1-s2.0-S034181620200139X-main

Ephemeral gully erosion in northwestern Spain

M. Valcarcela, Ma–. T. Taboadab, A. Pazb,*, J. Dafontea

aEscuela Politecnica Superior de Lugo, University of Santiago de Compostela, Lugo, SpainbFacultad de Ciencias, University of Corunna, Corunna, Spain

Received 20 October 2000; received in revised form 25 October 2001; accepted 10 December 2001

Abstract

This study aimed to describe types of ephemeral gullies and to determine their origin, evolution

and importance as sediment sources in A Coruna province (northwest Spain). Ephemeral gullies

and/or rills have been measured in a representative sample of medium-textured soils, most prone to

crusting, developed over basic schist. This sample consisted of 11 small sites, ranging from 0.63 to

7.34 ha. A case study of concentrated (rill + gully) erosion in a 0.47-ha catchment with coarse-

textured soils developed over granite was also reported. The mean slope of the sites studied ranged

from 6.1% to 16.8%. Main periods when soil surface was poorly covered were late spring (maize

seedbeds) and late autumn–early winter (grassland and winter cereal seedbeds). Case studies where

fields were left bare in winter were also investigated. Soil incision and channel formation were

observed even with relatively low rainfall intensities when the soil surface was sealed, but also after

a single short intense rainfall event on recently tilled surfaces. Concentrated erosion took place

mainly on seedbeds and newly tilled soils in late spring and by autumn or early winter, but gullies

also appeared in other seasons when the soil surface was left bare. In most of the cases studied,

ephemeral gully erosion caused significant soil losses, ranging between 2 and 5 m3/ha for a single

season to locally, over 25 m3/ha. Gully development was significantly affected by agricultural

operations, such as lineal elements often acting as initial axes of concentrated erosion. Main gullies

tended to reappear at the same position.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Concentrated erosion; Ephemeral gully; Crusting; Tillage; Atlantic Spain

1. Introduction

Regions along the Atlantic coast in northern and northwest Spain are characterized by

humid, temperate climate. Rain intensities are moderate to low, like in other Atlantic

0341-8162/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

PII: S0341 -8162 (02 )00139 -X

* Corresponding author. Tel.: +34-981-167000; fax: +34-981-167065.

E-mail address: [email protected] (A. Paz).

www.elsevier.com/locate/catena

Catena 50 (2003) 199–216

areas in Western Europe, extending from northern Portugal to the Scandinavian

countries.

Mean rainfall for the autonomous community of Galicia (northwest Spain) is in the

range of 1400–1500 mm. Summers are often characterized by low total rainfall depths and

dryness, even though thunderstorms with high-intensity rainfall are more frequent in this

season (Font-Tullot, 1983). Thus, because of the water deficit in summer, the rain regime

of Galicia presents, to some extent, transitional features between Atlantic and Mediterra-

nean conditions. Rain erosion rates are expected to be moderate to high in a global

perspective (Dıaz-Fierros and Dıaz de Bustamante, 1980; Gabriels, 2000).

Soil degradation and erosion is a matter of concern in areas of Western Europe devoted

to intensive agriculture. A number of field surveys demonstrated that concentrated flow

erosion is widespread in different regions of the Loes belt, for example, north and

northwest of France (Auzet et al., 1993; Ludwig et al., 1995, 1996), South Downs in

England (Boardman, 1990), central Belgium (Govers, 1987, 1991; Poesen, 1989; Poesen

et al., 1996; Nachtergaele and Poesen, 1999), the Limburg province in the Netherlands

(Kwaad, 1991) and southwest Germany (Baade et al., 1993).

Concentrated flow erosion was also described in other European regions with different

agricultural systems and soil types, such as in the Scandinavian countries (Uhlen, 1986;

Oygarden, 1996; Hasholt et al., 1997) or in the Lake Leman area (Vansteelant et al., 1997).

More recent case studies for this type of erosion were also reported for two Iberian regions

located in the transition zone between Atlantic and Mediterranean climate, southern

Navarra (Casalı et al., 1999) and northeast Portugal (Figueiredo de et al., 2000). These

transitional regions are also characterized by low to moderate rain intensities, contrasting

with typical Mediterranean environments where high intensity rains are more frequent.

The importance of ephemeral gullies as sediment sources for the climatic conditions

prevailing in Western Europe has also been stressed (Poesen, 1989; Poesen and Govers,

1990; Poesen et al., 1996).

In Galicia, large, nonagricultural areas may be seriously affected by forest fires in dry

summers. Thus, because of concern for soil degradation and water erosion after soil

burning, earlier erosion research focused on forest and on shrubland (Dıaz-Fierros et al.,

1982, 1990). However, runoff and erosion studies in agricultural land in northwest Spain

have only been a recent matter of interest (Dafonte, 1999; Valcarcel, 1999). In fact, as in

most of the Western Europe regions, attention has been given to agricultural soil erosion in

Galicia, not because of concern for loss of soil fertility, but rather because of nutrients

originating from the cultivated area causing water eutrophication. Starting in Spring 1997,

we measured rill and ephemeral gully volumes in successive campaigns (Valcarcel, 1999).

Despite the relative lack of information on soil erosion, large agricultural areas of

northwest Spain, particularly the Ordenes Basin (A Coruna province), are affected by

soil degradation and soil loss.

In the intensive agricultural areas of northern Europe, most of the concentrated

erosion surveys were conducted on catchments ranging from about 3 to 200 ha (Auzet et

al., 1993; Ludwig et al., 1995, 1996; Vandaele and Poesen, 1995; Souchere et al., 1998).

However, there are also areas in Europe where traditional agriculture is still important or

where both traditional and intensive management systems are found side by side. Runoff

generation and sediment production measurements have also been conducted at the small

M. Valcarcel et al. / Catena 50 (2003) 199–216200

catchment (0.3–3.2 ha) scale (Oygarden, 1996) or even at the field level (Hasholt et al.,

1997; Vansteelant et al., 1997). At this small scale, gully erosion in the uppermost part

of a dry valley or along linear features in valley side or hillslopes has received little

attention.

In Galicia, traditional agricultural systems were characterized by the small size of fields

and by a complicated system of terraces and border features separating the fields. For

several centuries, thousands of kilometers of walls acting as terraces have been constructed

at the property boundaries and they have been thought to be an important element in

erosion control. In recent years, in some areas, properties have been redistributed,

increasing the average field size and facilitating intensive farming practices. The extent

to which agricultural land rearrangement measures may or may not increase erosion has

not been investigated.

The aim of this work was to present partial results on the contribution of ephemeral

gullies to concentrated flow erosion. Both traditional and reallocated landscapes were

investigated. We focused on the rates and extent of ephemeral gully erosion and on the

description of the gully types at the small catchment and slope unit level in a period of two

consecutive years.

2. Site description and methods

The study area was located in northwest Spain, near the Atlantic coast, at a 30-km

radius of A Coruna. Concentrated flow erosion was surveyed in 13 sites comprising only

agricultural land. The study sites were situated around A Coruna, Laracha, Meson do

Vento and Betanzos (Fig. 1). The altitude above sea level ranged from approximately 80 to

480 m. The investigation extended over two consecutive years, 1997–1998 and 1998–

1999, reported observations starting April 1997.

Fig. 1. Location of the study sites.

M. Valcarcel et al. / Catena 50 (2003) 199–216 201

2.1. Regional context: climate and soils

In the study area, the mean annual rainfall was about 1000–1500 mm, lower than the

average for Galicia, estimated in the range of 1400–1500 mm (Font-Tullot, 1983).

Rainfall measurements for three rain gauges, A Coruna, Mabegondo and Ordenes,

representative of the study area, are summarised in Table 1 for 1997–1998 and 1998–

1999. Mean annual rainfall in A Coruna was 1017 mm, this value increases with altitude

above sea level from coast to inland zones. The between-year variations in rainfall were

smaller than the variations between lowland and highland. However, the seasonal rainfall

for different years showed important variations.

In Mabegondo, the average rainfall at intensities higher than 10 mm/h was about 7% of

total precipitation (Valcarcel, 1999). Twenty-three events at intensities higher than 10 mm/

h were recorded in the 2-year study period. Most intense rains occurred in spring;

however, more short intense rainfalls were recorded in Spring 1997 than in Spring 1998.

The highest rain depth for a 24-h period was 52.1 mm on June 1, 1998, corresponding to a

return period of about 50 years.

The study sites are located in two contrasting geologic and pedologic conditions.

The two different soil parent materials characterizing the area are basic metamorphic

rocks, mainly basic schist, and granite bedrock. Metamorphic basic schist belongs to

the lithotectonic unit of the Ordenes Basin, the most common basic rock formation

found in the northwest of the Iberian Peninsula. Soils on basic schist are relatively

deep medium-textured, mainly loam and loamy silt, according to USDA criteria, and

rather fertile; this is attributed to an important alteration produced by weathering. Soils

on granite are shallow, coarse-textured, mainly sandy loam and loamy sand and less

fertile. Because of leaching losses induced by high precipitation, soils developed from

both parent materials are acidic, but generally pH is much lower on granite soils, as

expected due to the few basic ions in this acidic rock. Soils on both parent materials

are rich in organic matter, but comparatively more organic matter is accumulated on

the surface of soils on granite. Main soil types are classified as Umbrisol and

Cambisol, according to the World Reference Base for Soil Resources (FAO-ISRIC,

1994).

As shown in Table 2, basic schist and granite soils were unequally represented in the

sample. Most of the study sites were located in the soils on schist of the Ordenes Basin,

Table 1

Seasonal and yearly rainfall (mm) for the study period April 1, 1997–March 31, 1999

Period Coruna Mabegondo Ordenes

1997–1998 1998–1999 1997–1998 1998–1999 1997–1998 1998–1999

April 1– June 30 279.3 326.8 329.0 435.4 344.2 500.3

July 1–September 30 55.4 184.0 78.8 122.3 113.5 220.2

October 1–

December 31

479.2 243.8 475.5 214.1 717.9 269.2

January 1–March 31 147.1 443.0 138.4 365.1 217.2 428.1

April 1–March 31 961.0 1197.7 1021.7 1136.9 1392.8 1417.5


where surface crusting had been frequently observed (Taboada et al., 1999). The sample

for concentrated flow erosion surveying on medium-textured soils of the Ordenes Basin

comprised 11 sites. Two sites were on soils with granite underneath. (Table 2). Most of the

topsoil horizons in the study sites of the Ordenes Basin were silt-loam with high silt

contents (55–65%) and moderate clay contents (10–20%), but soils with loam or sandy

loam texture were also investigated in this area. Soil textures in the studied sites on granite

were sandy loam with more than 60% sand content.

2.2. Selection of study sites

In Galicia, as a consequence of land partitioning with time, the average agricultural

property size has been estimated at about 4–5 ha, making the agricultural production

system very unique and family oriented. Even after property redistribution, field sizes

remain much smaller than those in intensive agriculture regions of Spain and northern

Europe. Representative landscapes for traditional and intensive agriculture systems have

been the primary criteria for the selection of sites. Thus, concentrated flow erosion was

surveyed on two major types of landscapes, old agricultural landscapes with traditional

land uses characterized by terraces and extremely small fields, and sites with increased

field size originated by land redistribution, which are more suitable to mechanized

agriculture. Sites in the Mabegondo zone (Fig. 1) were also selected because they were

part of ongoing research programs or because previous erosion events were known.

Despite the complexity of the agricultural landscape, all 13 experimental sites were

selected as areas limited by permanent features, i.e. roads, ditches and water divides and/

or temporary barriers such as dead furrows and other linear elements of agricultural

Table 2

Main characteristics of study sites where concentrated flow erosion was measured

No. Site name Area Mean Percentage of area in slope class No. of Soil texture

and code (ha) slope

(%)< 5% 5–10% >10%

fields

Schist area of the Ordenes Basin

1 Mabegondo (M1) 2.54 6.1 30.7 69.3 – 3 Silt-loam

2 Linares (LI) 1.48 15.3 1.6 7.4 91.0 7 Loam

3 Mabegondo (M2) 0.63 13.1 – 22.4 77.6 1 Silt-loam

4 Laracha (LA) 0.84 12.3 10.6 29.2 63.1 2 Sandy loam

5 Mabegondo (M3) 5.39 11.9 1.7 33.8 64.5 3 Silt-loam


7 Abelar (AB) 2.89 9.2 10.9 67.7 21.4 1 Loam





Granite area near Coruna

12 Revoltas (RE) 0.47 7.8 5.4 94.6 – 3 Sandy loama

13 Rialta (RI) 0.07 16.8 – – 100 1 Sandy loama

a Soils with granitic bedrock.


origin. Thus, each site was well isolated by border features regarding surface runoff

generation.

Morphologically, the study sites were small catchments or hillslopes. Sites where a

small catchment was well defined and also some of the hillslopes were hydrological

isolated units as runoff water concentrates and conveys to one single outlet. The main

concentration line was either a segment of the valley floor or a linear feature of agricultural

origin (Auzet et al., 1993). However, in other hillslopes, runoff was diverted into more

than one outlet. In this case, the study site was divided into ‘‘slope units’’, defined by

Hasholt et al. (1997) as ‘‘small independent source areas with regard to surface runoff’’.

The surface area of the study sites extended from only 0.07 to 7.34 ha (Table 2). In turn,

most of the sites, and particularly those on old landscapes, were not homogeneous in terms

of crops and tillage as they were divided into small fields with different land uses. The

number of individual fields (or plots) within each site ranged between 1 and 7 (Table 2).

Altogether, 39 small fields were surveyed; their size varied between 0.025 and 4.68 ha and

the six smallest fields were less than 0.2 ha in surface area. This range of sizes is

representative of fields in A Coruna province.

2.3. Field survey

Data on topography and land use were collected from field surveys. Topographical

surveys were conducted using an Abney level from which a detailed topographic map was

elaborated for each study site. DTM were also elaborated using the software package

PCRaster (Karssenberg, 1996). Physical parameters shown in Table 2 (mean slope, percent

of slope for selected classes) were obtained from these maps. Most of the sites were

characterized by gentle slope, maximum slope values generally being less than 15%.

Nevertheless, mean slopes of two study sites were steeper than 15%.

Field observations were made after each important rainfall event and were particularly

frequent when the soil surface was uncovered. Following the methodology previously

described by Auzet et al. (1993) and Ludwig et al. (1995), field observations included land

use data monitoring and assessment of soil surface degradation state according to the

criteria first proposed by Boiffin et al. (1988).

Rill and gully systems were monitored. The position of each channel was located by

Abney level in each survey, allowing representation on the topographic map of each site.

Sediment production was evaluated by measuring channel volume (Govers, 1987; Ludwig

et al., 1995). Width and depth of rills and ephemeral gullies were measured every 2–5 m

and at specific points, where the section changed abruptly. Eroded volumes were

calculated from cross-sections and length of channel segments. The threshold between

rills and gullies was considered to be 1 ft2 (929 cm2) according to criteria proposed by

Hauge (1977) and discussed by Poesen et al. (1996).

2.4. Laboratory methods

Soil organic matter content and soil texture were determined by standard laboratory

methods as quoted by Valcarcel (1999). Bulk density of topsoil and subsoil was also

measured in some of the study sites following concentrated flow erosion episodes.


3. Results and discussion

Main dominant cultivations in the study sites were maize and grassland, but some small

fields were used for winter cereals, potatoes, orchards and rape; in addition, sometimes,

fields were left fallow during winter after maize. Rotations in the Ordenes Basin area were

maize–fallow, grassland–maize, maize–winter cereals. In the granite area, potatoes

followed rape or winter cereals. Winter fallow was also observed, both in old landscapes

and after land consolidation.

In all of the 13 study sites, soil losses by concentrated flow erosion were documented

during at least one survey date, over the 2-year study period. Cumulative erosion volumes

and rates due to ephemeral gully incision, during this period varied in these sites between

6.02� 10� 2 and 55.60 m3/ha. Fig. 2 shows concentrate flow erosion rates for 10 out of 11

study sites of the Ordenes Basin. Catchment coded M7 was kept out of this figure because

of the lowest concentrated flow erosion rate of 6.02� 10� 2 m3/ha cumulative, taken into

account a mean bulk density of 1500 kg/m3 for the eroded horizons; mean annual rates for

the 2-year study period varied between 5.01�10� 3 and 4.17 kg/m2/year.

The most important types of arable land where concentrated flow erosion was observed

in the 11 surveyed sites of the Ordenes Basin were:

– tilled surfaces, prepared for sowing maize in later spring.

– tilled surfaces, prepared for sowing winter cereals or prairies in autumn.

However, the occurrence of soil incision in land left fallow after maize all over the winter

(e.g. sites M1 and AB) has not been significant, even if overland flow was frequently

observed in such soil surfaces. This may be attributed to soil surface compaction during

harvesting.

In the two study sites of the granite area, concentrated flow erosion was caused by one

single summer rainstorm in August 1997. The soil surface of both sites was freshly tilled

just before the rainstorm. Previous crop was potato. Soil loss rates were as high as 49.96

and 31.71 m3/ha for site numbers 12 (Revoltas) and 13 (Rialta), respectively.

Fig. 2. Temporal evolution of concentrated (rill + gully) erosion in 10 sites of the Ordenes Basin.


Medium-textured soils of the Ordenes Basin have been found to be prone to crusting.

Sedimentary crusts with a very low saturated conductivity ( < 5 mm/h) develop from

freshly tilled surfaces after cumulative precipitation of about 150–200 mm. Soil surface

kinetics was also found to depend on organic matter content and initial roughness of the

tilled surface (Taboada et al., 1999). This means that after a structural crust has developed,

the infiltration capacity during heavy rains is an order of magnitude lower than peak rain

intensity. Therefore, runoff production by overland flow was likely to be the dominant

process causing incision and concentrated flow erosion.

Fig. 2 also shows important differences in concentrated flow erosion rates between the

two study years. This was due, both to the temporal distribution and intensity of rainfall

and to the proportion of soil covered by crops in a given season, which was imposed by

the rotation schemes. For example, there were clearly important differences in concen-

trated flow erosion rates between later Spring 1997, and the same period of 1998. The

most important soil losses for individual campaigns conducted during the 2-year study

period were monitored in later Spring 1997. The volume of soil lost in study site numbers

1, 2, 3, 4 and 5 (i.e. Mabegondo, M1, M2, M3, Linares (LI) and Laracha (LA)) was in the

range between 10.79 and 55.59 m3/ha. Unusually heavy rainfall events occurred,

recurrently, on May 25, May 26, May 30 and June 2, 1997. Daily rainfall amounts of

35.8, 18.7, 18.9 and 24.4 mm, respectively, were recorded in Mabegondo during these

dates. These four events, with a precipitation total of 97.8 mm caused heavy erosion in the

seedbed of maize fields, already degraded to some extent by about 50 mm previous

rainfall. Subsequently, surface crusting and channel erosion could be well documented in

1997. During 1998, maize was also sown in the same sites (numbers 1 to 5); however, by

the end of June 1998, less than 10% of the surface had a structural crust, and consequently

total soil erosion, including sheet erosion was not significant. Even if cumulative rainfall

between April 1 and June 30 was higher during 1998, concentrated flow erosion in that

period was negligible.

There were also important differences between ploughed soils and seedbeds. No

significant concentrated flow erosion was found with high amounts of rainfall after

mouldboard plough in maize fields during early Spring 1997 and 1998 (e.g. sites M1 and

M2 at Mabegondo). In this case, the absence of runoff generation may be attributed to the

important storage capacity in depressions at the soil surface and was associated with the

high roughness produced by ploughing (Kamphorst et al., 2000). However, because of the

low surface roughness, autumn-tilled surfaces, left bare all over the winter (e.g. site M5 at

Mabegondo), produced high rates of concentrated flow erosion as shown by measurements

at the end of the winter.

Grassland and winter cereal sowed in autumn originated surfaces with low roughness

values, also prone to crusting where runoff was frequent. Examples were sites M6 and M8

at Mabegondo (Fig. 2). However, once soil cover reached its maximum, grassland

prevented totally concentrated flow erosion. Because a temporal prairie protects the soil

surface for a length of 3 or 4 years, concentrated flow erosion risk was highly reduced

when rotations included grassland and permanent pastures.

Thus, conventional tillage practices for seedbed preparation, both during spring and

autumn, have most enhanced concentrated flow erosion. In contrast, the maintenance of

grassland continuous cover completely prevented their formation.


3.1. Patterns of gullying

Ephemeral gullies were found in 6 out of 11 surveyed sites on the Ordenes Basin area

and in one out of two sites on the granitic area (Table 3). Two main patterns of gullying,

matching those previously described by Ludwig et al. (1995, 1996) and Poesen et al.

(1996), were found: (i) gullies located on topographical features, such as thalwegs in the

valley bottom of small-sized catchments and (ii) gullies associated with linear man made

permanent or temporal agricultural features, mainly depression lines in hillslopes. Based

on field observations, examples are next described.

An example of gullying associated with agricultural lineal depressions, thus

characterized by a nonuniform spatial distribution, is shown in Fig. 3. This ephemeral

gully was located at the boundary of a hillslope, limited by a road (site M5 at

Mabegondo). The slope length promoting concentrated flow was 325 m long. It was

first observed by the end of October 1997; it developed further and it was erased in

early Spring 1998. However, it reappeared at the field border already in later Spring

1998 and also one more time during Autumn 1998–Winter 1999. The contribution of

the main channel (gully or rill + gully, depending on the season) to concentrated flow

erosion in this site was always higher than 90%. This result points at the relative

importance of gullying on concentrated soil erosion and for conveying water from the

crusted surface to the outlet.

An aerial view showing ephemeral gullying due to flow concentration into a topo-

graphically predetermined depression at the valley bottom of a small catchment (M1 site at

Mabegondo) is shown in Fig. 4. This pattern of gully was also observed, but not frequently

because zero-order catchments were poorly represented in the sample due to the small

dimensions of the study sites. The site was well isolated all around the perimeter and

Table 3

Soil losses by ephemeral gully erosion and gully characteristics during the period 1997–1999

No. Site name

and code

Survey dates Ephemeral

gully position

in valley

Losses by

gullying

(m3/ha)

Gully/rill +

gully(%)

Mean cross-

section (m2)

Width–

depth

ratio

Schist of the Ordenes Basin area

1 Mabegondo (M1) May 1997 Bottom 0.72 10.78 0.158 6.57

June 1997 Bottom+ side 3.81 35.26 0.179 11.97

2 Linares (LI) June 1997 Bottom+ side 2.46 5.68 0.149 4.24

4 Laracha (LA) June 1997 Bottom 26.14 61.63 0.168 2.24

5 Mabegondo (M3) June 1997 Bottom+ side 2.94 26.04 0.131 2.97

8 Mabegondo (M5) November 1997 Side 3.94 65.97 0.156 3.35

January 1998 Side 5.28 76.11 0.149 2.44

May 1998 Side 0.17 18.42 0.131 2.14

11 Mabegondo (M8) July 1998 Side 5.07 22.46 0.165 3.44

January 1999 Side 1.31 43.28 0.261 1.64

Granite area near Coruna

12 Revoltas (RE) August 1997 49.96 43.27 0.160 4.77

Volumetric losses by gullying are for surface units.


runoff water generated upslope was conveyed and concentrated into a topographical

depression at the valley bottom.

An example of a dendritic rill pattern along rows and wheel tracks, which gradually

converge into a gully along a micro-topographical depression at the field border of a

hillslope is shown in Fig. 5. This ephemeral gully system was observed in later Spring

1998 in site M8 at Mabegondo. Again, this gully pattern is located in a pre-existing lineal

structure initiated by tillage.

Fig. 3. Ephemeral gully in valley side, along a dead furrow associated with a field border.


In traditional agricultural landscapes with small fields separated by terrace walls, such

as in Linares (LI) and Laracha (LA), a particularly adapted ephemeral gully pattern was

found. In these cases, overland flow in the fields above the terrace wall was observed to

produce rill– interrill erosion, whereas gully erosion occurred in fields below the wall.

Ephemeral gullying started at the most upslope part of the second field. Because wall-sides

between neighbour fields located along the slope remained stable and firm, no bank

erosion was produced.

An example of parallel rill patterns, which was observed in site M5 at Mabegondo, is a

gentle hillslope. This parallel network showed no convergence into a gully. Similar rill

patterns had been also previously described in other regions (Govers, 1987; Ludwig et al.,

1995; Casalı et al., 1999). In this case, the channel network was imposed by very close

wheel tracks separated by a regular distance of 1.80 m (Valcarcel 1999).

In summary, most of the surveyed ephemeral gullies appeared to be governed by

overland flow as a mere deepening of previous linear features, mainly in valley side

topographic location. In some cases, gullies were promoted by convergent concentrated

flow in drainage lines of small zero-order catchments, i.e. in valley-bottom. Singular

gullies may appear in terraced, old agricultural landscape.

The importance of the spatial position of runoff contributing areas on concentrated flow

erosion in areas of intensive agriculture has frequently been stressed (Ludwig et al., 1995,

1996). The possibility of identifying runoff collectors in order to analyze the spatial

variability of erosion at the small spatial scale used in this study was also investigated. A

case study is presented in Fig. 6, where both DTM and a schematic representation of

concentrated flow pathways and ephemeral gully location for study site M1 (Mabegondo),

Fig. 4. Aerial photograph showing ephemeral gullying in valley bottom.


at the end May 1997, are shown. It is apparent when comparing DTM and sketch that the

natural way of conveying excess water has been strongly modified by agricultural land use.

The small catchment comprises three fields, separated from each other by two unpaved

roads. The detailed DTM shows that the first two fields located upslope are concave,

whereas the field at the bottom of the catchment has a topographical depression delineating

a thalweg. This example confirms that overland flow from areas upstream causes ephemeral

gullying in a different field. In other words, it was illustrated how runoff produced in a place

may cause concentrated flow erosion in another place. The case study also shows how local

infrastructures and the agricultural framework influenced gully development in a small

catchment. However, further work is needed to test the hypothesis of the relationship

between concentrated flow erosion and hydrographical structure at the study scale.

Thus, depending on rain intensities, on soil erodibility and on agricultural factors

governing the water flow pattern, ephemeral gullies developed at small surface scale.

Fig. 5. Gully along a field border as a result of downward convergence of a dendritic rill network.


Spatial variability of ephemeral gully erosion was to a large extent dependent on both

topographical and anthropogenic factors. Agricultural operations significantly affected

gully development, as lineal features of the landscape often act as initial axes of erosion.

3.2. Relative contribution of ephemeral gully to sediment production

Ephemeral gully volumes and main characteristics are shown in Table 3. Taking into

account all the surveyed surfaces of the Ordenes Basin, the relative contribution of gully

erosion to concentrated flow sediment production varied between 0% (four sites without

gullying) and 76%. That percentage varied also throughout the year and between erosion

events.

In this area, ephemeral gully erosion caused significant soil losses. In most of the sites,

where gullying was observed, the range of soil loss by this type of concentrated flow

Fig. 6. DTM and schematic representation showing main lines of flow concentration and gully erosion during late

spring events in a site matching a small zero-order catchment. (Contours are in meters above sea level).


erosion during the different measurement epochs was between 2 and 5 m3/ha. Minimum

and maximum values in these six sites with gully erosion were 0.17 and 26.14 m3/ha,

respectively.

Taking into account the whole surveyed surface in the Ordenes Basin, rates of

ephemeral gully erosion and concentrated flow erosion were calculated as shown in Table

4. The total surveyed surface for concentrated flow erosion was 36.8 ha. Mean cumulative

soil losses computed for this surface, over the 2-year study period, due to ephemeral gully

erosion were 2.12 m3/ha. Concentrated flow erosion by rills plus ephemeral gullies was in

this surface, 5.89 m3/ha for the 2-year period. This means that ephemeral gully erosion was

on average 26.4% of concentrated sediment production during the 2-year study period.

Mean ephemeral gully erosion rate of 1.06 m3/ha/year matches well with results

obtained in other regions with humid climate (Vandaele and Poesen, 1995; Poesen et al.,

1996), even if it was somewhat lower than rates measured in areas of intensive agriculture

in northern Europe. However, this rate is much lower than values found in areas with

Mediterranean climates (Poesen et al., 1996; Alba de et al., 1998). Also, both concentrated

flow erosion rates and the relative contribution of rill and gully volumes in the Ordenes

Basin were in agreement with data reported for other regions with Atlantic climate. Gully

erosion rates in the medium-textured soils of the Ordenes Basin are low or moderate in a

global perspective. Critical periods for ephemeral gully formation were late spring and

autumn–early winter, as for concentrated flow erosion.

Results also indicated the recurrence of ephemeral gully erosion. Ephemeral gullies in

sites M5 and M8 at Mabegondo showed a trend to reappear at the same position. In site

M5, three recurrent channel incision episodes were observed in the period of November

1997–January 1999. They were strong enough to originate ephemeral gully development.

Notice the important total soil loss rates in the winter period of November 1997–January

1998 (5.28 m3/ha), when the highest relative contribution of ephemeral gully erosion to

soil losses by concentrated erosion was recorded. Important soil losses associated with

gullying were also more than one time observed in the site M8 at Mabegondo since 1998.

An ephemeral gully was first observed in later Spring 1998 under maize, while this site

was before used for grassland. The soil surface was prepared as a seedbed for prairie in

Autumn 1998 and in January 1999 an ephemeral gully was again incised in the same

position. In M8, the rate of soil loss was smaller in the winter period than in the later

spring one, but the contribution of ephemeral gullying to total erosion was about double as

high in winter than in later spring.

Concentrated flow erosion measurements made more than one time within one erosive

period allowed assessing the evolution of the relative contribution of ephemeral gullies as

a function of cumulative rainfall. Results are shown in Table 3 for site M1 during later

Spring 1997 and for site M5 during Autumn 1998–Winter 1999. They clearly show that

Table 4

Average erosion losses by rills and gullies in the Ordenes Basin sample during the period 1997–1999

Surveyed

surface (ha)

Rill + gully

(m3/ha)

Rill erosion

(m3/ha)

Gully erosion

(m3/ha)

Gully/rill + gully

(%)

36.8 8.01 5.89 2.12 26.4


the relative contribution of ephemeral gullies to total concentrated soil losses was

increasing as the rainfall increased. The rise in the importance of ephemeral gully soil

losses as a function of the number of erosive rainfall events during a single season may be

partly attributed to the increased degradation of structure with time.

The observed gully pattern in the granite area near Coruna, which is also shown in

Table 3, will be discussed next. As before mentioned, in August 1997 a local heavy

rainstorm with a total precipitation total of 29.8 mm caused channel incision in arable land,

tilled with rotative implements after potato harvest in the outskirts of Coruna. The surface

area of this field was 0.437 ha. Though there were scarce dimensions for producing

overland flow, soil loss rates were about 50 m3/ha. Ephemeral gullies represented 43.27%

of total soil losses. This result is more striking, if it is taking into account that previous to

the rainstorm causing concentrated flow soil losses, the topsoil was not degraded by

crusting, and the saturated hydraulic conductivity of coarse-textured soils on granite is

very high. However, the sandy loam soil is thought to be very prone to develop

hydrophobia when dry, which might explain the low water infiltrability during the

rainstorm and the formation of a sheetwash regime on the dry topsoil. Excess water

was concentrated along linear elements located very closely at distances of about 1.2 m

and finally originating devastating gullying following wheeltracks.

3.3. Main characteristics of the gully systems

Mean ephemeral gully widths, depths and cross-sections are shown in Table 3. Cross-

sections oscillated between 0.13 and 0.26 m2. Average values of width–depth ratio were in

the range between 1.63 and 11.97. Broad gullies had been considered to be more harmful

for water quality because of relatively higher losses of topsoil rich in fertilizers and organic

matter, which may cause nonpoint source loads in streams. In general, a trend was

observed for higher width–depth ratios by heavy rains in spring and smaller ratios in the

winter season.

The higher width–depth ratio was recorded in the thalweg gully developed near the

outlet of catchment M1 at Mabegondo. This was due to a significant higher erodibility in

topsoil with regard to subsoil because of the presence of a plow pan. Mean bulk density

was 1380 kg/m3 in the topsoil versus 1650 kg/m3 in the subsoil. Consequently, the surface

horizon of about 20 cm in depth was eroded, but not the subsoil as shown in Fig. 7.

Channel sections near the outlet were more than 1.6 m wide, whereas they were frequently

15–20 cm deep. Table 2 also shows that the smallest width–depth ratios were recorded for

site LI, near Laracha; in this case, the ephemeral gully bed was frequently located on the

bedrock and gully depths of the order of 50 to 60 cm were measured.

Ephemeral gullies formed by incision along linear elements generally showed large

sections in zones with high slope, so that a gradual decrease from the maximum cross-

section, both toward the head and downstream, occurred. This variation from head-cut

to outlet of the gully may be attributed to the small flow rates at the upstream and

saturation of the transport capacity downslope, where sedimentation initiates (Casalı et

al., 1999).

Frequently, management systems in old agricultural landscapes included a number of

measures for runoff control. Concentrated flow may be routed away from agricultural


lineal features such as furrows into concrete channels along roads. The effect would be

subdividing the runoff contributing area, thus concentrating flow volume and avoiding

water from flowing in uncontrolled manner on agricultural land. Runoff routing is an

elemental measure that probably would have reduced concentrated flow erosion, both on

small catchments (site M1) and hillslopes (site M5) in the study conditions. Contouring,

grassed waterways and tillage direction management would also reduce overland flow

concentration and minimize erosion.

4. Conclusions

In the medium-textured soils of the Ordenes Basin, occurrence of concentrate flow

erosion was related to surface crusting. Human impact is demonstrated through variations

Fig. 7. Detail of an ephemeral gully over a plow pan.


caused by crop rotation and tillage procedures. In a representative sample of cultivated

land of the Ordenes Basin, mean cumulative concentrated flow erosion rate over a 2-year

period was 8.01 m3/ha. On average, ephemeral gully erosion contributed 26.4% (i.e. 1.06

m3/ha/year) to the concentrated flow erosion. Ephemeral gully caused by a heavy summer

rainstorm was described also in coarse-textured soil on granite. Concentrated erosion may

transport large amounts of sediment to streams, unless buffer zones between the eroded

surface and the permanent watercourses were present. Conventional tillage practices and

seedbed preparation enhanced concentrated flow erosion and gully occurrence, whereas

the maintenance of vegetation cover completely prevented soil surface incision and

channel formation. The study of the development of the gully system through time

showed that main gullies tend to reappear at the same position.

Acknowledgements

This paper has been produced with the support of research projects HID96-1085-C02-

01 funded by CICYT and PGIDT99MA10303 funded by Xunta de Galicia, respectively.

We thank two referees for their annotations and comments.

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