Erosion rates in Alicante and Murcia (Spain). An overview of results

Erosion rates in Alicante and Murcia (Spain). An overview of
results, methods and scales of study of the last two decades
Carolina Boix-Fayos1, Martínez-Mena, M.1, Calvo-Cases, A.2, Castillo, V.1
1
Centro de Edafología y Biología Aplicada del Segura, CSIC, Murcia, Spain.
Departamento de Geografía Universidad de Valencia, Valencia, Spain.
CarolinaBoix@hotmail.com
2
1 Introduction
Land degradation involves how one or more land resources (soil, water, vegetation,
rocks, air, climate, relief) have changed for the worse (Stocking and Murnaghan,
2001). Soil erosion has been classified as one of the types of soil degradation, within
a wider context of land degradation. The interest of the international community to
study soil erosion and degradation processes is linked to the interest of learning
more about desert ecosystems held between 1950 and 1970 by the UNESCO
(Kassas, 1999). It is also linked to the use of the “desertification” concept, seen as
an environmental hazard, in 1974 by the UN General Assembly that culminated in
the Conference on Desertification held in Nairobi in 1977.
The characteristics of the climate, lithology and land use history of
Mediterranean ecosystems, in particular those on subhumid and semiarid zones,
make these areas very vulnerable to soil erosion by water, which implies the removal
of the fertile soil layer. In Spain this was translated in an interest of quantifying
erosion rates and understanding the processes involved in soil erosion and
degradation already since the end of the 70s. Since the 80s, the studies on soil
erosion carried out in Spain and particularly in the Murcia and Alicante provinces,
have been useful to achieve (i) a wide set of relative values on erosion rates at
different scales and (ii) a progress in understanding soil hydrological behaviour, soil
erosion mechanics and other geomorphological processes related to soil erosion and
degradation.
An intensive work on this subject has been carried out in Alicante and Murcia
in the last twenty years using a variety of methods and scales of work by different
research groups. An overview of research carried out in those regions is provided by
Cerdà (2001) for the work done on water erosion in Alicante (and the rest of the
Valencia region), by Calvo et al. (2003) for erosion data obtained on limestones in
different parts of the Mediterranean basin, and by Romero Díaz (2002) in an
extensive review of erosion research done for the province of Murcia. Most of this
research is based on field experiments under natural or simulated rainfall, in some
cases with long periods of observation, but also big efforts have been dedicated to
the development and application of physical, statistical and parametrical models.
Here a synthesized overview of both achievements erosion rates and the
functioning of erosion processes studied during the last twenty years is presented.
2 Different approaches to estimate erosion rates
From the variety of methods used, the available erosion rates published can be
classified in 3 big groups according to the methods used (Table 1). All the methods
have been applied at different scales; plots of various sizes have been used for
41
experiments under natural and simulated rain. Models have been also applied at the
catchment and the plot scale.
Table 1. General classification of methods used to study erosion processes in the
provinces of Alicante and Murcia
Field data/rain types
Rainfall simulation experiments
Experiments/data under natural rain
Model estimation
Scale of study
Micro plots (0.24 - 2 m2)
Meso plots (5 – 20 m2)
Closed plots (15-328 m2)
Open plots (1-2 m2)
Catchments (0.06-130 km2)
Check-dams
Reservoirs
Types of models
Parametrical models
Statistical models
Physical models
Erosion rates on different lithologies and estimated using different methodologies in
both Alicante and Murcia provinces are summarised in Figure 1. It is important to
realize that with the methodology, the spatial and temporal scales of study vary.
2.1 Field data
As it can be seen in Figure 1 field data on erosion rates are collected in different
ways, resulting in different erosion rate estimates. Erosion rates derived from rainfall
simulation experiments carried out in microplots (0,24 m2) with approximately 55
mmh-1 of rainfall intensity on limestones vary from 0 to 6.61 gm-2h-1 in Alicante
under dry soil conditions and from 1.86 to 47.28 g m-2h-1 with antecedent soil
moisture close to field capacity (Boix-Fayos et al., 1998, 1999). The erosion rates
found in Murcia with a rainfall intensity varying from 31 to 56 mmh-1 on bigger plots
(2 m2) located on Quaternary colluvium show higher average erosion rates (42.2 g
m-2h-1 with antecedent soil moisture always above 0.15 cm3cm-3) (Martínez-Mena
et al., 2002). The simulation experiments done on marls in Alicante produced
average erosion rates between 0.04 and 23.73 g m-2h-1 under dry soil conditions
(Cerdà, 2001) and in Murcia an average of 80.9 g m-2h-1 with antecedent soil
moisture ranging from 0.15 to 0.22 cm3cm-3 (Martínez-Mena et al., 2002).
The collection of data during several years in closed or open plots under
natural rain conditions allows estimating erosion rates per year. Rates between 0 and
8 tnha-1yr-1 in closed plots have been found in Alicante on marls (Bautista, 1996,
1999). In Murcia between 0.012 and 1.84 tnha-1yr-1 on limestones (Castillo et al.,
1997, Romero Díaz et al., 1998, Romero Díaz and Belmonte, 2000); between 0.006
and 2.4 tnha-1yr-1 on marls (Francis, 1986, Albaladejo et al., 1991); 2 tnha-1yr-1
on marls and sandstones (Romero Díaz et al., 1988).
In open plots under natural conditions two plots (328 and 759 m2) with a mixture
lithology of limestone, dolomites, marls and alluvial deposits in Murcia were
monitored during several years, registering soil losses of 0.85 and 2.99 tnha-1yr-1
(Martínez-Mena et al., 2001).
42
100
Erosion rates gm-2h-1
90
Simulated rainfall
80
70
60
50
40
30
20
10
0
1
3
5 7 9
Alicante
11 13 15 17 19 21 23Murcia
25 27 29 31
33 35 37 39 Murcia
41 43
Alicante
Marls
Limestones
25
Natural rainfall
Erosion rates tnh-1yr-1
20
15
10
5
0
1
Alicante
5 9
Murcia
Murcia
13 Murcia
17 21 25 29 33 Alicante
37 41 45 49 53 57 61
65Alicante
69
Closed plots
Open plots
Reservoirs
Figure 1. Erosion rates from studies done in the areas of Alicante and Murcia since
the late 80’s by different research groups. Different methodologies are used,
classified as rainfall simulation experiments (above) and data collected at different
scales under natural rain (below). (The data presented are derived from Bautista et
al., 1996, 1999; Boix et al., 1994, 1998; Bouma and Imeson, 2000; Calvo-Cases et
al., 1991, 2003; Castillo et al., 1997; Cerdà et al., 1995, Cerdà and Navarro, 1997;
Cerdà, 1993, 2001; Imeson et al., 1998; Martínez-Mena et al., 2001, 2002; Llovet et
al., 1994; López-Bermúdez et al., 1998; Verstraeten et al., 2003; Sánchez et al.,
1994).
43
Also in open plots located on limestone in Alicante the erosion rates varied from 0.2
to 1.47 in most of the plots with two exceptions, two plots which a high sediment
yield, 4.74 and 14.30 tnha-1yr-1, respectively (Calvo-Cases et al., 2003).
Several catchments: Rambla Salada (Murcia) (López Bermúdez et al., 2000), El
Picarcho (Murcia) (Castillo et al., 2000) and Cocoll (Calvo-Cases et al., 2002) are
monitored and their hydrological and erosional response is being studied at the
moment.
López Bermúdez and Gutiérrez Escudero (1982) reported 8.8 tnha-1yr-1 of
erosion rate derived from the study of the sedimentation in 9 reservoirs of the
Segura river (Murcia). Data reported by Avendaño Salas et al. (1997) show an
average of 3.32 tnha-1yr-1 for sedimentation rates in reservoirs of Murcia and
Alicante, with the exception of the Guadalest reservoir (Alicante) with a much higher
sediment accumulation rate (27.03 tnha-1yr-1) . Summarising, erosion rates
between 1.2 and 10.7 tnha-1yr-1 (Cenajo reservoir, Murcia) using bathymetric
methods are reported by different authors as pointed out by Romero Díaz (2002).
Lately some work on the estimation of erosion by means of the sedimentation behind
check-dams is being developed by Castillo Sánchez et al. (2002).
The erosion rates estimated in plots under natural rain are approximately
within the same range as the ones reported from the reservoirs. However the results
derived from rainfall simulation experiments must be used as relative values to
compare between different soil and rain conditions, they vary also very much
depending on the antecedent soil moisture conditions. The extrapolation of the
erosion rates derived from rainfall simulation experiments to higher spatial and
temporal scales is always difficult and not realistic. They simulate one event but it is
difficult to predict how many events are likely to happen in a certain period.
2.2 Estimations based on models
Models of different type have been developed and calibrated in the context of the
research on soil erosion done in Murcia. Among the models that have been applied
are the statistical model Fournier (Martínez Fernández, 1986; Conesa García, 1989),
physical models as SLEMSA (Albaladejo and Stocking, 1989), EUROSEM (Albaladejo
et al., 1994), GAMES (Conesa García, 1989) and SHETRAN (Bathurst et al., 1996).
The vegetation submodel of MEDRUSH (Kirkby, 2002) has been tested using data
from different Mediterranean areas, including Murcia with successful results.
Yet recently, other parametrical and statistical models (Verstraeten et al., 2003) are
being investigated and applied for catchments of Alicante and Murcia, and calibrated
with sediment accumulation rates in reservoirs
The parametric model USLE has been applied in Murcia and Alicante giving as
result the Map of Erosion Status of the Segura and the Júcar catchments by the
Ministry of Environment (ICONA, 1988) and recently reviewed in the Summary of the
National Map of Erosion Status (Dirección General de Conservación de la Naturaleza,
2003). The USLE has been applied for agricultural soils (Ortiz Silla et al., 1999) in
Murcia, obtaining erosion rates between 0 and 50 tha-1yr-1 .
The application of the USLE in different subcatchments of the Segura river
(Murcia) gave as a result erosion rates between 30.2 and 80.4 tha-1yr-1 (López
Bermúdez, 1986, 2002), and between 4.5-30 Romero Díaz (1992). The RUSLE was
applied by Méndez García in Murcia (1997) and it resulted in erosion rates of 17 tha1yr-1.
The General Forestry Plan of the Valencia Region recently published
(Generalitat Valenciana, 2003) includes actual erosion and a potential erosion map
44
based on the USLE for the whole Valencia region (including Alicante province). They
estimated that about 9% of the Alicante province has very high erosion rates (>100
tha-1yr-1), 6.68 % has high erosion rates (40-100 tha-1yr-1), 11.51 % shows
moderate erosion rates (15-40 tha-1yr-1), 18.5 % shows low erosion rates (7-15
tha-1yr-1), 43.61 % shows very low erosion rates (< 7 tha-1yr-1) and the remaining
10% is not classified.
According to the field data the erosion rates found in Alicante or Murcia never
reached on limestone or marls 100 tha-1yr-1 . For instance the catchment of the
river Xaló (subcatchment number 12 in Figure 2) is classified as having high erosion
rates by the USLE. On the contrary all the experiments conducted within the
catchment showed maximum erosion rates (with antecedent soil moisture higher
than field capacity) of 6.5 gm-1h-1 and the maximum erosion found in the
monitored erosion plots under natural rain was 1.01 tha-1yr-1. A discrepancy
between the field measurements and the estimations based on the USLE seems
evident.
The USLE tends to overestimate erosion rates for Mediterranean conditions,
where most of the sediment mobilization takes place during extreme intense rainfall
of a high return period. The low values of the erosion data obtained at field scale
may be explained according to Martínez-Mena et al. (2001) by the lack of extreme
rainfall events during the observed period and by factors linked to land
characteristics (significant rock cover and clay content) and land use, both of which
are important factors in controlling the intensity and frequency of overland flow and
surface wash erosion.
Figure 2. Potential erosion (left) and actual erosion maps based on the USLE included
in the General Forestry Plan of the Valencia Region (Source: Generalitat Valenciana,
2003) (Dark areas: high erosion, grey: medium erosion, light grey: low erosion).
3 Hydrological behaviour and sediment movement within catchments
The effort dedicated in the last years to study soil erosion at different scales and
under different environmental conditions has provided us with a better insight in the
45
processes behind soil erosion. The progress achieved may be translated in a better
understanding of (i) infiltration processes and runoff generation mechanisms at the
soil profile, slope and catchment scales, and (ii) in the sediment movement in
different microenvironments.
Among the main conceptual achievements it is important to point out: the
definition of runoff generation models more appropriated to Mediterranean
conditions; the designation of thresholds for runoff generation; the definition of
models of soil water redistribution; the establishment of conditions and controls for
sediment detachment and movement; and the characterization of the change in the
controlling factors of soil erosion and degradation under different environmental
characteristics (climatic or human-induced).
Exemplified with part of the work done in Alicante and Murcia, Martínez-Mena
et al. (1998) found a soil organic matter threshold for the functioning of different
mechanisms of runoff generation. In this way, they explain how infiltration-excess
overland flow occurs in more degraded areas with low organic carbon content
(>0.5%) and low infiltrability (>5 mmh-1) and saturation-excess overland flow is the
dominant process in less degraded areas with a higher organic carbon content (>2
%).
Calvo-Cases et al., (2003) defend that the traditional models of runoff
generation defined for humid ecosystems must be adapted for the Mediterranean
conditions. A hydrological disconnection between the slope segments occurs and
runoff generation on Mediterranean limestone slopes takes place according to two
models: Hortonian discontinuous runoff model where runoff of hortonian type is
generated. Runoff can be easily reinfiltrated in adjacent vegetated patches or
downslope, and never reach the river bed, this model takes place in the most
degraded slopes or during high intensity rain events; and a Mixed runoff generation
model in less degraded slopes or in previously wetted soils, where infiltration excess
runoff as well as saturation excess runoff can happen on the same slope, but also in
a discontinuous way in the space. In both cases the slopes behave as a patchwork of
runoff and run-on areas, where the size of the runoff and run-on patches depends on
the climatological conditions, as it has also been shown for other areas in the
Mediterranean (Lavee et al., 1998)
With respect to the soil water redistribution within the soil profile, this takes
place in an uniform pattern during the infiltration in shallow crusted soils, while in
deeper soils with higher contents of organic matter or soils under vegetation, water
infiltrates following a non-uniform pattern indicating a macropore flow, reaching
deeper wetting fronts and allowing higher water storage within the soil profile. This
has been demonstrated by Bergkamp et al. (1998) in Murcia and by Calvo-Cases et
al. (2003) in Alicante.
Furthermore with respect to the spatial soil water redistribution, the
presence/absence of vegetation influences the spatial variability of soil water and the
factors regulating such variability. When vegetation is scarce the soil texture and
slope angle are the factors to be considered. However, when there is a good vegetal
cover, the factors to be considered are those which favour its presence (aspect and
profile curvature) (Gómez-Plaza, 2001).
Conditions for sediment movement tested by Martínez-Mena et al. (2002)
have been observed to be detachment-limited during high intensity storms and
transport-limited during medium intensity events, in colluvial soils. However in
marls, the erosion was determined by the limited quantity of available sediment. In
marls it has been also seen that when different erosion processes, i.e., rill erosion
and mass movements are compared, relatively more sediment is eroded from
46
badland areas susceptible to mass movement (Bouma and Imeson, 2000). These
authors also explain how before saturation of the surface layer dynamic soil
properties play a major role in the erosion process, whereas after saturation and
increase of the runoff the increasing importance of flow hydraulics results in an even
more complex erosion process in these badland areas.
Thresholds of rain intensity of over 15 mm h-1 has been considered in these areas as
“erosive rainfall” taking into account the total soil loss and transport capacity of the
overland flow (Martínez-Mena et al, 2001).
The production of sediment under different climatological conditions has
shown differences: in subhumid areas total runoff can be higher or similar to the
total runoff of semiarid areas; however sediment concentrations and sediment yield
are always much lower, indicating better structured and less degraded soils (CalvoCases et al., 2003).
The study of the relations between soil surface characteristics and soil
hydrological and erosive response has allowed identifying the controlling factors of
the processes.
Kirkby et al., (1996) defined the relations between the different feedback
mechanisms which control the desertification cycle. When the climatic conditions
become more arid the first cycle to be affected is the organic feedback cycle, with a
lowering of the vegetation cover and less addition of organic matter to the soil,
causing a decrease of water retention capacity and an increase of runoff and
sediment yield. From this moment the structural cycle is the one controlling the
erosion process, depending on the soil structural characteristics (aggregation,
porosity, bulk density) and the morphological characteristics of the soil surface
(sealing, crust, stoniness) (Boix-Fayos, 1999). However other authors have found
that within environments with very low levels of organic carbon content, this still is
important for soil erodibility. Some experiences have demonstrated that soil organic
carbon content of less than 1% still has an important effect on aggregation
(Martínez-Mena, et al. 1998).
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