Agriculture, Ecosystems and Environment 107 (2005) 199–210
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Impact of erosion in the taluses of subtropical orchard terraces
V.H. Durán Zuazo a,b,*, J. Aguilar Ruiz c, A. Martı́nez Raya b, D. Franco Tarifa d
b
a
Centro de Investigación y Formación Agraria de Granada, Soil and Irrigation, Apdo. 2027, 18080-Granada, Spain
Erosion and Soil and Water Conservation Group, Wageningen University, Nieuwe Kanaal 11, 6709 PA Wageningen, The Netherlands
c
Departamento de Edafologı́a y Quı́mica Agrı́cola, Universidad de Granada, C/Severo Ochoa s/n, 18071-Granada, Spain
d
Finca Experimental ‘‘El Zahorı́’’ Ayuntamiento de Almuñécar, Plaza de la Constitución 1, 18690 Almuñécar (Granada), Spain
Received 15 April 2004; received in revised form 18 September 2004; accepted 23 November 2004
Abstract
The coast of the provinces of Granada and Malaga (SE Spain) are economically important areas for the subtropical fruit
cultivation. The climate is characterized by heavy periodic rainfall, which is one of the main factors responsible for soil erosion
in this agroecosystem. However, the erosion depends on a host of factors, including soil, topography, cropping and soilconservation techniques. The most widely taken soil-conservation measure taken on steeply sloped coastal mountains in the
zone is terracing. We hypothesise that despite these soil-conservation measures erosion remains a major problem in these steep
uplands. Soil loss and runoff were evaluated over a 2-year period (2001–2002) on the taluses of terraces, in this zone of intense
subtropical orchard cultivation. The experimental erosion plots (4 m 4 m in area) were located on a terrace of 214% (658)
slope at 180 m in altitude. The results indicated that soil loss occurs from rainfall runoff depositing topsoil at the foot of the
terrace. The average annual soil loss by erosion from the taluses of the orchard terraces was 9.1 Mg ha1 year1, with a runoff of
100 mm year1 and a rain erosivity index (EI30) of 219.7 MJ mm ha1 h1. Therefore, under these conditions the terraces had a
high risk of rockslide and slump, causing environmental and agricultural damages. The runoff coefficients ranged from 6 to 31%,
depending on the intensity and energy of the rainfall events. The present study highlights the severity of erosion in taluses of
orchard terraces of southeast Spain and reflects the urgency of planning strategies to protect these structures against chronic
destruction.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Almuñécar; Erosion plots; Orchard terraces; Soil loss; Terrace protection
1. Introduction
Terracing, an agricultural technique for collecting
water and reducing soil erosion, has an ancient history
* Corresponding author. Tel.: +34 958 267311;
fax: +34 958 258510.
E-mail address: [email protected] (V.H. Durán Zuazo).
of transforming landscapes into stepped agroecosystems in many mountainous regions of the world
(Goudie, 1986; Denevan et al., 1987; Sandor et al.,
1990; Hillel, 1991; Xing-guang and Lin, 1991; Treacy
and Denevan, 1994; Zurayk, 1994; Beach and
Dunning, 1995; Gardner and Gerrard, 2003). The
main purpose of these structures in the past as well as
present has been to increase the usefulness of steep
0167-8809/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.agee.2004.11.011
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V.H. Durán Zuazo et al. / Agriculture, Ecosystems and Environment 107 (2005) 199–210
slopes. In addition, they also may be used to boost the
agricultural potential of slopes that could be cultivated
without levelling (Wadswort and Swetnam, 1998).
However, terracing is not always effective in reducing
soil loss, according to Purwanto (1999) and Van Dijk
(2002), who report that in West Java the soil loss
amounted to 40 Mg ha1 year1. These authors attribute the ineffectiveness of the terracing to the sparse
vegetation and soil surface cover, particularly on the
terrace riser sections (Critchley and Bruijnzeel, 1995).
Throughout the Mediterranean region, as in the
traditional dry-land farming in southeast Andalusia (S
Spain), soils on high sloping lands cultivated for
thousands of years have been gradually degraded by
soil erosion (Tello, 1999; Campos, 1993). Currently,
terracing continues, sometimes through heavy financial investment, resulting in major alterations in the
soil profile. However, the presence of taluses without
vegetal protection has led to destructive erosion,
causing slumping and collapsing of terraces.
Today, on these steep terraces, intensive irrigated
agriculture has established subtropical crops including
avocado (Persea americana Mill.), mango (Mangifera
indica L.), loquat (Eriobotrya japonica L.), custard
apple (Annona cherimola Mill.), litchi (Litchi chinensis Sonn.) and others (Calatrava, 1998; Durán
et al., 2003). The detached soil from the talus
accumulates on the platform of the terrace below,
hindering manual fruit harvesting and orchard maintenance. In this sense, talus erosion, making terrace
reconstruction necessary, poses a serious economic
challenge for farmers.
Soil erosion in the study area is currently one of the
most damaging effects of human activity, particularly
farming. That is, when agricultural crops replace
native vegetation, the natural cycle is altered, and soil
and its nutrients can be readily transported by erosion
and runoff (Kosmas et al., 1997; Durán et al.,
2004a,b). Particularly in present agricultural practices
under semiarid Mediterranean conditions, the vegetative cover plays a fundamental role by scattering the
runoff and buffering its erosive power (Andreu et al.,
1998; Bochet et al., 1999; Pardini et al., 2003;
Casermeiro et al., 2004).
The objective of the present study was to determine
the impact of erosion on the taluses of subtropical
orchard terraces, evaluating its agricultural damage in
steep areas of intense fruit production in southeast
Spain. This information on the erosive nature of
rainfall could be useful in the future as a guide for
regional soil-conservation planning.
2. Materials and methods
2.1. Site descriptions and soil characteristics
The study was performed on a south-facing terrace
of a mango orchard located some 7 km north of the
Mediterranean coast at Almuñécar (Granada, SE
Spain) at the experimental farm ‘El Zahorı́’
(368480 0000 N, 38380 000 W) and at elevation of 180 m
a.s.l. (Fig. 1). The study terrace, representative of
those commonly found in the study area, is a reverse-
Fig. 1. Location of the experimental site of Almuñécar in southeast Spain.
V.H. Durán Zuazo et al. / Agriculture, Ecosystems and Environment 107 (2005) 199–210
201
Fig. 2. Orchard terraces for subtropical farming.
sloped bench-terrace type with a toe drain measuring
160–170 m long. The platform was 2–3 m wide and
the talus 3–5 m high. The platform had a single row of
bearing mango trees (M. indica L. cv. Manzanillo
Nuñez) spaced 3 m apart. The terrace, as in the rest of
the orchard, was managed according to the usual local
practice of not tilling the soil between trees, irrigating
with a drip system, and harvesting the fruit by hand
(Fig. 2).
Local temperatures are subtropical to semi-hot
within the Mediterranean subtropical climatic category (Elias and Ruiz, 1977). The average annual
rainfall in the study zone is 449.0 mm.
The soils, formed from weathered slates, vary in
depth, and some are rocky, providing generally very
good drainage, especially in the fill used to construct
the platforms. The soils of the zone are Typical
Xerorthent (Soil Survey Staff, 1999), with 684 g kg1
of sand, 235 g kg1 of silt and 81 g kg1 of clay,
containing 9.4 g kg1 of organic matter, and 0.7 g
kg1 of N, with 14.6 mg kg1 P and 178.7 mg kg1
assimilable K (MAPA, 1994).
2.2. Experimental setting
Three erosion plots (4 m 4 m each) with bare soil
devoid of vegetation were laid out on an orchard
terrace at 214% (658) slope (Fig. 3). Each erosion plot
consisted of a galvanized enclosure, drawer collector,
sediment and runoff collector, and tanks for storing
runoff.
2.3. Measurements and statistical evaluation
During the 2-year monitoring period (2001–2002)
the soil loss and runoff from the plots were collected
and measured after each rainfall event. The rainfall
data were collected from a local meteorological
station (<100 m from the plots) at the experimental
farm ‘El Zahorı́’. For each storm, the average intensity
[I = (total rain/total time) (mm h1)] and maximum
intensity at 30 min (I30) was calculated. From this data
the kinetic energy was calculated with units of energy
per unit area and precipitation (Eq. (1)). The kinetic
energy (KE), a widely used indicator of the potential
ability of rain to detach soil and splash, is related to I
as a logarithmic function (Wischmeier and Smith,
1978; Brandt, 1990):
KE ðJ m2 mm1 Þ ¼ 210 þ 89 log10 I
(1)
By analysis of variance (ANOVA), the means for
soil loss and runoff were compared, and differences
between individual means were tested using the LSD
test at P < 0.01. Linear regressions were fitted among
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Fig. 3. Experimental erosion plots in the taluses of orchard terraces.
runoff, soil loss, and rainfall parameters (maximum
intensity [I30] and the erosivity index [EI30] proposed
by Wischmeier and Smith (1978)).
3. Results
3.1. Rainfall characteristics, kinetic energy, and
erosive index
The total rainfall for the first and second year was
484.3 and 504.9 mm, respectively (Table 1), ranging
from 8.7 mm in 2001 to 190.2 mm in 2002. These
values were very similar to the rainfall average
(449.0 200.6 mm), calculated for the previous 39
years with data from the main meteorological station
in the study area (Motril, 1986). The maximum
intensity at 30 min (I30) during the monitoring period
ranged from 4.0 to 52.8 mm h1, displaying high
annual and inter-annual fluctuations.
The erosivity index (EI30) during the two monitoring periods varied considerably, ranging from 0.03 to
38.7 MJ mm ha1 h1 for the first year and from 0.04
to 275.2 MJ mm ha1 h1 for the second year.
During the study period, 68% from the total rainfall
fell in autumn and winter, while the spring rainfall was
24%. Therefore, the average monthly rainfall energy
and erosivity throughout the 2-year monitoring period
in the experimental area increased considerably during
November and February (Fig. 4).
The cumulative values for kinetic energy and
erosivity index throughout the rainfall events of the 2year monitoring period are shown in Fig. 5. Energy
values increased during October–December, representing the main erosive factor, i.e. heavy rainfall.
Rainfall during spring had a lower average kinetic
Table 1
Statistical characteristics of rainfall during the study period
2001
Average
S.D.
Maximum
Minimum
Total
Events
2002
Rainfall (mm)
I30 (mm h1)
EI30
(MJ mm ha1 h1)
Rainfall (mm)
I30 (mm h1)
EI30
(MJ mm ha1 h1)
60.5
37.1
124.9
8.7
484.3
8
13.9
11.3
40.7
4.7
111.6
8
11.6
12.9
38.7
0.03
92.6
8
72.1
67.8
190.2
9.7
504.9
7
16.5
16.9
52.8
4.0
115.4
7
49.5
100.7
275.2
0.04
346.8
7
I30, maximum intensity at 30 min; EI30, erosivity index.
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203
Table 2
Monthly mean soil loss and runoff
Fig. 4. Average rainfall energy and erosivity during the study period
(2001–2002).
Month
Soil loss (g m2)
Runoff (mm)
V
I
II
X
IX
III
XII
XI
24.7a
54.4b
126.1c
132.9d
137.5dc
141.0e
152.1f
355.9g
6.7a
5.5a
11.0d
9.8cd
8.3b
21.1e
9.4bc
27.8f
Year
2001
2002
112.3a
168.9b
11.7a
13.2b
**
**
**
**
**
**
ANOVA
Month
Year
Month–year
Values with different letters within the columns are statistically
different al level 0.01 (LSD).
**
Significant at P < 0.01.
Fig. 5. Cumulative rainfall energy and erosivity as a function of
cumulated rainfall during the study period.
energy than during autumn and winter, and therefore
lower erosive potential.
3.2. Soil loss and runoff
Table 2 presents the results concerning the monthly
rainfall effect on the average soil loss and runoff
during the experiment. The soil transported by runoff
in the plots was greater during November, coinciding
with the highest rainfall energy and erosivity
(2.9 MJ ha1 and 155.6 MJ mm ha1 h1, respectively) in the experimental area. Therefore, in most
cases the soil loss was closely related to runoff,
depending primarily on the rainfall amount and
intensity. Significantly greater amounts of soil loss
and runoff were recorded during the second year than
the first, due to the rainfall erosivity reached in this
period (346.8 MJ mm ha1 h1).
Table 3 presents detailed data on soil loss, runoff,
and rainwater intercepted by the talus soil for each
erosive event during the study period, disregarding the
evaporation because the runoff occurred in a very short
time. The average total soil loss and runoff were
greater during the second year (1351 g m2 and
106 mm, respectively). The heaviest rainfall events
(131.8 and 190.2 mm) during this year appeared to
account for this increase, having the strongest rainfall
energy and erosivity. Therefore, the heaviest storms
during the second year coincided with the highest
amount of eroded soil and runoff, as for example
related to the rainfall event of 190.2 mm, being
698.3 g m2 and 50.3 mm, respectively. During the
first year, this relationship held for soil loss but not for
runoff, which amounted to 21.8 mm as a maximum
rate with 69.4 mm of rainfall.
The average soil loss and runoff from the taluses
of orchard terraces at the plot level was 1125 g
m2 year1 and 100 mm year1, respectively. However, under the experimental conditions, a cultivated
hectare (100 m 100 m) of mango trees on steeply
sloped lands (658) would have 18 terraces 100 m long
with an average talus height of 4.5 m, for a total of
8100 m2 (4.5 m 100 m 18) of unprotected taluses
(bare soil), while the orchard terrace of approximately
3 m wide (platform) and 100 m long would cover
5400 m2 (3 m 100 m 18). Therefore, the average
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Table 3
Soil loss and rainfall water in erosion plots during the two-monitoring periods
Soil loss (g m2)
Year
Rainfall event
(mm)
2001
54.3
8.7
69.4
47.6
70.2
124.9
20.2
89
484.3
94.8 2.86
1.7 1.14
60.0 1.19
49.5 1.65
256.6 5.51
258.8 3.76
13.4 0.95
163.2 3.01
898.1
Total
15.6
131.8
79.1
19.9
9.7
190.2
58.6
504.9
14.0 1.25
250.6 5.28
221.9 3.29
18.4 1.59
7.0 1.28
698.3 7.32
140.9 3.19
1351.1
Average
494.6
1124.6
Total
2002
Surface runoff
(mm)
Rainwater intercepted
by talus soil (mm)
Runoff
coefficient (%)
Intercepted by
talus soil (%)
9.6 0.76
1.7 0.24
21.8 0.74
13.3 0.70
12.4 0.69
19.1 0.68
5.3 0.46
10.5 0.73
93.5
44.7
7.0
47.6
34.3
57.8
105.8
14.9
78.5
390.8
18
19
31
28
18
15
26
12
19
82
81
69
72
82
85
74
88
81
1.3 0.15
20.3 0.78
20.5 0.72
4.3 0.34
0.6 0.11
50.3 1.14
8.4 0.63
105.7
14.3
111.5
58.6
15.6
9.1
139.9
50.2
399.2
9
15
26
21
6
26
14
21
91
85
74
79
94
74
86
79
395.0
20
80
99.6
The values after the symbol ‘‘’’ is the standard deviation.
soil loss from the taluses of orchard terraces on steep
slopes areas would be 9.1 Mg ha1 year1.
Runoff coefficients (water running off the plot as a
percentage of the total volume of rainwater) for
rainfall events ranged from 6 to 31%, depending on the
nature of the events, for an average of 20% during the
monitoring period.
As in the case of soil loss and runoff, the rainwater
intercepted by the talus soil (rainfall minus runoff) was
greater during 2002, consistent with greater amount of
rainfall during this year. The highest rate reached in
the first and second year was 105.8 and 139.9 mm,
respectively. In this sense, the soil texture (sandyloam) and the rock matrix acting as a partial cover
(30–40%) of the talus could aid in intercepting a
considerable amount of water even during rainfall
events with high erosion risk.
Fig. 6 shows the average soil loss, runoff and
rainwater intercepted by the talus soil, displaying the
high variability in response to erosive rainfall events,
which is in line with energy and erosivity index
throughout the 2-year monitoring period (Fig. 4). It is
evident from Fig. 6 that, with greater monthly rainfall,
the average soil loss and runoff rates were higher and
vice versa, reflecting a close relationship between
these values over the study period.
The impact of raindrops on bare talus caused rill
and interrill erosion, detaching the topsoil, accumulating as runoff in many small channels, and uncovering
the rocky soil matrix (Fig. 7). This rocky matrix
provided temporary protection until the next heavy
rainfalls, where rapidly moving runoff cut wider and
deeper channels, delivering eroded soil to the next
terrace. In some extreme cases the runoff removed soil
and exposed the root system of trees, causing severe
stress, particularly in summer (Fig. 8). A major
problem is that the eroded soil does not leave the
terrace system entirely, but rather accumulates in the
Fig. 6. Average monthly soil loss (SL), runoff (RF) and intercepted
rainfall water by soil (IRW) during the monitoring period.
V.H. Durán Zuazo et al. / Agriculture, Ecosystems and Environment 107 (2005) 199–210
205
Fig. 7. Rill erosion and rock fragments in the taluses of orchard terraces.
next terrace down, burying the toe drain, irrigation
lines and tree trunks.
The soil-detachment risk (soil slumps) during the
autumn and winter months was greater, since soil
moisture was relatively high due to the frequent
seasonal rains and exacerbated by the force of gravity
on such a steep slope (214%).
3.3. Erosive parameters related to soil loss and
runoff
Table 4 presents the relationships between rainfall
characteristics, soil loss and runoff, showing that
rainfall depth was highly correlated with soil loss and
runoff (P < 0.01), but not with the runoff coefficient
Fig. 8. Slumps of the talus from orchard terraces.
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Table 4
Spearman rank correlation coefficients between rainfall characteristics and soil loss and runoff
Period
R-SL
R-RF
R-RFc
SL-RF
I30-SL
I30-RF
EI30-SL
EI30-RF
2001–2002
0.92**
0.91**
nsa
0.91**
0.70**
0.70**
0.91**
0.88**
R, rainfall depth; SL, soil loss; RF, runoff; RFc, runoff coefficient; I30, maximum intensity at 30 min; EI30, erosivity index.
a
Not significant.
**
Significant at P < 0.01.
Fig. 9. Cumulative soil loss (SL) and runoff (RF) as a function of
cumulated erosivity index during the study period.
on an event basis. Therefore, the soil loss was
significantly related to runoff, as mentioned above.
Significant relationships were found between the
maximum rainfall intensity (I30), soil loss, and runoff.
Similarly, EI30, soil loss and runoff were significantly
correlated in the study zone. The rainfall and runoff
coefficient presented a random trend for the two
variables, without any degree of significance.
The average cumulative soil loss and runoff in
relation to rainfall erosivity over the 2 years is shown
in Fig. 9. During the study period the soil loss and
runoff at plot level was 11.5 Mg ha1 and 106.2 mm,
respectively.
4. Discussion
The high annual and inter-annual differences in
rainfall quantity and intensity reflected the typical
seasonal pattern of the Mediterranean region with
rainfall concentrated in autumn and winter months. As
pointed out by Imeson (1990), the main characteristics
affecting the vulnerability of the Mediterranean area to
erosion are intense rainfall after a very dry summer
and pronounced short- and long-term fluctuations in
rainfall quantity. The rainfall kinetic energy and
erosivity index (EI30) (Wischmeier and Smith, 1978)
throughout the months of the 2-year monitoring period
showed a similar trend in the study area, increasing in
values considerably during autumn and winter
months, due to high erosive rainfall in the Mediterranean area (Ramos and Porta, 1993, 1994; López and
Romero, 1993; Salles et al., 2002).
Terracing necessarily results in topographic change
because its primary purpose is to create stable flat
surfaces for agriculture on steep terrain otherwise
unsuitable for sustained farming. Very few studies
have measured the soil erosion and runoff from taluses
of orchard terraces in southeast Spain, and virtually
none have examined the temporal patterns of these
alterations. According to Albadalejo et al. (1988),
erosion rates of 0–3 Mg ha1 year1 in the Mediterranean area are negligible and rates of 3–10 Mg
ha1 year1 are low. Although the average soil loss
from our taluses was 9.1 Mg ha1 year1, it is crucial to
take into account that, given the continuous loss of soil
and the fragility of the talus, this amount of erosion
can lead to slumping and collapse of the terraces.
The results of our study reveal that the traditional
method of planting trees in orchard terraces combined
with taluses with bare soil resulted in erosion during
the rainfall period, as in other agroecosystems of Spain
(Romana, 1992; Pallares, 1994). In all recorded
rainfall erosive events the soil loss was consistently
related to runoff, the amount of loss depending
fundamentally on the rainfall quantity and intensity.
The energy of rainwater arriving at the talus surface
was the most critical factor in soil-detachment rates.
Although the sandy-loam soil of the talus was able to
absorb a large amount of rainwater (Moldenhauer and
Long, 1964), intense rainfall generated substantial
amounts of runoff and sediments simultaneously.
The possibility of rainwater exfiltration from the
talus was negligible due to the inclination (5–78) of
platform terrace sloping towards the toe drain
(reverse-sloped bench terrace).
Crust development was weak on our talus because
raindrops struck the soil at an acute angle (vertical
V.H. Durán Zuazo et al. / Agriculture, Ecosystems and Environment 107 (2005) 199–210
component), and thus with low kinetic energy per unit
area of surface (Poesen, 1986). The raindrops
detached soil particles and, together with splash
transport, washed out rills, carrying the eroded soil
away from the talus and hampering crust formation
(Fig. 7). Nevertheless, the variability in soil loss and
runoff from the talus could not be ascribed to the
rainfall energy and erosivity alone, but involved the
cover of rock fragments exposed by the continued rill
and interrill erosion. It is well known that in
Mediterranean environments the rock-fragment cover
affects the intensity of various hydrological and soildegradation processes such as surface sealing,
infiltration, evaporation, runoff generation, runoff
energy, dissipation, and erosion by water (Brakensiek
and Rawls, 1994; Poesen and Levee, 1994; Van
Wesemael et al., 1996; Cerda, 2001). Under our study
conditions the rock cover intercepted large quantities
of rainfall (El Boushi and Davis, 1969) and absorbed
part of it, especially where the rock fragments were
weathered (Childs and Flint, 1990). Thus, the rock
fragments in the talus could retard ponding and
surface runoff.
In the erosion plots, our solid sediment collector
revealed large soil fragments, clearly signalling
terrace destruction over the time (Fig. 8). In addition,
in the orchard terraces with fruit trees—especially
avocado and custard apple, which have a greater
canopy and could cover the bare talus more
completely than could mango—the leaves tend to
channel and pool raindrops and thereby increase their
erosive force. When falling from 3 or 4 m onto the
unprotected talus (throughfall), these large drops
could splash soil particles many centimetres away. On
the other hand, the depth and extension of the root
system of the fruit trees influences terrace stability. In
this regard, Khan (1960) reported that 80–90% of
mango roots grow to a depth of 100 cm with a lateral
reach of 180 cm. However, under our conditions, in
which the soils of the terraces are shallow and the drip
irrigation encourages superficial roots, the weak
anchorage of the root system does not stabilize the
talus (Durán et al., 2003).
Although the rock fragments provide a measure of
protection for the bare soil, the absence of vegetative
cover significantly increased the chronic terrace
destruction, and thus such as cover could help retain
the soil on the talus. In this sense, Estalrich et al.
207
(1992a,b, 1997) and Durán et al. (2004a) pointed out
the importance of cover crops for minimising the
erosion rates on steep slopes.
Thus, to avoid the rill formation in the bare soil of
the taluses, control measures should be undertaken,
given the high costs of terrace reconstruction. Many
authors have reported the general benefits of vegetal
covers against the water erosion (Bochet et al., 1999;
Pardini et al., 2003; Martı́nez et al., 2001, 2002; Dunjó
et al., 2004). The challenge is to find and employ
conservation practices best adapted to the specific
situation of orchard terraces, in order to reduce the
impact of soil erosion on unprotected taluses. For
example, planting annual grasses (e.g. Lolium sp.) or
legumes (e.g. Trifolium sp.), rather than waiting for
natural colonization (Artemisia campestris, Convolvulus arvensis, Cynodon dactylon, Cyperus longus,
Amaranthus blitoides, Festuca granatensis, Agrostis
castellana, Dactylis sp., Bromus sp., etc.), which can
provide rapid control of the erosion on taluses. Our
previous experimental results indicated that growing
groundcover of aromatic scrubs (Thymus serpylloides,
Salvia officinalis, Rosmarinus officinalis, etc.) under
these conditions is one of the most effective means of
reducing runoff and erosion (Martı́nez et al., 1993,
2001, 2002; Durán et al., 2004b). Taluses covered by
thyme and sage registered 64 and 53% less sediment
production, respectively, than on the bare taluses,
while runoff diminished by 40 and 30%, respectively
(Durán et al., 2004a). In addition, the plant covers, by
decreasing runoff and soil loss, regulated the nutrient
flow at the same time as releasing nutrients from litter
in a biological cycle that is absent from bare soil (Smil,
1999; Kumar and Goh, 1999). Moreover, on bare soil
subjected to intensive cultivation, high application
rates of fertilizers (NPK) often results in the transport
of nutrients by sediments and runoff from agricultural
uplands, causing water and soil pollution (Guimera,
1991; Stumpf, 1991; Palis et al., 1997; Durán et al.,
2004a; Ramos and Martı́nez, 2004). Thus, erosion
control is a gain not only for the farmer but also for the
environment.
5. Conclusions
The main conclusion from this study is the
significant impact of rainfall on soil erosion and
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V.H. Durán Zuazo et al. / Agriculture, Ecosystems and Environment 107 (2005) 199–210
runoff at the talus level of orchard terraces, causing an
average soil loss and runoff of 9.1 Mg ha1 year1
and 100 mm year1, respectively. Raindrops detach
soil particles on the talus and cause splash transport,
washing out rills and carrying eroded soil away from
the talus. Once deposited at the foot of the talus,
eroded soil may plug the toe drain, exacerbating runoff
and causing re-transport of the sediments, especially
under heavy rain. Thus, on the basis of our
experimental results, we conclude that the planting
of groundcovers should be encouraged, particularly on
the fragile taluses of orchard terraces in order to
minimize the amount of soil loss by erosion, thus
avoiding slumping and promoting stability. Also, the
groundcovers should preferably be permanent, such as
aromatic scrubs, although certain annual plants could
be useful during periods with high erosion risk
(autumn and winter), under these conditions (214%
slope).
Terrace reconstruction can require nearly as much
labour and investment as the initial terrace, costs that
could be at least partially offset by the use of plant
covers on taluses, as recommended in the present
study. Finally, the potential pollution from fertilizers
on intensively farmed terraces can be reduced by
erosion control, providing benefits both for agriculture
and the environment.
Acknowledgements
This study was supported by a Postdoctoral
Fellowship Programme of the IFAPA, Consejerı́a de
Innovación, Ciencia y Empresas of Junta de Andalucı́a, Spain. Thanks are also due to the anonymous
reviewers for their comments on an earlier version of
this manuscript.
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