Soil & Tillage Research 105 (2009) 77–87
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Soil & Tillage Research
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Relationships between rainfall characteristics and ephemeral gully erosion in a
cultivated catchment in Sicily (Italy)
A. Capra a,*, P. Porto a, B. Scicolone b
a
b
Department of Agro-forestry and Environmental Sciences and Technologies, Mediterranean University of Reggio Calabria, Località Vito, 89060 Reggio Calabria, Italy
Via Mascalucia 5, 95125 Catania, Italy
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 2 October 2008
Received in revised form 10 March 2009
Accepted 25 May 2009
Recent research has shown a lack of long-term monitoring for detailed analysis of gully erosion response
to climate characteristics. Measures carried out from 1995 to 2007 in a wheat-cultivated area in Raddusa
(Sicily, Italy), represent one of the longest series of field data on ephemeral gully, EG, erosion. The data set
collected in a surface area of almost 80 ha, permits analysis of the influence of rainfall on EG formation
and development. Ephemeral gullies formed in the study area were measured on a yearly scale with a
Post-Processing Differential GPS for length and with a steel tape for the width and depth of transversal
sections. Ephemeral gully formation was observed for 8 years out of 12, which corresponds to a return
period of 1.5 years. The measurements show strong temporal variability in EG erosion, in agreement
with the rainfall characteristics. The total eroded volumes ranged between 0 and ca. 800 m3 year1, with
a mean of ca. 420 m3 year1, corresponding to ca. 0.6 kg m2 year1. Ephemeral gully erosion in the
study area is directly and mainly controlled by rainfall events. An antecedent rainfall index, the
maximum value of 3-days rainfall (Hmax3_d), is the rain parameter which best accounts for EG erosion.
This index is used here as a simple surrogate for soil water content. An Hmax3_d threshold of 51 mm was
observed for EG formation. The return period of the Hmax3_d threshold is almost the same as the return
period for EG formation. Although a mean of seven erosive rain events were recorded in a year, EG
formation and development generally occur during a single erosive event, similarly to other semiarid
environments. The most critical period is that comprised between October and January, when the soil is
wetter and the vegetation cover is scarce. Empirical models for EG eroded volume estimation were
obtained using the data set collected at this site. A simple power-type equation is proposed to estimate
the eroded volumes using Hmax3_d as an independent variable. This equation shows an R2 equal to 0.67
and a standard error of estimation of 0.79.
ß 2009 Elsevier B.V. All rights reserved.
Keywords:
Catchment scale
Ephemeral gully erosion
Linear erosion
Mediterranean wheat areas
Threshold rainfall
1. Introduction
Gully erosion is often the main source of sediment on a
catchment scale. The growing interest in gully erosion in the last
few decades is explained by the fact that offsite impacts of soil
erosion can only be assessed at the catchment level (Valentin et al.,
2005). In Belgium gully erosion produces from 40 to 60% of total
soil losses with an average value of 5 Mg ha1 year1 (Poesen et al.,
1996a). In a study conducted in Sicily (Italy) Capra et al. (1994a,b)
and Capra and Scicolone (1996) showed that the gully density was
strongly correlated to rill and interrill erosion and was the best
parameter to use to classify the catchment erosion hazard in eight
small catchments. A survey in 22 Spanish catchments (Poesen
et al., 2002) indicates that specific sediment yield increases when
the frequency of gullies increases. For catchments where no gullies
* Corresponding author. Fax: +39 0965 312681.
E-mail address: [email protected] (A. Capra).
0167-1987/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.still.2009.05.009
were observed, the mean specific sediment yield was
0.74 Mg ha1 year1, whereas in catchments with numerous
gullies the mean specific sediment yield was one order of
magnitude larger, i.e. 9.61 Mg ha1 year1. In the Warragamba
catchment (New South Wales, Australia), sediment yields from
gullied catchments were at least one order of magnitude higher
than ungullied catchments (Valentin et al., 2005).
Gully erosion is defined as the erosion process whereby runoff
water accumulates and recurs in narrow channels, and removes
the soil from these channels to a considerable depth (Poesen et al.,
2003). Different gully types, such as classic or permanent, bank and
ephemeral gullies are described in the literature.
Permanent gullies are often defined for agricultural land as
channels resulting from erosion and caused by the concentrated
but intermittent flow of water usually during and immediately
following heavy rain, deep enough (usually >0.5 m) to interfere
with, and not to be obliterated by, normal tillage operations (Soil
Science Society of America, 2001). Bank gullies develop whenever
concentrated runoff crosses an earth bank (Poesen et al., 2003).
78
A. Capra et al. / Soil & Tillage Research 105 (2009) 77–87
Different definitions of ephemeral gully (EG) have been given in
the literature. According to the most common, EGs are channels
which occur between two opposite slopes, typically masked, but
not completely obliterated, by normal tillage (Laflen, 1985; Foster,
1986; USDA, 1992). According to the Soil Science Society of
America (2001) EGs are ‘‘small channels eroded by concentrated
flow that can be easily filled by normal tillage, only to reform again
in the same location by additional runoff events’’. It is generally
accepted that rills are more common in planar elements of
watersheds, EGs on valley bottoms or within swales (Haan et al.,
1994; Casalı́ et al., 1999, 2006), but Poesen et al. (2003) highlight
that they observed EGs not only in natural drainage lines but also
along landscape linear elements. Casalı́ et al. (1999, 2006) define
EGs as incised channels of various sizes, formed in small valleys
(swales) on agricultural soils by the scouring of concentrated
surface runoff during rain events, which are usually refilled by
farmers shortly after the rains, but often reappear in the next rainy
season. Some authors use a threshold cross-sectional area or depth
to distinguish EGs from rills and classic gullies (Poesen et al.,
1996b, 2003).
Poesen et al. (2003) emphasise the difficulty of defining a clear
cut between EGs and rills, and between EGs and classic gullies and
river channels. In fact, the transition from rill to EG to gully to river
channel erosion represents a continuum, and any classification of
hydraulically related erosion forms into separate classes is, to some
extent, subjective (Poesen et al., 2003). The observation, in a field
survey (Capra and Scicolone, 2002), of linear erosion elements that
can be classified as rills in their top part, EGs in the middle part and
permanent gullies in the bottom part makes further classification
difficult.
Ephemeral gully erosion generally occurs in cultivated soils
during seedbed preparation, planting and crop establishment
periods, when the soil is scarcely protected by vegetation. The
damage it causes includes onsite loss of productivity, inconvenience and increases in the cost of farming operations, and loss of
land value. Furthermore, EGs are effective links for transferring
runoff and sediment from upland to permanent channels where
they cause offsite sedimentation and problems of water quality
(Poesen et al., 2003; Valentin et al., 2005).
Previous studies in different climate and land use conditions
indicate soil losses due to this kind of erosion as varying from 10 to
100% of the total soil loss in agricultural watersheds (see amongst
others Foster, 1986; Spomer and Hjelmfelt, 1986; Thomas et al.,
1986; Thomas and Welch, 1988; Auzet et al., 1990; Vandaele,
1993; Casalı́ et al., 1999, 2006; Poesen et al., 2003; Wilson et al.,
2008). Field-based studies evidenced annual soil losses due to EG
erosion ranging between 2 and 90 m3 ha1 year1 (Poesen et al.,
1996b; Casalı́ et al., 1999; Capra and Scicolone, 2002; Øygarden,
2003; Valcárcel et al., 2003; Cheng et al., 2006).
Ephemeral gully erosion, like other types of gullies, is a
threshold phenomenon in terms of flow hydraulics, rainfall,
topography, pedology and land use (Poesen et al., 2003; Valentin
et al., 2005). Data available on threshold rains is scarce, usually
restricted to small areas and examined over short time periods.
Threshold rains of 14.5–22 mm have been observed on cropland in
various study areas in Belgium, France, Northern Thailand, Spain
and the UK (Poesen et al., 2003). In Navarra (Spain) the minimum
conditions able to promote EG erosion were a total depth of 17 mm
and a peak rate of 54 mm h1 (Casalı́ et al., 1999). Nachtergaele
et al. (2001a) analysed EG formation over a 15-year period in
central Belgium and found a critical rainfall height of 15 mm in
winter and 18 mm in summer. In Normandy (France), a rainfall
height of 28.5 mm and a max 6-min intensity of 15 mm h1, and a
rainfall height of 21.6 mm and a max 6-min intensity of 98 mm h1
promoted rill and EG formation in a cropped area in December and
May, respectively (Cerdan et al., 2002). Chaplot et al. (2005)
observed that the rainfall threshold for linear erosion (rill and
gully) in a 0.62 km2 catchment in Laos was about 50 mm total rain
with a minimum rainfall intensity of 100 mm h1. The different
threshold rain values are attributed to different states of the soil
surface as affected by tillage operations and previous rains (Poesen
et al., 2003).
In addition to rainfall height and intensity, rainfall erosivity
indices and antecedent rainfall indices can be used to show the
influence of rainfall in erosion processes. One of the most common
rainfall erosivity index is the R-factor proposed by Wischmeier and
Smith (1978). Equations used to calculate the R-factor will be
explained in the next section (see Section 2). Previous soil moisture
before any rainfall event influences runoff generation (Descroix
et al., 2002; Castillo et al., 2003) and, therefore, soil erodibility
(Morgan, 2005; Casalı́ et al., 1999). Antecedent rainfall indices can
be used as surrogate for soil water content (Descroix et al., 2002;
Castillo et al., 2003). The cumulative 24-h rainfall (Woodward,
1999) and the cumulative 3-days or 5-days rainfall (Capra and
Scicolone, 2002; NRCS, 2003) have been previously used as
antecedent rainfall indices.
The main objective of this paper is to analyse the relationships
between rainfall characteristics and EG erosion in a small
catchment in Raddusa (Sicily, Italy) using measures carried out
in a 12-year period in an area of about 80 ha of wheat.
Fig. 1. Location and map of the study area.
Fig. 2. View of some fields in the monitored area.
A. Capra et al. / Soil & Tillage Research 105 (2009) 77–87
79
2. Materials and methods
2.1. Site description
The area surveyed (Figs. 1 and 2) is located in a small catchment
with a surface area of about 1.35 km2, in central Sicily, Italy,
belonging to a national (Italian) network of experimental
catchments for the hydrogeological protection of hill and
mountain environments.
The area is located at an altitude between 305 and 483 m above
sea level and the mean slope of the area is 16.4%. The dominant soil
association is Vertic Xerocrepts, which is very common in Sicily
(Fierotti, 1997). The main textural classes are silt clay loam and
clay (sand 33–39%, silt 22–38%, clay 29–45%); the gravel content is
negligible; the bulk density ranges between 1140 and
1170 kg m3; the organic matter content of the soil ranges from
0.98 to 1.13%. The soil profile does not show delimited horizons
until a depth of 1.2 m.
The climate is Mediterranean with a rainy temperate winter
and a warm dry summer. The annual rainfall during the period
1971–2007 shows a mean of about 500 mm, and a standard
deviation of about 205 mm. More than 80% of the rainfall is
concentrated in the October–May period.
The main crop in the area is durum wheat (ca. 80 ha); other
crops are olives, citrus fruit and grapes. The fields, separated by
roads, stones or torrents, slope in the direction of the maximum
length (Fig. 2) and they are mainly tilled up and down.
Some active EGs occur in the observed area during the rainy
season. They are erased by filling with soil from areas adjacent
to the channel using ordinary tillage equipment (generally, a
cultivator harrow 0.30–0.35 m deep and a plough 0.5 m deep).
The EGs frequently recur in the same places during the next
rainy season. In the wheat surface, the farmers do not move soil
or level the EGs from sowing to harvest and the EGs are erased
during tillage operations from July to October, before sowing.
The main farming operations consist of two tillages in summer
or early autumn with a cultivator and one ploughing operation
carried out every few years (generally a minimum of 3 years). In
fallow areas and orchards, on the other hand, tillage and EG
filling are practically continuous. The EG measures therefore
only concerned the wheat surface because the tillage operations
in fallow areas and orchards disturb channel formation and
monitoring.
The area has been monitored for EG formation and development on a yearly scale since 1995 (Capra et al., 2002).
Measurement at the event level in a microbasin inside the
catchment started in 1999 (Capra and Scicolone, 2001).
2.2. Rainfall characterisation
Considering the yearly distribution of the rainfall and the
described crop management, the October–May period is considered to be the hydrologic year in this study.
Until 1998, rainfall data was measured by a rotational rain
gauge located about 600 m away from the study area, and later by
an automatic station installed in the basin, at 1-h intervals. A
comparison of the rainfall measured in a period of 4 months shows
a good agreement between the two instruments (Fig. 3).
The rainfall of the hydrologic year is characterised by total
height, maximum 1-h intensity and the number of rainy days (days
with total rainfall 1 mm). Considering the importance of the soil
moisture content for EG formation (Casalı́ et al., 1999; Castillo
et al., 2003; Poesen et al., 2003) and the results of previous analyses
in the study area (Capra and Scicolone, 2001, 2002) the cumulative
24-h and 3-days rainfall were calculated as simple surrogates for
soil moisture content.
Fig. 3. Relationship between precipitation heights (mm) measured by rotational
gauge and automatic station.
The erosivity of the rainfall is expressed by the number of
events and the rainfall erosivity factor, R-factor. The computation
of the R-factor (Wischmeier and Smith, 1978; Morgan, 2005) for an
event, Re, is defined as:
Re ¼ E I30
(1)
where Re: event R-factor, MJ mm ha1 h1; E: kinetic energy of the
rain event, MJ ha1; I30: maximum 30-min intensity, mm h1.
The kinetic energy of a rain event, E, is estimated according to
Wischmeier and Smith (1978) as:
E¼
n
X
ei
(2)
i¼1
ei ¼ e0i hi
(3)
e0i ¼ 0:119 þ 0:0873 log Ii
(4)
1
where E: kinetic energy of the rain event, MJ ha ; ei: kinetic
energy for each increment, MJ ha1; n: number of successive
increments of the event; hi: height of each increment, mm; e0i :
kinetic energy for each increment and for each height unit of the
rain, MJ ha1 mm1; Ii: intensity of each increment, mm h1.
It is assumed that an event is erosive if the height of rainfall is
equal to or more than 13 mm or the intensity in 15 min is equal to
or more than 6 mm (Wischmeier and Smith, 1978).
Both the maximum value of Re (Rmax) and the sum of the Re of all
the erosive events in the hydrologic year (Rt) were calculated.
The return period T of the different rainfall characteristics was
estimated by the empirical frequency values.
2.3. Field measurements and description of ephemeral gully erosion
According to the discussion on EG definition (see Section 1) and
the agricultural use and soil management practices in the study
area, in this paper we consider an EG as a channel of different sizes,
mainly (but not only) located in swales, refilled by tillage
equipment normally used on farms in the area studied. Furthermore, the term EG system is used in this paper to indicate the
whole of the main branch and the interconnected tributaries of an
EG.
The ephemeral gully measurements were made at the end of
May each year and then the surveyed EGs resulting from all erosive
A. Capra et al. / Soil & Tillage Research 105 (2009) 77–87
80
events in the rainy season (from October to May). Both the main
branch of each EG and its tributaries (if present) were measured. A
Post-Processing Differential GPS with a planimetric accuracy of
12 cm was used to measure the spatial co-ordinates of points
located about every 5 m of channel in the longitudinal direction.
Cross-sections were measured about every 20 m of channel, or
whenever any change in the EG section, or the presence of tributaries,
was observed. As it was possible to assimilate the cross-section of the
EGs observed in the study area to a trapezium (or a rectangle), the
channel widths (upper and lower) and depths were measured with a
steel tape graduated every 5 mm. All the measures were made by the
same expert operator.
The length of the EGs was computed by the measured point coordinates; the eroded volumes were computed using the end area
method (average of two successive cross-sections times the
distance between them):
V¼
n
X
n
X
i¼1
i¼1
Vi ¼
Ai1 þ Ai
Li
2
(5)
where V: volume of eroded soil of an EG, m3: n: number of
segments; Vi: volume of eroded soil within the segment, m3; Ai1:
downstream cross-sectional area of the segment, m2; Ai: upstream
cross-sectional area of the segment, m2; Li: distance between
adjacent cross-sections, m.
2.4. Relationships between ephemeral gully erosion and rainfall
characteristics
The relationships between EGs and rainfall were investigated
using correlation matrices and regression analysis.
Correlation matrices of the Pearson correlation coefficient (R)
were computed using the EXCEL1 software to show whether the
EG characteristics observed are linearly related to rainfall factors.
Student’s t-probability test was used to display the significance of R
for a probability level (P) 0.05 (Maidment, 1992).
Multiple regression analysis was performed using the STATGRAPHICS1 software in a stepwise procedure. It allowed
independent variables to be individually added or removed from
the model at each regression step, evaluating the changes in the
coefficient of determination (R2). The efficiency of the models
obtained was tested by the standard error of estimation (SEE) and
mean absolute error (MAE) parameters (Maidment, 1992;
Statgraphics1 5 Plus, 2000):
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u n
u1 X
2
SEE ¼ t
ðY Y 0i Þ
n i¼1 i
(6)
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u n
u1 X
ðY i Y 0 Þ
MAE ¼ t
i
n i¼1
(7)
where Y: measured value; Y0 : estimated value; n: number of
observations.
A well-performing regression model should have an SEE close to
zero and low MAE values.
3. Results and discussion
3.1. Characteristics of rainfall
Table 1 shows the high variability of the rainfall characteristics
in the 12 hydrologic years, particularly as regards rainfall intensity
(Imax) and the R-factor (both Rt and Rmax), whose standard deviation
is very high. In the observation period, the total height (Ht) ranged
from a minimum of 175 mm in 2000–2001 to a maximum of
709 mm in 1995–1996, with a mean value equal to 415 mm. The
mean number of rainy days (Dr) was 45, with a minimum of 22 in
2000–2001 and a maximum of 66 in 1995–1996. The maximum 1h intensity (Imax) varied from 8 to 124 mm h1, with a mean of
37 mm h1.
A mean number of seven erosive events was observed in the
study area. The total R-factor (Rt) ranged from 77 in 2001–2002 to
3560 MJ mm1 ha1 h1 in 2005–2006. The maximum values of Re
(Rmax) ranged from a minimum of 30 in 2001–2002 and a
maximum of 1701 MJ mm1 ha1 h1 in 2005–2006. More than
80% of the events registered in a year had a low erosivity
(Re 100 MJ mm1 ha1 h1), and, generally, only one, which
occurred in the months between October and January, had high
erosivity (see Table 1 for Rmax values and Fig. 4 as an example for
two of the observed years).
Table 2 shows the return period (T) of the rainfall characteristics
considered. Due to the high number of rainy days, the rainfall was
exceptionally high in 1995–1996 (T = 24 years). The maximum
values of Imax and Hmax3_d were recorded in 1999–2000 (T equal to
ca. 16 and 22 years, respectively) whereas the maximum number
of erosive events was observed in 2003–2004 (T = ca. 16 years), and
Table 1
Main characteristics of rainfall and erosive events in the study area: total height (Ht), rainy days (Dr), maximum 1-h intensity (Imax), 24-h height (Hmax24_h), 3-days height
(Hmax3_d), number of erosive events, total R-factor (Rt), maximum values of R in a year (Rmax), and date when Rmax was observed.
Hydrologic yeara
Rainfall
Erosive events
Ht
(mm)
Dr (n)
Imax
(mm h1)
Hmax24_h
(mm)
Hmax3_d
(mm)
Events (n)
Rt (MJ
mm1 ha1 h1)
Rmax
(MJ mm1 ha1 h1)
Date of
Rmax
1995–1996
1996–1997
1997–1998
1998–1999
1999–2000
2000–2001
2001–2002
2002–2003
2003–2004
2004–2005
2005–2006
2006–2007
709
224
518
211
491
175
221
389
475
504
560
501
66
45
47
24
47
22
32
56
49
55
47
48
n.m.
n.m.
n.m.
22
124
8
9
14
43
30
60
25
47
51
112
30
91
29
28
29
104
65
161
79
101
51
116
37
189
29
31
42
117
68
185
88
n.m.
n.m.
n.m.
6
8
3
3
9
10
8
7
9
n.m.
n.m.
n.m.
301
1038
213
77
296
1494
1023
3560
1008
n.m.
n.m.
n.m.
112
451
97
30
76
1130
536
1701
458
n.m.
n.m.
n.m.
11/13
11/29–12/06
10/30
01/16
01/11
10/15
03/04
10/22
12/23–25
Mean
Standard deviation
415
170
45
13
37
37
69
42
88
56
7
3
1001
1076
510
565
a
From October 1st to May 31st; n.m. = not measured.
A. Capra et al. / Soil & Tillage Research 105 (2009) 77–87
Fig. 4. Erosivity indices of different events, Re, in 2003–2004 and 2004–2005.
the maximum Hmax24_h (T = ca. 22 years), Rt and Rmax (T = ca. 16
years) in 2006–2007. The rainy seasons 1998–1999, 2000–2001,
2001–2002 and 2002–2003 were dryer (T = ca. 1 year for most of
the characteristics considered).
The results of the correlation matrix between the rainfall
characteristics (Table 3) show that the best correlations are: Ht
with Dr and Hmax24_h; Hmax3_d with Imax and Hmax24_h; and Imax, Rt
and Rmax with Hmax24_h (significant at P 0.001). According to the
results listed in Table 2, the correlation between Imax and the other
characteristics is not statistically significant, with the exception of
Hmax3_d. The number of erosive events in a year is well correlated
(P 0.05) with both Ht and Dr. Both the erosivity indices Rt and
Rmax are well correlated with Hmax24_h.
3.2. Characteristics of ephemeral gullies
Tables 4 and 5 show the main characteristics of the ephemeral
gullies which developed in the study period as regards each year
and the whole period 1995–2007, respectively. The total number,
channel length and eroded volume equal to 181 EG systems,
20,355 m and 3338 m3, respectively, were surveyed in the whole
81
study period (Table 5). The total number of measured crosssections was 1328, thus resulting in a mean interval between
consecutive sections of about 15 m in the longitudinal direction. A
comparison performed on two EGs between cross-sections
measured at mean intervals of 15 and 2 m showed differences
in the eroded volumes of about 5%.
Like the rainfall events, the data in Table 4 confirms the great
temporal variability in the occurrence of EG erosion as observed
in other environments (Poesen et al., 2003; Chaplot et al., 2005;
Valentin et al., 2005). Ephemeral gully formation occurred 8
years out of 12, with a frequency corresponding to 67% of the
years covered by the survey and a return period of 1.5 years. No
EG erosion occurred in the rainy seasons 1998–1999, 2000–2001,
2001–2002 and 2002–2003. The erosion was minimum in 1996–
1997 when a total of four active EGs, 530 m of channels, and
about 20 m3 of soil loss were observed (Table 4). The maximum
number and total length of active EGs, equal to 46 and 6190 m,
respectively, were detected in 1999–2000, whereas the maximum total eroded volume, equal to ca. 800 m3, occurred in
2003–2004, when the EGs were wider and deeper than in 1999–
2000 (Table 4).
Averaging the results over the 12 years, then including the years
with zero EG erosion, the total number, length and eroded volume in
the examined area are 15 EGs year1, 1696 m year1 and 278
m3 year1, respectively. The corresponding values averaged over the
wheat surface area are 0.19 EGs ha1 year1, 21 m ha1 year1 and
3.5 m3 ha1 year1 (about 0.4 kg m2 year1), respectively. The
mean eroded volume in the 8 years when EGs were active is equal
to 0.6 kg m2 year1.
In the study period, with the exception of 1999–2000, EG
formation and development occurred during a single event, mainly
in the months from October to January, as observed in ephemeral
gully systems and sediment yield in agricultural watersheds in
Navarra (Casalı́ et al., 1999, 2008).
Table 2
Return period (years) of the main characteristics of rainfall and erosive events in the study area: total height (Ht), rainy days (Dr), maximum 1-h intensity (Imax), 24-h height
(Hmax24_h), 3-days height (Hmax3_d), number of erosive events, total R-factor (Rt), and maximum values of R in a year (Rmax).
Hydrologic yeara
1995–1996
1996–1997
1997–1998
1998–1999
1999–2000
2000–2001
2001–2002
2002–2003
2003–2004
2004–2005
2005–2006
2006–2007
a
Rainfall
Erosive events
Ht
Dr
Imax
Hmax24_h
Hmax3_d
Number of events
Rt
Rmax
24.0
1.4
4.8
1.1
2.2
1.0
1.3
1.6
1.8
3.4
8.0
2.7
24.0
1.4
1.6
1.1
1.8
1.0
1.3
8.0
3.4
4.8
2.2
2.7
n.m.
n.m.
n.m.
1.6
16.3
1.1
1.2
1.4
3.6
2.6
5.8
2.0
1.6
1.8
7.8
1.4
3.4
1.1
1.0
1.3
4.7
2.2
21.6
2.7
2.7
1.6
3.4
1.3
21.6
1.0
1.1
1.4
4.7
1.8
7.8
2.2
n.m.
n.m.
n.m.
1.4
2.0
1.1
1.2
3.6
16.3
2.6
1.6
5.8
n.m.
n.m.
n.m.
1.6
3.6
1.2
1.1
1.4
5.8
2.6
16.3
2.0
n.m.
n.m.
n.m.
1.6
2.0
1.4
1.1
1.2
5.8
3.6
16.3
2.6
From October 1st to May 31st; n.m. = not measured; unit of measurement as in Table 1.
Table 3
Correlation matrix for the rainfall characteristics: total height (Ht), rainy days (Dr), maximum 1-h intensity (Imax), 24-h height (Hmax24_h), 3-days height (Hmax3_d), number of
erosive events, total R-factor (Rt), and maximum values of R in a year (Rmax).
Ht
Dr
Imax
Hmax24_h
Hmax3_d
Number of events
Rt
Rmax
a
Ht
Dr
Imax
Hmax24_h
Hmax3_d
Number of events
Rt
Rmax
1
0.865
0.570a
0.808
0.769
0.754
0.732
0.722
0.865
1
0.332a
0.464a
0.453a
0.743
0.406a
0.416a
0.570a
0.332a
1
0.599a
0.887
0.379a
0.452a
0.413a
0.808
0.464a
0.599
1
0.894
0.463a
0.963
0.961
0.769
0.453a
0.887
0.894
1
0.461a
0.793
0.762
0.754
0.743
0.379a
0.463a
0.461a
1
0.391a
0.428a
0.732
0.406a
0.452a
0.963
0.793
0.391a
1
0.967
0.722
0.416a
0.413a
0.961
0.762
0.428a
0.967
1
Not significant (at P 0.05).
A. Capra et al. / Soil & Tillage Research 105 (2009) 77–87
82
Table 4
Main characteristics of the ephemeral gullies measured in the rainy seasons from 1995–1996 to 2006–2007 in the study area.
1995–1996
Total
Meanb
Minimumb
Maximumb
Standard deviationb
1996–1997
Total
Meanb
Minimumb
Maximumb
Standard deviationb
1997–1998
Total
Meanb
Minimumb
Maximumb
Standard deviationb
Systemsa
(n)
Measured
cross-sections (n)
Tributaries
(n)
Length (m)
Main
Tribut.
System
Upper
Lower
Medium
19
182
10
3
35
8
16
0.84
0.00
6.00
1.80
2319
122
29
306
93
900
180
16
477
188
3219
169
29
782
183
0.39
0.24
0.57
0.09
0.21
0.12
0.29
0.06
0.30
0.18
0.42
0.07
0.23
0.01
2.54
0.56
169.77
8.94
0.10
38.65
12.34
530
133
92
157
29
0.52
0.30
1.55
0.32
0.30
0.10
1.40
0.34
0.41
0.30
1.48
0.32
0.10
0.01
0.35
0.12
20.13
5.03
1.40
10.07
3.69
4
32
23
6
5
6
1
0
0
0
0
0
530
133
92
157
29
Width (m)
Depth (m)
Eroded
volume (m3)
133
4
3
7
1
3
0.09
0.00
2.00
0.39
1670
52
9
138
34
23
12
12
12
0
1694
53
9
138
35
0.65
0.10
2.20
0.42
0.31
0.10
1.00
0.23
0.48
0.16
1.60
0.31
0.16
0.02
0.80
0.17
206.63
6.46
0.23
41.21
10.28
355
8
3
20
4
19
0.41
0.00
3.00
0.88
5473
119
20
369
68
718
72
13
161
50
6190
135
20
369
80
0.57
0.10
5.00
0.53
0.20
0.04
0.80
0.15
0.38
0.07
2.55
0.30
0.16
0.02
1.00
0.18
634.52
13.79
0.67
91.06
17.78
266
8
2
63
11
13
0.39
0.00
7.00
1.39
3297
100
16
331
77
572
143
5
399
177
3869
117
16
730
129
0.65
0.05
3.10
0.49
0.32
0.05
1.80
0.27
0.49
0.05
2.15
0.35
0.20
0.01
1.65
0.25
809.55
24.53
0.19
284.70
52.60
97
8
2
16
5
3
0.25
0.00
1.00
0.45
818
68
6
169
54
87
29
9
46
19
0.52
0.10
2.40
0.43
0.28
0.07
1.14
0.23
0.40
0.11
1.77
0.33
0.19
0.01
1.10
0.23
118.72
9.89
0.06
43.61
13.72
193
7
2
14
4
7
0.27
0.00
1.00
0.45
2652
102
9
255
58
235
34
18
49
12
2908
112
9
255
62
0.90
0.16
4.45
0.76
0.30
0.04
1.15
0.26
0.60
0.14
2.31
0.46
0.31
0.02
1.20
0.25
774.83
29.80
0.53
280.51
55.60
79
10
4
15
3
5.00
0.63
0.00
2.00
0.92
903
113
36
185
53
137
46
12
94
43
1040
130
36
185
54
1.21
0.33
3.55
0.95
0.53
0.12
2.07
0.60
0.87
0.24
2.69
0.75
0.47
0.03
1.54
0.44
603.58
75.45
5.21
174.74
59.53
1998–1999 (No EG erosion)
1999–2000
Total
Meanb
Minimumb
Maximumb
Standard deviationb
46
2000–2001, 2001–2002,
2002–2003,(No EG erosion)
2003–2004
Total
Meanb
Minimumb
Maximumb
Standard deviationb
2004–2005
Total
Meanb
Minimumb
Maximumb
Standard deviationb
2005–2006
Total
Meanb
Minimumb
Maximumb
Standard deviationb
2006–2007
Total
Meanb
Minimumb
Maximumb
Standard deviationb
a
b
33
12
27
8
905
75
6
169
60.33
Main branch and connected tributaries.
Between EG systems when not specified, or between main or tributaries.
In the different years, both EG systems comprising a main
branch alone and those with a main branch and one or more
tributaries were active, with the exception of 1996–1997, when no
tributaries were observed. The mean number of active tributaries
in the study area was eight per year, with a minimum of 0 in 1996–
1997, and a maximum of 19 in 1999–2000 (Tables 4 and 5). The
mean number of tributaries per EG system was 0.36, ranging
between a minimum of zero and a maximum of seven (Table 5).
The mean length of active EG systems, main branches and
tributaries were equal to 116, 101 and 74 m, respectively; the
mean eroded volume per EG system was ca. 22 m3 (Table 5). The
width ðwÞ and depth (d) of tributaries were lower than those of the
main branches: the mean values of w and d were equal to 0.74 m
ðwÞ and 0.37 m (d) and 0.32 m ðwÞ and 0.18 m (d), for the main
branches and the tributaries, respectively. The mean width of the
upper, lower and medium sections of the EG systems were 0.68,
0.31 and 0.49 m, respectively (Table 5); ca. 80% of the measured
sections showed a mean width lower than 0.8 m.
The mean depth of EG systems was 0.23 m (Table 5); ca. 80% of
the measured sections had a depth of less than 0.5 m, which
A. Capra et al. / Soil & Tillage Research 105 (2009) 77–87
83
Table 5
Synthesis of the main characteristics of ephemeral gullies in the rainy seasons from 1995–1996 to 2006–2007 in the study area.
Systemsa (n)
Total valuesb
Total
Mean
Minimum
Maximum
Standard deviation
181
23
4
46
14
Measured
cross-sections (n)
Tributaries (n)
1328
166
23
355
107
66
8
0
19
7
Length (m)
Width (m)
Main
Tribut.
System
17,662
2,208
530
5,473
1,638
2673
382
23
900
345
20,355
2,544
530
6,190
1,904
Upper
Lower
Depth (m)
Medium
Eroded
volume (m3)
3337.73
417.22
20.13
809.55
319.85
Mean valuesc
Mean
Minimum
Maximum
Standard deviation
8
4
10
2
0.36
0.00
0.84
0.27
101
52
133
28
74
12
180
64
116
53
169
36
0.68
0.39
1.21
0.26
0.31
0.20
0.53
0.10
0.49
0.30
0.87
0.18
0.23
0.10
0.47
0.11
21.74
5.03
75.45
23.41
Minimum valuesc
Mean
Minimum
Maximum
Standard deviation
3
2
5
1
0.00
0.00
0.00
0.00
27
6
92
28
12
5
18
4
27
6
92
28
0.17
0.05
0.33
0.10
0.08
0.04
0.12
0.03
0.15
0.05
0.30
0.08
0.02
0.01
0.03
0.01
1.05
0.06
5.21
1.74
Maximum valuesc
Mean
Minimum
Maximum
Standard deviation
22
6
63
22
2.75
0.00
7.00
2.75
239
138
369
239
177
12
477
177
348
138
782
348
2.85
0.57
5.00
2.85
1.21
0.29
2.07
1.21
1.87
0.42
2.69
1.87
1.27
0.35
2.54
1.27
120.57
10.07
284.70
120.57
Depth
Eroded volume
a
b
c
Main branch and connected tributaries.
Between years.
Between years and EG systems.
Table 6
Correlation matrix for the morphometric characteristics of ephemeral gullies.
Number of tributaries
Length
Width
Main
Tributaries
System
Upper
Lower
Mean
Number of tributaries
1
0.279
0.891
0.689
0.210
0.249
0.214
0.416
0.420
Length
Main
Tributaries
System
0.279
0.891
0.689
1
0.761
0.854
0.761
1
0.964
0.854
0.964
1
0.183a
0.024a
0.202
0.177a
0.085a
0.217
0.178a
0.015a
0.195a
0.349
0.590
0.517
0.522
0.460
0.563
Width
Upper
Lower
Mean
0.210
0.249
0.214
0.183a
0.177a
0.178a
0.024a
0.085a
0.015a
0.202
0.217
0.195a
1
0.807
0.975
0.807
1
0.911
0.975
0.911
1
0.676
0.634
0.678
0.689
0.609
0.679
Depth
Eroded volume
0.416
0.420
0.349
0.522
0.590
0.460
0.517
0.563
0.676
0.689
0.634
0.609
0.678
0.679
1
0.679
0.679
1
a
Not significant (at P 0.05).
represents the maximum depth of the tillage operations in the
area, so this arable layer is easier to erode. A limited number of
cross-sections located at the bottom of the valley reached depths
greater than 2 m, evolving into classic gullies. Although channels of
such width and depth are not usually considered ephemeral, the
data was taken into consideration in this study because, as
discussed in Section 1, such types of systems constitute a
continuum from the hydrological point of view and because
farmers in the area continue to fill these EGs when the deeper
segments are short in length. The filling is performed by moving
soil from adjacent areas and passing through the EGs several times
with ordinary farm equipment to level the soil.
The morphometric characteristics of the EGs are generally
positively correlated with each other (Table 6). Apart from the
obvious correlations (e.g. that between main branch or tributary
length and system length), the higher correlation coefficients show
that in particular the length of the tributaries increases when the
length of the main branch increases. Similarly, a wider EG tends to
be deeper. A physical explanation of these correlations can be
found by observing the steps of the EG formation and development
during six consecutive erosive events in 1999–2000 in a
microbasin located inside the study area (Capra and Scicolone,
2001). The first trace of the EG, only a few meters long, appeared at
the foot of the slope after the first erosive event. The following four
events caused an increase in the EG length, width and depth. Only
during the last event the formation of tributaries was observed
whose depth and width were less than those of the main EG. The
tributaries only appeared when the length of the main EG reached
the maximum allowed by the drainage area and length of the
catchment, thus explaining the good correlation between the
length of the main branch and the length of the tributaries. But, as
explained before, the tributaries are less wide and deep than the
main branch, thus explaining the poor correlations between the EG
system length and width and depth.
Length and width, on the other hand, are the morphometric
features that are worst correlated with the other EG characteristics,
with the obvious exception of eroded volume (see Table 6).
The significant correlation between EG length and eroded
volume (Table 6) confirms the feasibility of using EG length to
estimate the eroded volumes, as proposed by several authors
A. Capra et al. / Soil & Tillage Research 105 (2009) 77–87
84
Table 7
Parameters of the correlation equations between EG system eroded volume, V (m3)
and total length Ls (m) ðV ¼ a Lbs Þ in the different years.
1995–1996
1996–1997
1997–1998
1999–2000
2003–2004
2004–2005
2005–2006
2006–2007
a
b
R2
Data (n)
P
0.0017
0.9735
0.0180
0.0050
0.0049
0.0109
0.0179
0.0023
1.5898
0.2901
1.3885
1.5382
1.6530
1.4381
1.4503
2.0787
0.801
0.007
0.548
0.653
0.707
0.701
0.630
0.839
19
4
17
46
33
12
26
8
0.001
ns
0.001
0.001
0.001
0.001
0.001
0.001
R = coefficient of determination; P = probability level; ns = not significant.
(Casalı́ et al., 1999; Nachtergaele et al., 2001a,b; Capra et al., 2005).
The use of an equation-type other than the linear type, e.g. a
power-type as suggested by the cited authors, improves R2. Table 7
shows that the coefficients of the power-type equations are
different in the different years, probably depending on rainfall
characteristics (see next Section). The regression equations show a
high significance, the only exception being 1996–1997 when the
number of EG was very low. Using all the data, the power-type
equation (Eq. (8a)) shows an R2 equal to 0.577, higher than the R2 of
the linear equation performed with the same data set (R2 = 0.317).
The performed power-type equation is:
V ¼ 0:0106 L1:4393
s
R2 ¼ 0:577ðP < 0:001Þ
(8a)
where V: volume of eroded soil in an EG system, m3; Ls: length of
the EG system, m; similarly, Eq. (8b) could be used to estimate the
total eroded volume in a year in the environment considered:
V t ¼ 0:057 L1:113
st
R2 ¼ 0:534ðP 0:05Þ
(8b)
3
where Vt: total eroded volume in a year, m ; Lst: total length of EG
systems active in a year, m.
The efficiency of Eq. (8b) in eroded volume estimation will be
examined and compared with other equations in Section 3.3.
3.3. Relationships between ephemeral gully erosion and rainfall
characteristics
Fig. 5. Relationships between maximum cumulative 3-days rainfall and yearly
ephemeral gully system number, length and eroded volume.
gullies were wider and deeper than previous years in 2005–2006,
when the maximum values of Hmax24_h, Rt and Rmax were monitored.
No tributaries were formed in 1996–1997, when Hmax3_d was equal
to the minimum in the 8 years with EG erosion.
The correlation matrix between rainfall characteristics and
total yearly values for EG number, length and eroded volume
shows that EG development in the considered environment is
mainly affected by the maximum cumulative 3-days rainfall and
by the maximum intensity of 1-h rainfall (Table 8). Threshold
values equal to 51 mm for Hmax3_d (Fig. 5) and 25 mm h1 for Imax
(Fig. 6) are necessary for EG development; both the threshold
values had a return period of 1.6 years.
Considering the significant correlation between Hmax3_d and
Imax, the best simple equations that could be used for yearly total
eroded volume (Vt) estimation are:
Hmax 3 d 51 mm
A simple comparison between Tables 1 and 4 shows that EG
formation and development are directly controlled by rainfall
events. The 4 years when active EGs were not observed (i.e. 1998–
1999, 2000–2001, 2001–2002 and 2002–2003) were dryer. In these
years, almost all the rainfall characteristics were the lowest in the
12-year period, lower than mean values, with a return period lower
than 2 years and very low erosivity (Rmax < 120 MJ mm1 ha1 h1).
The maximum number and total length of the EGs were recorded in
1999–2000, when the cumulative 3-days rainfall (Hmax3_d) and the
intensity (Imax) were maximum. The maximum eroded volume was
observed in 2003–2004, which shows the maximum number of
erosivity events and high values of both Rt and Rmax. The ephemeral
2:31
V t ¼ 0:00556 Hmax 3 d
R2
¼ 0:669ðP 0:05Þ
Hmax 3 d < 51 mm
(9a)
Vt ¼ 0
(9b)
where Vt is expressed in m3 and Hmax3_d in mm.
Multiple regression analysis was performed using Vt as a
dependent variable and the rainfall characteristics in Table 1 as
independent variables. Considering the limited number of years
with active EGs (eight) the stepwise procedure was applied using
three dependent variables at a time and testing all the possible
combinations. The only parameter selected by the procedure was
Hmax3_d.
Table 8
Correlation matrix between the total yearly values of ephemeral gully number, length and eroded volume and rainfall characteristics: total height (Ht), rainy days (Dr),
maximum 1-h intensity (Imax), 24-h rainfall (Hmax24_h), 3-days rainfall (Hmax3_d), number of events, total R-factor (Rt), and maximum values of R in a year (Rmax).
Ht
Dr
Imax
Hmax24_h
Hmax3_d
Number of events
Rt
Rmax
Number of systems
Number of tributaries
0.683
0.621a
0.435a
0.401a
0.915
0.907
0.755
0.639
0.921
0.856
0.420a
0.514a
0.589a
0.431a
0.639
0.492a
Total length
Main
Tributaries
Systems
Eroded volume
0.626a
0.567a
0.622a
0.776
0.385a
0.372a
0.384a
0.457a
0.946
0.867
0.939
0.646
0.695
0.603a
0.688
0.902
0.911
0.811
0.902
0.871
0.454a
0.507a
0.462a
0.587a
0.517a
0.330a
0.506a
0.765
0.553a
0.481a
0.548a
0.819
a
Not significant (at P 0.05).
A. Capra et al. / Soil & Tillage Research 105 (2009) 77–87
85
Fig. 6. Relationships between maximum intensity of 1-h rainfall and yearly ephemeral gully system number, length and eroded volume.
Of the morphometric characteristics of the EGs, the minimum
values of the lower width and the maximum values of the upper
width are significantly correlated with Hmax3_d (R = 0.687, P 0.05
and R = 0.749, P 0.05, respectively). The maximum depth, on the
other hand, only shows a significant correlation with Imax
(R = 0.759, P 0.05).
To evaluate the influence of the rainfall characteristics on the
length–volume equations, a simple regression analysis was
performed using the coefficients a and b of Eqs. (8a) and (8b)
(Table 7) as dependent variables. Again, Hmax3_d is the rain
characteristic best correlated with the exponent b (R = 0.731;
P 0.05); the coefficient a seems to be independent of rainfall
characteristics.
Finally, multiple regression analysis was performed again
including both the EG length and the rain characteristics as
independent variables.
The best equations selected by the stepwise procedure were:
V t ¼ 2794:47 0:1676 Lm þ 10:7464 Hmax 3 d
þ 289:561 Ne
(10)
V t ¼ 1061:85 0:0779 Lm þ 34:6686 Hmax 24 h
0:9940 Rt
(11)
where Vt: total eroded volume in a year, m3; Lm: total length of the
EG main branch in a year, m; Hmax24_h: maximum of cumulative
24-h rainfall, mm; Hmax3_d: maximum of cumulative 3-days
rainfall, mm; Ne: number of erosive events in a year; Rt: total Rfactor, MJ mm1 ha1 h1.
Table 9 shows the parameters of the described simple and
multiple regression equations (Eqs. (8b), (9a), (10) and (11)).
Eq. (9a) is the most efficient for the standard error of estimation
(SSE) and shows a mean absolute error (MAE) similar to the more
complicated Eq. (11) (Fig. 7). Eq. (9a) allows the total yearly eroded
volume of the EG to be estimated without any EG field
measurement. Obviously it does not supply any information about
the position of the EG in the watershed. On the contrary, the use of
Eqs. (8b), (10) and (11) requires the measurement of the EG length.
The EG length measurements by a GPS allow to individuate the
location of the EGs in the field.
The fact that the cumulative 3-days rainfall is the rainfall
characteristic best correlated with EG erosion, confirms the
importance of initial soil water content in runoff generation in
semiarid environments (Castillo et al., 2003). Descroix et al. (2002)
studied the influence of the previous soil moisture to model runoff
yield in North-Western Mexico. The results of this study have
shown that the use of an antecedent rainfall index as a surrogate
for soil water content significantly improves the rainfall runoff
relationship. In Navarra (Spain) early winter, when the soil is
Table 9
Performance coefficients of the models to estimate yearly total eroded volume of
the ephemeral gullies in the Raddusa study area.
Independent variable
R2
R2-adjusted
P
SEE
MAE
Lt (Eq. (8b))
Hmax3_d (Eq. (9a))
Lm, Hmax3_d, Ne (Eq. (10))
Lm, Hmax24_h, Rt (Eq. (11))
0.535
0.669
0.999
0.999
0.457
0.614
0.999
0.999
0.0393
0.0131
0.0074
0.0030
0.940
0.792
3.272
1.310
0.649
0.588
1.373
0.515
Lst = total length of EG systems active in a year, m; Lm = total length of the EG main
branch in a year, m; Hmax24_h = maximum of 24-h rainfall, mm; Hmax3_d = maximum of 3-days rainfall, mm; Ne = number of erosive events in a year; Rt = total Rfactor, MJ mm1 ha1 h1; R2 = coefficient of determination; R2-adjusted = adjusted
coefficient of determination; P = probability level; SEE = standard error of the
estimation; MAE = mean absolute error.
Fig. 7. Standard error of estimation (SSE) and mean absolute error of estimation
(MAE) for the different tested models.
86
A. Capra et al. / Soil & Tillage Research 105 (2009) 77–87
wetter, was found to be the most critical period for ephemeral
gully formation (Casalı́ et al., 1999).
4. Conclusions
The experimental catchment in Raddusa (Sicily), belonging to a
national (Italian) network of experimental catchments for the
hydrogeological protection of hill and mountain environments, has
been monitored for several years for ephemeral gully erosion in
cultivated fields. The area can be considered representative of wide
wheat zones in southern Italy and other Mediterranean areas.
Ephemeral gully erosion is not an infrequent event in the
examined area. From 1995 to 2007 EG formation was observed 8
years out of 12, which corresponds to a return period of 1.5 years. A
mean of 23 EG systems, with mean length, width and depth equal
to 116, 0.49 and 0.23 m, respectively, were recorded in the 8 years
when active EGs were observed. The mean eroded volume was
equal to ca. 420 m3 year1, corresponding to about
0.6 kg m2 year1.
Ephemeral gully erosion in the study area is directly and mainly
controlled by rainfall events. The height, intensity and erosivity of
the rainfall have a role in EG formation and development. The
positive correlation between the different EG morphometric
characteristics means that an erosive event acts on the width,
depth and length of an EG.
Ephemeral gully characteristics presented great interannual
variability, in agreement with the rainfall characteristics. An
antecedent rainfall index, the maximum cumulative 3-days rainfall
(Hmax3_d), used as a simple surrogate for soil water content, is the
rain characteristic which best explains EG erosion in the
environment considered. An Hmax3_d threshold of 51 mm was
observed for EG formation. This result accounts for the great
importance of initial soil water content in runoff generation, as
observed in other semiarid environments. The return period of the
Hmax3_d threshold is almost the same as the return period for EG
formation.
Although a mean of seven erosive rain events were recorded in a
year, EG formation and development generally occur during a
single erosive event, as in to other semiarid environments. The
most critical period for EG formation is that comprised between
October and January, when the elevated soil water content
facilitates runoff development and the almost bare soil surface
with emergent wheat plants erodes most intensely.
Considering the lack or difficulty of use of entirely physically
based models, simple empirical models for EG eroded volume
estimation were obtained using the data set collected at this site. A
simple power-type equation is proposed to estimate the eroded
volumes using Hmax3_d as an independent variable. This equation
performs better than the eroded volume–total length equation
already proposed by several authors in different environments. The
benefit of the eroded volume–Hmax3_d equation as compared with
the eroded volume–total length equation is that it is not necessary
to perform any field measurement of the EGs. The disadvantage is
that it does not permit the EGs to be located and therefore
precludes the implementation of conservation practices.
Data collected in a microbasin inside the study area recently
equipped with a Venturi canal for measurement of the flow-rate of
an EG, and with TDR for soil moisture content measurements, will
be used to clarify and complete the research described in this
paper.
Acknowledgements
This research was supported by different projects, mainly the
European Commission MWISED in the past, and, more recently, the
Italian Ministry of University and Scientific Research PRIN 2007.
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