Homepage | DESERTLINKS Homepage
| © DESERTLINKS 2004
English-EN | Español-ES | Italiano-I | Ελληνικά-GR | Portuguese-PT
|Desertification Indicator System for Mediterranean Europe|
of the physical environment
g Description of reasons for degradation of the physical environment and why it is an issue in the context of desertification
Author: Brian Irvine B.Irvine@geography.leeds.ac.uk
Introduction. Land degradation and desertification are processes characterised by the deterioration in land quality in terms of its capability to functionally support a selected land use and associated flora and fauna. Desertification may be considered as extreme land degradation where land loses much of its natural productivity. Land degradation is usually associated with sparse vegetation of low biodiversity. As the soil becomes prone to erosion vegetation becomes less likely to grow back in a positive feedback loop. Usually the extremes are associated with regional and climatic trends, which may threaten large areas. Some areas are more sensitive to degradation than others. Land degradation may have been observed for some time and no mitigation action has been taken, whereas in other areas remedial actions have been set in place to try and alleviate certain problems and reduce the risk. Physical factors driving land degradation vary as the physical environment changes. Areas at risk from erosion may be unaffected by salinisation as drainage and climatic parameters may vary.
Soil Erosion (Hillel 1991,
F. Basso et al, 2002, C. Kosmas et al, 2002). Soil erosion is one of the
main physical processes of land degradation, and in Europe is generally
important in both its economic costs and the areas affected. Soil erosion
is a natural process associated with rainstorms and gully and rill formations,
but its rate has been greatly accelerated by human activity, primarily
agricultural tillage and grazing pressure. Erosion progressively removes
the topsoil, which has the best structure and contains most of the organic
matter and nutrients.
On-site impacts of soil erosion are
directly in loss of material, and indirectly in loss of nutrients. Where
soils are already thin, erosion may lead to an irreversible removal of
the medium for plant growth. Elsewhere, erosion removes the most fertile
topsoil, which holds most of the organic matter and nutrients, and farmers
incur costs for remedial cultivation and additional fertilisers. Off-site,
eroded material may be re-deposited in buildings, roads, reservoirs and
watercourses, and nutrients in runoff and sediment increase eutrophication
and may lead to algal development. Off-site effects of soil erosion may
prove to be more costly and already threaten implementation of EU Water
Framework Directives, particularly for nitrates.
Action to control erosion requires
both assessment and remedial strategies, and effective action requires
considerable investment of time and money. Objective assessment is needed
at a broad scale, to determine where erosion is most severe, and where
resources should be concentrated for remediation and for detailed studies.
Local assessment is then required to identify which areas are worth conserving
and to account for individual factors in each area. Remedial strategies
include changes in land use at regional scales, identification of sensitive
areas for action at local scales, and detailed conservation planning at
Salinisation (L. Postiglione
2002, A.J. Conacher and M. Sala. 1998 ). In general, the more
saline a soil, the more limited the vegetation that it supports. Soil
salinisation is the accumulation of natural and artificial salts in the
soil. Soil salinisation can happen in a number of ways.
Some vegetation grows better on slightly saline soils, though there are limits beyond which vegetation dies back. If a soil is gradually becoming more saline but is likely only to reach an equilibrium level of salinity at which much of the present ecosystem will persist, then arguably this does not pose a major land degradation risk. However, if there is a likelihood of salinisation continuing unchecked, or if it is unlikely that the present ecosystem will cope, then arguably there is a high land degradation risk. Irrigated areas are prone to salinisation as salts are more readily drawn upwards from the shallow water-table and the quality of the irrigation water may be in question.
Fire (A. Dalaka et al 2002, A.J. Conacher and M. Sala. 1998 , A.J. Conacher and M. Sala. 1998 ). Forest fires are part of the Mediterranean ecosystem. Post-fire areas are prone to erosion and a high risk of land degradation. However, given the correct conditions regeneration will occur. Post-fire rehabilitation has been recognised as a crucial factor during this sensitive period. Large forest fires have a significant effect on vegetation cover but can also change soil properties (physical and chemical) enhancing water repellence (hydrophobic soils) and destroy habitat, infrastructure and life. Human activity has been identified as a major driver in the imbalance that has reduced forest areas and increased fuel load and the number of forest fires in the Mediterranean. Land abandonment can intensify fire risk.
Vegetation degradation (A.J. Conacher and M. Sala. 1998 ). Vegetation degradation is defined as, "the temporary or permanent reduction in the density, structure, species composition or productivity of vegetation cover". The long period of human activity in the Mediterranean Basin has greatly limited the areas of natural or indigenous vegetation. Pressure on the natural vegetation of the region has arisen from changing agricultural practice, fire and livestock grazing, and the feed-back with the loss of bio-diversity. Vegetation dynamics are driven by climatic conditions, soil conditions and management practice (crop selection, harvesting, grazing and fire), with non-natural vegetation being less resilient to high climatic variability, climatic extremes and soils of the region, which are themselves under threat. Thus, over exploitation and management practices may result in vegetation degradation and tend to enhance land degradation risk.
Productivity/biodiversity loss (L. Postiglione 2002, A. Ferrara 2002). Loss of agricultural productivity leads to reduced vegetation cover and/or loss of net farm income. Reduced vegetation cover may be offset by the application of fertilisers. However, the effect of this may be observed initially in net farm income or with respect to future in-stream water quality. Productive agriculture has replaced natural vegetation with vegetation that is less resilient to regional climatic stresses and soil conditions. Crop and livestock production may not be in harmony with existing patterns of vegetation growth, thus vegetation cover and production are in a continual state of stress. Reduced productivity may well drive land-use practice and land use change which are themselves regarded as principle drivers of land degradation in Mediterranean environments.
Lower Inner Alentejo, Portugal
The state and degree of land degradation in the municipality of Mértola is a result of the interactions between the physical characteristics of the landscape and human action in processes of soil destruction and degradation. These have resulted in an increase in water erosion and consequent soil fertility and biodiversity loss.
Human pressure on the natural resources (soil, vegetation and water) resulting from agriculture has progressively increased over time, reaching a maximum between the beginning and middle of the twentieth century, and resulting in the present high degree of land degradation.
Results obtained experimentally at the Vale Formoso Erosion Experimental Centre confirm this picture. For more than 40 years, different agricultural practices and cultures (cereals, legumes, pastures) have been analysed and measured as contributory factors to the process of soil erosion.
However, knowledge about the physical characteristics of the area allowed the identification of the specific natural conditions that favour soil degradation by erosion:
For more than a century there has been unsuitable and insensitive land management in this area, accentuated by political decisions of strictly economic character, affecting the distribution of the population and contributing to unsuitable land use, particularly for agriculture.
The main reason for colonizing and clearing the land was the significant increase of population, mostly as a result of the establishment of the São Domingos mine, which led to the division of the existing communal land into two distinct phases. The result of this division was a serious and widespread degradation of the vegetation cover by fire, clear cutting and fuel production (wood and charcoal), as well as the increase in soil erosion by agricultural labour on extremely thin soils on steep slopes.
The negative impact of human activity progressively increased by means of large scale and intense activity, benefiting from new technology introduced from the British industrial revolution (ploughs, machinery), and later by the application of the internal combustion engine (tractors and other machinery) providing far more efficient and intense soil mobilization and agricultural exploitation. For consecutive decades there were strong incentives for this agricultural activity from political sources (wheat protection in the late nineteenth century, the wheat campaign and agricultural production campaign from the 1930s onwards) and currently by the Common Agricultural Policy.
The Mértola municipality thus exhibits serious degradation of its natural resources, combined with economic disadvantages that directly affect the people that live there and dedicate themselves to dry cereal cropping, in most of the cases below any level of economical rationality.
Climatic characteristics and human activities have affected degradation processes in the Guadalentín valley. In particular, dry summers, mild winters, low precipitation (<300 mm), high annual insolation (2900 h/year), strong evaporation (900-1200 mm/year) and an average temperature of 18ºC, plus inappropriate human action in the environment contribute to degradation processes in the area. The main processes are as follows:
Salinization. The major processes that cause salt accumulation in soils are related to climate and irrigation. The climate acts directly through high evaporation rates, and indirectly as the driving force behind soil salinization associated with irrigation. The potential for year-round crop production in the Guadalentín valley is limited only by water deficit, and this is now overcome using irrigation. However, once irrigation waters have been applied, strong evaporation forces draw water and accumulating salts back to the surface. The salt accumulation and translocation is caused predominantly by irrigation, depending on water quantity and quality. Irrigation using contaminated groundwater has induced soil salinization and has led to aquifer overexploitation, releasing gas and salts. The use of waters with high salt concentrations, or water from purification plants which do not include a tertiary treatment, has led to degradation and reduction of the agricultural production capacity.
The salts in the aquifers originate from the Miocene substrata, where sulphate-chloride salts dissolve with the carbonates originating with CO2, whose appearance is related to the active tectonic fault system and its mobilization, is a consequence of the decrease in hydrostatic pressure in the detrital aquifer. Over time, the ion concentration of the water has increased and it carries an increasing risk of soil salinization. Statistical studies proves that changes in the salinity of the waters used for irrigation, agricultural practices, the use of fertilisers, and salts supplied by runoff from the surrounding areas are the essential factors that have led to the dramatic reduction in soil quality. The problems in the Guadalentín valley are amplified by having insufficient water to leach out accumulated salts, and by the economy of the areas being dependent on agriculture. The highest salinities are found under flood irrigation methods, with the valley displaying a complex spatial distribution of soil salinity. Soils under irrigation display a higher ion concentration, higher salinity and different ion composition than found on semi-natural or disused sites. For all soil profiles within the irrigated floodplain area, irrigated and disused, calcium and sulphate are the dominant ions. Conversely, the semi-natural sites at the edge of the floodplain that have no agricultural use had a prevalence of calcium and carbonate ions and yet a lower salinity and total ion concentration.
The Guadalentín valley, with its secondary soil and water salinization is a more recent phenomenon and is a good example of at-a-point soil salinization resulting from irrigation. The solute inputs from irrigation and rainfall are much higher than the solute outputs of plant uptake or leaching in the soil. This salt accumulation leads to a reduction in the economic value and productive capabilities of the land. As a system, therefore, the Guadalentín valley is unstable economically and hydrologically, with the soil and aquifer resources being utilized at an unsustainable rate.
Soil and vegetation degradation. There are few alternatives to natural vegetation (Mediterranean macchia, Stipa tenacissima, etc.) in non-irrigated areas. Olive or almond trees are the alternatives that farmers tend to choose. Land use in the upper Guadalentín catchment consists mainly of rainfed cereals and almonds, scrubland and forest. Semi-natural vegetation and diverse cropping systems have been converted into monocultures with low tree densities, leaving the soil unprotected. The need for rainfall to penetrate deep into the soil to sustain almond monocultures in semi-arid climates requires loose, bare topsoil between the trees and thus large areas of bare soil are exposed on hillslopes resulting in high erosion risks.
The low plant density combined with frequent tillage increases the vulnerability of the orchards to soil erosion, and specific planting and water conservation strategies are required to supplement the irregular rainfall. Such soil and water conservation measures need to be adapted to local topography and soil conditions. During the rapid expansion of almond plantations into marginal areas, traditional soil and conservation techniques have been lost and increasingly entire hillslopes have been remodelled using heavy machinery and planted in a uniform manner. Very steep slopes are abandoned or remain covered with semi-natural matorral.
Tillage is generally shallow and is carried out twice a year in order to increase the infiltration capacity of the soil and to eliminate weeds competing for the limited soil water. Tillage depth in almond plantations varies between 10 and 20 cm depending on the local topography. The frequent tillage leads to the development of a dry mulch layer and forces the almond trees to root deeper into the soil or weathered bedrock.
The erosional response is primarily determined by soil aggregate stability and topographical properties. A greater proportion of finer particles in the eroded material than in the soil matrix indicates selective erosion and the transport of finer material. Studies have concluded that the annual vegetation and plant residues prevent the formation of soil crusts which hamper infiltration, and at the same time reduce overland flow velocities. Land use here is more important than the soil properties with respect to soil erosion.
Productivity loss. Although the inter-annual variability in crop yields is large, the 5-year running mean indicates a general decrease in crop yields from 800 kg/ha in the 1960s to 400kg/ha in the 1990s. The yield decrease is in agreement with the extensive production methods and the expansion of almond plantations into marginal areas.
Fire. Fire and erosion are closely related. The variability in erosion is associated with fire severity. Forest fires have increased in frequency over the last few decades, and as a consequence, fire-prone Mediterranean gorse shrublands have significantly expanded. In mature plant communities, even under moderate weather conditions, the flammability and capacity to generate severe fires make these plant communities extremely sensitive to erosion and land degradation processes.
Under Mediterranean conditions, where severe wildfires occur mainly in summer and torrential rainfall events are frequent in autumn, conservation measures such as planting herbaceous seedlings are necessary in order to prevent irreversible soil degradation in the first vegetation regeneration phase after the fire.
Agri Basin, Italy
Degradation of the physical environment in the Agri Basin assumes different forms, reflecting its peculiarities and variations in physical and environmental characteristics, as well as socio-economic dynamics and land use. The Agri Valley is divided into three homogeneous sub-areas: Upper, Middle and Lower Agri.
Studies conducted in the Agri Basin (F.Basso et al., 2002) show that soil loss in the hilly areas of the Basin was occurring every year, as a continuous process, and it is slightly more than 1 t ha-¹ year-¹, within a range of 0.40-4.15 t ha-¹ year-¹. The data obtained in the experimental plots during the period 1990-1995 show that the most significant soil losses corresponded to a small number of rainfall events. These events were characterized by an intensity of between 13.2 and 52 mm h-¹ in summer or between 2 and 4.6 mm h-¹ in winter when the soil surface was exposed and before the crops had grown to provide an effective cover. All the observations demonstrate that slope angle is a significant factor for soil erosion and carrying out tillage following the contours rather than downslope is important in reducing the soil erosion risk.
According to Postiglione (L. Postiglione,
2002) soil salinization in the Mediterranean Basin can derive from
pedogenetic processes or it may be related to excessive evapotranspiration,
sea-water infiltration, salt concentration of irrigation water or other
anthropogenic causes such as overgrazing and deforestation in semi-arid
environments, the excessive use of chemical products and the contribution
(via the air or water) of pollutants emitted by industry. In the lower
part of the Agri Basin, the area particularly affected by salinization,
more factors combine to cause the problem. However, constant or increasing
salinity is chiefly caused by the use of highly saline irrigation water,
compounded by excessive evapotranspiration in dry areas. As a vital resource
for agriculture irrigation water may be extracted from surface water (springs,
rivers, streams) or groundwater (phreatic boreholes, artesian wells).
In the case of surface water resources, springs are sometimes salt-rich
since if the water passes through rock layers and saline or sodic soils
where there is an excess of sodium which has stayed in situ during the
pedogenetic process, or because there is sea-water infiltration. Such
infiltration frequently occurs in some aquifers when well supplies are
over-abstracted or when the groundwater fails and is not recharged due
to a shortage of rain during the winter. Both types of process may leave
a void to be filled by sea-water infiltration. Of course, the situation
becomes even more serious when excessive abstraction means that wells
have to be sunk deeper, with the consequent risk of reaching saline groundwater.
The main cause of deforestation and forest degradation in the Agri Basin is forest fire (Ferrara, 2004). Fires, in terms of both frequency of occurrence and areal extent, are a serious problem in this environment, already subject to degradation and desertification phenomena. Just to give some information about the size and occurrence of forest fires in the Agri Basin, in the period 1990-1995 about 1327 hectares of forest, mainly represented by high stands of broad-leaved trees, were affected by 304 fires. This affected 3.88% of the total forested area of the valley and 13.41% of the total number of fires registered in the region. Fires in Agri Basin follow a clear pattern that is found in many other Mediterranean Regions in that the highest numbers of fires are found in areas having the lowest forest index. In the Agri Basin fires occur mainly in areas with forest indices up to 20% that tend to be dominated by coppices, i.e. a forest area of rather poor productivity.
The spread of a durum wheat monoculture,
mainly due to Common Agricultural Policy support, has aggravated the land
degradation process and changed the traditional landscape. This agricultural
specialization process has also brought about a loss of traditional genetic
material, while promoting exotic cultivars that are much more demanding
in terms of soil conditions and cause depletion of soil resources. Only
recently has the re-introduction of local traditional cultivars been observed.
The island of Lesvos is a typical example in the Mediterranean basin of an area that has been subjected to intensive land use for thousands of years. The physical environmental changes that have taken place have been caused by natural events and human action. An analysis undertaken using historical and archaeological documents and recent soil and vegetation survey data for the last five thousand years has shown dramatic changes in the physical environment (Marathanou et al., 2000). As the Table below shows, in the ancient period forested and pasture land covered 50% and 23% of the total area respectively, while the agricultural land was restricted to about one quarter of the area (22%). In contrast, by 1886 forested land had greatly decreased to less than half (23.9%), while pastures increased from 23% to 37.2% in 1886. Of course, pasture land has increased on the island by replacing the low profit agricultural land.
Table: Land use distribution (area %) in the ancient period and in 1886
Land cultivated with cereals or orchards was significantly reduced from 18% in the ancient period to 9.1% in 1886. The greatest reduction occurred in hilly areas where the production was very low due to high soil degradation resulting from erosion. The reduction in cereal cultivation was higher in the western part of the island. The presence of very steep slopes (usually greater than 35%) combined with erodible soils formed on pyroclastic material favoured high erosion rates and very shallow soils were formed. The area cultivated with vines remained almost unchanged representing a small portion of the land (about 2%). According to the existing information, the area cultivated with vines increased especially during the Byzantine and Ottoman periods but later declined.
Great changes in land use have been observed on the island during the last century. As the Table below shows, olive groves were largely expanded, changing from 26.9% to 41.2% of the total area. Additionally, deciduous oak forests have expanded significantly from 2.2% to 7.1% of the area due to production of oil used in the leather industry existing in the island in the previous decades. The total area occupied by pine forests remained almost unchanged, even though the geographical distribution has changed. Pine forests have replaced deciduous oak forests or pastures due to their higher ability to recover after fire. Their expansion was influenced by soil depth and parent material. The increase of the area occupied by the previous land uses was partially compensated by the reduction of areas used for pastures (the area decreased from 37.2% to 22.6%.)
Table: Land use distribution measured in 1886 and 1996
During the last century, great changes did not occur only in the area occupied but also in the geographical distribution of the various types of land uses. Olive groves were removed from some areas for various reasons and they were expanded in other areas with more fertile soils. Olives were mainly removed from hilly areas with very steep slopes and highly degraded soils where they had both low productivity and low plant canopy. The low plant canopy increased the vulnerability of the trees to frost hazards during the cold period of the year. In a few areas, mainly located in the vicinity of villages with relatively deep soils, olive groves were replaced by other crops producing higher income, such as tobacco and vegetables. Today, most of these areas are used for pasture.
The expansion of olive plantations in the last century occurred mainly in hilly areas with lower frost hazard and relatively fertile soils. The frequency of expansion was higher in moderately deep (33.4%) to deep soils (55.4%) located mainly on steep slopes (35.6%). Most of these areas have been terraced in order to protect the soil from erosion and to increase the productivity of the olives. Crescent-shaped terraces have been carefully constructed for individual trees. The soil was removed from other places to fill these terraces. In the last few decades the value of such terraces has decline markedly because of the difficulty to access them and to mechanically cultivate them. At present several of these areas have been abandoned, and some of the terraces have collapsed causing rapid removal of the soil by runoff water. Maintaining such terraces appears to be a very expensive practice compared to most other alternatives for soil erosion control. Considering that such terraces protect very valuable soil for preserving the olive trees and protect a very sensitive environment, these agricultural structures should be protected.
The olive groves which have been in existence for a long time are, in many cases, located on moderately deep (33% of area occupied by olives) to deep (60.4%) soils, but very steep slopes (42.9%) prevail under this type of land use. The soils were protected from erosion due to the construction of thousands of kilometres of terraces. The above findings clearly demonstrate the importance of land use planning and management of hilly areas sensitive to soil erosion.
The area of annual crops has largely decreased especially in the post 1950 period. This is attributed both to the low productivity of the land and the massive migration of the local people to other areas of the country. Moreover, significant increase in the number of animals, basically sheep, was seen, especially after the mid-60s, and more recently due to European Union policies and subsidies.
Soil survey data showed that the various types of land use have greatly affected land degradation. Soils under pastures are the most degraded on the island today. The soils under this type of land use are very shallow (depth <15 cm) to shallow (depth 15-30 cm) covering 46.3% and 40.0% of this area, respectively. Deep soils under pasture are highly limited. The analysis of land use evolution shows that most of this land was under pine or oak forest in the ancient period. People began to clear the forests or the natural vegetation in the ancient period or later in order to raise crops or to graze animals. Measures of soil conservation were insufficient and erosion stripped away the topsoil. The soils remaining in these areas have limited subsurface layers such as bedrock (pyroclastics, marble, volcanic lava) and under hot and dry climatic conditions rainfed agriculture or natural forest can not be economically supported. In contrast, areas that have remained for long periods under pine forest or olive groves are relatively well protected, with soil depths greater than 75 cm. Part of the areas under oak forest today has been subjected to recent land use change, therefore any conclusion on the effect of land use on land degradation must be undertaken in relation to the previous land use.
Soils under pastures have the highest degree of erosion among the various types of land use. These soils are severely to very severely eroded in an area covering 63.6%. The rest of the soils under pasture are moderately (25.5%) or slightly (7.3%) eroded. Areas with soils under oak forest are better protected from erosion than the previous land use. An area of 29.2% of the soils is severely eroded, while 50.1% is characterised as moderately eroded. Areas with olive groves are relatively well protected from erosion under the existing management practices. Severely eroded soils cover only 11.1%, while very severely eroded soils are almost absent in the area. The majority of the soils are moderately eroded (63.7%), while 13.6% are slightly eroded. The rest (10.6%) of the area covered with olives trees is well protected from erosion and found mainly in plain areas. Areas with pine forest are the best protected from soil erosion on the island. Moderately eroded soils cover 58.1% and slightly eroded 30.4%.
of how the indicators inter-relate
Areas sensitive to land degradation
can be identified through the Environmental
Sensitivity Index at the catchment scale or by the Regional Degradation
Index at a courser regional resolution.
Land use in the diagram is assumed to be driven by climatic, soil and topographic suitability; world prices; subsidies and historic land-use (inertia). Agricultural patterns and grazing pressure out of phase with climatic conditions may reduce Vegetation cover and vegetation productivity. If high vegetation productivity is unmanaged, fuel load may increase (Fire frequency).
Low productivity may initiate Land
use evolution or a change in production methods.
Vegetation degradation resulting from
agricultural practice or fire will enhance surface runoff and increase
High erosion rates reduce soil fertility,
Soil depth and
restricting or reducing vegetation productivity and tending towards land
erosion control measures, Runoff
water storage). Early indications of soil erosion may be the formation
of rills and gullies on-site or the deposition of eroded material off-site.
Reduced vegetation productivity reduces
land cover directly. Reduced Net
farm income may become apparent, forcing change in land-use and practice
area), though this may be restricted through Water
availability and Water
Further financial input may be required
to maintain income from the land (EU
production subsidies). Land
abandoned from agriculture may be an issue. External pressures from
non-stationary climate (Rainfall,
index) may add further pressure to the land resource and land use
Land degradation; vegetation and soil degradation, may not be irreversible but may be sensitive to future practice and activities.
to simple or more complex models which demonstrate this inter-relation
Since the early 1990s, erosion risk models and indicators have been developed through successive EC funded research projects. The Regional Degradation Index (RDI), which has been expanded in the Pan-European Soil Erosion Assessment (PESERA1), offers a methodology to assess regional soil erosion risk. The RDI is based on concepts developed in MEDALUS II and offers an explicit theoretical response based on a simple and conservative soil erosion model. The model makes use of land-use, topographic, soil and climatic data.
The RDI model combines ground cover, surface crusting, runoff and sediment transport, to give an estimate of water and sediment delivered to stream channels. A model schematic is shown above. Modelled erosion risk is consistent with finer scale erosion models for flow strips, and is integrated across the frequency distribution of storm magnitudes. The model partitions daily precipitation into Hortonian and saturation overland flow, subsurface flow and evapo-transpiration. Hortonian overland flow, which is mainly responsible for soil erosion, is generated with respect to local soil and sub-surface moisture characteristics. The emphasis of the PESERA-RDI model is the prediction of hillslope erosion, and the delivery of erosion products to the base of each hillslope. Channel delivery processes and channel routing are explicitly not considered.