Akitsu Kimoto

Li Changhua

Takahisa Mizuayma

Taro Uchida

Yuko Asano

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The Limitation of Using Cesium-137 for Investigating Soil Erosion
in Devastated Hills in Southeastern China
Taro Uchida1*, Takahisa Mizuayma1, Akitsu Kimoto1, Yuko Asano1 and Li Changhua2
1Graduate School of Agriculture, Kyoto University, Kyoto, 6068502, Japan
2Chinese Academy of Science, Beijing, 100101, China
*E-mail: uchy@kais.kyoto-u.ac.jp
1 Introduction
The forest vegetation in the central part of Jiangxi Province, south China is subtropical coniferous
forest.  Most of the forest consists of Pinus massoniana, and are strongly devastated due to deforestation
resulting from cutting and litter removal by the inhabitants for fuel use, and also from over-grazing.
There are, therefore, many devastated hills where bare soil is exposed.  Consequently, both the direct
runoff percentage and the sediment yield are very high in the headwater catchments around this region.
This very high erosion rate causes damage downstream impeding water traffic through obstruction due to
the aggradation of the riverbed.  These problems are commonly found in south China.  In the devastated
hills in south China, although evaluation of soil erosion rate is one of the most important factors for basin
management, very few information have been obtained in south China.
The 137Cs technique have used for estimating net (ca. 40 years) soil flux in many environments (e.g.,
Walling and Quine, 1990) and for determining spatial patterns of soil loss and sediment accumulation
(Walling and Quine, 1991; Chappell, 1999).  The technique has been used widely because the 137Cs
technique overcomes a number of the problems of monitoring soil erosion and deposition rate over the
medium-term.  In this respect, it can be thought that the 137Cs technique offers the greatest potential for
measuring net soil flux and for evaluation of spatial variability of soil erosion in devastated hills where
information about soil erosion rate over the medium-term are limited.  The studies in Loess Plateau have
supported the potential of the 137Cs technique (e.g., Zhang et al., 1998). Recently, the 137Cs technique have
been used for estimating net soil flux in Chinese agricultural environments (e.g., Higgit et al., 2000).
However, the 137Cs technique has not been used for the evaluation of soil erosion characteristics in
devastated hills.  Thus, to examine the applicability of the 137Cs technique for the evaluation of soil flux
from the devastated hill, we measured the spatial variability of 137Cs activity, the sediment yield in terms
of catchment scale and surface erosion depth, and carried out in-situ sprinkling experiments at Dahou
experimental watershed, central part of Jiangxi Provience, China.
2 Dahou experimental basin
This study was conducted at the Dahou experimental watershed.  The watershed is located in Dahou
Village of Jiangxi Province in southern China (115°28′E, 26°30′N).  The altitude of this watershed ranges
from 340 m to 420 m above sea level.  The dominant tree species is Pinus massoniana.  The bedrock is
weathered granite, and there is no significant organic soil generation.  Annual precipitation is 1,870 mm
and the mean annual temperature is 17.6 .  The stream channel depth ranges from 0.4 m to 2.0 m and
width ranges from 0.2 m to 2.0 m (Figure 1).
The experimental hillslopes were classified into two based upon vegetation and topography (Figure 1).
The lower part of the hillslope within laterally 10 m—20 m from the stream (Figure 1) is relatively gentle,
ranging from 15 to 25 degrees.  Vegetation on the lower part is denser than on the upper part.  The mean
height of Pine trees is 3.6 m and the mean density is 0.8 trees/m2 at the lower part.  Moreover the land
surface is covered by a microbiotic crust (<1 mm) with sand and silt grains.  Removal of microbiotic crust
increased saturated hydraulic conductivities four times larger than before the removal (Uchida et al.,
2000).
12th ISCO Conference                                                                                                  Beijing 2002
22
In contrast, the gradient of the upper part of the hillslope is steeper, ranging from 25 to 40 degrees.
The mean height of the Pine trees is 1.6 m and the mean density is 0.5 trees/m2.  There is no ground cover
vegetation and weathered granite is exposed at the surface.  Gullies develop parallel to each other on the
hillslope.  The gully runs almost perpendicular to the slope connecting the lower and upper parts of the
hillslopes.  The width and depth of the gullies are 2.0 m —5.0 m and 0.5 m—2.0 m, respectively.
Fig. 1 Schematic illustration describing the mechanism of the sediment production and
transport in Dahou Experimental watershed
3 The mechanism of the sediment production and transport in Dahou watershed
To understand the surface erosion processes in Dahou Experimental watershed, we measured
sediment yield and surface erosion depth in terms of catchment scale, and carried out in-situ sprinkling
experiments, focusing on the spatial variability of surface conditions and sediment sources (Kimoto et al.,
1998; Uchida et al., 2000).  The results of sediment yield measurements at the end of headwater
catchment suggest that surface water is capable of transporting more material than is supplied by
detachment.  That is, the sediment discharge process is detachment-limited.  In-situ sprinkling
experiments indicate that in the lower part of the hillslope where the surface is covered by microbiotic
crust, the ratio of surface runoff to rainfall is high, but sediment yield is small.  In comparison, in the
upper part of the hill slope where there is no microbiotic crust, the ratio of surface runoff to rainfall is low,
but sediment yield is large.  Measurements of surface erosion depths showed that the surface erosion
depth is almost zero at the lower part of hillslope, indicating that microbiotic crusts provide a high degree
of protection against soil erosion, even though the surface runoff ratio is large at the lower hillslope.
From all of these investigations, a conceptual model describing the mechanism of the sediment
production and transport in this watershed is shown in Figure 1.  On the upper part of the hillslope as
weathered bedrock with no vegetation cover, a large quantity of unstable sediment is detached, although
overland flow is relatively small compared to that on the lower part.  While, on the lower part of the
hillslope, the detachment of sediments is small because of the microbiotic crusts even though the surface
runoff is much greater than that of the upper part.  When overland flow occurs, detached sediments from
the upper part are transported to the gully.  Sediment and water concentrate in the gully, and the
sediments are transported to the lower part along the gully.  Since the ratio of surface runoff to rainfall is
very large, even though the lower part of the hillslope is relatively gentle, the sediment transport capacity
of the flow along the gully is not limited. Consequently, deposition does not occur on the lower part of the
hillslope and the sediment is transported to the stream.  The upper part of the hillslope with no vegetation
is the source of the sediment, while the lower part of hillslope with microbiotic crust is the main source of
surface runoff water and the transit zone of sediment (Figure 1).
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4 Method
Soil samplings were conducted at three slope positions (Locations 1, 2 and 3: Figure 1).  At each
location, four 100 cm3 soil samples were collected from shallow five depths (0 cm—2 cm, 2 cm—5 cm,
5 cm—10 cm, 10 cm—20 cm, 20 cm—30 cm) and two samples was collected from deeper soils using a
core sampler.  These samples were divided into fine soil (grains < 2 mm in diameter), gravel (> 2 mm),
roots, and litter.  The 137Cs activity in fine soil was measured through direct gamma spectrometry using a
high purity gamma detector and a multichannel analyzer..
To evaluate carbon content, soil samplings were collected from two sampling points per location.  At
each sampling point, a 400 cm3 soil sample was collected from each of five depths (0 cm—5 cm, 5 cm—
10cm, 10 cm—20 cm, 20 cm—30 cm, and 30 cm—50 cm) using a core sampler.  These samples were
divided into fine soil, gravel, roots, and litter.  Fine soil was ground into powder and total carbon content
was determined using an NC analyzer (NC-900, Shimadzu).
5 Results
The total 137Cs activity in soil layer at the lower part of hillslope was lower than that at the upper part
(Figure 2).  There was significant difference in 137Cs concentrations in the soil profile between the upper
and lower part of hillslopes (Figure 3).  The 137Cs concentration at the lower part of the hillslope
decreased with the increasing of thesampling depth and the 137Cs concentrated predominantly in the top
10-cm of the soil profile.  The 137Cs activity in soil layer below 30 cm was not detected (Figure 3).  In
contrast, the 137Cs activity in the upper part was not related to the sampling depth (Figure 3).  The 137Cs
concentration in the top 10cm of the soil profile at the upper hillslope was lower than that at the lower
hillslope (Figure 3a), while the 137Cs concentration in soil layers from 20 cm to 100 cm depths at the
upper hillslope was higher than that at the lower hillslope (Figures 3b and 3c).
Fig. 2 Total 137Cs activity in soil layer at Location 1 (L1), 2 (L2) and 3 (L3)
Fig. 3 The depth distribution of 137Cs activity at Location 1 through 3.  The fine lines show
the estimated 137Cs activity using Takenaka’s (1998) equation.
24
The carbon content in soil layer was very low, ranging from 0.1% to 1.3 % (Figure 4).  The carbon
content at the lower part of the hillslope exponentially decreased with increasing of the depth (Figure 4a).
The carbon content in the top-20 cm soil was relatively high, compared to the deeper soil (Figures 4b and
4c).  Anyway, the carbon content at the upper part of the hillslope was extremely low (< 0.3 %).
Fig. 4 The depth distribution of carbon content
6 Discussions
6.1 Total 137Cs activity
Since we cannot find undisturbed sites near Dahou watershed, the baseline of the 137Cs at Dahou has
not been clarified.  Higgit et al. (2,000) showed that the baselines of the 137Cs at two sites in southeastern
China (Shaxian, Fujian and Yujiang, Jiangxi) were about 4,000 Bq m–2.  The 137Cs input was proportional
to average annual precipitation (e.g., Lance et al., 1986).  The annual precipitations in Shaxian (1,643 mm)
and Yujiang (1,733 mm) were similar to that of Dahou (1,870 mm), suggesting that the baseline of the
137Cs at Dahou was similar to that of Shaxian and Yujiang.  This indicates that 70 % of 137Cs was removed
from the lower part of the hillslope, suggesting that the net erosion rate over the medium-term at the
lower part of hillslope was very high.
Moreover, comparison with the total 137Cs activity on the upper part of the hillslope demonstrates
that the net erosion rate over the medium-term at the lower part of hillslope was greater than that at the
upper part.  However, erosion depth measurements indicated that a current erosion rate at the lower part is
almost zero (Uchida et al., 2000).  This confliction was probably caused by the spatial variability of the
infiltration rate.  A part of the fallout 137Cs did not penetrate into the soil layer at the lower part of the
hillslope, since the microbiotic crust promote surface runoff.
6.2 Profile of 137Cs activity
The 137Cs profile in the lower part of the hillslopes is similar to the characteristic exponential curve
of undisturbed areas worldwide (e.g., Walling and Quine, 1991; Chappell, 1999).  This concurs with the
previous conceptual model in this watershed (Figure 1), indicating that the small activity of 137Cs at the
lower part of the hillslope may not indicate the great erosion rate over medium-term.  While, the 137Cs
profile in the upper part of the hillslopes is similar to the characteristic of cultivated or depositional sites
(Walling and Quine, 1991), although the soil erosion depth measurements indicate that the upper part of
hillslope is a main source of discharged sediment.
Takenaka et al. (1998) reported that since most of cation exchangeable capacity has arisen from soil
organic matter, the spatial distribution in 137Cs activity could be characterized by the carbon content in an
undistributed forest.  In both parts, the pattern of 137Cs profile was similar to that of the carbon content.
Takenaka et al. (1998) also reported that the 137Cs activity (Cs) in red pine forest soil can be described by
Cs=92.1exp(–6.48exp(–0.691C)), where C is carbon content.  Using Takenaka’s equation and the
observed carbon content, the estimate 137Cs activity was shown in Figure 3.  The estimated 137Cs profile in
the lower part of the hillslopes was 1.5 times greater than the observed 137Cs profile, while estimated 137Cs
25
profile in the upper part was one order of magnitude smaller than that of observed.  This result supports
the previous suggestion that the rate of 137Cs penetration into soil layer at the lower part of the hillslope
was smaller than that of the upper part.
At the upper part of the hillslope, the penetrated 137Cs was not fully absorbed by the topsoil, since the
carbon content of the topsoil was extremely low.  Thus, large amount of 137Cs penetrated to the deeper
soil.  In contrast, the penetrated 137Cs at the lower part of the hillslopes was readily adsorbed by the
topsoil with relatively high carbon content.
7 Conclusions
At the lower part of the hillslope, the microbiotic crust did not allow considerable 137Cs penetration,
since the microbiotic crusts promote surface runoff (Figure 5).  The penetrated 137Cs was readily adsorbed
by the topsoil with relatively high carbon content.  Thus, the 137Cs activity exponential decreased with the
depth.  Since the erosion rate was small, we can see the exponential decrease of 137Cs activity.  In contrast,
at the upper part of the hillslope with no vegetation cover, the penetrated 137Cs was greater than that of the
lower part (Figure 5).  The penetrated 137Cs was not fully absorbed by the topsoil, since the carbon content
of the topsoil was extremely low.  Thus, large amount of 137Cs moved to the deeper soil.
The results of this study indicated that the 137Cs technique is difficult to use for the evaluation of soil
flux from the devastated hillslopes where the carbon contents were extremely low and the infiltration
capacity varied.  Additional approaches (i.e., in-situ experiments and measurements of erosion depth)
and/or information (i.e., spatial variability of infiltration capacity) are necessary for describing surface
erosion process and quantifying soil flux on the devastated hills.
Fig. 5 Schematic illustration describing the dynamics of the fallout 137Cs
Acknowledgement
We are gratefully to Mr. R. Tateno, Mr. T. Fukuyama and Dr. Y. Onda for assistance in conducting
the laboratory analyses.
References
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The Limitation of Using Cesium-137 for Investigating Soil Erosion in Devastated Hills in Southeastern China

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The Limitation of Using Cesium-137 for Investigating Soil Erosion in Devastated Hills in Southeastern China

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