Lessons and experience of soil conservation in Spain
Artemi Cerdà
Departament de Geografia, Universitat de Valencia. Spain.
[email protected].
1. Environment and Agriculture
The survival and development of humankind has been closely related to agriculture
productivity. Most of the European soils were deforested, burnt and ploughed
thousand years ago in order to increase the food supply. During millenniums, the
increase of agriculture production was related to the crop surfaces (Ponting, 1991).
The maintenance of soil productivity was based on organic farming until the XVIII
century, and a long history of traditional management was accumulated. During the
XIX and the XX century farmers productivity increased due to mineral fertilization
application, without the enlargement of the farm surfaces. Soil management was
suddenly changed in order to exploit the soil fertility as the nutrients recharge was
easily found on chemical fertilizer. The use of powerful tractors, increasing the soil
mineralization, and herbicides and pesticides also allowed the farmers to increase the
productivity, but this also led to a decrease on flora and fauna diversity (McNeill,
2003). During the second half of the XX century, consciousnesses of the
environmental problems triggered by the modern agriculture were developed by
small groups of farmers. During the last decades, the environmental crisis -including
food quality crisis- became more evident. Some of the environmental problems are
summarized here:
1. Salinization due to the large scales irrigation systems developed in Eastern
Europe and Central Asia, but also in the Ebro valley and the coastal areas of
the Mediterranean Spain.
2. Soil pollution by excessive use of fertilizers and pesticides. Heavy metals,
organic pollutants and artificial radionuclides are also found on soils, where
chemical fertilization was used.
3. Soil acidification where the industrial emission is clearly related to the
decrease of the pH of the prone soil. The lack of organic matter increases the
soil acidification.
4. The current chemical agriculture induce that the soil becomes a source of
pollution, acidification and salinization instead to be a filter of the water and
air.
5. Soil degradation. The reduction of organic matter, continuous tillage, and the
development of crust show the lack of soil structure, aggregate stability and
soil porosity, which is related to the decrease of soil biological activity, and of
soil fertility. The conventional agriculture reduces as much as possible the
vegetation cover, increases the tillage frequency and avoids the use of
organic matter such as manure or compost. Then, the crust development and
soil compaction results in an increase in surface runoff and soil erosion.
This paper is focussed on the effect of modern agriculture on soil erosion in Spain,
and on the past and present soil conservation practices developed. A state-of-the-art
55
was done to know the soil on agriculture land uses. The data were collected after a
review of the data published by researchers, and the set of data is compared to the
soil erosion rates found under natural scrub, herbs or forest covered soils in order to
assess the effect of agriculture on soil erosion. Some examples of different
agriculture management are also shown. Traditional farming used many strategies to
reduce water and soil losses. Some of them are reviewed here. Conservation
agriculture and organic farming are new views of how to manage in a more friendly
environmental and sustainable agriculture.
2. Agriculture or agricultures?
The main objective of agriculture is to produce food and fibres. However, the way to
reach the objective is different between farmers. Then, there are at least four
agricultures:
1. The conventional or modern agriculture was developed on the north-western
European countries and is less than two centuries old. Most of the Mediterranean
countries did not applied the modern agriculture until the beginning of the XX
century, and in most of the Spanish rain fed agriculture areas, the chemical fertilizer
did not arrive until the 60’s. Conventional agriculture is based on the use of mineral
fertilizers, herbicides and pesticides, which results in the lowering of the soil organic
matter and fertility, reduced biodiversity, increased CO2 emissions, etc.
2. Conservation agriculture refers to several practices which permit the management
of the soils for agrarian uses, altering its composition, structure and natural
biodiversity as little as possible and defending it from degradation processes. Nontillage (direct sowing), minimum tillage, incorporation of crop residues and
establishment of cover crops in perennial woody crops or in between successive
annual crops, are some of the techniques which constitute conservation agriculture.
Usually, it is assumed that conservation agriculture includes any practices which
reduces changes or eliminates soil tillage and avoids residues burning to maintain
enough surface residues throughout the year. Then, farmers try to protect the soil
from rainfall erosion and water runoff, soil aggregates are stabilized, organic matter
and the fertility level naturally increases, and less surface soil compaction occurs.
Furthermore, the contamination of surface water and the emissions of CO2 to the
atmosphere are reduced, and biodiversity increases.
3. Traditional agricultural is based on risk reduction, the year round vegetative cover
of soil, diversity of crops, complexity as the natural systems (interaction between
plants, weeds pathogens, insects…..), high net energy yields due to the low inputs
and high degree of self-sufficiency. This system was applied for millenniums, and can
be classified into the following basic categories: (i) shifting cultivation, slash and
burn agriculture, (ii) nomadic pastoralists, (iii) continuous cultivation, and (iv) mixed
Subsistence Farming.
4. Organic farming (also called biological or ecological) is an agricultural production
system that minimizes the use of synthetically produced fertilizers, pesticides,
growth regulators, and livestock feed additives. To maintain soil productivity and
fertility and to control weeds and pests, organic farming relies primarily on crop
56
rotations, crop residues, animal manure, legumes green manure, and biological pest
control. Organic farming can be considered as a part of sustainability, and is an
ecological production management system that promotes and enhances biodiversity, biological cycles and soil biological activity.
Around the world, there is growing interest in finding alternatives to the industrial
farming methods that have emerged during the 20th century (Lampkin, 1991;
Brown, 2003). The deleterious effects of pesticides, inefficient fossil fuel usage,
chemical fertilizer inputs, genetic monocultures and factory farming of livestock have
become increasingly apparent. One approach is to build upon traditional methods
which evolved over the first 10,000 years of agriculture. These produced a
tremendous variety of domesticated crops and livestock, and systems of farming.
While some systems proved to be environmentally destructive, many were not and
were able to sustain diverse cultures for centuries. Unfortunately, within the last
generation, much of the know-how of traditional systems has been lost, especially in
the more industrialized countries. There is still a vast store of farming know-how in
many of the less developed countries. Researchers are beginning to appreciate that
many traditional farmers in the developing world are still practicing farming methods
that are in balance with the surrounding ecosystems; stable, sustainable and highly
efficient. Farmers, who have sometimes been portrayed as ignorant and not
adaptive, have actually been utilizing very sophisticated methods of agricultural
production for centuries. These farming systems can perhaps help the developed
world to grow food with fewer chemical inputs, slow erosion, control pests, decrease
our dependence on fossil fuels and feed an expanding global population.
3. The erosion rates under natural and agriculture conditions
Soil erosion rates under Spanish environmental conditions are very high due to the
climatic and cultural conditions. The Iberian Peninsula and especially South-eastern
Spain are characterized by very erodible soils, developed over soft rocks and under
an aggressive climate. But, probably the most important factor is the human activity,
which is at least 5,000 year old and has triggered soil and vegetation degradation
processes. Grazing, mining, burning, road construction, ploughing are some of the
negatives influences of the humankind. Nonetheless, the agriculture activities
developed the conditions to reach the highest erosion rates. Soils were burned and
ploughed; vegetation was degraded and sometimes eliminated.
Although,
agriculture lead to the highest erosion rates in the world, and the Iberian Peninsula is
affected by soil degradation and erosion processes little is known about the effect of
the agriculture activities on soil erosion.
57
Author
Year
Erosion rate
Plot size
(Mg ha-1 year-1)
(m x m)
Soil conditions
Agriculture
Cuadros et al.,
1993
10.88
6 x 24
Almonds plough
García-Ruiz
1996
10.00
3 x 10
Artica (shifting agriculture)
García-Ruiz
1996
5.20
3 x 10
Barley
García-Ruiz
1996
15.50
3 x 10
Fallow
Rodríguez Mart.-Conde
1996
9.69
5x5
Ploughed
Rodríguez Mart.-Conde
1996
13.42
5x5
Ploughed
Bienes & Torcal
1997
4.31
4 x 20
Bare
De Alba
1998
7.30
10 x 25
Ploughed
Padrón et al.,
1998
28.50
8 x 25
Ploughed
Rodríguez Mart-Conde
1998
17.04
5x5
Ploughed
Rodríguez Mart-Conde
1998
18.00
5x5
Ploughed
Rodríguez Mart-Conde
1998
17.70
5x5
Ploughed
Martínez Raya et al.,
2001
5.17
6 x 24
Legums
Non-Agriculture
Martínez Raya et al.,
2001
0.38
6 x 24
Scrubland
García-Ruiz
1996
1.10
3 x 10
Scrubland
Bienes & Torcal
1997
0.05
4 x 20
Scrubland
Bochet et al.,
1998
0.04
< 1 m2
Rosmarinus sp.
Andreu et al.,
1998
0.01-0.4
40 x 8
Scrubland
Padrón et al.,
1998
0.10
8 x 25
Natural vegetation
Pinus radiata
Padrón et al.,
1998
0.00
8 x 25
Bienes et al.,
2000
0.0002-0.15
4 x 20
Scrubland
Andreu et al.,
2001
0.14
25
Fire affected woodland
Cantón et al.,
2001
0.02
0,24
Scrubland
López Bermúdez
1996
0.09
10 (8) x 2
Scrubland
Puigdefábregas et al..,
1999
0.16
2x8
Scrubland
Table 1. Erosion rates measured by means of closed plots of different sizes on agriculture
fields and on non-agriculture (forest, scrubland and grasslands) slope plots.
The expected high erosion rates under the Spanish environmental conditions were
confirmed by the measurements done in the field. Some data collected on closed
plots under agriculture and natural conditions shows that erosion rates were much
lower under natural conditions (Table 1). Data collected by Cuadros et al., (1993), de
Alba, (1994; 1998), García-Ruiz (1996), Rodríguez Martínez-Conde (1996; 1998),
Bienes and Torcal (1997), Padrón et al., (1998), Bienes et al., (2000), Martínez Raya
et al., (2001), also demonstrated that erosion rates were much higher under
cultivation than under natural vegetation cover. The studies of López Bermúdez et
al., (1996), Puigdefábregas et al., (1996), Bochet et al., (1998), Andreu et al.,
(1998) and Cantón et al., (2001) on scrubland, grasslands, or even under fire
affected woodlands demonstrated that the erosion rates can be very low in Spain.
Erosion rates lower than 0.4 Mg ha-1 year-1, and even lower than 0.1 Mg ha-1 year-1
were measured by different researchers on a wide range of plot sizes (Table 1). The
comparison of the data collected on agriculture fields and the nearby scrubland,
grasslands or forest show that cultivation can result in 10 times more soil loss
(García Ruiz, 1996), at least 2 times greater (Bienes et al., 2000), 13 times greater
(Martínez Raya et al., 2001) and infinitive much higher on ploughed land (28.5 Mg
ha-1 year-1) than on Pinus radiata cover (0 Mg ha-1 year-1) (Padrón et al., 1998). The
58
data reviewed shows that the fallow land is the worse management because it
produces 3 times more erosion than the barley fields under the Pyrenees
environmental conditions (García Ruiz, 1996) and 5 times more soil loss than on the
cereal plough fields of the Central Spain (de Alba, 1994). Sometimes, the crop
determines the erosion rates, such as Martínez Raya et al. (2001) found on legumes
(5.17 Mg ha-1 year-1) and wheat fields (1.65 Mg ha-1 year-1).
Figure 1. Soil erosion in an Almond orchard after 45 mm of rainfall during one hour. Valencia.
4. Agriculture and erosion. A state-of-the-art
The objective of this section is to know the erosion rates under agriculture land. The
data was collected from international, national, regional and local publications.
Scientists expected to find an extensive data-base of soil erosion in Spain, mainly
focused on soil erosion under agriculture conditions. But, data is scarce, difficult to
find due to local publications, focuses on natural vegetated covered soils and with
few years of measurements. Data was collected during the last two decades, and 34
papers were found until 2002, which is a very poor balance. Moreover, sometimes
different papers used the same data set. In fact, less than ten scientifically sound
researches can be highlighted in Spain related to the erosion processes on
agriculture land. Most of the research carried out was conducted on plots (18
publications), simulated rainfall (9 publications), Gerlach traps (3 publications) and
topographical measurements (3 publications)
The research was carried out mainly by Spanish researchers, and in sites
located nearby the Universities or the research centre of the Spanish Council for
Scientific Research (CSIC, Consejo Superior de Investigaciones Científicas). We
should highlight the research groups of the Universidad de Murcia (López Bermúdez,
Romero, Alonso, Belmonte, etc) and CEBAS-CSIC in Murcia (Albaladejo, Castillo,
Martínez-Mena) which have a wide experience on the studies of soil erosion under
semi-arid climatic conditions (Albadalejo et al., 1991; López Bermúdez et al., 1991).
The García-Ruiz (1996) research group in Zaragoza focused mainly on the Pyrenees
and is a well-known research group (see also the publications of Lasanta and RuizFlaño). Other research groups developed the research on the olive fields (Rodero et
59
al., 2000), almond (Cuadros et al., 1993) or legumes-cereal fields (Martínez Raya et
al., 2001).
Other groups are Saturnino de Alba in the Estación Experimental from the
Centro de Ciencias Medioambientales in Toledo (de Alba et al., 1994), Bienes et al.,
(2000) in the experimental research stations of Marchamalo and el Encín in
Guadalajara, and Rodríguez-Martínez Conde (1998) in Galicia. Much less research
was done on the erosion control strategies; however we should highlight the work of
Albaladejo et al., (2000) in Murcia and Ingelmo et al., (1999) in Valencia.
Some of the research carried out on the agriculture effects on soil erosion in Spain is
coming from the studies on the erosional effects of land abandonment during the
second half of the twentieth century. Some examples are the works carried out by
Rodríguez et al., (1991), Cerdà (1997), Molina and Rubio (1998), Lasanta et al.,
(2000), Francis (1986) o Ries et al., (2000).
The importance of the soil erosion problem within the rain fed agricultural
land of Spain is due to the: (i) soil management has have a main objective the
removal of the vegetal cover in order to avoid the water competence from weeds or
other plant cover. As a consequence the soil erosion is high. (ii) The agriculture land
in Spain is mainly devoted to rain fed crops. From 18,515,000 ha of agricultural land,
15,150,000 ha are rain fed crops. During millenniums, large parts of the Spanish
territory had a very low vegetation cover due to the ploughing effect. Recently, also
herbicides are responsible of the bare and crusted soils which favoured the formation
of runoff and accelerated the soil losses. Conservation agriculture is only applied in
some areas were the farmers are applying herbicides during one season. Then,
usually the soils were intensively ploughed, the weeds were eliminated and the
vegetation cover reduced to the crop. Some of the crops usually are caducifolius
(vineyards, almonds) or they growth for 3-5 months (sunflower, barley, wheat), and
as a consequence the soil rest bare and/or ploughed for, at least, half a year.
The data reviewed show the results of the most relevant research carried out
in Spain on soil water erosion on agriculture fields during the last 30 years. The
research done is still poor, such is indicated by the number of publications related to
soil erosion on agriculture fields; around 1 per year. Moreover, the researches,
sometimes, show preliminary research without any definitive conclusion. However,
there are some relevant works that we should review because they will give us some
explanations of the soil erosion problem on the agriculture land in Spain.
Period
10/9509/97
Erosion rate
(Mg ha-1 year-1)
Author
Year
Region
Casalí et al.,
1997
De Alba et al.,
1998
Navarra
Castilla La
Mancha
23/08/1995
39.00*
Mart.-Casasnovas et al.,
2001
Cataluña
06/2000
207*
13.30
Table 2. Erosion rates measured by means of topographical measurements.* one event.
4.1 Topographical measurements
The data generated by means of topographical measurements such as erosion pins
or Digital Elevation Models, always show high erosion rates. This is due to his
application on areas of extremely high erosion rates or after high magnitude events.
An example is shown by the work of Casalí et al. (1997), Navarra, where they found
60
high erosion rates on gullies developed on agriculture land (13.3 Mg ha-1 year-1).
These values seem to be low for a gullied area although high intensity rainfall events
were not measured. This is probably due to the high dependence of the erosion
processes under Mediterranean conditions of the high magnitude events. When
intense thunderstorms take place the erosion rates reach extremely high rainfall
intensities and volumes. Data such as was presented by De Alba (1998) in Toledo
and Martínez Casasnovas et al., (2001) in Lérida show the highest erosion rates
during one event. Martínez Casasnovas et al., (2001) have shown that a rainfall
event of 205 mm during 135 minutes removed 207 Mg ha-1. This is the erosion rate
estimated for ten years period at the study area (Table 2).
4.2 Plots
The Gerlach collectors (no bounded plots) were used mainly on vegetated soils, but
some authors also applied this methodology to agriculture fields in order to compare
with the non-cultivated areas. The soil erosion rates measured in Murcia by Belmonte
et al., (1999) and previously by López Bermúdez (1989) inform that the erosion
rates were below 1 Mg ha-1 year-1, although these values are related to the time
from the abandonment. The erosion rates measured in Murcia reached values close
to 3 Mg ha-1 year-1 after land abandonment (Table 3). García Ruiz et al. (1994) paid
attention to the importance of the soil erosion rates due to the piping on irrigated
areas, where the removal of sediments is done as solute and suspended sediment
yield.
Author
Year
Region
Period
Erosion rate
(Mg ha-1 year-1)
López Bermudez
1989
Murcia
1986
1.81
López Bermudez
1989
Murcia
1986
3.18
López Bermudez
1989
Murcia
1986
2.71
López Bermudez
1989
Murcia
1986
1.78
García Ruiz et al.,
1994
La Rioja
1993
3.00
Romero et al.,
1995
Murcia
04/1993-12/1993
0.35
Table 3. Erosion rates measured with Gerlach traps on agricultura fields.
The most successful method to measure erosion rates on agriculture land in Spain,
such as in other countries, was the bounded plots, with a Gerlach collector or an
automatic recorder. This is because they are easily built on bare and recently
ploughed soils, and also because the influence of the Soil Conservation Service
research done in USA for the Universal Soil Loss Equation. They got success also
because the author can easily measure the size of the plots, although the borders
disturb the natural drainage network and disturb the soil. Moreover, the sediment
exhaustion can results in an artificial reduction of the erosion rates.
Land use
Runoff coefficient
(%)
Sediment concentration
(g l -1)
Erosion rate
(Mg ha-1 year-1)
10.00
Artica
3.9
0.7
Cereal fertilizado
3.1
0.4
5.20
Barbecho
4.5
1.6
15.50
Matorral denso
1.4
0.2
1.10
Table 4. Erosion rates measured on 30 m2 plots in the Aísa experimental station in the Central
Spanish Pyrenees, 1200 mm of mean annual rainfall (García Ruiz, 1996).
61
The research developed by the Pyrenean Institute of Ecology from the Spanish
Council for Scientific Research (García Ruiz, 1996) on the Pyrenees, demonstrated
that the abandoned fields under the wet conditions (> 1000 mm y-1) where the
vegetation cover is easily recovered show very low erosion rates, 5 times lower than
the cereal farming, 10 times lower than the traditional shifting-burning agriculture
called Artica, and 15 time lower the fallow land. The pioneer works of Lasanta and
Sobrón (1984), demonstrate that the erosion rates under cultivation can be very low
(< 1 Mg ha-1 year-1) in La Rioja vineyards, although the experimental period was
shorter than one year and rainfall intensities and volumes were very low. The short
period of measurements is sometimes the highest handicap of the research done
under Mediterranean conditions, due to the overestimation of the values if the high
magnitude-low frequency events take place or they underestimate the erosion rates
(Table 3).
Land use
Runoff coefficient
(%)
Erosion rate
(Mg ha-1 year-1)
Barley
25.3
1.04
Tillage
31.4
1.84
Shrubs
17.0
0.05
Table 5. Erosion rates measured on 20 m2 plots in Murcia, 300 mm of mean annual rainfall
(López Bermúdez et al., 1991).
On south-eastern Spain, López Bermúdez et al. (1991), demonstrated that
the ploughed and fallow land show erosion rates of 1.84 Mg ha-1 year-1, meanwhile
they were lower when cereal covered the soil (barley, 1.04 Mg ha-1 year-1), and
much lower when soil was covered by Matorral (005 Mg ha-1 year-1). The vegetation
cover is shown as the main factor to explain the erosion rates under rainfed
agriculture fields of Spain. This is why after the land abandonment and the recover
of vegetation the erosion rates are reduced (Francis, 1990; Ruiz Flaño, 1993). This
positive effect of vegetation was checked by Andreu et al. (1994) on plots of 320 m2,
on slopes on different vegetation covers.
Land use
Runoff coefficient
(%)
Erosion rate
(Mg ha-1 year-1)
10.88
Tillage
3.9
No-Tillage
6.1
2.94
Shrubs
1.4
1.11
Table 6. Erosion rates on almonds orchards measured on 144 m2 plots in Andalucía, 831 mm
of mean annual rainfall during the two year study period (Cuadros et al., 1993).
In Andalucía, Cuadros et al. (1993) found that the Almond orchard have a soil
loss of 10.88 Mg ha-1 year-1 under the traditional ploughing, meanwhile under the
zero tillage is reduced to 2.94 Mg ha-1 year-1. In Castilla-La Mancha, cereal show a
erosion rate of 2.39 Mg ha-1 year-1 under fallow land, meanwhile the cereal under
ploughing the erosion rates were reduced to 0.48 Mg ha-1 year-1 and 0.67 Mg ha-1
year-1 with no-tillage and direct sowing (de Alba et al., 1994). Under abandoned
fields the erosion rates were lower (0.17 Mg ha-1 year-1) due to the recovery of the
natural vegetation. Similar results were found by Bienes and Torcal (1997) where the
bare soil reached 4.31 Mg ha-1 year-1, barley 0.06 Mg ha-1 year-1 and on soils with
natural vegetation cover the erosion rates were 0.5 Mg ha-1 year-1.
62
Erosion rate
(Mg ha-1 year-1)
Land use
No-tillage
0.69
Tillage
0.48
Fallow
2.39
Abandoned
0.17
Table 7. Erosion rates on Cereal field in Central Spain (Toledo), (de Alba et al., 1994).
The available data on Spain demonstrate the ploughed land have the greater erosion
rates, which will reduce the soil fertility. Rodríguez Martínez-Conde (1996; 1998)
measured in Galicia very high erosion rates, with 17.58 Mg ha-1 year-1 in 1996 and
1581 Mg ha-1 year-1 in 1997. Although the data obtained until now inform that the
zero-tillage can be the best solution to the problem of erosion in the Spanish
agriculture land some research studies are contradictory. Martínez-Raya et al. (2001)
found on the Andalucía olive orchard that the no-tillage triggered the erosion rates
until 28.03 Mg ha-1 year-1, meanwhile the traditional farming reduce the erosion
rates to 9.08 Mg ha-1 year-1 and the soil with vegetation cover were reduced to 1.56
Mg ha-1 year-1. However, usually the researchers found that the vegetation cover is
the most efficient soil protection. Bienes et al. (2000) measured in Aragón erosion
rates on ploughed fields of 4.5 Mg ha-1 year-1, which were reduced to 1.7 Mg ha-1
year-1 on cereal orchards and 1.5 Mg ha-1 year-1 were the straw is not removed.
Soil erosion rates under agriculture fields are extremely variable as a
consequence of different management. Belmonte et al. (1999) measured rates of
19.5 Mg ha-1 year-1 under abandoned fields affected by rilling and 0 Mg ha-1 year-1 on
cereal covered fields (Table 8). This confirms the contradictory results of different
data sets, because under abandoned conditions soil stability and reduction of the
erosion rates are usually found.
Land management
Runoff coefficient
Sediment concentration
Erosion rates
(%)
(g l -1)
(Mg ha-1 year-1)
1.0
Control
3.9
8.5
Cereal
5.7
20.8
2.1
Rilling
10.4
80.5
15.6
Ploughing
7.9
16.9
2.8
Table 8. Erosion rates measured on 16 m2 plots (1995-1997) under different land uses and
managements in Murcia (300 mm h-1) (Belmonte et al., 1999a).
Other strategies to recover the soil and avoid the soil erosion processes can be found
when Urban Solid Refuse are applied to cover the soil and increase the organic
matter (Albadalejo et al., 1991; 2000), and also sewage sludge (Ingelmo et al.,
1999). Both research studies demonstrated that soil erosion is reduced when the
doses are high enough.
Solid Urban Refuse
Erosion rates
(Mg ha-1 year-1)
0 Mg ha-1
4.26
65 Mg ha-1
0.83
130 Mg ha-1
0.13
195 Mg ha-1
0.08
260 Mg ha-1
0
Table 9. Erosion rates measured on 75 m2 plots (1989-1990) under different Solid Urban
Refuse addition in Murcia (300 mm h-1) (Albaladejo et al., 2000b).
63
Due to the lack of information, there are some controversial results on soil erosion
under agriculture fields. An example is the study of Giráldez et al. (1989), using the
Universal Soil Loss Equation (Wishmeier and Smith, 1958), which show the soil
losses for different crops under conventional and conservation agriculture. It is
surprising that always the conservation agriculture results in lower erosion rates
except on olive orchards (table 10). Francia et al. (2000) also found that the modern
ploughing (5.2 Mg ha-1 year-1) increase the soil erosion rates in comparison to the
traditional ploughing (1.3 Mg ha-1 year-1) and the vegetation cover (0.41 Mg ha-1
year-1).
Tillage
Non-Tillage
Wheat
58.5
4.94
Sunflower
70.5
7.35
Sugar beet
58.9
38.3
Beans
46.3
7.34
Olive
68.0
76.5
Table 10. Estimated soil erosion rates (Mg ha-1 year-1) by means of the Universal Soil Loss
Equation (USLE) between 1953-1987 in Córdoba distinguishing between five crops under
tillage and non-tillage strategy (Giráldez et al., 1989).
The comparison between plots, study areas, and erosive events is difficult under the
high spatial and temporal variability of the Mediterranean rainfall and soil
characteristics. Rainfall simulation experiments can solve some of the problems due
to the controlled conditions of the rainfall. Differences between rainfall simulators,
previous soil conditions and duration of the experiments avoid comparison amongst
rainfall simulators (Cerdà, 1999). The pioneer work of Francis (1986) demonstrated
that the erosion rates were high in the abandoned fields of Murcia. The rainfall
simulator of Cerdà et al., (1997) has been used by many authors in Spain and
comparison is suitable amongst experiments. Rodríguez et al. (1991) found low
erosion rates on the abandoned fields of northern Alicante. Arnáez et al. (1996)
found higher erosion rates on the abandoned fields from the Pyrenees, and Cerdà
(1997 a and b) demonstrated that vegetation cover recovery is the key factor to
avoid the soil loss after land abandonment.
Molina and Rubio (1998) measured in Valencia high erosion rates under
crusted soils; meanwhile on ploughed and abandoned soil the erosion rates were
lower than 1 Mg ha-1 h-1. Lasanta et al. (2000) found on the Ebro valley soil erosion
rates under different cover always lower than 1 Mg ha-1 h-1, with rainfall intensities of
50 mm h-1. Ries et al. (2000), under the semi-arid conditions of Aragón and Rodero
et al. (2000) in the Olive orchards of Andalusia, measured very low erosion rates
under rainfall thunderstorms of 10-year return period.
Little measurements were done at watershed scale related to the effect of
agriculture, although the available data always show greater sediment yield where
agriculture takes place (Marqués and Roca, 1985; Marqués, 1991).
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4.3 Implications and discussion
The available data demonstrates that the erosion rates are much higher when
high magnitude events are measured. This is why the research stations should be
maintained for long periods. The work of Martínez Casasnovas et al. (2001) is the
best example of the concentration of soil losses during few hours. Sometimes, the
substantial differences between two years of measurements (0.8-24 Mg ha-1 year-1)
are due to the torrential rainfall of one autumn thunderstorm (Marqués, 1991).
The comparison of the data generated on soil erosion in Spain is difficult due
to the different sizes of the plots used to measure the soil losses. The plot sizes
ranged from 3 to 320 m2 on closed plots, which make the comparison amongst the
data from different researches very difficult. This is because the erosion processes
are highly dependent on the scale. Future agreement on applied methods is required
to improve the knowledge on soil erosion processes in Spain. This should be
encouraged by the researchers and the funding institutions such as the RESEL (Red
de Estaciones Experimentales para el Estudio de la Erosión y la Desertificación; Rojo
and Sánchez, 1997) as previous work presented (López Bermúdez et al., 1993). The
use of different plot sizes is important to study the runoff generation processes and
sediment productions within a watershed (Ceballos, 1997). However, the comparison
of the runoff and erosion rates between sites is avoided by the different sizes of the
plots.
The volume of published data until now is too scarce to show definitive
conclusions. However, the investment on research by local, regional, national and
international institutions resulted on many interim reports and data-bases but were
not published. As a consequence, farmers and policy-makers know little about the
effect of agriculture management on soil losses. As an example we can say that now
many institutions promoted the abandonment of land without any restoration or
planning (Lasanta et al., 2000), and moreover, the Common Agriculture Policy from
the European Union promote the fallow land, which has been demonstrated as the
management that provoke the highest erosion rates (de Alba, 1998).
Figure 2. Soil erosion in an Almond orchard in South-eastern Spain (Murcia) after 30 mm of
rainfall during one hour.
65
The comparison of different data amongst zones and climates do not demonstrated
that under agriculture conditions the semiarid areas of Spain have higher erosion
rates than the humid Spain. In fact, the highest erosion rates found in Spain on
cultivated fields were found in Galícia (13-19 Mg ha-1 year-1) after Rodríguez
Martínez-Conde and in the Pyrenees (5-15 Mg ha-1 year-1) after García Ruiz (1996).
However, on the fields from the arid areas of south-eastern Spain the erosion rates
measured were lower, sometimes lower than 1 Mg ha-1 year-1. Only in some soil
managements, such as the almonds (Cuadros et al., 1993), or abandoned field
affected by rilling (Belmonte et al., 1999) are found erosion rates higher than 10 Mg
ha-1 year-1. Average values, and under agriculture fields the measurements taken on
the humid areas of Spain show higher erosion rates than the semiarid areas. Then,
the myth that the semiarid Spain has the highest erosion rates is not confirmed
under agriculture land.
Figure 3. Conservation agriculture at La Costera, Valencia. The vegetation cover reduces the
soil losses and improves the soil quality.
5. Erosion control on agriculture land. A Challenge for the future
The soil erosion rates under agriculture land are not sustainable due to the
low soil recovery. The usually high soil losses measured in Spain are a consequence
of human activity, climate conditions and vegetation cover. Farmers usually reduce
the vegetation as much as possible to avoid crops water competition, remove the
rock fragments to easily plough the land, tillage is more intense, herbicides are
widely used, etc.
The modern agriculture is characterized by a high income of fertilizer,
mechanization, pesticides and herbicides. However, in Spain, until the 50-60’s the
agriculture was in many places a traditional agriculture, where fertilization depends
on the manure. Herbicide and pesticides did not exist and tillage was done by mules
and horses. This traditional agriculture developed some strategies to avoid high
soil losses:
1. Most of the cultivation was located on the bottom valleys of the mountains,
where sediments and water were collected. In fact, some fields were built to trap
sediment and water to develop a most fertile soil on the bottom of the valley.
66
2. Agriculture recycled the organic waste, even from the cities. This is the reason
of the high organic content of some agriculture soils under semi-arid conditions,
were irrigation and manure resulted on fertile soils.
3. Ploughing was superficial due to the low power traction of horses or mules.
Bulls were also used on deep soils and were deeper ploughing was necessary. Soil
were much less disturbed than now.
4. Strategies against erosion were developed on sloping areas where soils are
very fragile:
a) Terraces were built everywhere to collect sediments and water. The area
affected by terraces construction is located mainly on the Mediterranean coast,
where mountainous areas can be found. Usually the terraces were built with stones
from the fields or carried by mules and donkeys from nearby. Right now, terraces
are usually abandoned and the break down of the walls can increase the sediment
available on the slopes.
b) On marl steep slopes terraces were built sometimes without a stone-wall,
but grass, bushes or trees as a hedge were allowed to grow in order to stop the
sediment transfer between terraces. The use of herbicides resulted during the last
two decades in the degradation of the terraces border and hedges, which also were
a natural biodiversity resource.
c) Drainage systems. Sometimes, the collection of runoff on a field can be
dangerous because it can overflow the terrace and destroys the wall and the
terrace systems. This is because the terraces have slight contrary slopes to
concentrate the ponding and runoff far away from the wall. The runoff is collected
in a drainage system that avoids the destruction of the terraces and the removal of
the sediments. This drainage organization was developed to avoid terraces to
collapse but also to favour the development of ponds to collect water and
sediments. Under these circumstances, terraces have some ponds were sediments
are collected. Farmers emptied the ponds in order to increase the storage capacity,
and favour the deep infiltration of water. And later the sediments were spread on
the soils.
d) Mixed trees and cereal farming. This favours a reduction on the bare soil on
the plots, and as a consequence reduces the runoff and erosion rates. Rotation
system was also used to reduce the soil losses by runoff and erosion.
Figure 4. Plots from the El Teularet-Serra de Enguera Soil Erosion Experimental Station
devoted to study the effect of land management on soil degradation and erosion.
67
5. Irrigation systems to collect water as well as sediments. This was usual nearby
the rivers, and has resulted on the enrichment of agriculture soil on the alluvial
plains, but also on alluvial fans and terraces.
The present agriculture also developed some strategies in order to reduce the
erosion rates and improve the soil quality. Some of them are:
1. Conservation agriculture. Under semi-arid conditions, the weeds should be
removed because they can reduce the water available for the crops. The use of
herbicides strategically allows the use of weeds or green manure during part of the
year.
2. Organic farming strategies are diverse and developed to avoid soil erosion and
degradation. Mulching by straw, triturated pruned branched and weeds, manure and
green manure incorporation, are currently been introduced in Spain. These practices
are being widespread due to the subsidence of the European Union, but they still
cover an area lower than 4 % of the agriculture land of Spain.
3. Solid urban refuse and sewage sludge as an organic amendment that reduced
the erosion rates is being used in agriculture land.
4. The improvements on land management on agriculture land in Spain will be
advised by the Scientific Research. The studies on soil water regimes, runoff,
erosion, soil quality, vegetation cover, etc., will shed light on the most economical
and environmental sustainable land management. An example is the work developed
by the Department of Geography at the University of Valencia on the El TeularetSierra de Enguera Soil Erosion Experimental Station by means of 52 plots (13 x 4
subplots of 1, 2, 4 and 16 m2), where different land uses and land managements are
monitored in order to know the soil and water losses.
5. The use of Agro-textiles also will reduce the soil losses and will avoid the use
of herbicides.
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