gorges Dam. However, reliable baseline data on soil erosion and sediment delivery for small agricultural catchments in this area are relatively scarce. There is an urgent need to establish catchment sediment budgets, to quantify sediment generation, transport, deposition and

Using 137Cs Tracing Methods to Estimate Soil Redistribution

Rates and to Construct a Sediment Budget for a Small

Agricultural Catchment in the Three Gorges Reservoir Region,

China

JU Li1,2, WEN An-bang1*, LONG Yi1, YAN Dong-chun1, GUO Jin1,2

1 Key Laboratory of Mountain Surface Processes and Ecological Regulation, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, China

2 University of Chinese Academy of Sciences, Beijing 100049, China

*Corresponding author, e-mail: wabang@imde.ac.cn; First author, e-mail: juyongmei@126.com

© Science Press and Institute of Mountain Hazards and Environment, CAS and Springer-Verlag Berlin Heidelberg 2013

Received: 05 November 2012 Accepted: 02 April 2013

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output. A sediment budget provides useful data for the design and implementation of conservation strategies and land use management policies (Walling et al. 2002; Walling and Collins, 2008). The complex nature of soil erosion processes, their spatial and temporal variability, and observational and instrumentation problems make it difficult to obtain reliable data on sediment budgets in a short term study using traditional methods (Collins and Walling 2004; Brown et al. 2009). New approaches such as those involving high-precision topographic measurements, numerical modeling and environmental radionuclides have been developed to overcome these problems (Walling et al. 2002; Faleh et al. 2005; Walling, 2006; Erkens et al. 2006; Viparelli et al. 2011). In the TGRR, many of the small reservoirs and ponds used for water storage and irrigation, trap the sediment originating from their catchments areas and the resulting deposits provide a useful basis for estimating soil erosion rate and sedimentation processes in this area.

Environmental radionuclides (more particularly 137Cs) have been frequently used as chronometers for sediment deposition in lakes and reservoirs to establish the sedimentation rate and the long term average sediment yield from the upstream catchment, thereby avoiding the time consuming process of monitoring the sediment load (Zapata 2003; Benninger et al. 1997; Appleby and Oldfield 1983; Xiang et al. 2002). It is possible to obtain this information for different time intervals by dating sediment cores retrieved from the sediment deposits using specific time markers and core correlation techniques (Zhang and Walling 2005).

The chronology of sediment cores retrieved from various depositional water bodies can be determined using fallout 137Cs (half-life 30.2 yr), which was deposited across the landscape in close association with precipitation, as a result of atmospheric nuclear testing in the 1950s-1970s. In most environments, 137Cs is immediately adsorbed by the fine fractions of the surface soil when it reaches the land surface. Two dating markers, 1954 (the first appearance of 137Cs in sediment cores retrieved from water bodies indicates the year the isotope was first detected in measurable amounts) and 1963 (the peak occurrence in sediment cores indicates the year of maximum fallout), can be

identified in the sediment horizons at most depositional sites. It is, however, now generally recognized that the 1954 marker is unreliable, due to the possibility of downward diffusion or migration of the 137Cs (Ritchie and McHenry 1990; Loughran and Elliott 1996; He et al. 1996; Zapata 2003; Zhang 2006; Begy et al. 2009). In addition to providing information on the sedimentation rate in a water body, Zhang and Walling (2005) have also demonstrated how the 137Cs profile within the sediment deposit can be used to estimate the erosion rate in the upstream catchment.

The objective of this study is to illustrate the use of 137Cs as a tracer to reconstruct the recent sedimentation rate in a pond, to estimate the erosion rate in the upstream catchment using the profile of 137Cs activity in the sediment core, to construct the sediment budget for the study catchment and to provide a basis for understanding the erosion processes operating in similar landscapes.

1 Study Area

We selected a small, representative agricultural catchment for this study. The study catchment (0.0495 km²) is located in Zhongxian County in the central part of the Three Gorges Reservoir Region of China (30°25'N, 108°09'E) (Figure 1). The area is underlain by horizontal sandstones, siltstones and mudstones of the Jurassic rocks of the Shaximiao Group (J2s). Elevation ranges from 233 to 290 m, with a simple morphology. The average slope is around 36% in the upper parts of the catchment and around 15% in the lower parts. Local farmers reported that the original long slopes were divided into plots with shorter lengths and narrow widths in order to control water erosion. Soils are purple soils, known as Orthic Entisols (Chinese Soil Taxonomic System), Regosols (FAO Taxonomy) or Entisols (USDA Taxonomy) (He et al. 2009), derived from the products of rapid weathering of the Jurassic rocks of the Shaximiao Group (J2s), which are rich in phosphorus and potassium, and have a pH of 7.1. The area has a subtropical southeast monsoon climate with distinct seasons, and a mean annual temperature of 19.2°C. Mean annual precipitation is 1,150 mm, about 70% of which occurs from April

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to October. The land use in the study catchment is directly

linked to the topography. The upper parts of the sloping cultivated lands (approximately 40% of the catchment area) are used for upland crops (e.g. corn, beans), while the summits of the hilly areas are forest land. The lower level terrace lands (accounting for 29 % of the catchment area) have been for rice cultivation for centuries.

The catchment drains into an artificial pond constructed in 1956 at the outlet of the catchment. At the time of the study the pond had a water surface area of 2,390 m2 and a storage capacity of 7,300 m3. The dam height is 5 m and the bank slope is about 30°. According to information provided by the local water conservancy technical personnel, the trap efficiency of the pond is not less than 90%, which means almost all the sediment delivered from the upstream catchment was deposited in the pond. This had never been cleared since the pond was built.

2 Methods

2.1 Sampling and measurements

Sediment samples were obtained from a core taken from the central part of the pond in March 2012, using a steel core tube (2 m long and

attached to a rod, internal diameter 9.6 cm, with a thin sleeve lining the inner wall of the tube). The tube was hammered vertically into the pond sediment deposits and was subsequently kept vertical during transport to the laboratory, in order to prevent disturbance or loss of material. The core was collected from a water depth of 2.5 m, and the sampling depth was 84 cm. Because of the homogeneous composition of the deposited sediment, it was impossible to identify flood couplets in the profile, and the core was sectioned at ~6 cm increments.

Two soil cores (A and B) were taken from two paddy fields using a 7.8 cm diameter core tube to a total depth of approximately 45 cm. The cores were subsequently sectioned at ~7 cm increments using a cutter. A total of 7 and 6 samples were obtained from cores A and B, respectively.

All samples were dried in an oven at 80°C, disaggregated, passed through a 2 mm mesh sieve and weighed prior to the measurement of the 137Cs activity of the < 2 mm fraction of each sample by gamma-ray spectrometry using an HPGe detector at 662 keV. The sample weight and the counting times were typically about 300 g and 25,000 s, respectively. The measurement precision was ca. ±6% at the 95% level of confidence.

The particle size distribution of sediment samples was measured using a Malvern Mastersizer 2000. In total, 27 samples were

Figure 1 The location of the study area and the land use in the study catchment.

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analyzed for 137Cs and 14 samples for grain size composition.

2.2 Calculation of surface erosion rate

Zhang and Walling (2005) proposed a model for using the 137Cs depth distribution in the sediment profile of a lake or pond to estimate the surface soil erosion rate for the eroding area in the contributing catchment. The model is described below:

Compared with the atmospheric 137Cs fallout before 1970, the 137Cs fallout after 1970 is insignificant, and thus the variation of the 137Cs activity in the sediment profile of a lake or reservoir after 1970 primarily reflects the variation of the 137Cs activity of sediment mobilized from the surrounding catchment and transported to the lake or reservoir.

For a cultivated catchment, the surface soil erosion rate (h, cm yr-1) within the eroding areas can be estimated by Equation (1):

1m m0 = [1 - ( ) ]nnh H C C (1)

where H (cm) is the depth of the plough layer; and Cm0 and Cmn represent the mean 137Cs concentration in the soil (Cm) at the beginning (Cm0) and end (Cmn) of the period of n years, respectively.

In a cultivated catchment, the sediment deposited in the lake or reservoir will have been eroded from the surface of the cultivated land and from the channel and other subsurface sources in the upstream catchment. In most situations it is reasonable to assume that the relative contributions from surface and subsurface sources will have remained essentially constant over the period of n years and that the grain size composition and 137Cs content of the deposited sediment are similar to those of the eroding source materials. The relationship between the 137Cs concentration within the sediment core at a given depth (d) at the beginning of the period under consideration (e.g., 1970,) (Cd0) and at the time when the core was collected (Cdn), is represented by Equation (2):

m d

m0 d0

= n nC CC C

(2)

Consequently, Equation (1) can be modified to: 1

d d0 = [1 - ( ) ]nnh H C C (3)

Attention now focuses on the upper part of the profile. By establishing the depth of the 1963 peak in 137Cs concentration, and assuming a constant annual rate of sediment accretion (between the 1963 peak and the surface of the sediment), the depth in the core representing 1970 can be estimated.

3 Results

3.1 The 137Cs profile in the sediment core

The depth distribution of 137Cs concentrations for the core retrieved from the pond is shown in Figure 2(A). In order to eliminate the effects of downcore changes in sediment bulk density on the shape of the 137Cs distribution, as well as to improve the accuracy of the calculation results, cumulative mass depth was used (He et al. 1996). The 137Cs activity extends to a depth of 90.40 g cm-2

in this core and varies with depth. The maximum 137Cs concentration with an activity of 9.78 Bq kg-1 is found at a depth of 73.34 g cm-2. This appears to be related to temporal changes in its global fallout and the occurrence of peak fallout in 1963. Above this level, the concentration decreases rapidly towards the sediment surface. The sediment profile depicted in Figure 2(A) shows that there is only one well-defined and relatively narrow 137Cs activity peak in the lower part of the profile corresponding to 1963, and that the 137Cs concentrations are relatively constant in the upper part (0~20 g cm-2) of the sediment profile.

From the vertical profile of sediment particle size for the same core, it was noted that silt (73.1-87.4%) dominated the sediment, and that clay (8.5~14.1%) and sand (0.7~16.7%) accounted for much smaller proportions (Figure 2(B)). Within the profile, clay has a relatively homogeneous vertical distribution, while the distribution of silt and sand varied little with depth except in the middle portion of the profile. In the bottom layer, the particle size is coarser than for other depth increments, and 137Cs was not detected within this layer. This indicated that the sediment found below a depth of 90.40 g cm-2 was the original base of the reservoir. We also found that the variation of 137Cs activity was linked to the relative importance of fine sediment particles.

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The 137Cs activity in the upper part of the core reflects the history of sediment input from the catchment, while the peak 137Cs activity in the lower part of the core is most likely associated with the peak fallout of 137Cs in 1963 from atmospheric nuclear testing，which reflects the general pattern of the atmospheric fallout input (Xiang et al. 2002; Navas et al. 2011). Even though there may be post-depositional redistribution, most of this post-depositional movement, such as that caused by diffusion and bioturbation, will not change the position of the peak 137Cs horizon (Ritchie et al. 1975; Putyrskaya et al. 2009).

3.2 The sedimentation rate in the pond

In the sediment profile from the pond, the maximum 137Cs activity was observed at a depth of 73.34 g cm-2, indicating that the sediment from depths ranging between 0 to 73.34 g cm-2 was deposited between 1963 and 2011 (Edgington 1991; Walling et al. 2003; Zhang 2006). The average deposition rate was calculated by dividing the

depth of 73.34 g cm-2 by the time (49 years). Consequently, the average sedimentation rate since 1963 was estimated to be 1.50 g cm-2 yr-1. Based on the frustum formula (Zhao and Wang 2009), which incorporates water surface area, water depth, bank slope, bulk density and deposition depth, the estimated total mass of sediment deposited in the pond since 1963 was estimated to be 1,553 t (Table 1). The high average sedimentation rate is consistent with the high erosion rate of the upstream catchment and the high catchment-lake surface area ratio.

3.3 Erosion and deposition rates in the catchment

From Equation (3), the surface erosion rate of the eroding areas was calculated. A value of 0.29 cm yr-1 was estimated for the value of Cdn/Cd0=0.55 and H = 20 cm (Li et al. 2009), which corresponds to an average soil erosion rate from the eroding areas of approximately 3,770 t km-2 yr-1 (assuming the average soil bulk density is 1.3 g cm-3). This is consistent with previous knowledge about the erosion rate of sloping cultivated land in this area (Li et al. 2012).

Figure 3 presents the depth distribution of 137Cs concentration in the sectioned cores collected from paddy fields in this catchment. The 137Cs distribution depths extend to 40 cm, and the 137Cs inventory is higher than the local 137Cs reference inventory of 1,296 Bq m-2 (adjusted to 2012, Li et al. 2009). This confirms that deposition has occurred in the paddy fields. According to our measurement results, the depth of the plough layer in paddy fields is about 30 cm. It means that about 10 cm has accumulated over the last 50 years, and thus the deposition rate in the paddy fields is 0.2 cm yr-1. This is equal to a soil deposition rate of 2,600 t km-2

yr-1 (assuming the average soil bulk density is 1.3 g cm-3).

In this small catchment, based on the land use conditions and our observations, the land used for housing and forest produced very limited amounts of sediment. So the main eroding zone of this catchment is the sloping cultivated land and the paddy fields located in the lower part of the catchment are an aggrading zone. The erosion rate of approximately 3,770 t km-2 yr-1 calculated from Equation (3) is representative of the average

Figure 2 The depth distributions of 137Cs (A) and particle size (B) for the sediment profile from the pond

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erosion rate for the sloping cultivated land located within the higher parts of the catchment. This reflects the fact that surface erosion within the catchment is not likely to occur over all or most of the catchment but will be concentrated within parts of the catchment. This is consistent with existing knowledge that cultivated slopes are the major soil erosion area and the major river sediment source in the TGRR.

3.4 The sediment budget

Based on the results presented above, the sediment budget of the study catchment was established as follows. In order to take account of the different time base of the estimates of the various components of the budget, the sediment budget was constructed for the period 1970-2011. The gross erosion since 1970 was estimated to be 3,135 t by using the 137Cs profile for the sediment core collected from the pond. This value has been derived as the product of the average erosion rate and the area of the sloping cultivated land. The same approach was used to estimate the deposition in the paddy fields of the lower slopes and the total

sediment accumulation (estimated as the product of the mean deposition rate and the area of the paddy fields) within the catchment since 1970, which is 1,561 t. The sediment budget should show a balance between sediment mobilization, deposition and storage and sediment output. The net soil loss from the watershed was estimated to be 1,574 t by subtracting the sediment deposited in the paddy fields from the gross erosion. This value should be similar to the alternative estimate of sediment output provided by the mass of sediment deposited in the pond since 1970. The latter value is 1,375 t (Table 1). There is quite close agreement between the two estimates of sediment output from the catchment, although the estimate derived from the mass of sediment deposited in the pond is smaller. The difference between the two values could reflect additional conveyance losses within the channel system during delivery of sediment to the catchment outlet (Porto et al. 2011), which

would result in the estimate of net soil loss being an overestimate, or a pond trap efficiency of less than 100%, which would result in the value of sediment output derived from the mass of sediment deposited in the pond being an underestimate. The good agreement between the two estimates of sediment output from the study catchment lends confidence to the overall sediment budget and suggests that the model developed by Zhang and Walling (2005) provides an effective alternative approach for estimating gross soil loss from catchments, such as those in the TGRR, from the 137Cs depth profile of a

Figure 3 Depth distribution of 137Cs concentrations of paddy fields of the catchment

Table 1 Distribution of bulk density and sediment mass of each layer in the profile

Cumulative mass depth (g cm-2)

Bulk density (t m-3)

Sediment mass (t)

0-5.25 0.75 112.52 5.25-9.15 0.80 85.50 9.15-14.21 0.84 107.46 14.21-19.89 0.95 121.22 19.89-26.92 1.17 148.89 26.92-33.92 1.17 148.48 33.92-41.47 1.26 159.49 41.47-49.35 1.31 165.37 49.35-56.85 1.25 157.38 56.85-64.89 1.34 168.26 64.89-73.34 1.41 176.57 73.34-81.81 1.41 176.09 81.81-90.41 1.43 178.12 90.41-99.55 1.52 188.81

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sediment core retrieved from a pond, rather than from soil cores collected within the catchment (Walling et al. 2002; Faleh et al. 2005; Fang et al. 2012). The latter approach commonly involves a considerable number of cores, in order to provide a representative result, and will therefore prove more time consuming and costly.

Based on the estimates of net soil loss (1,574 t) from the catchment and the mass of gross erosion (3,135 t) from the sloping cultivated land, the sediment delivery ratio (SDR) of the catchment is estimated to be ~0.5. Existing knowledge of the magnitude of SDRs in the TGRR suggest that values are likely to be in the range 0.3-0.8 (Jing et al. 2010). The value of SDR obtained in this study is consistent with this range, since the SDR can be expected to be influenced by many factors, including the drainage area, land use and soil properties (Vigiak et al. 2012).

The importance of sediment deposition within the study catchment demonstrated by the sediment budget and the associated SDR of ~0.5 reflects two important controls. First, it is well known that the sediment yield at the outlet of a watershed may not reflect the actual soil erosion rates within the watershed (Walling 1983; Nearing et al. 2005), because delivery of eroded soil to the outlet of the watershed is commonly reduced by deposition. In this study, the topography is relatively simple and the upper parts of the catchment are steeper than other parts. Erosion is therefore likely to occur primarily from cultivated land in this zone. Secondly, according to the local land use tradition, the original long hill slopes have been subdivided into several slope segments, characterized by short slope lengths and narrow slope widths. This will reduce soil loss by water and promote on-slope deposition. The lower parts of the leveled terraces with gentle slope gradients have been used for planting rice for centuries. They greatly increase the potential for deposition and storage. Runoff from upslope areas will be intercepted by the paddy areas and a substantial proportion of the suspended sediment mobilized by erosion from the upper hill slopes will be deposited in the paddy fields before reaching the pond (catchment outlet), which means that the paddy fields intercept much of the sediment mobilized by erosion. Furthermore, the difference between gross and net soil loss will reflect the fact that surface erosion within the

catchment is not likely to occur over most of the catchment, but be concentrated within a relatively small area. This is consistent with existing knowledge, which indicates that cultivated slopes are the primary area affected by soil erosion and the primary source of river sediment in the TGRR (Lu and Higgitt 2000).

4 Conclusions

This study of soil redistribution and the sediment budget in a small agricultural catchment in the TGRR has demonstrated that the 137Cs dating method described is an effective means of determining the rate of sediment accumulation in small ponds with high deposition rates, such as those in the TGRR. This study also successfully exploited the potential for using the 137Cs profile in the pond sediment to estimate the gross erosion rate in the upstream catchment.

The average sedimentation rate estimated since 1963 is 1.50 g cm-2 yr-1 and the corresponding amount of sediment deposited is 1,553 t. The gross erosion rate from the sloping cultivated land and the sedimentation rate within the paddy fields were estimated to be 3,770 t km-2 yr-1 and 2,600 t km-2 yr-1, respectively. The mass of sediment mobilized by erosion and the mass of sediment deposited within the catchment during the period 1970-2011 are estimated to be 3,135 t and 1,561 t, respectively, and the sediment delivery ratio of the catchment of the same period is therefore ~0.5.

The high land surface soil erosion rate and relatively low SDR indicate that the sloping cultivated lands are the primary soil erosion area and the paddy fields are aggrading zones in this catchment. The data also suggest that the typical land use pattern (with the upper parts representing sloping cultivated land and the lower parts paddy fields) plays an important role in reducing the sediment yield from typical agricultural catchments in Three Gorges Reservoir Region.

Acknowledgements

This work was funded by National Key Technology R&D Program (Grant No.2011BAD31B03), the Action Plan for West

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Development of Chinese Academy of Sciences (Grant No. KZCX2-XB3-09), the National Natural Science Foundation of China (Grant Nos.41201275, 41101259, 41001163), Western Light-Western Doctor of CAS. The authors would also like to thank Prof. Xin-bao Zhang from Institute of

Mountain Hazards and Environment, Chinese Academy of Sciences, the anonymous reviewers and editors for constructive comments on the manuscript. The language editing by Prof. Iain Taylor from the University of British Columbia is highly appreciated.

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