Catena 63 (2005) 132 – 153
www.elsevier.com/locate/catena
Gully erosion: Impacts, factors and control
C. Valentin a,*, J. Poesen b, Yong Li c
b
a
IRD-IWMI-NAFRI, BP 06 Vientiane, RPD Laos
Physical and Regional Geography Research Group, K.U. Leuven, Redingenstraat 16, 3000 Leuven, Belgium
c
Institute of Agro-Environment and Sustainable Development, Chinese Academy of Agricultural Sciences,
No12 Zhongguancun South Street, Beijing 100081, PR China
Abstract
Gully erosion attracts increasing attention from scientists as reflected by two recent international
meetings [Poesen and Valentin (Eds.), Catena 50 (2–4), 87–564; Li et al., 2004. Gully Erosion Under
Global Change. Sichuan Science Technology Press, Chengu, China, 354 pp.]. This growing interest
is associated with the increasing concern over off-site impacts caused by soil erosion at larger spatial
scales than the cultivated plots. The objective of this paper is to review recent studies on impacts,
factors and control of gully erosion and update the review on dgully erosion and environmental
change: importance and research needsT [Poesen et al., 2003. Catena 50 (2–4), 91–134.]. For the
farmers, the development of gullies leads to a loss of crop yields and available land as well as an
increase of workload (i.e. labour necessary to cultivate the land). Gullies can also change the mosaic
patterns between fallow and cultivated fields, enhancing hillslope erosion in a feedback loop. In
addition, gullies tend to enhance drainage and accelerate aridification processes in the semi-arid
zones. Fingerprinting the origin of sediments within catchments to determine the relative
contributions of potential sediment sources has become essential to identify sources of potential
pollution and to develop management strategies to combat soil erosion. In this respect, tracers such as
carbon, nitrogen, the nuclear bomb-derived radionuclide 137 Cs, magnetics and the strontium
isotopic ratio are increasingly used to fingerprint sediment. Recent studies conducted in Australia,
China, Ethiopia and USA showed that the major part of the sediment in reservoirs might have come
from gully erosion.
Gullies not only occur in marly badlands and mountainous or hilly regions but also more globally
in soils subjected to soil crusting such as loess (European belt, Chinese Loess Plateau, North
America) and sandy soils (Sahelian zone, north-east Thailand) or in soils prone to piping and
tunnelling such as dispersive soils. Most of the time, the gullying processes are triggered by
* Corresponding author.
E-mail address: [email protected] (C. Valentin).
0341-8162/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.catena.2005.06.001
C. Valentin et al. / Catena 63 (2005) 132–153
133
inappropriate cultivation and irrigation systems, overgrazing, log haulage tracks, road building and
urbanization. As exemplified by recent examples from all over the world, land use change is
expected to have a greater impact on gully erosion than climate change. Yet, reconstructions of
historical causes of gully erosion, using high-resolution stratigraphy, archaeological dating of pottery
and 14C dating of wood and charcoal, show that the main gully erosion periods identified in Europe
correspond to a combination not only of deforestation and overuse of the land but also to periods
with high frequency of extreme rainfall events.
Many techniques have proved to be effective for gully prevention and control, including
vegetation cover, zero or reduced tillage, stone bunds, exclosures, terracing and check dams.
However, these techniques are rarely adopted by farmers in the long run and at a larger spatial scale
because their introduction is rarely associated with a rapid benefit for the farmers in terms of an
increase in land or labour productivity and is often contingent upon incentives.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Gully erosion; Land use change; Sediments; Soil crusts; Erosion control; Reservoirs
1. Introduction
Soil erosion has been recognized as the major cause of land degradation worldwide.
In the past decades, priority of research has been given to address agricultural issues at
the plot scale and thus to rill and inter-rill erosion. More recently, gully erosion has
attracted a growing interest as reflected by two recent international conferences: one in
Leuven, Belgium (Poesen and Valentin, 2003) and one in Chengdu, China (Li et al.,
2004). This is explained by an increasing concern for off-site impacts of soil erosion
that can be tackled only at the catchment scale. It is now well recognized that
increased exploitation of land resources in upper parts of catchments results in
increased sediment yield and elevated nutrient loads in runoff that reduce water quality
and availability to downstream users. Furthermore, control of sedimentation in
reservoirs requires that all the potentially significant sediment sources and sinks are
known. Recent studies (e.g., Wasson et al., 2002; Krause et al., 2003; de Vente et al.,
2005; Huon et al., 2005) indicate that gully erosion is often the main source of
sediments. Gully erosion has been long neglected because it is difficult to study and to
predict. Gully processes have a three-dimensional nature affected by a wide array of
factors and processes. Although gully erosion is commonly triggered or accelerated by
land use change (e.g., Chaplot et al., 2005a,b) and/or extreme climatic events, it often
results also from a long antecedent history that cannot be overlooked when attempting
to understand spatial erosion patterns. Moreover, many gullies grow initially rapidly to
large dimensions (e.g., Nachtergaele et al., 2002; Vanwalleghem et al., 2005a; Thomas
et al., 2004), making effective control technically difficult or prohibitively expensive.
This is why studies on gullying processes (Gomez et al., 2003) as well as modelling
attempts (Siepel et al., 2002; Gimenez et al., 2004; Sidorchuk, 2005) remain scarce.
The objectives of this paper are to review recent studies on impacts, factors and control
of gully erosion and to update the review on dGully Erosion and Environmental Change:
Importance and Research NeedsT (Poesen et al., 2003).
134
C. Valentin et al. / Catena 63 (2005) 132–153
2. Human and environmental impacts on gully erosion
2.1. Farming systems
Soil erosion is often blamed for drastic reduction of soil fertility. Govers et al. (2004),
based on the alarming results from one-time desurfacing experiments that attempt to
mimic inter-rill erosion, note that gradual erosion has a much weaker effect on crop
productivity than the sudden removal of a significant proportion of the topsoil. In the case
of gully erosion, the usually unpredictable impacts are often more serious and flashy
(Table 1). They also include the loss of available land and an increase of labour costs. To
combat gully erosion, most upland rice farmers fill gullies with wood and trash (i.e. weeds
removed from cropland). This helps in limiting the growth of existing gullies. In a slash
and burn system of northern Laos, this filling of gullies has been estimated at 11 days per
year representing 5% of the total work time spent in fields, (namely, US$13 per hectare; de
Rouw et al., 2003; Pelletreau, 2004; Table 2). The gully filling activity requires even more
specific time, corresponding to 7% of the total work time (15 days) marketed at US$18.
The development of deep gullies perpendicular to the contour also constrains the
farmers’ choices with respect to parcel use patterns and thus influences fallow/cultivated
plot mosaics. In Southeast Asia, deforestation started on the gentler slopes before
encroaching whole hillslopes. As a result, the limits between fallow and cultivated plots,
nearly on the contour, delineated plots with rather short slope length (b 50 m) where fallow
plots can trap the sediments from cultivated fields located more upslope. Once the gullies
develop along the main steep slope, they form new limits between parcels, increasing the
parcel length and thus accelerating water erosion rates in a feedback loop (Figs. 1 and 2).
2.2. Hydrological functions
Gullies are often blamed for enhanced drainage and accelerated aridification processes
(e.g., Eitel et al., 2002; Daba, 2003). For instance, in the arid region of the Negev highlands of
southern Israel, gully incision erodes alluvial sediments and loess deposited along the
valleys. The agricultural fields and the main floral biomass are limited to narrow valleys. The
gullies concentrate the runoff into narrow channels, preventing the floodwater from
irrigating the whole width of the valley. The change in irrigation efficiency of the valley
bottom is reflected in an 80% reduction in biomass and a significant loss in the agricultural
Table 1
Crop yield losses and costs due to rill and gully erosion in a slash and burn system of upland rice in northern Laos
(after Dumas de Rauly, 2003; Pelletreau, 2004)
Mean crop yields (Mg ha 1)
(a) Production loss (Mg ha 1)
(b) Estimated revenues loss (a crop prices), in kipa
Estimated revenues loss (b / exchange rate), in US$
Estimated revenues loss in percentage of normal revenues
a
Upland rice price: 1300 kip kg
1
; 1 US$ = 10 000 kip.
Rill
Gully
1.381
0.552
717 600
71.76
29%
1.219
0.714
928 200
92.82
37%
C. Valentin et al. / Catena 63 (2005) 132–153
135
Table 2
Calculated specific labour costs for gully control for upland cultivation, in a slash and burn system of upland rice
in northern Laos (after de Rouw et al., 2003; Pelletreau, 2004)
Activity
Clearing (field preparation and
second clearing)
Weeding
Placing weeds inside gullies
Placing woods inside gullies
Placing weeds inside cut trees
Gully treatment
Weeds treatment
Total annual cropping work
a
Annual
number
of days
Calculated labour
cost per year (kip)a
67
804 000
74
7–8
3–4
7–8
11
15
210
888 000
90 000
42 000
90 000
132 000
180 000
2 520 000
Calculated labour
cost per year (US$)
Percentage of total
annual cropping
labour (%)
80
31.9
89
9
4.2
9
13.2
18
252
35.2
3.6
1.7
3.6
5.2
7.1
100.0
1 working day = 12 000 kip.
potential of the region (Avni, 2004, 2005). In the Ethiopian highlands, the development of
gullies has led to an enlarged drainage of the intergully areas, resulting in soil moisture
decrease and a corresponding crop yield reduction on plots located near the gully walls
(Nyssen et al., 2004c). In severely crusted environments, gully bottoms are the main runoff
water transmission sources to recharge ground water, which may be a crucial issue in semiarid environment as exemplified in southern Niger (Leduc et al., 2001; Esteves and Lapetite,
2003). This rule suffers some exception as recently observed in northern Burkina Faso where
electrical resistivity mapping survey indicated that deep infiltration processes were not
occurring below the gully situated on the hillslope (Descloitres et al., 2003).
2.3. Sediment production
Fingerprinting the origin of sediments within catchments to determine the relative
contributions of potential sediment sources has become essential to identify sources of
potential pollution and to develop management strategies to combat soil erosion.
Moreover, control of sedimentation in large reservoirs requires soil conservation at the
catchment scale. In large, heterogeneous catchments, soil conservation planning needs to
be based on the identification of potentially significant sources and sinks and of the
major sources of sediment reaching the reservoir. Tracers such as carbon, nitrogen, the
nuclear bomb-derived radionuclide 137 Cs, magnetics, the strontium isotopic ratio, and
the neodymium isotopic ratio are increasingly used to fingerprint sediment. In tropical
northwestern Australia, about 96% of the sediment in the Lake Argyle reservoir has
come from less than 10% of the catchment, in the area of highly erodible soils formed
on sedimentary rocks of Cambrian age. About 80% of the sediment in the reservoir has
come from gully and channel erosion (Wasson et al., 2002). Similarly, in a 1.2 km2
gullied catchment in southeastern New South Wales, multi-parameter fingerprinting of
sediment deposited in successive downstream pools has identified gully walls
responsible for between 90% and 98% of the pool sediment when the grazed pasture
surface was the only other potential source (Krause et al., 2003). In the outer
Warragamba catchment, southern New South Wales, sediment yields from gullied
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C. Valentin et al. / Catena 63 (2005) 132–153
Slope
m m-1
Estimated inter-rill erosion
Mg ha-1 yr-1
100 m
Fallow
0
100 m
0.80
Crop
0.40
5
a) first stage: situation prevailing about 40 years ago
Crop
Fallow
5
0
b) second stage: situation prevailing about 20 years ago
Crop
Fallow
7
c) third stage: prevailing current situation
Fig. 1. Change of mosaic patterns along hillslope due to gullying in northern Laos; impact on estimated inter-rill
erosion. Vertical lines represent gully channels.
catchments of 29, 52, and 510 ha were at least one order of magnitude higher than from
the ungullied catchments at around 1 Mg ha 1 year 1 (Armstrong and Mackenzie,
2002). Sediment tracers were also used to quantify erosion from cultivated fields and
identify major source areas of channel bottom sediment within the Wildhorse Creek
drainage, an intensively cropped tributary of the Umatilla River in northeastern Oregon,
USA. Most channel-bottom sediment was of sub-surface origin with much of it likely
coming from channel and gully banks (Nagle and Ritchie, 2004). In the Chinese Loess
Plateau, rill and gully erosion contribute between 60% and 70% of all sediments (Li et
al., 2003; Zhu and Cai, 2004). A similar proportion (70%) has been reported for
northwestern highland Ethiopia (Bewke and Sterk, 2003).
3. Factors controlling gully erosion
Gullying is a threshold-dependent process controlled by a wide range of factors. We
limit this review to a few of factors, which were reported recently.
C. Valentin et al. / Catena 63 (2005) 132–153
137
Fig. 2. Rapid changes in crop/fallow mosaics associated with gullies. Example in the Houay Pano catchment,
northern Laos. Photos: Management of Soil Erosion Consortium.
138
C. Valentin et al. / Catena 63 (2005) 132–153
3.1. Topographic thresholds
3.1.1. Slope gradients and soil crusts
Gullies are common features of mountainous or hilly regions with steep slopes. Recent
examples have been reported from the French Alps (Esteves et al., 2005; Mathys et al.,
2005), Slovakia (Stankoviansky, 2003), Morocco (Naimi et al., 2003; Mohamed et al.,
2004), Ethiopia (Daba, 2003; Nyssen et al., 2002; Bewke and Sterk, 2003; Nyssen et al.,
2004a,b,c), Kenya (Jungerius et al., 2002), Laos (Chaplot et al., 2004, 2005a,b),
peninsular Malaysia (Sidle et al., 2004), New Zealand (Gomez et al., 2003a,b), Idaho
(Istanbulluoglu et al., 2003) and Ecuador (Podwojewski et al., 2002; Vanacker et al.,
2003). Steep slopes favour high runoff velocity and thus rill and gully initiation but, given
climatic conditions, they can produce lower runoff volumes than gentle slopes, as recently
shown in northern Thailand (Janeau et al., 2003). This is caused by the lower crusting rate
on steep slopes as compared to lower slopes due to a lower impacting kinetic energy and
a continuous erosion of the surface seal (e.g., Poesen, 1986). Because soil crusts mainly
develop on gentle slopes generating higher runoff, the slope threshold for rill initiation
can be very low for seriously crusted soils (about 1% in the loamy plateaux of
southeastern Niger; Valentin et al., 1999).
3.1.2. Slope and critical drainage area
Considering that for a given slope (S), a critical drainage area (A) is necessary to
produce sufficient runoff to concentrate and initiate gullying, thresholds lines (S = aA b )
have been recently produced by scientists with the constant a and the exponent b
depending on environmental characteristics (Poesen et al., 2003). Using a global
positioning system (GPS) to measure the morphology and the location of gullies in a
small catchment near Suide, Shaanxi Province, representative for the loess plateau in
China, Wu Yongqiu and Cheng Hong (2005) established the critical relationship of
S = 0.1839A 0.2385. Topographic threshold conditions for hillslope gully initiation in
cultivated land in the Chinese loess plateau plot above those needed to initiate ephemeral
gullies in cultivated land under Mediterranean and European conditions (e.g., Poesen et
al., 1998). The values of AS 2, considered as an indicator for the gully initiation point range
between 41 and 814 m2, are much smaller than those commonly observed by Montgomery
and Dietrich (1992), which range between 500 and 4000 m2. These two threshold
relationships (S and AS 2) are suggested as indexes for the position of hillslope gully heads
from DEM in small watersheds on the Loess Plateau of China. In the same province of
China (Chunhua county), Guanglu Li et al. (2004) observed that shallow gullies (0.3–2 m
deep) occupy 75% of the lower and middle areas of the slope and rill erosion in the upper/
middle areas. They reported critical slope gradients of 28, 58 and 88 for rills (b 0.3m deep),
shallow gullies (0.3–2 m deep) and deep gullies (N 2 m deep), respectively.
3.2. Soil and lithologic controls
3.2.1. Soil/lithologic/geomorphology factors
Patterns and rates of gully network development as well as network geometric
configuration are highly controlled by soil/lithological properties (Bryan, 2004). Field
C. Valentin et al. / Catena 63 (2005) 132–153
139
observations (e.g., Planchon et al., 1987) showed that landform development could
often be related to tectonically induced compressional or tensional forces. These can
form fractures or cracks in rocks without causing actual tectonic movement or
displacement. These fractures act as structurally weakened starting points for weathering
processes. The resulting sub-surface concavities concentrate throughflow, which tends to
accelerate the eluviation of soil particles, lowering thus the soil surface. These
depressions, which can also be old landslide scars, become sites for surface flow
concentration and thus gully erosion. As observed by Avni (2004) in the Negev
highlands, gullies can also be generated by a natural dynamic change related to the
long-term process of readjustment of the present geomorphologic system to the
Holocene climate.
3.2.2. Soil crusting
Soil crusting has an ambivalent effect on gully development. Soil crusts can delay
the initiation of gullies (e.g., Zhu and Cai, 2004) due to their stronger shear strength
as compared to non-crusted soils. Yet, headcuts occur often at points where cracks
have developed in surface crusts (Prasad and Römkens, 2004). Because soil crusts
favour runoff generation and concentration downslope, soils prone to crusting are
generally eroded not only by sheet but also by gully erosion. Even for very gentle
slope gradients, rills and gullies develop in the regions where crusting is a common
problem, such as in the loess belts of China (e.g., Hessel and van Asch, 2003; Hessel
et al., 2003; Li et al., 2003, 2004; Ming Bin et al., 2003; Guanglu Li et al., 2004;
Wu and Cheng, 2005; Zhu and Cai, 2004; Fig. 3), Europe (e.g., Nachtergaele et al.,
2002; Nachtergaele and Poesen, 2002; Bork, 2004; Govers et al., 2004; Vanwalleghem et al., 2004; Vanwalleghem et al., 2005b; Fig. 4) and North America (Thomas
et al., 2004). Because of the scarcity of vegetation, soils of the arid and semi-arid
regions are subjected to crusting and thus runoff production and gullying as
exemplified in Spain (Martinez-Casasnovas et al., 2003; Poesen and Vandekerckhove,
2004), South Africa (Kakembo and Rowntree, 2003), and New South Wales (Erskine
et al., 2002; Krause et al., 2004). Soil crusts explain why gullies can affect very
sandy soils in the Sahel (e.g., Descloitres et al., 2003; Fig. 5). In addition to loess
and sandy soils, alluvial sediments are highly susceptible to gullying (e.g., Avni,
2004, 2005) as well as exposed marls (e.g., Naimi et al., 2003; Esteves et al., 2005;
Mathys et al., 2005).
3.2.3. Piping
Although most studies have linked gully morphology to surface water flow, an
increasing attention is paid to the significance of piping and tunnelling (e.g., Bryan and
Jones, 2000) along with the influence of soil chemistry on soil hydrological pathways.
Dispersive soils with sodic layers are prone to the development of pipes that turn into
rills or gullies when their roofs collapse as observed by, e.g., Faulkner et al. (2004).
Such process can also occur in soils that are non-sodic but very eluviated (e.g., Planchon
et al., 1987) or rich in smectite clay minerals (Bork, 2004; Nyssen et al., 2004a).
Channel morphology features are therefore often dictated by sub-surface seepage
processes.
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C. Valentin et al. / Catena 63 (2005) 132–153
Fig. 3. Gullies in the Chinese Loess Plateau, Yangjuangou catchment (36842V N, 109831V E;. 2.02 km2; mean
annual rainfall is 550 mm; 1025–1250 m asl) near Yan’an city, northern Shaanxi province in northwest China.
The gully density is 2.74 km km 2. The slope gradients range between 108 and 308. The typical particle size
distribution of this soil is 20% sand, 55% silt and 25% clay by weight. Water erosion problems are the result of
runoff scouring, deforestation, and cultivation of steep slopes up to 408 and the extremely high erodibility of the
loess soils when lacking vegetation cover (Li, 1995). The crops grown on the farmland are potatoes, beans and
millet. In the hilly areas of the Chinese Loess Plateau, the gully erosion contributes 60–90% of total sediment
production on agricultural land (Li et al., 2003). Photo. Yong Li.
3.3. Land use change
3.3.1. Present land use changes
Natural gullying processes are accelerated by the intensification of farming systems.
The depletion of the soil organic matter reduces the soil structural stability and favours
crusting, runoff production and gully erosion (e. g., Valentin, 2004). In mountainous
regions, annual cropping has been reported as intensifying rill and gully erosion
processes (e.g., Chaplot et al., 2005a) as well as in vineyards in Mediterranean regions
(Martinez-Casasnovas et al., 2003). Irrigation channels can also favour gully erosion
(e.g., Vanacker et al., 2003; Nyssen et al., 2004b; Poesen and Vandekerckhove, 2004).
Overgrazing is also often reported as one of the main drivers of gully erosion in
rangelands (e.g., Podwojewski et al., 2002; Gomez et al., 2003b; Nyssen et al., 2004c;
Fig. 6). Mieth and Bork (2005) report present gullying rates exceeding 190 Mg ha 1
year 1 due to sheep grazing on Easter Island (Rapa Nui, Chili). Although a long fallow
period is considered a means in restoring soil structural stability and thus reducing
C. Valentin et al. / Catena 63 (2005) 132–153
141
Fig. 4. Permanent gully (up to 2 m deep) which developed as an ephemeral gully under cropland in the European
loess belt (central Belgium, March 2001; photo: J. Poesen). Due to soil profile truncation in this concentrated flow
zone, very erodible calcareous loess is present at less than 1 m from the soil surface, explaining the fluting
phenomena at the gully head and the relatively large dimensions of this gully.
gullying hazards, it has been recently observed in the sandy Sahelian soils that soil
crusts develop during fallow periods as a result of dust deposition and colonisation by
blue green algae (Valentin et al., 2004), enhancing thus gully development. Under these
circumstances, tillage limits water erosion but exacerbates wind erosion (Rajot et al.,
2003). Similar crusting processes might explain why widespread abandonment of
communal cultivated fields in South Africa has been associated with gully initiation and
intensification (Kakembo and Rowntree, 2003). Irrigation water flowing without control
over bare abandoned fields can also trigger serious gully erosion, as shown in the oasis
of San Pedro de Atacama in northern Chile (Bork, 2004; Fig. 7).
3.3.2. Roads and construction sites
The acceleration of gully erosion cannot be ascribed solely to agricultural and
pastoral activities. An increasing number of studies focus on gully development due to
forestry activities, road construction and building activities in urban environments. The
main sediment sources created by selective harvesting of tropical rain forests come
from building access roads and log haulage tracks. When unchecked, these tracks
develop into gullies that continue to erode long after logging (Douglas and Pietroniro,
2003).
Although obvious in the field, the impact of road building on gullying is rarely analysed
(e.g., Reid and Dune, 1996). While damage by runoff to the road itself can remain limited,
off site effects are often important. The roads induce a concentration of surface runoff, a
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C. Valentin et al. / Catena 63 (2005) 132–153
Fig. 5. Rills and gullies formed on very sandy soils in northern Burkina Faso. Left: Oursi during the dry season,
early December, photo: C. Valentin. Right: Dori under rainfall conditions, early July, 0.4 m broad, 0.20 m deep,
photo: O. Ribolzi.
diversion of concentrated runoff to other catchments, and an increase in catchment size,
which enhance gully development after road building (Nyssen et al., 2002). To limit the
risks roads should be designed in a way that keeps runoff interception, concentration and
deviation minimal. Techniques must be used to spread concentrated runoff in space and
time and to increase its infiltration instead of directing it straight onto unprotected slopes.
Yet, as mentioned by Jungerius et al. (2002), wherever such measures are designed, they
become rapidly outdated in developing countries because a new road attracts settlement.
Deterioration of surface drainage and erosion start at unforeseeable points where people
settle.
C. Valentin et al. / Catena 63 (2005) 132–153
143
Trails converted in rills and gullies
a
Gullies in overgrazed upper catchment
Gullies in cultivated lower catchment
b
Fig. 6. Gullies in the Potsheni catchment (Kwazulu-Natal, South Africa). a) Cattle, which is excluded from the
cropped field during the rainy season, concentrates on the upper zone of the catchment. b) Gullying downslope is
also favoured by sub-surface flow. Photo: C. Valentin, November 2004.
Changes in drainage patterns associated with urbanization can result in gullying
(e.g., Archibold et al., 2003) but gullies often take place where there are illegal
settlements without urban infrastructure, such as sanitation and paved roads (Guerra,
2004).
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C. Valentin et al. / Catena 63 (2005) 132–153
Fig. 7. Soil erosion rates over the last two centuries (1800–2000 AD) in sites in South Africa, USA, Chile, China
and Germany. Absolute rates of soil erosion differ significantly in the research areas. The most sensitive areas
have steep slopes with silty and sandy soils (such as loess-derived soils) and high precipitation intensities when
the vegetation cover density is low, after Bork (2004).
3.3.3. Past land use and climate changes
In many places of Europe, gullies can be observed under old forest, often with quite
considerable dimensions. Given the high infiltration rates under the present conditions,
these gullies were formed under other land use and climate circumstances. Recent
detailed analyses of soil profiles using high-resolution stratigraphy, with archaeological
dating of pottery and 14C dating of wood and charcoal, enabled the reconstruction of the
historical causes of gully erosion in a variety of environments. Three main periods of
gully erosion have been identified in Europe (e.g., Dotterweich et al., 2002; Schmitt et
al., 2003, in Germany; Stankoviansky, 2003 in Slovakia; Vanwalleghem et al., 2005b).
Most gullies were formed during periods of extensive forest clearance and expansion of
farmland associated with extreme rainfalls in the 14th century, between the end of the
16th century and the 1730s and during the Little Ice Age at the turn of the 18th and
19th centuries. Some of these gullies are clearly associated with historical roads as
C. Valentin et al. / Catena 63 (2005) 132–153
145
shown recently in Belgium (Vanwalleghem et al., 2004, 2005b). Sidorchuk and Golosov
(2003) estimate that since the late 17th century, 2 106 gullies longer than 300 m have
formed mobilizing about 4 109 m3 of sediment. Most of it, about 97%, has been
redeposited and accumulated, infilling small valleys with sediments 5–6 m thick. In the
Negev highlands region, about 10% of the land with agricultural and pastoral value has
been lost since the 7th century due to gully erosion accelerated by inappropriate land
management (Avni, 2004, 2005). The degradation of the vegetated valley bottoms in the
upper Murrumbidgee River, NSW, Australia by introduced stock in the 1840s and 1850s
triggered a massive phase of gully erosion, increasing the sediment flux out of the
catchment by a factor of about 200. In comparison, multi decadal variations in rainfall
caused less than a twofold increase in the sediment transport capacity of the river (Olley
and Wasson, 2003). Overgrazing is also blamed to have triggered serious gully erosion
at the turn of the 19th and 20th century in Easter Island (Mieth and Bork, 2005) and in
northern Namibia (Eitel et al., 2002). In the Transkei (Republic of South Africa) the
population density and the number of cattle grew dramatically in the third decade of the
20th century, as a result of the apartheid system. With the resulting reduced vegetation
cover density and the increased concentration of activities along paths, much more
rainwater could concentrate and infiltrate in the deeper swelling and shrinking clay soil
layers. Following intensive sub-surface erosion, gullying rates exceeded 60 Mg ha 1
year 1 (Bork, 2004, Fig. 7). In Germany, investigations of more than 2300 sites
provided evidence that, on average, rill and gully erosion rates increased from 2 to 6
Mg ha 1 year 1 since the 1950s (Bork, 2004, Fig. 7). Because no significant increase
in the frequency or intensity of heavy rainfalls has been recorded over this period, such
a rapid increase must be ascribed to recent land use change. Due to subsidies and world
market, crops were cultivated with a low cover density during the erosive early summer
months. Field sizes increased as a result of the land reallocation. Soil conservation
practices such as terraces and hedges that had often existed since the last period of
intensive soil erosion (the late 18th century) were abandoned. The use of heavy
agricultural equipment lead to soil compaction and thus reduced infiltration capacities
(Bork, 2004).
3.4. Climate change
There is little information on how gully systems may respond to climatic change (Li
et al., 2004). Where annual rainfall is known to have decreased significantly over the
last decades (e.g., Sahel, western Australia), no concurrent decrease has been observed
for high-intensity rain events (e.g., Yu and Neil, 1993). During the protracted droughts,
the associated vegetation decays leaving large areas unprotected from splash and
subjected to soil crusting. Runoff tends to increase and concentrate, thus, promoting
gully erosion. A drier climate in the semi-arid zone is thus expected to foster rill and
gully development (e.g., Valentin, 2004). Under cold conditions, global warming is
expected to increase the frequency of freeze–thaw cycles exacerbating the risk of
gullying as shown in southern Saskatchewan, Canada, (Archibold et al., 2003) and in
southern Norway (Øygarden, 2003). More generally, land use change is expected to
have a greater impact on gully erosion than climate change. The lessons derived from
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C. Valentin et al. / Catena 63 (2005) 132–153
historical erosion show however that the main gully erosion periods correspond not only
to rapid land use changes associated with denudation but also to a higher frequency of
high-intensity rainfall. Global warming associated with the extension of grazed and
cropped areas should put more regions at high risk of gully erosion in the future, with a
particular threat on the semi-arid zones.
4. Prevention and control of gully erosion
4.1. Vegetation cover
Aboveground vegetation is known to favour water infiltration and to protect soil from
erosion. To be effective, the last intercepting vegetation layer must be near the soil
surface. Intercepted drops by tall trees without understorey can be larger and can have
higher kinetic energy than non-intercepted drops, favouring soil crusting, runoff
generation and gully initiation (Fig. 8). Gully retreat is often controlled by the inherent
strength of the tree root mat that binds the surface soils until the undercut trees finally
collapse (e.g., Archibold et al., 2003). The increasing effects of plant roots on soil
resistance to concentrated flow erosion mainly depend on the characteristics of effective
roots (fibrils less than 1 mm in diameter) distributed densely in the depth 0–30 cm (Li et
al., 1991). Plant roots reduce gully erosion in improving soil physical properties such as
structural stability and infiltrability (Li et al., 1992; Li, 1995). It was also recently shown
in the European loess belt that an increase in root density of different cereal and grass
Fig. 8. Gully developed in sandy soils in a tree plantation of Eucalyptus sp., north-east Thailand. Note the absence
of understorey. Photo: C. Hartmann.
C. Valentin et al. / Catena 63 (2005) 132–153
147
plants results in an exponential decrease of concentrated flow erosion rates (Gyssels and
Poesen, 2003). In the Chinese loess plateau, an increase in grassland and forestland by
42% and a corresponding decrease in farmland by 46% reduced sediment production
mainly due to gully erosion by 31% in the catchment (Li et al., 2003, 2004). The mean
figure for sediment production declined by 49% for a terraced hillslope and by 80% for
a vegetated hillslope compared with a cultivated hillslope. These data demonstrate the
effectiveness of terracing and perennial vegetation cover in controlling sediment delivery
at a hillslope scale.
In a small (29 km2) steepland catchment of New Zealand, gully erosion was triggered
by conversion to pasture early in the twentieth century and 48% of the sediment
generated by gully erosion between 1950 and 1988 was stored in the channel along the
stream. This situation was ameliorated by reforestation that commenced in 1962. The
amount of sediment contributed from gullies declined by 62% as the forest became
established; but even if the amount of sediment generated by gully erosion continues to
decline, it likely will be many decades before the sediment is released from storage in
the channel (Gomez et al., 2003b). This illustrates that even under vegetation cover it
may take a long time to rehabilitate gullied land. From a land management perspective,
the success of tree plantings, to mitigate gully erosion, depends on the stage of gully
development and particularly on whether or not mass movement erosion has begun
(Betts et al., 2003). Where mass movement assisted by excessive groundwater pressure
is the main process leading to uncontrollable gully expansion, a particular attention must
be paid to the stabilization of eroding riparian areas and swales, especially on the lower
slopes of agricultural fields as suggested by Nagle and Ritchie (2004) in north-eastern
Oregon.
4.2. Soil conservation works
As mentioned by Armstrong and Mackenzie (2002), the impacts of sediment trapping
and grade stabilisation works on sediment yields mainly depend on the activity of the gully
being treated and the mobility of the bed sediments. At the catchment scale, it is often the
combination of widespread conservation measures not only in the gullies (check dams) but
also in the intergully zone (stone bunds, exclosures) that leads to a decrease of soil erosion
rates as observed in the northern Ethiopian highlands (Nyssen et al., 2004a,b). With
respect to check dams these authors noticed that they need a spillway, apron, concave plan
form (when looking downslope) and they need to be built at vertical intervals and with
heights that result in a negative slope gradient of the line connecting the top of the spillway
and the foot of the upstream dam. The frequent collapse of dams (39% after 2 years) is
strongly associated with drainage area (A) and slope gradient of the soil surface near the
gully (S), the product of these factors (S A) being a proxy for runoff energy (Nyssen et
al., 2004a).
4.3. Constraints to the adoption of conservation strategies
Although innovative conservation strategies have proved to be effective in
controlling gully erosion (see above), they are rarely adopted by farmers in the long
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C. Valentin et al. / Catena 63 (2005) 132–153
run and at large scale, with few exceptions: e.g., in the U.S.A. where grassed
waterways are established in cropland to control ephemeral gully erosion or in the
Ethiopian highlands where check dams are installed to prevent the development of
permanent gullies in rangeland and cropland (Nyssen et al., 2004a,c). Neither scientists
nor policy-makers have a clear understanding of the reasons why these strategies are
accepted or not (e.g., Govers et al., 2004). Clearly, to be acceptable to the farming
community these strategies need to be associated with a rapid benefit in terms of land
or labour productivity. It occurs that if land needs to be closed off to grazing, people
with no other source of feed for the cattle have to sell their livestock (FAO, 2002).
Similarly, if the household workload needs to be increased or new land needs to be put
under cultivation, as is often the case for agro-forestry practices, the burden particularly
falls on women. In addition, projects may be making considerable progress on
reducing soil erosion rates and increasing water conservation through adoption of zero
tillage but still continue to rely on applications of herbicides. In other cases, improved
organic matter levels in soils may lead to increased leaching of nitrate to groundwater
(FAO, 2002). The willingness to adopt new improved soil and water conservation
measures is often related to the perception of the danger of gully erosion by the
farmers. In the case of the Hararghe highlands, eastern Ethiopia, for instance, this
perception is significantly correlated to severity of water scarcity. Moreover, successful
implementation of an improved or new measure of soil and water conservation
measure is contingent upon the availability of incentives, primarily fertilizers (Daba,
2003).
5. Conclusions
(1) Gully erosion is not a process limited to badlands, mountainous and hilly regions but
a global and serious cause of land degradation affecting a wide variety of soils prone
to crusting and/or piping.
(2) Gully erosion results not only from surface flow but also often from sub-surface
flow.
(3) Under many circumstances gully erosion is the main source of sediment at the
catchment scale.
(4) Gully erosion is most often triggered or accelerated by a combination of
inappropriate land use and extreme rainfall events.
(5) Once formed gullies can continue to generate sediment long after the triggering
causes have ceased.
(6) Although many strategies to prevent and combat gully erosion have proved to
be effective, they are rarely adopted by farmers in the long run and at a large
scale.
(7) Research priorities should include sub-surface flow erosion processes, prediction
models, and the causes of adoption or not of conservation strategies by the
farmers.
(8) A global research network should be established to assess the global state of gully
erosion and to monitor gully erosion in selected long-term bench mark sites.
C. Valentin et al. / Catena 63 (2005) 132–153
149
Acknowledgements
This paper is a contribution to the Soil Erosion Network of the Global Challenge and
Terrestrial Ecosystem Core Research Programme, which is part of the International
Geosphere-Biosphere Programme. The authors wish to thank their institutions (Institut de
Recherche pour le Développement, International Water Management Institute, K.U.,
Leuven, FW0-Vlaanderen and the Chinese Academy of Agricultural Sciences and
National Natural Science Foundation of China) for supporting various research projects
related to gully erosion in Europe, Southeast Asia, China, South America and Africa.
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