Published online August 3, 2006
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
Partitioning the Contributions of Sheet and Rill Erosion Using Beryllium-7
and Cesium-137
Ming-Yi Yang,* D. E. Walling, Jun-Liang Tian, and Pu-Ling Liu
flected in their grain size selectivity. Young and Onstad
(1978), for example, reported the results of erosion investigations on loam, silt loam, and loamy sand soils in
Minnesota and showed that the soil eroded from interrill
areas had a higher sand content and lower clay content
than the bulk soil and the sediment eroded from rills.
Contrasts between the two erosion processes, relating
to the grain size composition of the mobilized sediment
and the depth within the surface soil horizon from which
the sediment is mobilized, are also likely to result in
contrasts in the nutrient and contaminant content of the
mobilized sediment. The precise timing of the transition
from sheet to rill erosion and the relative importance of
the two erosion processes may thus represent key features of the erosion processes operating during a storm
event. An improved understanding of these features and
their controls must be seen as an important requirement
for refining existing physically based erosion prediction
models. For example, several frequently used physically
based erosion models, such as the LISEM model (De
Roo et al., 1996), do not distinguish between interrill
and rill erosion. In the LISEM model, all erosion is
assumed to be rill erosion, although rills are not explicitly simulated. Other models, such as the WEPP
model (Nearing et al., 1990), do distinguish interrill and
rill erosion, but this model uses a simple steady-state
continuity equation to describe the transport of suspended sediment within the rills.
Any attempts to investigate further the timing of the
transition from sheet to rill erosion and the relative efficacy of the two processes faces problems because existing monitoring techniques are generally inadequate for
this purpose. In particular, it is difficult to document
temporal variation of rates of sediment mobilization or
soil loss associated with the two distinct processes during
an event. The conventional approach to documenting
the relative importance of interrill and rill erosion on
hillslopes is to establish an erosion plot and to measure
the total soil loss from that plot and the volume of the
rills generated during the event. If it is assumed that the
amount of soil mobilized by rill erosion is directly equivalent to the product of the rill volume and the bulk
density of the soil horizons into which the rills have cut,
then the amount of soil mobilized by interrill erosion
can be calculated by subtracting the estimate of the
mass of soil mobilized by rill erosion from the total
soil loss from the plot. However, this approach provides
no information on the timing of the transition from sheet
to rill erosion or progressive changes in the relative
importance of the two processes during an individual event.
ABSTRACT
An understanding of the relative contributions of sheet and rill
erosion to the total soil loss associated with individual events and
of changes in their relative importance during an event is important
for developing more effective soil erosion prediction models and for
predicting the physical and geochemical properties of the eroded
sediment in relation to those of the in situ soil. It is difficult to obtain
information on variations in the relative importance of sheet and rill
erosion during an event using traditional monitoring techniques. The
use of environmental radionuclides, particularly beryllium-7 (Be-7)
and cesium-137 (Cs-137), as sediment tracers offers an alternative
approach to obtaining this information. Because of their contrasting
depth distributions, the two radionuclides provide a means of distinguishing sediment eroded from the soil surface by sheet or interrill
erosion and that eroded from greater depths by rill erosion. The approach was tested using simulated rainfall experiments undertaken on
two small erosion plots located in Yichuan County, Shaanxi Province,
China, representative of cultivated land and uncultivated land previously under forest. The results confirmed that the development of
rills within the plots was clearly marked by changes in the Be-7 activity
of the sediment eroded from the cultivated plot and changes in the
Be-7 and Cs-137 activity of the sediment eroded from the uncultivated
plot. By relating the changes in the Be-7 and Cs-137 activity of the
eroded soil to information on the depth distribution of the two radionuclides in the soils of the two plots and the mass of soil eroded, it was
possible to estimate the relative contributions of the two erosion processes to the total soil loss and their changes through an event. This
information provided a basis for investigating the influence of temporal variations in the relative contributions of sheet and rill erosion
during an event on variations in the physical and geochemical properties of the sediment eroded from the plot during the event.
S
is a complex time-variant process. During the early stages of a heavy rainfall event that
causes erosion, sheet erosion occurs as a result of splash
detachment and detachment and transport by shallow
sheet flow. As the event proceeds, the flow frequently
becomes concentrated, and rills may develop. At this
time, the erosion processes include interrill and rill
erosion. Sheet erosion processes are dominant on the
interrill area, whereas rill erosion is dominant within the
rills. Interrill and rill erosion are characterized by different hydraulic conditions, and these may in turn be reOIL EROSION
M.-Y. Yang, J.-L. Tian, and P.-L. Liu, State Key Laboratory of Soil
Erosion and Dryland Farming on the Loess Plateau, Institute of Soil
and Water Conservation, Chinese Academy of Science and Ministry
of Water Resources, Northwest Sci-Tech University of Agriculture
and Forestry, Yangling, Shaanxi, China 712100; D.E. Walling, Department of Geography, University of Exeter, Amory Building, Rennes
Drive, Exeter EX4 4RJ, UK. Received 8 Sept. 2005. *Corresponding
author ([email protected]).
Published in Soil Sci. Soc. Am. J. 70:1579–1590 (2006).
Soil & Water Management & Conservation
doi:10.2136/sssaj2005.0295
ª Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
Abbreviations: Be-7, Beryllium-7; Cs-137, Cesium-137; LISEM, Limburg Soil Erosion Model; WEPP, Water Erosion Prediction Project.
1579
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
1580
SOIL SCI. SOC. AM. J., VOL. 70, SEPTEMBER–OCTOBER 2006
There is a need to develop improved methods for
investigating the time-variant behavior of sheet and rill
erosion, including the transition from sheet to rill erosion and intra- and inter-event changes in the relative
importance of the two erosion processes. Environmental
radionuclides have proved to be valuable in providing
new approaches to assessing erosion rates and in providing new insights into the spatial patterns and processes involved (e.g., Walling, 1998; Walling et al., 1999;
Zapata, 2003), and scope exists to exploit their potential
further in obtaining an improved understanding of the
relative importance of interrill and rill erosion processes
and of variations in their contribution to the total soil
loss during an event. Walling and Woodward (1992)
were among the first to point to the potential for using
the contrasting depth distributions of beryllium-7 (Be-7)
and cesium-137 (Cs-137) in cultivated and noncultivated
soils to trace or fingerprint fluvial suspended sediment
derived from cultivated and noncultivated areas and
from different levels within the soil. Wallbrink et al.
(1999), Whiting et al. (2001), and Matisoff et al. (2002)
developed this approach further by including excess
lead-210 (Pb-210 excess) as a further tracer and using
mathematical functions to describe the depth distributions and numerical mass balance or mixing models to
establish the relative proportions of the sediment derived from surface or sheet erosion or from rill and gully
erosion. The focus of their work was primarily catchment scale assessment of suspended sediment sources,
but Wallbrink and Murray (1993) also identified the potential for using Be-7, Cs-137, and Pb-210 excess measurements to study the erosion processes operating on a
small plot and to investigate the transition from sheetdominated erosion to rill-dominated erosion. In this
case, however, the interpretation of changes in the radionuclide activity of the eroded sediment was essentially
qualitative. We attempt to develop this previous work
further by focusing on small plots and the relative efficacy of sheet and rill erosion and by using numerical
mixing models to document variations in the relative
contributions of sheet and rill erosion to the total sediment output from the plot during an event. Previous
work using mixing models has commonly attempted
only to use the radionuclide measurements to provide a
temporally lumped estimate of the relative importance
of surface erosion and rill erosion (e.g., for an individual event). If samples of the sediment output from an
erosion plot are collected, measurements of the Be-7
and Cs-137 activity of the eroded sediment and analysis
of temporal variations in those activities can provide an
effective means of establishing the timing of rill initiation and changes in the relative contributions of sheet
and rill erosion to the sediment output through time. An
experimental investigation involving the monitoring of
two small erosion plots under simulated rainfall, one
located in a cultivated area and the other in an adjacent
uncultivated area, was undertaken to explore further the
potential of this approach. The investigation was undertaken in Yichuan County, Shaanxi Province, in the Loess
Plateau region of China (Fig. 1).
Fig. 1. Location of the study area.
BACKGROUND TO THE APPROACH
Beryllium-7 is a short-lived environmental radionuclide of natural origin with a half-life of 53 d. It is
produced in the upper atmosphere by the spallation of
nitrogen and oxygen by cosmic rays. The nuclear reaction produces BeO or Be(OH)2, which diffuses through
the atmosphere until it attaches to an atmospheric aerosol. Subsequent deposition to the land surface occurs
as wet and dry fallout (Olsen et al., 1985; Rosner et al.,
1996; Ródenas et al., 1997), although available measurements suggest that Be-7 fallout is primarily associated
with precipitation (Murray et al., 1992, 1993; Wallbrink
and Murray, 1994). In most environments, Be-7 fallout
reaching the soil surface is rapidly and strongly fixed by
clay particles in the surface soil, and its occurrence is
restricted to a shallow surface layer, within which Be-7
activities exhibit a rapid exponential decline with depth.
Measurements reported by Wallbrink and Murray (1993)
indicated that Be-7 is rarely found below a depth of
10 mm, although Blake et al. (1999) reported a maximum depth of 24 mm. Its restriction to a shallow surface
layer reflects its fallout origin and its short half-life.
Although other fallout radionuclides, such as Cs-137 and
excess Pb-210, are commonly found to much greater
depths in uncultivated soils due to downward migration
caused by processes such as bioturbation, these processes operate over time scales that are much longer
than the residence time of Be-7 inputs because of the
short half-life of this radionuclide. Although tillage
mixing of cultivated soils could transfer Be-7 to greater
depths than those indicated previously, the associated
activities are low and generally below detection and
rapidly reduce further due to the short half-life of the
radionuclide. Typical depth distributions of Be-7 in uncultivated and cultivated soils for a location in Western
Europe are provided in Fig. 2a and 2b, respectively. Because of this characteristic depth distribution and, more
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
YANG ET AL.: PARTITIONING CONTRIBUTIONS OF SHEET AND RILL EROSION
1581
Fig. 2. Typical Be-7 and Cs-137 depth distributions in uncultivated (a, c) and cultivated (b, d) soils in western Europe.
particularly, because it is found only close to the soil surface, Be-7 offers considerable potential as a tracer of
sheet erosion. Soil mobilized from the surface by sheet
erosion should be characterized by relatively high Be-7
activity, although the activity progressively declines if
erosion rates are high and sediment is subsequently mobilized from the lower part of the exponential depth
distribution. In contrast, sediment mobilized by rill erosion is characterized by much lower activities, because
rills commonly cut down into the soil below the Be-7 rich
surface layer, and much of the mobilized sediment is
therefore likely to contain little or no Be-7 activity.
In contrast to Be-7, Cs-137 is a man-made radionuclide, primarily associated with the atmospheric testing
of nuclear weapons during the period from the mid
1950s until 1963, when the Test Ban Treaty was signed.
Fallout occurred globally, although the amounts received were much less in the southern hemisphere than
in the northern hemisphere (Walling, 2002). Additional
fallout occurred in some areas of the world in 1986 as a
result of the Chernobyl accident, but this was mainly
restricted to regions adjacent to Chernobyl. Again in
contrast to Be-7, Cs-137 fallout has essentially ceased,
and there is no replenishment of existing inventories.
Nevertheless, with its half-life of 30.3 yr, significant
quantities of this radionuclide remain in the surface soils
of most areas of the world, and measurable Cs-137 activities are found to much greater depths than for Be-7
as a result of bioturbation or mixing of the plow layer
by tillage. Typical depth distributions of Cs-137 in uncultivated and cultivated soils for a location in Western
Europe are provided in Fig. 2c and 2d. In the case of
uncultivated soils (Fig. 2c), the depth distribution of Cs-
137 is commonly characterized by a maximum activity at
or near the surface and an exponential decline in activity
below the surface, with most of the Cs-137 inventory
being found within the upper 10 cm of the soil profile
(Walling et al., 1995; He and Walling, 1997). Because of
the mixing induced by tillage, the depth distribution of
Cs-137 activity in cultivated soils (Fig. 2d) is commonly
characterized by only limited variation of activity with
depth within the plow layer. Below the base of the plow
layer (i.e., the plow depth), Cs-137 activities rapidly decline to zero, except in areas of deposition where the
depth to which Cs-137 occurs increases due to a gradual
extension of the depth distribution by surface accretion
(Walling et al., 1995; He and Walling, 1997). The depth
distributions of Be-7 and Cs-137 exhibit important contrasts, and these can be exploited to develop an improved understanding of the relative contributions of
interrill and rill erosion to the sediment mobilized during storm events.
THE FIELD EXPERIMENT AND
LABORATORY ANALYSIS
The site selected for the field experiment was located
within the Tie Longwan Forestry Center in Yichuan
County, Shaanxi Province, China (Fig. 1). The ready
availability of a water supply for the rainfall simulator
was a key consideration in selecting this location. The
area is characterized by Heilu soils formed from the
deep Malan loess, a common soil in the Loess Plateau.
Two slopes representative of cultivated land and a recently cleared area within a plantation of Simon Poplar
(Populus tomentosa) were selected for installation of the
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
1582
SOIL SCI. SOC. AM. J., VOL. 70, SEPTEMBER–OCTOBER 2006
erosion plots. The cultivated area was last tilled 6 mo
before the experiment, and the soils had remained bare
during the intervening period. On the cultivated slope, a
plot 3 m long 3 1.5 m wide with a slope of 168 was established, and a similar plot 5 m long 3 1.5 m wide with a
slope of 288 was established on the recently cleared
forest slope. The plots were constructed using rigid thin
plastic sheet, 20 cm high, to delimit the upper boundary
and the two sides of the plot. A trough made of steel
sheet was installed across the lower boundary of the plot
to trap and collect the output of runoff and sediment. To
prevent subsoil that was disturbed during the installation of the plastic sheets and the metal trough from
being mobilized during the experiment, the plastic
sheets and the trough were carefully installed into slots
opened in the soil, and PVC glue was applied to the
plastic sheets to bond to the loosened soil. The trough
discharged into a plastic container. Depth-incremental
soil samples were collected from sites close to the newly
installed plots using sectioned cores, to characterize the
depth distribution of Be-7 and Cs-137 activity within the
soils of the plots immediately before the experiments. In
the case of the plot within the area of cleared forest, the
sampling depth was restricted to 10 cm due to the presence of tree roots. The plot experiments were undertaken in May 2002. The experiment preceded the main
wet season and was timed to ensure that the spatial
distribution of Be-7 inventories within the plots was
uniform. The lack of heavy rainfall within the preceding
months meant that no surface runoff, and thus no soil
redistribution, had occurred on the plots in the recent
past to cause spatial variability in the Be-7 inventory.
Because of the short half-life of Be-7, spatial variability
in Be-7 inventories within the plots, associated with surface runoff and soil redistribution earlier in the year,
would have been rendered insignificant by radioactive
decay. Information concerning the particle-size composition and organic C content of the soils from each of the
plots is presented in Table 1.
A side-spray rainfall simulator, with the maximum
spray height set at 7.5 m, was used to generate rainfall
over the plots. The water applied to the plots contained
no Be-7 or Cs-137. A mean rainfall intensity of 1.22 mm
min21 was used on the cultivated plot, and this was increased to 2.22 mm min21 for the uncultivated plot. The
intensities used are representative of erosive storm
events in the study area, with higher rainfall intensities
being required to generate surface runoff from the
uncultivated soil on the cleared slope, which was characterized by higher infiltration rates. The drop-size distributions produced by the rainfall simulator were
representative of local rainfall at the intensities used.
Table 1. The particle-size composition and organic C content of
the top 20 cm of the soil on the cultivated soil and the top10 cm
of the soil on the uncultivated plot.
Particle size
Plot
Cultivated
Uncultivated
Sand
10.87
8.27
Silt
Clay
Organic C content
80.45
82.83
%
8.68
8.90
2.62
3.54
Rainfall was generated over each of the two plots for
30 min. Plastic containers, with a capacity of 17 L, were
used to collect the runoff and associated sediment at the
bottom of the plots. When the first container was full, it
was replaced by another container, and this sequence
was maintained until the runoff ceased. Rills developed
naturally on the cultivated plot, but they occur more
rarely on recently cleared land, and it was necessary to
initiate rill formation on the plot established within the
area of cleared forest. This was achieved by using a
spade to create a shallow (,10 cm) rill at the base of the
plot during the rainfall application. Once initiated, this
rill propagated upslope to generate a rill network. The
rills developed on the cultivated plot were characterized
by a maximum width and depth of 7.8 cm and 16.5 cm, respectively, whereas on the uncultivated plot the equivalent dimensions were 8.3 cm and 8.8 cm, respectively.
The sediment was recovered from the individual
containers by settling and decantation and was air-dried,
weighed, and disaggregated before measurement of its
Cs-137 and Be-7 activity by g-spectrometry. These measurements were undertaken using a low background
hyperpure coaxial germanium detector coupled to a
multichannel analyzer system (EG&G ORTEC, Oak
Ridge, TN). Cesium-137 was measured at 661.6 keV
with a count time of 28 800 s, and Be-7 was measured at
477.6 keV with a count time of 86 400 s. The detector was
calibrated using standards of a known activity. The
measured Cs-137 and Be-7 activities were converted to
activities at the time of sampling using the appropriate
decay constants, and the resulting values were typically
characterized by a precision of approximately 66% at
the 95% level of confidence. Measurements of the grain
size composition and organic C content were undertaken
on a representative composite sample of surface soil
collected from each plot and on the sediment recovered
from the individual containers. Grain size measurements
were made using a laser granulometer (Micromeritics,
Norcross, GA), with all samples pretreated with hydrogen peroxide to remove organic matter and chemically
dispersed using sodium hexametaphosphate before
analysis. Organic C was measured by pyrolysis using a
C/N analyzer (Carlo Erba, Milan, Italy).
RESULTS AND DISCUSSION
The Vertical Distribution of Beryllium-7 and
Cesium-137 in Soils
The vertical distributions of Be-7 and Cs-137 measured in the soils adjacent to the two erosion plots immediately before the experiment are portrayed in Fig. 3
and 4. Because the plots were established in representative locations close to the sampling points, these depth
distributions can be viewed as representative of those
within the plots. The data presented in Fig. 3 show that
the Be-7 depth distribution was similar in the cultivated
(Fig. 3a) and uncultivated (cleared forest) soils (Fig. 3b).
The Be-7 activity was limited to the upper 10 mm of the
soil profile, and its depth distribution was characterized
by a surface maximum and a rapid exponential decrease
below the surface. The reduced inventory associated
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
YANG ET AL.: PARTITIONING CONTRIBUTIONS OF SHEET AND RILL EROSION
1583
Fig. 3. The vertical distributions of Be-7 activity in soils representative of the cultivated (a) and uncultivated (b) plots. The exponential depth distributions described by Eq. [2] and [3] have been superimposed on the vertical distributions of Be-7 activity.
with the cultivated soil, relative to the uncultivated soil,
is likely to reflect its recent cultivation, which would
have mixed the Be-7 inventory existing in the soil at that
time into the plow layer, producing activities below
the level of detection. The current Be-7 activity found at
the surface would therefore have accumulated over a
shorter period than in the case of the uncultivated plot.
The depth distributions of Cs-137 activity associated
with the same cores are presented in Fig. 4. The core
from the uncultivated site (Fig. 4b) was characterized by
a broad peak in the Cs-137 profile close to the surface,
with the maximum activity occurring at a depth of 4 cm
below the surface. Cesium-137 activity showed a slow
exponential decrease below the peak, with measurable Cs-137 activity to a depth of | 10 cm, the maximum
depth to which the core extended due to the presence
of roots. In the case of the cultivated area (Fig. 4a), the
Cs-137 activity was substantially lower than that found
in the uncultivated soil, due to mixing of the fallout input
into the plow layer, and relatively constant down to the
base of the plow layer at approximately 20 cm, again due
to mixing. Below this level, Cs-137 activity declined
rapidly. The reduced Cs-137 inventory associated with
the cultivated soil can be accounted for by the erosion occurring over the past approximately 45 yr, which
would have progressively reduced the inventory, with
soil eroded from the surface being replaced by soil from
below the original plow layer containing little or no
Cs-137 and incorporated into the plow layer by tillage.
Grain Size Characteristics of the Soils
and Sediments
Information on the primary grain size composition
of the mineral sediment eroded during the individual
events and the equivalent grain size distribution of the
surface soil on the two plots is presented in Fig. 5, which
compares the grain size distribution of a representative
composite sample of surface soil with the spread of the
particle size distributions associated with the samples of
eroded sediment collected from the plots. Overall, the
eroded sediment had a similar grain size composition to
that of the original soil, although there is some evidence
that the eroded soil was slightly coarser than the parent
soil, particularly for the cultivated plot. The influence of
the initiation of rilling on the grain size composition of
the eroded sediment is shown in Fig. 6, where the grain
size distributions of soil collected from the plot before
and after rill initiation are compared. In the case of the
uncultivated plot (Fig. 6b), there seems to be no significant difference in grain size composition between the
two sediment samples, but in the case of the cultivated
plot (Fig. 6a), there is some evidence to suggest that
the soil eroded after rill initiation was coarser than
that eroded before rill initiation. The tendency for the
eroded sediment to coarsen after rill initiation shown by
the cultivated plot (Fig. 6a) differs from the findings of
Young and Onstad (1978), who reported that the sediment associated with interrill erosion on loam, silt loam,
Fig. 4. The vertical distributions of Cs-137 activity in soils representative of the cultivated (a) and uncultivated (b) plots. The depth distribution
described by Eq. [10] has been superimposed on (b).
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
1584
SOIL SCI. SOC. AM. J., VOL. 70, SEPTEMBER–OCTOBER 2006
Fig. 5. A comparison of the range of the grain size distributions of sediment discharged from the cultivated (a) and uncultivated (b) plots during the
simulated rainfall events with the mean grain size distribution of the surface soil within the individual plots.
and loamy sand soils in Minnesota was significantly
coarser than that generated by rill erosion. The lack of
any shift in grain size composition after rill initiation on
the uncultivated plot may reflect the higher (2.22 mm
min21) rainfall intensity used on this plot, as compared
with the cultivated plot (1.22 mm min21).
Temporal Changes of Beryllium-7 and Cesium-137
Activities in Sediment
Figures 7 and 8 present plots of the Cs-137 and Be-7
activities associated with the sediment collected from
the outlets of the two plots during the course of the
simulated rainfall events. Significant contrasts exist between the temporal trends exhibited by the two radionuclides. For the cultivated plot (Fig. 7), Cs-137 activities
remained essentially constant throughout the event at
about 4.0 Bq kg21, whereas Be-7 activities decreased
from an initial value of 37.4 Bq kg21 to a final activity of
10.3 Bq kg21. This marked decline in Be-7 activity
through the event cannot be linked to a grain size effect
because the clay and fine silt content of the eroded soil,
with which the Be-7 is preferentially associated, did not
change significantly through the event. Rather, it reflected the characteristic depth distribution of Be-7 in
cultivated soil, and more particularly, the existence of a
shallow surface layer characterized by relatively high
Be-7 activity and a rapid decline in Be-7 activity immediately below the surface (Fig. 3a, b). As the event proceeded, erosion progressively removed the surface soil
and mobilized soil from beneath the surface, which was
characterized by a significantly lower Be-7 activity. The
precise timing and trend of the decline in Be-7 activity
during the event was also strongly influenced by the
Fig. 6. A comparison of the range of the grain size distributions of sediment discharged from the cultivated (a) and uncultivated (b) plots before and
after the initiation of rill development.
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
YANG ET AL.: PARTITIONING CONTRIBUTIONS OF SHEET AND RILL EROSION
Fig. 7. Variation of the Be-7 and Cs-137 activity of the sediment eroded
from the cultivated plot during the simulated erosion event. The error
bars represent the precision of the g spectrometry measurements at
the 95% level of confidence.
initiation of rill development approximately 11 min after
the beginning of the simulated rainfall event. By mobilizing soil from greater depths below the surface, rill
erosion would have caused a further reduction in the
Be-7 activity of the sediment, so that Be-7 activities declined further as the event proceeded and as the relative
contribution of rill-derived sediment to the total mass
of sediment mobilized by erosion increased. The small
fluctuations in the Be-7 activity of the eroded sediment,
which occurred after the appearance of the rills, are
consistent with this explanation because as the rills
deepened, the side walls became unstable through undercutting and small mass movements, and surface soil
containing higher Be-7 activity might have collapsed
into the rill, causing a short-term increase in the Be-7
activity of the eroded sediment. As the rills became
progressively wider and deeper, the impact of these collapses and associated small inputs of Be-7–rich sediment
in causing an increase in the Be-7 activity of the mobilized sediment was likely to have decreased. The nearconstant Cs-137 activity associated with the sediment
eroded from the cultivated plot is consistent with this
1585
explanation because Cs-137 activities were essentially
uniform within the upper 20 cm of the soil profile (see
Fig. 4a) due to mixing by tillage, and neither the progressive lowering of the surface nor the initiation of rills,
which did not extend below the 20-cm depth, could result in a change in the Cs-137 activity of the eroded soil.
The variation of Be-7 and Cs-137 activities in the soil
eroded from the uncultivated plot during the course of
the simulated storm event is shown in Fig. 8. The trends
shown by Be-7 activity are broadly similar to those
documented for the cultivated plot, but some important
differences are apparent in the trend of Cs-137 activity
through the event. First, the Cs-137 activities reported
for the eroded soil were 4–6 times greater than those for
the cultivated plot, and the overall range of the variation
was greater. This is consistent with the information on
the Cs-137 depth distributions for the cultivated and
uncultivated plots presented in Fig. 4a and 4b, which
show similar contrasts in Cs-137 activity. Second, although the trend is not well defined, it could be suggested that the Cs-137 activity of the eroded soil tended
to decline after the initiation of rills at the 13-min mark.
This decreasing trend is consistent with the Cs-137 depth
distribution for the uncultivated plot (Fig. 4b), where Cs137 activity decreased rapidly with depth below 4 cm. As
rills incised deeper into the soil, the Cs-137 activity in the
mobilized sediment would have declined. It could also
be suggested that there was some evidence of a small
increase in the Cs-137 activity of the eroded soil around
the time that the rill was artificially initiated. This could
reflect the precise form of the Cs-137 depth distribution
in the soils of the plot because the maximum Cs-137 activity occurred approximately 4 cm below the surface,
and sediment initially mobilized by the incising rill could
be expected to have higher Cs-137 activities than sediment mobilized from the surface by interrill erosion.
Because the variations in the Be-7 and Cs-137 activity of
the soil eroded from the two plots during the simulated
Fig. 8. Variation of the Be-7 and Cs-137 activity of the sediment eroded from the uncultivated plot during the simulated erosion event. The error bars
represent the precision of the g spectrometry measurements at the 95% level of confidence.
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
1586
SOIL SCI. SOC. AM. J., VOL. 70, SEPTEMBER–OCTOBER 2006
rainfall event described previously have been shown to
be closely linked to the erosion processes operating on
the plots, these data may provide further information
on the precise timing of the initiation of rill erosion and
on the relative contributions of sheet and rill erosion to
the total mass of sediment eroded from the plot at different times during the event and to the overall soil loss.
The basis for deriving such estimates is described below.
The Relative Contributions of Sheet and
Rill Erosion
Es,i 5 h9s 3 D 3 SP
The total amount of sediment eroded from the plots
during a specified time increment can be quantified by
weighing the sediment accumulated in the collecting
bucket during that period. Because Be-7 is found close
to the soil surface, it is associated with sediment mobilized by sheet erosion, and the activity of Be-7 in the
eroded sediment directly reflects the relative contributions of sheet and rill erosion. As the contribution of
rill erosion increases, the Be-7 activity in the eroded soil
declines. However, even in the absence of rill erosion,
the activity of Be-7 in the eroded sediment can be expected to decline through time as the erosion progressively mobilizes soil from deeper layers. The Be-7
activity in the surface horizons of the soil from the two
plots declines rapidly with depth, and the exponential
depth distributions are characterized by relaxation depths
(i.e., the depth at which the activity reduces to 1/e [0.36]
of that at the surface) of approximately 6 mm (Fig. 3). If
sheet erosion is the only process operating to mobilize
sediment from the plot, there is no deposition within the
plot, and the grain size composition of eroded sediment
is similar to that of the parent soil (see Fig. 5), the amount
of Be-7 in the eroded soil can be expressed in terms of
a simple mass balance:
OW 3C
hs
N
i51
i
Be,e,i
5 D 3 Sp 3 X0 aebh dh
[1]
Where Wi is the mass of sediment collected in the ith time
increment (kg), CBe,e,i is the Be-7 activity in the eroded
sediment in the ith time increment (Bq kg21), D is the soil
bulk density (kg m23), SP is the plot area (m2), hs is the
cumulative mean sheet erosion depth (m) over the plot,
h is the depth in the soil (m), and the coefficients a and b
describe the exponential depth distribution of Be-7 in the
upper horizon of the soil.
The relationships between Be-7 activity in the soil
CBe,s (Bq kg21) and soil depth h (m) established for the
cultivated and uncultivated plots, using the cores collected immediately adjacent to the plots, are given in
Eq. [2] and [3] for the cultivated and uncultivated soils,
respectively:
CBe,s 5 38:18e2202:75h (n 5 5, r2 5 0:975)
Where hs,i is the total depth of soil loss before time I, and
hs,i21 is the total depth of soil loss before time i 2 1.
Because the rills occupy a very small proportion of
the total area of the plot (e.g., in the experiment on the
cultivated plot, the final rill area occupied only 3% of
the total area), it is reasonable to assume that sheet
erosion occurs from the entire area of the plot, including
that occupied by rills. The amount of sheet erosion occurring in the ith time increment Es,i (kg) can then be
expressed as:
and the rill erosion amount in the ith increment can be
estimated as:
Er,i 5 Wi 2 Es,i
[3]
According to Eq. [1], the depth of soil loss in the ith time
increment should be:
h9s 5 hs,i 2 hs, i21
Si % 5 Es,i /Wi 3 100%
[7]
Sr % 5 Er,i /Wi 3 100%
[8]
and
The above assumption would seem to be reasonable,
bearing in mind that rills are likely to occupy only a
small portion of the total plot and that little deposition is
likely to occur within the small area covered by the plot,
particularly when the slope is relatively high.
Turning to the use of the Cs-137 measurements to
provide information on variations in the relative importance of sheet and rill erosion during the simulated storm
events, in the case of the cultivated plot, the Cs-137 is
uniformly distributed within the plow layer, and it is not
possible to distinguish sediment mobilized from different
depths within the upper soil profile and thus to discriminate sediment mobilized by sheet and rill erosion.
In the case of the uncultivated plot, measurable Cs-137
activities are found to much greater depths than Be-7,
and it is therefore not possible to discriminate sediment
mobilized by sheet and rill erosion, as is possible with
Be-7. However, it is possible to use the Cs-137 measurements to provide a general confirmation of the approach
used with the Be-7 measurements to discriminate sediment mobilized by sheet and rill erosion. In this case, the
Cs-137 activity of the eroded sediment can be used to
estimate total soil loss, and this value can be compared
with the measured soil loss. Additionally, Cs-137 could
offer the potential to discriminate sediment mobilized by
gully erosion because such sediment would be mobilized
from deeper layers in the profile and from below the
horizons containing Cs-137. For cultivated soil, the mass
balance equation for Cs-137 takes the following form:
OW 3C
i51
CBe,s 5 50:78e2295:86h (n 5 5, r2 5 0:999)
[4]
[6]
The proportions of sheet and rill erosion, respectively,
comprising the total erosion occurring in the ith time
increment, would be:
N
[2]
[5]
i
Cs,e,i
hs
5 D 3 SP 3 X0 f (h) dh
[9]
where Wi is the mass of sediment collected in the ith time
increment (kg), CCs,e,i is the Cs-137 activity in the eroded
sediment in the ith time increment (Bq kg21), D is the
soil bulk density (kg m23), SP is the plot area (m2), hs is
the cumulative mean erosion depth (m) over the plot, h is
1587
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
YANG ET AL.: PARTITIONING CONTRIBUTIONS OF SHEET AND RILL EROSION
the depth in the soil (m), and f(h) is the relationship
between Cs-137 activity and soil depth for the local soil
adjacent to the plot. Because Cs-137 is uniformly mixed
in the tillage layer (20 cm), that is:
f (h) 5 3:28
[10]
Eq. [9] can also be applied to the uncultivated plot, but if
SP is the plot area, it will underestimate soil loss because
interrill and rill erosion depths are averaged/lumped in
Eq. [9]. If only rill erosion is considered and if SP is the rill
area, the result should be more accurate. Rill area varies
(increases) during a rainfall event, and because it is
difficult to document this variation, it is difficult to
estimate the mass of sediment mobilized by rill erosion
during individual time increments. The soil loss associated with rill erosion at the end of the event can be
estimated because the final rill area can be measured
after the rainfall event. For uncultivated soil, the mass
balance equation for Cs-137 takes the following form:
OW 3 C
hs,i
N
i51
i
Cs,e,i
2 D 3 SP 3 X0 f (h) dh
hr
5 D 3 Sr,i 3 X0:01 f (h) dh
[11]
2
where Sp is the plot area (m ), hs,i is the total depth of
interrill erosion before time i (this value can be obtained
from Eq. [1]), Sr,i is the total rill area before time i (m2),
hr is the rill erosion depth (m), and f(h) in the uncultivated soil should be:
f (h) 5 4602564:07h5 2 3533653:84h4
1 690252:62h3 2 52640:13h2 1 1423:49h
1 14:89(n 5 10, r2 5 0:972)
[12]
21
where f(h) is the Cs-137 activity in the soil (Bq kg ), and
h is the depth within the soil (m).
In Eq. [11], the soil loss from the upper 0.01 m is assumed to represent interrill erosion, and the rills should
therefore been viewed as commencing below this depth
and thus as having a minimum depth of 0.01 m.
The results of applying the equations presented here
to estimate the contributions of sheet and rill erosion to
the sediment eroded from the experimental plots and
the variation of these contributions during the simulated
events are presented in Table 2 for the cultivated plot
and in Table 3 for the uncultivated plot. Figure 9 also
provides a graphical representation of the variation of
the amounts of sediment mobilized by sheet and rill
erosion during the simulated rainfall event on the cultivated plot. Table 2 and Fig. 9 indicate that rill erosion
started to make a significant contribution to the sediment output from the cultivated plot by about the ninth
minute after the commencement of runoff, and this is
consistent with the observation that the rill started to
develop on the cultivated plot 11 min after the commencement of runoff. For the uncultivated plot, Table 3
suggests that small amounts of rill erosion occurred before the artificial initiation of rill development 13 min
from the commencement of runoff. However, a major
increase in the amount of sediment mobilized by rill erosion occurred 14 min after the commencement of runoff,
and this is fully consistent with the known timing of
rill development on the uncultivated plot. As the simulated event proceeded, rill erosion became progressively
more important on both plots, accounting for as much as
66.3% of the sediment eroded from the cultivated plot
and 84.2% of the sediment eroded from the uncultivated
plot during the individual time increments. For the simulated rainfall events as a whole, rill erosion is estimated
to have contributed 54.3% of the total soil loss from the
cultivated plot and 61.4% of the total soil loss from the
uncultivated plot and can thus be seen to have dominated sediment mobilization on both plots.
The measurements of the mass of sediment eroded
from the two plots during the simulated rainfall events
and additional measurements of the volume of the rill
network formed on the plots made after the events provide a basis for confirming the consistency of the results
presented in Tables 2 and 3. The measurements of rill
volume made on the cultivated plot after the simulated
rainfall event showed that the rill developed on this plot
had a length of 231.4 cm and a mean width and depth of
5.8 and 13.4 cm, respectively. Measurements of the bulk
density of the soil into which the rill had incised provided a typical value of 1.1 g cm23. If it is assumed that
rills are characterized by V-shaped cross-sections, the
Table 2. Changes in the Be-7 and Cs-137 activity and amount of sediment eroded during the simulated erosion event on the cultivated plot
and associated estimates of changes in the importance of sheet and rill erosion during the event.
Runoff time
increment
Be-7 activity
in sediment
Min
0–5
5–7
7–9
9–11
11–13
13–15
15–17
17–19
19–21
21–23
23–25
25–27
27–29
Total output
37.39 6
29.82 6
23.12 6
13.44 6
15.54 6
15.77 6
17.36 6
14.94 6
14.84 6
10.35 6
10.09 6
12.85 6
10.31 6
Cs-137 activity
in sediment
Measured
sediment loss
Total erosion
estimated from Cs-137
507
517
1288
1441
1623
783
531
1595
965
2595
2730
1730
850
17155
502
575
1344
1760
1941
962
573
1638
1065
3099
3688
2391
1117
20655
21
Bq kg
2.28
3.3 6 0.22
1.96
3.6 6 0.22
1.65
3.4 6 0.21
1.26
4.0 6 0.22
1.39
3.9 6 0.22
1.44
4.0 6 0.23
1.44
3.5 6 0.21
1.41
3.4 6 0.21
1.32
3.6 6 0.21
1.16
3.9 6 0.22
1.09
4.4 6 0.24
1.36
4.5 6 0.25
1.15
4.3 6 0.23
Sheet erosion
estimated from Be-7
Rill erosion†
Sheet
erosion
501
416
823
551
736
369
279
736
453
874
930
777
313
7758
6
101
465
890
887
414
252
859
512
1721
1800
953
537
9397
98.8
80.5
63.9
38.2
45.3
47.1
52.5
46.1
46.9
33.7
34.1
44.9
36.8
45.2
g
Rill
erosion
%
† Rill erosion is calculated as the measured total sediment loss soil minus the sheet erosion estimated from the Be-7 measurements.
1.2
19.5
36.1
61.8
54.7
52.9
47.5
53.9
53.1
66.3
65.9
55.1
63.2
54.8
1588
SOIL SCI. SOC. AM. J., VOL. 70, SEPTEMBER–OCTOBER 2006
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
Table 3. Changes in the Be-7 and Cs-137 activity and amounts of sediment eroded during the simulated erosion event on the uncultivated
plot and their associated estimates of changes in the importance of sheet and rill erosion during the event.
Runoff time
increment
Be-7 activity
in sediment
Cs-137 activity
in sediment
Min
0–3
3–6
6–8
8–10
10–12
12–13
13–14
14–15
15–17
17–19
19–21
21–23
23–25
Total output
Bq kg
50.54 6 4.23
43.48 6 3.47
53.99 6 5.92
36.43 6 3.29
37.39 6 3.65
36.85 6 4.08
15.28 6 2.08
9.95 6 1.47
12.77 6 1.49
7.67 6 1.06
8.66 6 1.23
7.52 6 1.18
9.96 6 1.72
Measured
sediment loss
Total erosion
estimated from Cs-137
258
321
155
296
262
226
328
420
595
850
645
676
330
5362
280
329
249
344
347
366
417
377
492
703
534
584
380
5402
21
16.2 6 0.71
15.3 6 0.60
24.2 6 1.16
17.4 6 0.66
20.1 6 0.77
24.6 6 0.93
19.3 6 0.65
13.7 6 0.48
12.7 6 0.41
12.8 6 0.41
12.9 6 0.41
13.5 6 0.42
18.1 6 0.62
Sheet erosion
estimated from Be-7
Rill
erosion†
Sheet
erosion
258
278
168
218
199
171
103
86
157
136
117
107
69
2068
0
43
213
78
63
55
225
334
438
714
528
569
261
3294
100.0
86.7
108.5
73.7
76.1
75.5
31.5
20.5
26.5
16.0
18.1
15.8
21.0
38.6
g
Rill
erosion
%
0
13.3
28.5
26.3
23.9
24.5
68.5
79.5
73.5
84.0
81.9
84.2
79.0
61.4
† Rill erosion is calculated as the measured total sediment loss soil minus the sheet erosion estimated from the Be-7 measurements.
total soil loss from the cultivated plot estimated from the
Be-7 measurements of 9396 g, when combined with a
mean rill width of 5.8 cm, would represent an average rill
depth of 12.7 cm. This differs slightly from the measured
mean depth of 13.4 cm, but the difference is likely to
reflect deviations of rill cross-sections from the assumed
V-shape and the assumption that sediment mobilized by
rill erosion will have a zero Be-7 activity. It is only the
sediment below approximately the 10-mm depth that
contains 0 Be-7, and the resulting estimate of average rill
depth should therefore be 12.7 cm 1 1 cm (i.e., 13.7 cm).
This value is close to the measured average rill depth of
13.4 cm, particularly when the uncertainties associated
with the measured value are recognized. The equivalent
measured values for the uncultivated plot were a rill
length of 75 cm and a mean width and depth of 6.5 cm
and 5.6 cm, respectively. In this case, the characteristic
rill cross-section was rectangular. The value for average rill depth estimated from the Be-7 measurements
is 4.45 cm 1 1 cm. Again, the measured and predicted
values are similar.
Effect of Relative Contributions of Sheet and Rill
Erosion on Sediment Characteristics
The essentially unique information on the magnitude
and relative importance of sheet and rill erosion contributions to soil loss during an event, provided by the
Fig. 9. The mobilization of sediment by sheet and rill erosion during
the course of the simulated erosion event on the cultivated plot.
approach developed in this study and as reported in
Tables 2 and 3 and Fig. 9, provide a valuable basis for
investigating the influence of the two erosion processes
on the physical and geochemical properties of the sediment eroded from the plots. Several previous studies
have reported contrasts in the grain size composition of
sediment mobilized by the two erosion processes. In this
study, however, there was little variation in the grain size
composition of the sediment eroded from both plots
during the simulated rainfall events despite significant
changes in the relative contributions of sheet and rill
erosion, indicating that there was no substantive difference between the grain size composition of sediment
mobilized by sheet and rill erosion at this study site. This
is further confirmed by Fig. 10 and 11, which present
plots of the variation of the sand and clay content of the
sediment eroded from the plots during different time
intervals, and the variation in the relative contribution of
rill erosion to the eroded sediment for the uncultivated
and cultivated plots, respectively. In both cases, the onset of rill erosion and the subsequent increase in its relative contribution to the total mass of sediment eroded
during individual time increments has no clear influence
on variations in the grain size composition of the eroded
sediment. If the experiments were undertaken at other
locations where the soils contained a broader range of
grain sizes, greater contrasts in grain size composition
between sediment mobilized by sheet and rill erosion
might be expected.
The variation of the organic C content of the eroded
sediment is also plotted on Fig. 10 and 11. The onset and
increase in the relative importance of rill erosion could
be expected to influence the organic C content of the
eroded sediment through two mechanisms. The first reflects the different hydraulic conditions associated with
sheet and rill erosion. It is well known that eroded sediment is frequently enriched in organic matter due to the
preferential mobilization of low-density organic matter,
and this preferential mobilization should be sensitive to
the changing hydraulic conditions associated with the
onset and increasing importance of rill erosion. The
importance of preferential mobilization of organic matter could be expected to decrease as rill erosion becomes
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
YANG ET AL.: PARTITIONING CONTRIBUTIONS OF SHEET AND RILL EROSION
1589
Fig. 10. The relationship between changes in the grain size composition and organic C content of sediment mobilized during the simulated rainfall
event on the uncultivated plot and changes in the relative importance of sheet and rill erosion during the event.
more important. The second mechanism reflects the potential existence of a downprofile reduction in the organic C content of the soil, such that sheet erosion
mobilizes surface soil with a relatively high organic C
content, whereas rill erosion, which cuts deeper into the
soil profile, may mobilize soil with a lower organic C
content. In the case of cultivated soils, mixing of the
upper soil horizon by tillage is likely to reduce any contrast between the organic C content of the surface soil
and soil from deeper in the plow layer, but significant
contrasts between the organic C content of surface and
subsurface soil may exist in uncultivated soils. The results from the uncultivated plot presented in Fig. 10
provide clear evidence of a progressive reduction in the
organic C content of the eroded sediment as the simulated event proceeded and more particularly as rill erosion assumed increased importance. This reduction in
the organic C content of the mobilized sediment is likely
to primarily reflect the lower organic C content of sediment mobilized by rill erosion from deeper in the soil
profile. The results from the cultivated plot shown in
Fig. 11 provide less evidence of any decreasing trend in
the organic C content of sediment mobilized from the
cultivated plot during the simulated rainfall event. The
values at the beginning and end of the event are almost
identical despite an increase in the relative contribution
of rill erosion from zero at the beginning of the event to
.60% at the end of the event. This reduced variation in
organic C content shown by the sediment mobilized
from the cultivated plot largely reflects the impact of
tillage mixing in limiting the down profile decrease in
the organic matter content of the soil. There is some
evidence to suggest that the organic C content of the
eroded sediment increased during the early stages of the
event, as rill erosion was initiated, and this may reflect
depletion of the organic C content of the immediate surface layer by preceding erosion events and thus an initial
increase in the organic C content of the mobilized sediment as the rill started to mobilize sediment from immediately below the surface. As the rill cut into deeper soil
layers, there is some evidence that the organic C content
of the mobilized sediment declined.
CONCLUSIONS
Figures 9, 10, and 11 and Tables 2 and 3 present essentially unique information concerning the relative importance of sheet and rill erosion during a storm event
Fig. 11. The relationship between changes in the grain size composition and organic C content of sediment mobilized during the simulated rainfall
event on the cultivated plot and changes in the relative importance of sheet and rill erosion during the event.
Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.
1590
SOIL SCI. SOC. AM. J., VOL. 70, SEPTEMBER–OCTOBER 2006
and confirm the potential for using Be-7 and Cs-137
measurements to provide important new insights into
variations in the relative importance of these two erosion processes. Figures 7 and 8 show that the development of rills within the plots was marked by significant
decreases in the Be-7 activity of the sediment eroded
from the cultivated plot and changes in the Be-7 and
Cs-137 activity of the sediment eroded from the plot
established within the uncultivated area. By relating the
changes in the Be-7 activity of the eroded soil to information on the depth distribution of the radionuclides in
the soils of the two plots and the mass of soil eroded, it is
possible to estimate the relative contributions of the two
erosion processes to the total soil loss and their changes
through an event. The validity of the approach has been
demonstrated by comparing the estimates of the total
mass of sediment mobilized from the two erosion plots
by rill erosion during the simulated rainfall event provided by the Be-7 measurements with equivalent estimates based on measurements of rill volume undertaken
after the event. The two estimates show a high level
of agreement.
This new information provides a basis for investigating the influence of variations in the relative importance
of sheet and rill erosion on variations in the physical and
geochemical properties of the soil eroded from the plots
during the simulated rainfall event. The results presented in Fig. 10 and 11 confirm that, in the case of the
uncultivated plot, there seems to be no significant difference in grain size composition before and after rill initiation. However, in the case of the cultivated plot, there
is some evidence to suggest that the soil eroded after rill
initiation is coarser than that eroded before rill initiation. An improved understanding of the relative importance of sheet and rill erosion, its variation during an
event, and its potential influence on the physical and
geochemical properties of the mobilized sediment must
be seen as potentially providing an important input to
the development and refinement of physically based erosion prediction models capable of generating meaningful predictions of the amount and the properties of the
eroded sediment. Further work is required to apply the
approach described here to sites with contrasting soil textures, to obtain further information on the response of the
properties of eroded soil to spatial and temporal variations in the relative importance of sheet and rill erosion.
ACKNOWLEDGMENTS
The financial support for the work described in this paper
provided by the National Natural Science Foundation of
China (Grant Nos. 90502007, 40401032 and 40471079), the UK
Natural Environment Research Council (Research Grant
NER/A/S/2001/01076) and the International Atomic Energy
Agency (Technical Contract 12094) is gratefully acknowledged.
Thanks are also extended to Helen Jones for producing the
diagrams and to Mr. Art Ames and Mr. Jim Grapes for their
assistance in undertaking the laser granulometer and organic
C measurements. The paper has benefited from comments on
the original manuscript provided by three anonymous referees
and particular thanks are extended to SSSAJ Associate Editor
Dr John Zhang, for his many constructive suggestions when
finalizing the paper.
REFERENCES
Blake, W.H., D.E. Walling, and Q. He. 1999. Fallout beryllium-7 as a
tracer in soil erosion investigations. Appl. Radiat. Isot. 51:599–605.
De Roo, A.P.J., C.G. Wesseling, and C.J. Ritsema. 1996. LISEM: A
single-event physically based hydrological and soil erosion model
for drainage basins. I. Theory, input and output. Hydrol. Processes
10:1107–1117.
He, Q., and D.E. Walling. 1997. The distribution of fallout 137Cs and
210
Pb in uncultivated and cultivated soils. Appl. Radiat. Isot. 48:
677–690.
Matisoff, G., E.C. Bonniwell, and P.J. Whiting. 2002. Soil erosion and
sediment sources in an Ohio watershed using Beryllium-7, Cesium137, and Lead-210. J. Environ. Qual. 31:54–61.
Murray, A.S., J.M. Olley, and P.J. Wallbrink. 1992. Natural radionuclide behavior in the fluvial environment. Rad. Prot. Dosim. 7:
297–304.
Murray, A.S., A. Johnston, and P. Marrtin. 1993. Transport of naturally
occurring radionuclides by a seasonal tropical river, northern Australia. J. Hydrol. 150:19–39.
Nearing, M.A., L. Deerscoughl, and J.M. Laflen. 1990. Sensitivity
analysis of the WEPP hillslope profile erosion model. Trans. Am.
Soc. Ag. Eng. 33:839–849.
Olsen, C.R., I.L. Larsen, P.D. Lowry, N.H. Cutshall, J.F. Todd, and
G.T.F. Wong. 1985. Atmospheric fluxes and marsh-soil inventories
of 7Be and 210Pb. J. Geophys. Res. 90:10487–10495.
Ródenas, C., J. Gómez, L.S. Quindós, P.L. Fernández, and J. Soto.
1997. 7Be concentrations in air, rain water and soil in Cantabria
(Spain). Appl. Radiat. Isot. 48:545–548.
Rosner, G., H. Hötzl, and R. Winkler. 1996. Continuous wet-only and
dry-only deposition measurements of 137Cs and 7Be: An indicator
of their origin. Appl. Radiat. Isot. 47:1135–1139.
Wallbrink, P.J., and A.S. Murray. 1993. Use of fallout radionuclides as
indicators of erosion processes. Hydrol. Processes 7:297–304.
Wallbrink, P.J., and A.S. Murray. 1994. Fallout of Be-7 in south Eastern
Australia. J. Environ. Radioact. 25:213–228.
Wallbrink, P.J., A.S. Murray, and J.M. Olley. 1999. Relating suspended
sediment to its original soil depth using fallout radionuclides. Soil
Sci. Soc. Am. J. 63:369–378.
Walling, D.E. 1998. Use of Cs-137 and other fallout radionuclides
in soil erosion investigation: Progress, problems and prospects.
p. 39–62. In Use of Cs-137 in the study of soil erosion and sedimentation, IAEA-TECDOC-1028, IAEA, Vienna.
Walling, D.E. 2002. Recent advances in the use of environmental
radionuclides in soil erosion investigations. p. 279–301. In Nuclear
techniques in integrated plant nutrient, water and soil management.
IAEA C&S Papers Series 11/C, IAEA, Vienna.
Walling, D.E., Q. He, and T.A. Quine. 1995. Use of caesium-137 and
lead-210 as tracers in soil erosion investigations. p. 163–172. In
Tracer technologies for hydrological systems. IAHS Pub. 229, IAHS
Press, Wallingford, UK.
Walling, D.E., Q. He, and W. Blake. 1999. Use of Be-7 and Cs-137
measurement to document short and medium-term rates of waterinduced soil erosion on agricultural land. Water Resour. Res. 35:
3865–3874.
Walling, D.E., and J.C. Woodward. 1992. Use of radiometric fingerprints to derive information on suspended sediment sources. p. 153–
164. In Erosion and sediment transport monitoring programmes in
river basins. IAHS Pub. 210, IAHS Press, Wallingford, UK.
Whiting, P.J., E.C. Bonniwell, and G. Matisoff. 2001. Depth and areal
extent of sheet and rill erosion based on radionuclides in soils and
suspended sediment. Geology 29:1131–1134.
Young, R.A., and C.A. Onstad. 1978. Characterization of rill and
interrill eroded soil. Trans. ASAE 21:1126–1130.
Zapata, F. 2003. Field application of the Cs-137 technique in soil erosion and sedimentation studies– Introduction. Soil Tillage Res.
69:3–13.