Catena 99 (2012) 1–8
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Catena
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The effects of rainfall regimes and land use changes on runoff and soil loss in a small
mountainous watershed
Nu-Fang Fang a, b, Zhi-Hua Shi a, b, c,⁎, Lu Li d, Zhong-Lu Guo c, Qian-Jin Liu c, Lei Ai c
a
State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A & F University, Yangling 712100, China
Institute of Soil and Water Conservation of Chinese Academy of Sciences and Ministry of Water Resources, Yangling 712100, China
College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China
d
Hubei Water Resources Research Institute, Hubei Province 443600, China
b
c
a r t i c l e
i n f o
Article history:
Received 4 February 2012
Received in revised form 28 June 2012
Accepted 11 July 2012
Available online xxxx
Keywords:
Runoff
Soil loss
Land use change
Rainfall events
Mountainous watershed
a b s t r a c t
This paper analyzes runoff and soil loss in relation to the rainfall regimes and land use changes in a small
mountainous watershed in the Three Gorges Area (TGA) of China. Based on 10 years of rainfall measurements
and K-means clustering, 152 rainfall events were classified into three rainfall regimes. The mean statistical features of different rainfall regimes display a marked difference. Rainfall Regime I is events of medium amounts
(31.8 mm) and medium duration (1371 min). Rainfall Regime II is events with high amounts (54.0 mm), long
duration (2548 min), and an infrequent occurrence. Rainfall Regime III is events of low amount (22.2 mm),
short duration (494 min) and high frequency. Each rainfall regime results in differing levels of runoff and erosion
and Rainfall Regime I causes the greatest proportion of accumulated discharge (368.7 mm) and soil loss (4283 t).
In the different rainfall regimes, the values of the mean runoff coefficient and the mean sediment load were ordered as follows: Rainfall Regime II > Rainfall Regime I > Rainfall Regime III. These results suggest that greater attention should be paid to Rainfall Regimes I and II because they had the most erosive effect. In the Wangjiaqiao
watershed, the changes in land use primarily affected the paddy fields, where the cropland decreased significantly and the forest and orchards increased by 9.9% and 7.7%, respectively, during 1995–2004. The ANOVA shows
land use changes caused significant decreasing trends in the runoff coefficients (Pb 0.01) and sediment loads
(Pb 0.01). In order, the most sensitive response of runoff and erosion to land use was Rainfall Regime II> Rainfall
Regime> Rainfall Regime III. Rainfall characteristics are decisive for the relative importance of different storm
runoff generation mechanisms. The land use changes in the study watershed have considerably decreased runoff
and soil loss.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Soil erosion poses a serious problem for sustainable agriculture and
the environment. Erosion has direct and indirect impacts, with soil erosion often directly affecting the overall quality and fertility of the soil
mostly by loss of the A horizon (Gao et al., 2011; Olson et al., 1994).
The A horizon is rich in nutrients and organic matter, and its loss also
causes a decrease in soil fertility, land productivity sustaining and plant
growth and so causes land degradation (Ebisemiju, 1990). The indirect
impacts of soil loss include increased sedimentation and turbidity, increased levels of nutrients and pollutants that diminish water quality, siltation of dams and irrigation channels (Craswell et al., 1998; Duvert et al.,
2010; Hopmans et al., 1987; Pimentel et al., 1995), and a decrease in the
abundance of aquatic life (Danielsen and Schumacher, 1997).
Runoff and erosion processes are affected by many factors but among
these factors, rainfall and land use are the two most often researched
⁎ Corresponding author at: College of Resources and Environment, Huazhong Agricultural
University, Wuhan 430070, China. Tel.: +86 27 87288249; fax: +86 27 87671035.
E-mail address: [email protected] (Z.-H. Shi).
0341-8162/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catena.2012.07.004
(Wei et al., 2007). Runoff generation and sediment generation are considered to vary greatly amid various rainfall types. Morin et al. (2006)
found that complex interactions exist between the spatio-temporal distribution of rainfall systems and the hydrological response of watersheds. Nadal-Romero et al. (2008) suggested that local storm patterns
are important for determining the shape of the runoff hydrograph.
Cerdan et al. (2010) attributed a fundamental difference in runoff generation and sediment transfer according to different land cover types at the
plot level. Valentin et al. (2008) concluded soil erosion is predominantly
influenced by land use rather than environmental characteristics at the
plot scale and the catchment scale. The results of Gafur et al. (2003) suggest annual direct runoff from small watersheds under long term fallow
and/or perennial vegetation vary typically between 15 and 24% of the annual rainfall. The loss of soil material from watersheds during the
cultivation year may be six times greater than under fallow conditions.
Although responses vary widely between sites and situations, removal of
forest cover leads to higher water yield and reforestation leads to lower
water yields, whereas changes in storm and dry season flow regimes are
more variable and complex and depend on climate, soil properties and
the change in land use (Bruijnzeel, 1990; Calder, 2000). In conjunction
2
N.-F. Fang et al. / Catena 99 (2012) 1–8
with rainfall, there is also evidence that changes in land use have
influenced the hydrological regime of various river basins. The impacts
of these changes on small watersheds can be significant (Ashagrie
et al., 2006; Fohrer et al., 2001; Jones and Grant, 1996).
Currently, simulation models (Rose and Peters, 2001; Shen et al.,
2009), plot observations (Licznar and Nearing, 2003; Wei et al., 2007)
and rainfall simulators (Fu et al., 2011; Meerveld and Weiler, 2008)
are the prevalent methods used to evaluate the effect of land uses and
rainfall regimes on runoff and soil erosion. However, the advantages
and disadvantages of these approaches have been the subject of some
debate (Hartanto et al., 2003). Plot observations cannot describe
how a catchment responds to certain management practices. Micro
plots can be used to investigate on the contribution of splash and little
rain-impacted flow on sediment mobilization but can significantly overor underestimate, depending on the environmental context and the overall soil water erosion. Plots scale measurements are useful tools to evaluate interrill erosion but provide little information on the dominant erosive
processes and their interactions (Chaplot and Le Bissonnais, 2003, Chaplot
and Poesen, 2008). The sediment yield from a hillslope or a catchment is
likely to be less than the total sediment mobilized within it and that estimated from plots (Chaplot and Poesen, 2008). An evaluation of land use
change impacts on flooding requires identification of the relevant storm
runoff generation mechanisms for the specific catchment characteristics
and precipitation conditions (Bronstert et al., 2002). Model evaluation
has hitherto been limited (Favis-Mortlock 1998; Licciardello et al., 2009;
Tsara et al., 2005). Additionally, owing to the relatively high natural variability of most hydrological systems, extrapolating or generalizing the
results from simulation models to other systems can be challenging (DeFries and Eshleman, 2004). Identifying the hydrological
consequences of land use changes and rainfall regimes is complicated
and controlling soil erosion requires much more detailed and accurate
data (Elsen et al., 2003).
The Three Gorges Area (TGA) refers to the riparian counties along
the Yangtze valley between Yichang and Chongqing (Fig. 1). This area
is the site of the Three Gorges Project, which is the world's largest hydropower project. This project is controversial for several reasons (Chu and
Zhai, 2008; Dai et al., 2011), including the likely impact of sedimentation
on the operation and useful life of the reservoir. Because of long term
anthropogenic pressure, including overuse and inappropriate development, soil erosion is a serious issue in this area. The annual soil loss in
the TGA has been estimated at 157 million t, of which 40 million t are
delivered to the Yangtze River at a rate of 700 t km−2 yr−1 (Lu and
Higgitt, 1998; Lu et al., 2003). Following the construction of the Three
Gorges Dam (1994–2009), many farmers resettled in the surrounding
mountain areas and cultivated marginal lands, which are mostly on
steep slopes with soil of poor structure with generally high rates of soil
loss occur during intense storms. These factors combined mean that the
recent land use changes, coupled with climate, leave the TGA vulnerable
to continued soil erosion. Lu and Higgitt (2001) concluded that high sediment yields reflect severe soil erosion due to cultivation aligned with
steep slopes in 32 catchments in the TGA, and 60% of sediment is from arable land. Shi et al. (2004) also regarded arable land as the main sediment
source in a small watershed in the TGA. Therefore, when considering land
use changes and vegetation restoration, it is important to study the effects
of land uses and rainfall regimes on runoff and soil erosion, as this should
ultimately lead to a better management of agricultural ecosystems.
Small watersheds are a convenient scale for soil conservation planning because they are easily identified on maps and on the ground
and they allow for detailed descriptions of some ecosystem processes
and capabilities (RWCSCB, 1998). Prior to 1990, soil and water conservation measures were implemented without unified planning in the
TGA. However, since the 1990s, Integrated Small Watershed Management (ISWM) for soil conservation has developed in the TGA. Up to
2008, the central government invested 15.2 billion Yuan (RMB) (Liao,
2010), and the ISWM has been conducted in >5000 small watersheds.
Both soil erosion reduction and regional economic growth were
integrated based on sustainable land uses, including self-sufficiency in
grain production, the protection of woodlands, and the development
of trees for their economic yield to make full use of natural resources.
This study aims to formalize recent findings in the small Wangjiaqiao
watershed and investigates the runoff and erosion processes that were
subject to different rainfall regimes and land use changes. The main objectives of this study are the following: (1) to analyze the effects of land
use change on runoff and erosion; (2) to determine the response of runoff
and soil erosion to different rainfall regimes; and (3) to study the role of
different land use types on soil erosion control under different rainfall regimes. The natural resources, land use patterns, and population densities
in the Wangjiaqiao watershed are typical of the surrounding region (Bu et
al., 2008; Feng et al., 2008; Shi et al., 2004; Wan et al., 2010). Therefore,
the results derived from this small watershed within the TGA give an indication of possible trends occurring throughout the TGA.
2. Study area and methods
2.1. Study area
The study was conducted in the Wangjiaqiao watershed (31°5′ N.
to 31°9′ N., 110°40′ E. to 110°43′ E.), which lies in Zigui County of
Hubei Province, China. It is approximately 50 km NW of the Three
Gorges Dam and covers an area of 16.7 km 2 (Fig. 1). Elevations within
the watershed range from 184 to 1180 m, and the slopes range from
2° to 58°, with an average of 23°. The streams in the Wangjiaqiao watershed have a pinnate drainage pattern, and the length of the main
channel is approximately 6500 m. The parent materials of this area
are mostly Cretaceous or Tertiary purple shale, sandy shale, and sandstone, and they contain large quantities of iron and manganese oxides
in specific mineralogical forms. Two main soil groups occur in the study
watershed: purple soils derived from purple sandy shale and paddy soils
developed from the purple soil. According to the USDA Soil Taxonomy,
the purple soils and paddy soils are classified as Entisols and Aquepts, respectively. The average annual suspended sediment load is 8033 t/yr
(Fang et al., 2011). The climate is subtropical, with mean temperatures
between 11 and 18 °C. The annual precipitation averages 1016 mm, and
70% of this occurs between May and September. The Wangjiaqiao watershed has six villages and 4920 people. The population density is 295 person/km2, and the natural population growth rate is 0.66%.
2.2. Reconnaissance field surveys
The reconnaissance field surveys were carried out in 1995, 2000
and 2005. The watershed topographic map (scale 1:10 000) was used
in combination with 1995 and 1999 aerial photos and 2005 SPOT5 imagery. The land use units were delineated on the photographs and verified in the field (Fig. 2). Land use is mainly a function of elevation and
topography. The remnant forest patches exist primarily on steep, inaccessible peaks and slopes. Little natural vegetation is observed, and
most areas are covered by secondary vegetation under human influence.
The main agricultural crops are rice (Oryza sativa L.), maize (Zea mays L.),
and wheat (Triticum aestivum L.). Land use information from 2005 was
used in conjunction with the 2004 hydrological data because the last
stream measurements were taken in 2004.
2.3. Field and lab methods
A set of instruments consisting of a continuous recording rain gauge, a
water level stage recorder, and a silt sampler (bottle type) was used to record rainfall, streamflow, and sediment flow, respectively. The water
stage is measured every 15 min and then transformed into discharge by
means of the calibrated rating curve obtained through periodic flow measurements. The suspended sediment concentrations (SSCs) were determined by the gravimetric method. Suspended sediment samples were
taken only during rainfall–runoff events, and >10 samples were required
N.-F. Fang et al. / Catena 99 (2012) 1–8
3
Fig. 1. Location of the study watershed in the Three Gorges Area, China.
during each event. The water samples were vacuum filtered through a
0.45-μm filter, and the residue was oven dried at 105 °C for 24 h. The
weight of each dried residue and the sample volume provided the SSC
(g m−3). The suspended sediment load was then calculated from the
SSC and the water discharge data. Floods were identified in cases where
the increase in stream discharge exceeded 1.5 times the base flow
recorded at the beginning of the rainfall event (García-Ruiz et al., 2005;
Lana-Renault et al., 2007). Using the classical hydrograph separation
method of Hewlett and Hibbert (1967), runoff was separated into the categories of storm flow and base flow. On several occasions, malfunctioning
equipment prevented complete monitoring of storms. A hydrograph
separation was conducted on 152 events during 1995–2004, for
which reasonably complete records of the sediment concentrations
were obtained.
2.4. Clustering approach
A clustering approach was used to classify the rainfall events. A
clustering approach is a fundamental and important tool in statistical
analysis (Huang, 1998; Sibson, 1973). It aims to group objects based on
their similarities and has been widely used in various scientific fields
(Anderberg, 1973; Yeh et al., 2000). There are two methods of clustering:
4
N.-F. Fang et al. / Catena 99 (2012) 1–8
Fig. 2. Land use maps for 1995 (A), 2000 (B) and 2005 (C) in the Wangjiaqiao watershed.
the hierarchical method and the K-means method. The K-means clustering method is suitable for a large number of cases (Hong, 2003), and a
cluster number is required before classification. To determine the number
of clusters in a data set, numerous criteria have been proposed (Perruchet,
1983). In this study, the most suitable cluster number was chosen by trial
and error. The classification met the ANOVA criterion for a significant
level (Pb 0.001).
2.5. Statistical analysis
For each rainfall–runoff event, the characteristics of individual storms
were evaluated, considering their erosive characters. The flood events
have been subsequently characterized using three groups of variables
(Table 1). Three indices including runoff coefficient, surface runoff and
total suspended sediment load are used.
RC ¼ R=P
ð1Þ
R ¼ Q −BF
ð2Þ
where RC, R, P, Q and BL denote runoff coefficient, surface runoff, precipitation, total discharge and baseflow, respectively.
TL ¼ Q mean SSCmean
152 rainfall events were classified into three rainfall regimes. Rainfall Regime III occurred 94 times, with a total depth of 2050.3 mm. Rainfall Regime I occurred 46 times, with a total depth of 1349.1 mm. Rainfall
Regime II, however, was observed only 14 times, with a total depth
of 817.7 mm. The mean P and D decreased in the following order:
Rainfall Regime II > Rainfall Regime I > Rainfall Regime III. Rainfall
Regime III has the highest values of the mean I30, followed by Rainfall
Regimes II and I. The general characteristics of each Rainfall Regime
can be described as follows: Rainfall Regime I was composed of rainfall
events with a moderate P, a moderate D, and low I30. Rainfall Regime II is
the group of rainfall events with a high mean P and long D. Rainfall Regime III, however, has a low P and a short D.
Fig. 3 shows the shows the frequency distributions of the three
rainfall regimes during the study years. The total depths of the three
rainfall regimes in different years are shown in Fig. 3(A). Except
for 1995 and 2004, the total depths of Rainfall Regime II were lower
than the other regimes. This was because the frequency of Rainfall Regime II was the lowest of any regime; in fact, it did not even occur in
1998 and 2002. Fig. 3(B) shows that with a value of 31.8 mm (during
2003), Rainfall Regime III had the highest mean I30, followed by Rainfall
Regimes I and II. Fig. 4 shows the relationship between I30 and D of the
152 events and indicates that events with a large value of I30 rarely last
ð3Þ
where TL, Qmean and SSCmean denote total suspended sediment load,
mean discharge and mean flood SSC, respectively. All results were calculated with SPSS13.0 for windows (Hong, 2003).
3. Results
3.1. Rainfall regimes
The 152 rainfall events were divided into three groups with K-means
clustering. Three rainfall variables were used during this process including the depth (P), duration (D) and maximum 30 min rainfall intensity
(I30) (Table 2). Based on 10 years of rainfall measurements clustering,
Table 1
Flood variables and associated abbreviations used in the statistical analysis.
Rainfall related
variables
Total precipitation
(P, mm)
Duration of the event
(D, min)
Runoff related
variables
Runoff
(R)
Runoff
coefficient
(RC)
Mean rainfall intensity Total discharge
(Im, mm h−1)
(Q, m3 s−1)
Maximum
30 min Baseflow at the
beginning
rainfall intensity
(Qb, m3 s−1)
(I30, mm)
Suspended sediment related variables
Mean flood suspended sediment
concentration (SSCmean, g m−3)
Maximum flood suspended sediment
concentration (SSCmax, g m−3)
Total suspended sediment load
(TL, t)
N.-F. Fang et al. / Catena 99 (2012) 1–8
40
Table 2
Statistical features of different rainfall regimes.
Variables
Mean
SD
Variation
Sum
Frequency
(time)
I
P (mm)
D (min)
I30 (mm)
P (mm)
D (min)
I30 (mm)
P (mm)
D (min)
I30 (mm)
31.8
1371
3.7
54.0
2548
5.3
22.2
494
7.5
15.1
256
1.6
22.9
543
4.2
12.2
248
7.1
0.47
0.19
0.44
0.42
0.21
0.78
0.56
0.50
0.95
1349.1
59,770
44
817.7
37,632
15
2050.3
45,949
93
III
Rainfall event
30
I30 (mm)
Rainfall regimes
II
5
20
10
0
0
long. Events with I30 > 20 mm coincide with D b 1000 min. This is a decisive characteristic for different Rainfall Regimes.
1000
2000
3000
4000
D (min)
Fig. 4. Relationship between maximum 30 min intensities and durations of the 152
events in the Wangjiaqiao watershed.
3.2. Land use changes
Land use has been altered greatly between 1995 and 2005 in the
Wangjiaqiao watershed. Table 3 shows the areas of land use types
and the corresponding percentages. In 1995, forest covered 44.5% of the
study area, while cropland covered 23.3% (389.4 ha) and paddy field covered 19.8% (330.7 ha). The other land use types were relatively minor and
consisted of shrubland (3.2%), orchards (4.2%), rural residential land
(3.9%) and water bodies (0.7%). By 2005, the areas of paddy fields and
cropland had decreased significantly. Orchards, a type of non-farm land
use introduced to boost farmers' incomes, increased 2.8 fold and composed 11.9% of the watershed area in 2005. Important changes occurred,
for example, some steep lands with slope gradients of > 25° were
converted to forest. This change was related to the implementation
of ISWM for soil conservation in the TGA in the 1990s. During this
period, forest increased to 48.6% in 2000 and to 55.4% in 2005.
3.3. Soil and water loss of different rainfall regimes
The characteristics of runoff and soil loss under the three rainfall
regimes are indicated in Table 4.We found that the values of the RC and
the TL of the different rainfall regimes were as follows: Rainfall Regime
II>Rainfall Regime I>Rainfall Regime III. Rainfall Regime I created the
most accumulated sediment load (4283 t) with a high total discharge
(368.7 mm), far more than that in Rainfall Regime II and III. Rainfall Regime III, however, created a low mean sediment load with a large amount
of P (2050.3 mm). Although the accumulated discharge of Rainfall Regime I is more than Rainfall Regime II, the total surface runoff of these
two Rainfall Regimes are almost the same amount.
3.4. Tendency of soil and water loss under different rainfall regimes during
1995–2004
Decreasing trends in RCs and sediment loads were found for all
the three Rainfall Regimes during the years 1995–2004. These trends
are illustrated in Fig. 5, which shows a similar decline in both the runoff coefficients and the sediment loads for each Rainfall Regime. The
runoff coefficients and the sediment loads of Rainfall Regime II drop
dramatically. Rainfall Regime I decreases more gently than Rainfall Regime II. Rainfall Regime III, however, has only a slight decline.
An ANOVA with the RC and TL as the dependent variables and the
rainfall regimes and land use as independent variables was performed
Table 3
Changes in different land use categories as a percentage of the total watershed area.
Land use
Fig. 3. Characteristics of the three rainfall regimes in different years.
Forest
Orchards
Paddy field
Cropland
Shrub land
Rural residential
land
Water body
% of total (16.7 km2)
watershed area
Change (%)
1995
2000
2005
1995–2000
2000–2005
1995–2005
44.5
4.2
19.8
23.3
3.2
4.3
48.6
8.3
16.3
18.3
3.1
4.6
54.4
11.9
13.6
10.5
3.0
5.3
4.1
4.1
−3.5
−5
−0.1
0.3
5.8
3.6
−2.7
−7.8
−0.1
0.7
9.9
7.7
−6.2
−12.8
−0.2
1
0.7
0.8
1.3
0.1
0.5
0.6
6
N.-F. Fang et al. / Catena 99 (2012) 1–8
Table 4
Features of runoff and erosion in different rainfall regimes.
Rainfall
regimes
Runoff
Accumulated Mean
Q (mm)
RC
SD of
mean RC
Accumulated
TL (t)
Sediment
Mean
TL
(t)
SD of
mean TL
I
II
III
368.7
253.1
279.0
0.20
0.23
0.19
4283
2694
2792
97
180
30
247
278
115
0.21
0.24
0.14
(events during 1995–1999 as one land use, and events during 2000–2004
as the other land use). The two-way ANOVA (Table 5) revealed that TL
showed significant differences between different rainfall regimes (P=
0.009) and among different rainfall regimes (P=0.008). However, a significant difference in RCs was found only among different land use (P=
0.001). No statistically significant interactions between rainfall regimes
and land use were found in RC or TL.
4. Discussion
4.1. Effects of different rainfall regimes on runoff and soil loss
According to this study, Rainfall Regime III is the most frequent of
the rainfall events and has the maximum accumulated rainfall depth.
However, it has only slight erosive effects on the soil and it cannot
produce severe erosion and water loss. Rainfall Regime II constitutes
only a small portion of the total rainfall events, yet it produces nearly
the same sediment load as Rainfall Regime III. The mean SS load of Rainfall Regime II is almost three times that of Rainfall Regime I, and six times
that of Rainfall Regime III. It has a destructive effect on the soil surface.
Rainfall Regime I tends to induce a higher total runoff and total erosion
than the other regimes. This result emphasizes the importance of the
rainfall type as a major cause of runoff and erosion. From this standpoint,
the rainfall depth is the most important factor in predicting or indicating
the degree of soil erosion in the study area. Other studies also confirmed
that rainfall depth plays a vital role in both runoff and sediment generation (Berndtsson and Larson, 1987; Dunne and Black, 1970; Dunne et al.,
1991; Hewlett and Hibbert, 1967).
A large proportion of the runoff and sediment load was produced
by a small number of runoff events. This finding is illustrated in Fig. 6,
which shows the percentage of the accumulated runoff and the sediment
load as a function of the percentage of events. These results clearly emphasize the variability of runoff and sediment production in the study
watershed, and they are consistent with other studies of agricultural watersheds in that most of the sediment load is transported during a small
number of events (Estran et al., 2009; Lana-Renault and Regüés, 2009;
Mano et al., 2009). Many rainfall events with a small rainfall depth and
a short duration produced limited runoff and nearly no sediment, and
this is especially evident in Rainfall Regime III.
4.2. Effects of land uses on runoff and soil loss
Along with defining the surface depression storage, land use also
defines the proportion of the soil surface that is bare and therefore exposed to raindrop impact and crusting. This factor influences interception
and surface and subsurface water storage capacities directly. Cultivated
lands are thought to be the major contributors to sediment yield in this
area (Shi et al., 2004). Most soil loss occurs from June to August, which coincides with the relatively high frequency of rural activity (Fang et al.,
2011). Much like the main land use changes in the present study area,
when cultivated lands are converted to orchards or forest, the rural activity reduces. The canopies of orchards or forest can reduce the erosive
power of the raindrops (Sinun et al., 1992; Wiersum, 1984), and they
provide materials for soil cover on the forest floor. As a result, the energy of raindrops, which is dependent on the raindrops' size and velocity,
is reduced to almost zero when the raindrops reach the soil (Binkley
and Brown, 1993). Furthermore, their rooting systems will also hold
Fig. 5. Runoff coefficients (A) and suspended sediment load (B) under different rainfall regimes in the Wangjiaqiao watershed.
N.-F. Fang et al. / Catena 99 (2012) 1–8
characteristics. Oakes et al. (2012) suggested that sheet erosion is mainly controlled by rainfall characteristics and soil surface features.
Table 5
Two-way ANOVA of RC and TL with different rainfall regime and land use.
Factors
Rainfall regimes (A)
Land use (B)
Interaction (A × B)
RC
7
TL
F-ratio
P-value
F-ratio
P-value
2.504
11.195
0.968
0.085
0.001
0.382
4.891
7.322
1.844
0.009
0.008
0.162
soil particles effectively and make the soil more resistant to erosion.
Penetration by the roots and their subsequent growth can compact the
soil in the immediate vicinity (Greacen and Sands, 1980), thus increasing
its resistance to erosion. The soils of the watershed are mainly purple
soils developed from purple sandstone; they have high rock fragment
content and are rich in macropores (Fu et al., 2011). The occurrence of
macropores is closely linked to land use and agricultural management
practices. It has been argued that agricultural management, in particular
deep plowing and intense soil tillage — reduces the macroporosity of the
soil and thus contributes to increasing storm–runoff from agricultural
areas (Bronstert et al., 2002). Other studies confirmed that an increase
of forest area results in a significant reduction in discharge and sediment
(Niehoff et al., 2002; Wegehenkel, 2002).
4.3. Effects of land uses and rainfall regimes interaction on runoff and soil
loss
The dramatically decreasing trend shown under Rainfall Regimes I
and II, however, contrasts with the more gently decreasing trend in
Rainfall Regime III. This difference was mainly due to the fact that the
mean precipitation depth and the duration of a single rainfall event
under Rainfall Regime III were lower than those under I and II. In
order, the most sensitive response of runoff and erosion to land use was
exhibited by Rainfall Regime II, followed by Rainfall Regime I, and then
Rainfall Regime III. This indicates that rainfall characteristics are decisive
for the relative importance of different storm runoff generation mechanisms. The runoffs caused by storms of large rainfall depths are more
strongly influenced by land-surface conditions than floods caused by
precipitation with lower rainfall depths, especially when the rainfall intensity is in the same order of magnitude as the infiltration capacity of
the soil (Bronstert et al., 2002). The small number of runoff events that
produced a large proportion of the runoff and sediment load mainly occurred before the year 2000.
Table 5 shows that the land use changes define the trend of runoff
(P = 0.001) and sediment load (P = 0.008) change. Different rainfall
regimes show significantly different sensitivities of RC change. The results of Niehoff et al. (2002) show that the influence of land use conditions on storm runoff generation depends greatly on the rainfall event
Fig. 6. Percentage of accumulated runoff and suspended sediment yield in the Wangjiaqiao
watershed in relation to the number of events.
5. Conclusions
In this paper, three rainfall regimes were classified from 152 events
using K-means clustering based on rainfall depth, max-30 min intensity
and duration. Rainfall Regime I causes the greatest proportion of accumulated runoff and soil loss. The values of the mean runoff coefficient
and the mean sediment load of the different rainfall regimes were
as follows: Rainfall Regime II > Rainfall Regime I > Rainfall Regime
III. A shift from paddy fields and cropland to forest and orchards in
the Wangjiaqiao watershed is the most important land use change
between the years 1995 and 2004. The results indicated that the runoff
and soil losses of different land uses (land uses before and after 2000)
varied greatly under the different rainfall regimes. Apparently, the overall impact of the land use changes in the study period was considerably
decreased runoff and soil loss in the Wangjiaqiao watershed. The different rainfall regimes are decisive for the sensitivity of this decrease. Runoff caused by the storms of large rainfall depths, mainly under Rainfall
Regimes I and II, are more strongly influenced by land surface conditions than floods caused by precipitation with lower rainfall depths.
Acknowledgments
Financial support for this research was provided by the National
Natural Science Foundation of China (41071190 and 40901132), the
“Hundred‐talent Project” of the Chinese Academy of Sciences, the Key
Project of Chinese Ministry of Education (No. 108165), and the Program
for New Century Excellent Talents in University (NCET‐10‐0423).
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