Using Cs-137 fingerprinting technique to estimate sediment deposition and erosion rates from Yongkang depression in the karst region of Southwest China

land degradation & development

Land Degrad. Develop. 21: 474–479 (2010)

Published online 17 March 2010 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/ldr.983

USING CS-137 FINGERPRINTING TECHNIQUE TO ESTIMATE SEDIMENTDEPOSITION AND EROSION RATES FROM YONGKANG DEPRESSION

IN THE KARST REGION OF SOUTHWEST CHINA

X.-Y. BAI1,2,4, X.-B. ZHANG1,3*, H. CHEN2 AND Y.-B. HE1,4

1Key Laboratory of Mountain Environment Evolvement and Regulation, Institute of Mountain Hazards and Environment,Chinese Academy of Sciences, Sichuan 610041, PR China

2Institute of South China Karst, Guizhou Normal University, Guizhou 550001, PR China3State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guizhou 550002, PR China

4Graduate School of Chinese Academy of Sciences, Beijing 100029, PR China

Received 21 October 2009; Revised 21 January 2010; Accepted 21 January 2010

ABSTRACT

Understanding the erosion and deposition rates is very important for designing soil and water conservation measures. However, existingmethods of assessing the rates of soil loss present many limitations and are difficult to apply to in karst areas, and there is still very little data inthis areas. Karst depressions comprise geomorphologically important sources and sinks for sediments and can provide the long-term historyrecords of environmental changes. But there have been few similar studies focused on its sediments in the world. In this paper, the Cs-137technique was employed to estimate the sediment deposition rate of karst depression to assess the surface erosion. The results indicate that theaverage deposition rate, deposition amount and specific deposit yield for the Yongkang catchments since 1963 were estimated to be4�32mmy�1, 3�16 t y�1and 20�53 t km�2 y�1, respectively. The results obtained were consistent with the actual monitoring data of localrunoff plots, and confirm the validity of the overall approach. So it was suggested that the mean specific sediment yields of 20 t km�2 y�1 canbe representative of the soil loss rates in the regions. Copyright # 2010 John Wiley & Sons, Ltd.

key words: sediment production records; karst depression; caesium-137; erosion; PR China

INTRODUCTION

Soil erosion has been a global environmental problem, and

information on rates of soil loss is an important requirement

both for quantifying the problem and for developing

improved land management and soil conservation practices.

However, it is a pity that reliable data on soil erosion rates is

very limited in karst areas (Karst Research Group of the

Institute of Geology, 1987; Cao and Yuan, 2005; Ford and

Williams, 2007). One of the most important reasons is that,

existing traditional methods of assessing rates of soil loss

possess many limitations and are hampered by a range of pro-

blems, because there is a dual structure in karst environment

including ground and underground drainages (Xiong et al.,

2002;Wang et al., 2002, 2004a and 2004b; Ford and

Williams, 2007).

Although 137Cs tracer technique has been widely used in

homogeneous soils over the world (Zhang et al., 1990, 1997,

*Correspondence to: X.-B. Zhang, Institute of Mountain Hazards andEnvironment, Chinese Academy of Sciences, Chengdu, Sichuan, 610041,PR China.E-mail: [email protected]

Copyright # 2010 John Wiley & Sons, Ltd.

1998; Owens et al., 1999;Walling et al., 1999; Zapata, 2003;

Ritchie et al., 2004), it is also difficult to apply to limestone

soil of carbonate rock slopes. Besides the dual structure’

effects, another important reason is that, 137Cs is absorbed

by soil particles instead of stones. Bared bedrock, discon-

tinuous soil covering and various proportions of gravels and

different depth of soil layer result in strong heterogeneity of

the soil in karst slopes. So it will face the great challenge to

apply this technique in karst areas.

As we all know, lake sediments can provide the long-term

history records of environmental changes (Albrecht et al.,

1998; Zapata, 2003). We found that karst depressions are

widespread and numerous in many karst areas (Cao and

Yuan, 2005; Ford and Williams, 2007), as an unique and

marvelous landform, and there have been few similar studies

focused on its sediments by using 137Cs technique in the

world, although it as an excellent tracer has been also com-

monly used for determining sedimentation rates in various

deposition environments such as floodplains, lakes, estu-

aries, reservoirs, and deltas. So it is the core of this paper to

use 137Cs fingerprinting techniques to estimate sediment

deposition and erosion rates from karst depressions.

Figure 2. Geographical environment and sampling site.

ESTIMATING SEDIMENT DEPOSITION AND EROSION RATES 475

STUDY AREA

Yongkang depression is located near Yongkang Town, Libo

County, Guizhou Province, China, which is a typical closed

catchments (Figure 1). The drainage area of the catchments

is 0�154 km2, and the elevations varied between 860 and

1020m. Most of the catchments are underlain by limestone

of Late Carboniferous Datang Group (C1d). The local

climate is subtropical monsoon, with an average annual

rainfall 1470mm, and the average annual temperature is

198C. The rainy season (April–August) precipitation

accounts for 83 per cent of the annual precipitation, and

floods happen frequently in the wet season, while average

monthly precipitation is below 60mm in the dry season from

October toMarch each year. Most soils in the catchments are

clay limestone materials, which are the weathering products

from limestone. The depression with a length of about 420m

and a width of about 280m is entirely cultivated at present

and there is a sinkhole in the western part of the depression.

The natural vegetation of the study area is dominated by

calcicole shrubs and drought tolerant herbs which flourish on

the thin soils covering the slopes.

FIELD SAMPLING AND LABORATORY

MEASUREMENT

Sampling was undertaken at Luoyang Town next to the

Yongkang catchment for the local 137Cs reference inventory

in November, 2007. Ideally the core for the reference

inventory profile, which is used to estimate the total or

reference fallout, should be collected from flat undisturbed

land. However, it is difficult to find flat land which has not

been cultivated since the mid-1950s in the study area,

therefore, the core from a flat cultivated land was selected

for the purpose. The sampling site with an area of about

110m2 (20m� 6m) was a flat paddy terrace covering the

Quaternary red clay soil. 31 soil bulk samples and 1 soil

Figure 1. Geographical position of study area. This figure i


depth incremental sample were collected by grid sampling

with a spacing of 2m� 1�2m. Bulk samples were collected

using a 6�5 cm diameter core tube, which was propelled into

the ground manually to a depth of 30–40 cm. Sampling to

this depth ensured that the total 137Cs inventory of the profile

was measured. The method for collecting the depth

incremental samples was similar to the one for soil profile

collection in the depression.

In the same time, three incremental samples were

collected at different positions of Yongkang depression

(Figure 2). Angles and distances were measured from an

electronic total station to the boundary points of this

depression under survey, and the coordinates (X, Y, and Z) of

surveyed points relative to the total station position were

calculated using trigonometry. And then the depression area

of 610m2 was also calculated according to the coordinates

of the boundary points.

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LAND DEGRADATION & DEVELOPMENT, 21: 474–479 (2010)

476 X.-Y. BAI ET AL.

137Cs activity of the soil samples were measured in the

Isotope Laboratory of Department of Physics, Sichuan

University, China. All samples were air-dried, disaggre-

gated, passed through a 2mm sieve and weighed. The137Cs content of the <2mm fraction of each sample was

measured by g spectrometry using a hyper pure coaxial

germanium detector and multichannel analyzer system. The

samples have a weight of �250 g. 137Cs was detected at

662 keV and counting times were more than 3,3000 s,

providing results with an analytical precision of approxi-

mately �5 per cent at the 90 per cent level of confidence.

Particle size distributions of the sediment samples were

measured in the Soil Laboratory of Chengdu Institute of

Mountain Hazards and Environment, Chinese Academy of

Sciences, by a laser particle size analyzer with a measuring

range of 0�1–600mm, accuracy error �5 per cent and

repeatability error �3 per cent, respectively.

RESULTS AND ANALYSIS

137Cs Reference Inventory and the Plough Layer

Among the 32 samples, 137Cs inventories varied from 754�6–1273�8Bqm�2, with a coefficient of variation (CV) of 14�5per cent. The average value of 997�7Bqm�2 was considered

as the 137Cs reference inventory of the study area. The depth

of plough layer could be estimated from the 137Cs depth

distribution of the reference profile due to the similar tillage

method between the 137Cs reference site and the depression

in this study. Depth distribution of 137Cs in reference site was

illustrated in Figure 3. 137Cs was almost evenly distributed

within the 0–15 cm topsoil, while the 137Cs concentrations

Figure 3. Depth distributions of 137Cs in the reference profile.


ranged between 4�32 and 5�65Bq kg�1, with a mean value of

5�03Bq kg�1, and then 137Cs concentration decreased as the

depth increases from the depth of 15cm. The result of our

actual measurement using the rule was also about 15cm. And

the popular saying of local peasants holds true: the depth

of the plough layer is 15 cm. All these confirmed the depth of

local plough layer is 15 cm. So it is decided that the depth of

plough layer is 15 cm for the study field in this paper.

The Proof of Sediment

Depth distributions of 137Cs of the three profiles is shown in

Figure 4, and depth distributions of fine particle contents see

Figure 5. Among the three cores collected from the

depression field, 137Cs inventories were respectively

1585�40, 1736�13, and 2888�26Bqm�2, and their mean

was 2069�93Bqm�2. The local 137Cs reference inventory

was 997�70Bqm�2 in the same time. It was indicated that

deposition occurred in the depression because all the137Cs inventories of the three cores were greater than the137Cs reference inventory. In addition, each 137Cs distri-

bution of the three profiles is deeper than local 137Cs

reference depth and the plough layer. All these proved that

the deposition process has happened in the depression.

Calculating the Depth of Deposited Sediment137Cs has been widely used for dating of undisturbed soil

profiles such as lakes and reservoirs deposits (Albrecht et al.,

1998; Owens et al., 1999;Walling et al., 1999; Zapata, 2003;

Ritchie et al., 2004). The expected 137Cs depth profile,

characterized by a single well-defined peak in 137Cs activity

for the year of 1963 (Figure 6a) which the 137Cs maximum

fallout flux occurred and the mean deposition rate since 1963

can be calculated based on the depth of the peak in137Cs activity as following equation:

R ¼ H

n� 1963(1)

where, R is the deposition rate (cm y�1),H is the depth of the

peak in 137Cs activity (cm), and n is the sampling year.

The karst depression bottom land is often cultivated and

the deposited sediments will be mixed into the plough layer

by plough activities. The 137Cs distribution depth at a

deposition site is greater than the depth of the plough layer

and the inventory is usually greater than the local reference

inventory (Figure 6b). Assuming the 137Cs fallout pre-

cipitated totally on the ground in 1963, the sediment

deposition depth was derived from the following equation:

DH ¼ H � Hp (2)

where, DH¼ the sediment deposition depth since 1963

(cm);H¼ the total 137Cs distribution depth in a profile (cm);

Hp¼ the plough layer depth (cm);


Figure 4. Depth distributions of 137Cs in Yongkang depression profile.


Depth distributions of 137Cs of the three profiles are

shown in Figure 4.The deposition depths since 1963 of the

three cores were estimated according to the distribution

depths of 137Cs and the depth of the plough layer. However,

the second core maybe is more exact to reflect deposition

than the first and the third cores, because it located in the

centre of the depression bottom, deposited well and was

Figure 5. Depth distributions of fine particle c


more carefully sectioned. 137Cs concentration of the second

core was almost evenly distributed within the 0–34 cm top

soil, while the 137Cs concentrations ranged between 5�57 and7�29Bq kg�1, with a mean value of 6�27Bq kg�1, and then137Cs concentration decreased as the depth increases from

the depth of 34 cm. This indicated the deposition depth since

1963 is 19 cm, because local 137Cs reference depth and the

ontents in Yongkang depression profile.


Figure 6. Characteristic depth distributions of 137Cs in lakes deposits andcultivated soils. (a) In uncultivated land; (b) in cultivated land.

478 X.-Y. BAI ET AL.

plough layer is 15 cm. So the average deposition rates since

1963 is 4�32mmy�1.

Interpreting Sediment Production Records

Deposition amounts and specific sediment yields were esti-

mated according to the mean deposition depth of the core

since 1963.The area of sediments and the area of the drai-

nage are 610m2 and 0�154 km2, respectively, and depression

sediments had a mean bulk density of 1�2 tm�3. So the

specific deposit yield can be calculated by the following

equation:

R ¼ D � H � SA � ðn� 1963Þ (2)

where R is land surface erosion rate of the catchments

(t km�2 y�1), D is the soil bulk density (t m�3), H is the

sediment deposition depth since 1963 (m), S is the area of the

depression (m2), n is the sampling year (a), A is the area of

catchments (km2).

The mean deposition rate, deposition amount, and specific

deposit yield for the Yongkang catchments since 1963

were estimated to be 4�32mmy�1, 3�16 t y�1, and

20�53 t km�2 y�1, respectively. By the monitoring data of

six runoff plots during the period of July 2007–

February 2008, in the Chenqi Gully, Puding, Guizhou and

specific sediment yields were very low and ranged between

1�0 and 32�4 t km�2 y�1(Peng et al., 2009). The geology and

landscape conditions in the Chenqi Gully are similar to

Yongkang Depression. As above mentioned, it was

suggested that the mean specific sediment yields of


20 t km�2 y�1 can be representative for the soil loss rates

in the regions.

It is important to note, however, that sinkholes are widely

distributed in karst depressions (Ford andWilliams, 2007; Li

et al., 2008), and a depression usually has at least one

sinkhole. It may be a relative large estimating error due to

excess sediments lost from the sinkhole. However, the

sinkhole of this depression was blocked, and it is flooded

more than twice a year and its waterlogging time is about 7

days. For this frequently flooded depression, where flood

water drains very slowly and most of sediments will deposit

into depressions (Ford and Williams, 2007), except very

little sediments flow out through sinkholes. According to the

study result of detention time (Brune, 1953; Cao and Yuan,

2005), trap efficiency of Yongkang depression is about 78

per cent. But these needs to be further studied and maybe are

the nest research priority what should be done in the future.

CONCLUSIONS

The results presented in this paper indicated that Yongkang

depression of China is a representative and significant sink

of the soils being eroded from the upland slopes, and deposit

rates of sampling sites were estimated by 137Cs technique for

the period from 1963 to 2007. The mean deposition rate,

deposition amount and specific deposit yield for the

Yongkang catchments since 1963 were estimated to be

4�3 2mmy�1, 3�16 t y�1, and 20�53 t km�2 y�1, respectively.

The results obtained were consistent with the actual

monitoring data (Peng et al., 2009) of local runoff plots,

and confirm the validity of the overall approach. So it was

suggested that the mean specific sediment yields of

20 t km�2 y�1 can be representative of the soil loss rates

in the regions. The study reported above appears to offer

valuable potential to understand land surface erosion of

karst landform areas by estimating sediment deposition

rates of karst depressions for using the 137Cs technique,

rather than assessment of complicated soil erosion in stony

soils of carbonate rock slopes. All these conduce to our

understanding of soil loss and sedimentation in karst

environment.

ACKNOWLEDGEMENTS

The authors express their sincere thanks to the three

reviewers for their valuable suggestions and comments that

enabled us to improve the paper substantially. This research

work was jointly supported by the State Key Basic Research

Plan (Grant No. 2006CB403200), the National Key Tech-

nology R&D Programme (Grant No.2006BAC01A09 and

2008BAD98B07), and the National Natural Science

Foundation of China (Grant No. 40721002).



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Using Cs-137 fingerprinting technique to estimate sediment deposition and erosion rates from Yongkang depression in the karst region of Southwest China