**3. Methodology**

62 Soil Erosion Studies

Germany, Luxembourg, etc.) and to urban centres (Nunes, 2007). This exodus and subsequent abandonment of cultivated land was associated with the low rate of return from traditional crops, a result of the low productivity of dry farming systems (a Mediterranean climate, undulating relief, and poor, shallow soils), uncompetitive farm structures (with small, scattered plots), the peripheral location of the area, the lack of alternative employment sectors and the extensive presence of elderly farm owners. Furthermore, the socio-economic and political changes in Portugal in the 1970s that led to higher agricultural wages and migration from the countryside, made it difficult to maintain traditional management and manual shrub clearing, which was essentially based on low labour costs. During the period 1960-2001, the study area lost about 60% of its total population and more

According to SROA (Service of Agrarian Recognition and Management, 1951-56) statistics, in the middle of the last century cereal cultivation occupied about 55% of the total area of the Guarda district. Five decades later, the same crop only represented 10% of the total surface (CORINE Land Cover, 2000), meaning that approximately 80% of the cereal crop

Complete farm abandonment has resulted in enhanced natural secondary succession and the spread of shrub and woodland (Lasanta et al., 2009). In the first stage of land abandonment, after 4-5 years the dominant vegetation belongs mainly to the *Gramineae* family and forms a sparse herbaceous cover (Fig. 2). Perennial shrub communities, mainly dominated by nanophanerophytes such *Cytisus multiflorus* and *Lavandula sampaioana*, follow after two decades of farmland abandonment. As a result of the abandonment of cultivated land and the decline in forest land, shrub plant communities have become one of the most important vegetation types in the Iberian Peninsula (Casermeiro et al., 2004). Negligible areas are covered by recovering *Quercus pyrenaica*, indicating a lengthy period of abandonment of approximately 30-40 years. The *Quercus pyrenaica Willd*. wood is the characteristic autochthonous vegetation in the study area. The unmanaged accumulation of large quantities of fuel has led to a dramatic increase in forest fires and burnt areas

(Carvalho et al. 2002), and therefore to difficulties with *Quercus* regeneration.

Fig. 2. Dynamic evolution of vegetation in the study area following land abandonment

(adapted from Capelo, in Costa et al., 1998)

than 90% of its farmers.

area had been abandoned.

Six types of land use associated with traditional land use, land abandonment and subsequent plant recovery, and alternative land uses proposed by the CAP for marginal areas (extensification of land use and conversion of arable land to forest) were selected for the study.

1. Cereal crops (traditional land use)

In dry cereal systems with Mediterranean marginal soils, during March/April the 20-30 cm of top soil is turned over and remains without vegetation until sowing. This process is called laying fallow. The cereals are planted from October to mid November to make use of autumn precipitation for germination. Spring is the main growing season and mature cereals are harvested from June to early July before the onset of the hot, dry months.

2. Fallow land (traditional land use or short-term abandonment)

In a rainfed cereal system, fallow land is a traditional part of the cereal rotation system. During fallow cycles, land remains unseeded for 2, 3 or more years to enhance soil fertility and soil moisture availability for subsequent crops. No chemical fertilizers or manures are used, and the plant residue is kept in the fields. Fallow lands are usually used as grazing land in traditional land use agropastoral herding practices in the territory.

3. Shrub land (long-term abandonment)

When arable land is abandoned, a process of plant colonisation begins. This is a very complex process in which ecological conditions (both physical and biotic factors), human activity (the agricultural history of the fields' as well subsequent management, namely grazing, fire, clearing, etc.) and time interact. Before shrubby species, mainly *Cytisus spp.,*  proliferate, abandoned fields in central inland Portugal are invaded by herbaceous plants during the first years of abandonment. It can take more than 15-20 years for shrub land with a high percentage of ground cover to develop. Due to the accumulation of biomass in abandoned fields after recolonisation, there has been an increase in forest fires.

4. Recovering oak (very long-term abandonment)

The Pyrenean oak (*Quercus pyrenaica* willd.) is the autochthonous species in this area. Human activities over the centuries have led to considerable deterioration of the native arboreal

Soil Erosion Under Different Land Use and Cover Types in a Marginal Area of Portugal 65

from 10ºC and 13ºC during the wet season experiments to 21ºC and 22.5ºC in the dry season. The slope gradient varied from 0 to 20%. The number of simulations for each land use type varied from 4 to 10. Time to runoff (number of minutes between the beginning of simulated rainfall and runoff), runoff coefficient (rates for the relationship between rainfall intensity and runoff, in %) and total soil loss (in g m-2) were the evaluated parameters. Water and sediment samples were taken continuously (every 2 minutes) from

Fig. 3. Example of the plot used in the rainfall simulations (shrub and pastureland)

To understand the factors influencing runoff generation, the detachment of sediments from the plot and their removal to the outlet, explorative data analysis, correlation analysis and Principal Component Analyses (PCA) were performed using the SPSS 17.0 statistical package. One-way analysis of variance (ANOVA) and the Waller-Duncan multiple comparison procedure were performed on each soil property layer and for runoff and soil erosion to test whether the changes in land use and cover were statistically significant (pvalue <0.05). The *Spearman-Rho* correlation coefficient (Rs) was selected to estimate the correlation between the quantitative attributes of the soil surface and runoff and sediment production. This rank-correlation method is considered robust against outliers and non-

the beginning of runoff.

normal distribution of data.

vegetation characteristic of the region. The restoration of native vegetation is a very long process, disturbed by the regular occurrence of forest fires.

5. Afforested land (conversion of arable land to forest)

The main aim of Afforestation Regulation 2080/92 (Community Aid Scheme for Forestry Measures in Agriculture) was to reduce agricultural surpluses, but the EU also hoped that it would 'enhance forest resources'; 'provide greater ecological balance in countryside management'; and 'combat the greenhouse effect". The CAP measures for afforestation of marginal fields promote the use of a wide range of native species; however the main species selected in the area studied was *Pinus pinaster*. In Portugal, soil preparation before planting, often carried out by public works companies, involves the use of heavy machinery and deep ploughing techniques.

6. Pastureland (conversion of arable land into pasture - extensification of land use)

Recent EU agri-environmental measures support the maintenance of natural pastures or extensive cultivation of fodder crops (without deep ploughing and the use of fertilisers) and the livestock unit subsidy supports the maintenance of livestock. The study area was used for grazing cattle and the unit head per hectare was lower than 1. This value has been classified as light to moderate grazing (Rauzi & Smith, 1973; Van Haveren, 1983).

A total of 26 x 50m2 plots were created. In each plot, the percentage of plant cover (lichens + mosses, herbaceous + shrub canopy, litter cover and bare soil) were estimated at the end of the dry and wet seasons. The height of the vegetation was also determined (in cm). Two soil samples (0-10 cm) were collected and certain characteristics (grain size distribution, bulk density, soil organic matter, etc.) were determined. A Coulter LS Particle Size Analyzer was used for grain size analysis for fractions of < 2 mm. Dry bulk density and porosity were measured using the cylinder method. Soil resistance was assessed through the use of a pocket penetrometer. Soil moisture was determined by the gravimetric method. Organic matter content was determined by the Tinsley method (1950). Soil water repellency was measured using the Ethanol Method (MED), at the suggestion of Doerr et al. (1998). The ethanol concentrations used in this study area were 0, 1, 3, 5, 8.5, 13, 18, 24 and 36%, representing liquid surface tension intervals of approximately 5 dynes/cm (Coelho et al., 2005). A zero value corresponds to hydrophilic (or wettable) soil and 36% to extremely water repellent soils. These tests were carried out before the rainfall simulations.

A rainfall simulator similar to the one described and tested by Cerdà et al. (1997) was used to evaluate the main hydrological and erosional characteristics of the soil1. It consists of a springlink device placed 2 m above the soil. A small 0.24 m2 round plot is inserted carefully into the soil (Fig. 3). Rainfall simulations have a duration of 60 minutes and intensities of around 53-55 mm h-1. Tests were carried out over two years (2005 and 2006) under different plant cover and soil moisture conditions. Tests were carried out in August of 2005 and 2006, when there was a severe drought due to very low precipitation in the preceding three months (10 mm and 45 mm, respectively) and in April and November of 2006 after a very intense period of natural rainfall (160 mm and 300 mm, respectively, during the preceding months). The average monthly temperatures also differed, ranging

<sup>1</sup> It is important to note that although rainfall simulation tests on small surfaces are used worldwide, their results should only be considered for comparative purposes, especially in the case of sediment concentration (Lasanta et al., 2000; Pardini et al*.*, 2002). In fact, measurements on experimental plots are acceptable only for comparative purposes, that is, to assess the amount of overland flow and erosion in different environments and land use types. They cannot be accepted as absolute coefficients or rates.

vegetation characteristic of the region. The restoration of native vegetation is a very long

The main aim of Afforestation Regulation 2080/92 (Community Aid Scheme for Forestry Measures in Agriculture) was to reduce agricultural surpluses, but the EU also hoped that it would 'enhance forest resources'; 'provide greater ecological balance in countryside management'; and 'combat the greenhouse effect". The CAP measures for afforestation of marginal fields promote the use of a wide range of native species; however the main species selected in the area studied was *Pinus pinaster*. In Portugal, soil preparation before planting, often carried out by public works companies, involves the use of heavy machinery and deep

6. Pastureland (conversion of arable land into pasture - extensification of land use)

classified as light to moderate grazing (Rauzi & Smith, 1973; Van Haveren, 1983).

water repellent soils. These tests were carried out before the rainfall simulations.

A rainfall simulator similar to the one described and tested by Cerdà et al. (1997) was used to evaluate the main hydrological and erosional characteristics of the soil1. It consists of a springlink device placed 2 m above the soil. A small 0.24 m2 round plot is inserted carefully into the soil (Fig. 3). Rainfall simulations have a duration of 60 minutes and intensities of around 53-55 mm h-1. Tests were carried out over two years (2005 and 2006) under different plant cover and soil moisture conditions. Tests were carried out in August of 2005 and 2006, when there was a severe drought due to very low precipitation in the preceding three months (10 mm and 45 mm, respectively) and in April and November of 2006 after a very intense period of natural rainfall (160 mm and 300 mm, respectively, during the preceding months). The average monthly temperatures also differed, ranging

1 It is important to note that although rainfall simulation tests on small surfaces are used worldwide, their results should only be considered for comparative purposes, especially in the case of sediment concentration (Lasanta et al., 2000; Pardini et al*.*, 2002). In fact, measurements on experimental plots are acceptable only for comparative purposes, that is, to assess the amount of overland flow and erosion in different environments and land use types. They cannot be accepted as absolute coefficients or rates.

Recent EU agri-environmental measures support the maintenance of natural pastures or extensive cultivation of fodder crops (without deep ploughing and the use of fertilisers) and the livestock unit subsidy supports the maintenance of livestock. The study area was used for grazing cattle and the unit head per hectare was lower than 1. This value has been

A total of 26 x 50m2 plots were created. In each plot, the percentage of plant cover (lichens + mosses, herbaceous + shrub canopy, litter cover and bare soil) were estimated at the end of the dry and wet seasons. The height of the vegetation was also determined (in cm). Two soil samples (0-10 cm) were collected and certain characteristics (grain size distribution, bulk density, soil organic matter, etc.) were determined. A Coulter LS Particle Size Analyzer was used for grain size analysis for fractions of < 2 mm. Dry bulk density and porosity were measured using the cylinder method. Soil resistance was assessed through the use of a pocket penetrometer. Soil moisture was determined by the gravimetric method. Organic matter content was determined by the Tinsley method (1950). Soil water repellency was measured using the Ethanol Method (MED), at the suggestion of Doerr et al. (1998). The ethanol concentrations used in this study area were 0, 1, 3, 5, 8.5, 13, 18, 24 and 36%, representing liquid surface tension intervals of approximately 5 dynes/cm (Coelho et al., 2005). A zero value corresponds to hydrophilic (or wettable) soil and 36% to extremely

process, disturbed by the regular occurrence of forest fires. 5. Afforested land (conversion of arable land to forest)

ploughing techniques.

from 10ºC and 13ºC during the wet season experiments to 21ºC and 22.5ºC in the dry season. The slope gradient varied from 0 to 20%. The number of simulations for each land use type varied from 4 to 10. Time to runoff (number of minutes between the beginning of simulated rainfall and runoff), runoff coefficient (rates for the relationship between rainfall intensity and runoff, in %) and total soil loss (in g m-2) were the evaluated parameters. Water and sediment samples were taken continuously (every 2 minutes) from the beginning of runoff.

Fig. 3. Example of the plot used in the rainfall simulations (shrub and pastureland)

To understand the factors influencing runoff generation, the detachment of sediments from the plot and their removal to the outlet, explorative data analysis, correlation analysis and Principal Component Analyses (PCA) were performed using the SPSS 17.0 statistical package. One-way analysis of variance (ANOVA) and the Waller-Duncan multiple comparison procedure were performed on each soil property layer and for runoff and soil erosion to test whether the changes in land use and cover were statistically significant (pvalue <0.05). The *Spearman-Rho* correlation coefficient (Rs) was selected to estimate the correlation between the quantitative attributes of the soil surface and runoff and sediment production. This rank-correlation method is considered robust against outliers and nonnormal distribution of data.

Soil Erosion Under Different Land Use and Cover Types in a Marginal Area of Portugal 67

the shrubland, the soil cover of grass, lichens and mosses explained the differences observed between the dry and wet seasons, with values between 84 and more than 92%. In fact, during the different stages of vegetation succession, the development of soil cover, mainly the herbaceous, shrub and litter cover, depends on the length of time the land has been

The soil cover of recently abandoned fields increased around 30%between the dry and wet periods, mainly due to the development of lichens, mosses and grass. The pastureland shows a similar behaviour, with an increase by more than 20% between dry, hot and wet,

In contrast, the ploughed and afforested land revealed average annual values of less than 15%. As the cereal (mainly rye) is planted in the end of September or beginning of October, the crop is covering the ground before winter and continues to grow in the spring. Therefore

Table 2 summarizes the physico-chemical properties of soils for the different types of land use and vegetal covers. There were no significant differences in particle size distribution for the top 10 cm layer among the land cover types. A sandy loam texture was found in all the soils studied, in line with the same parent material on which they lie. In this layer, the soils revealed a very high percentage of sand fractions, over 70% of the total, and a low silt and clay fraction. In general, a sandy, coarse-textured soil drains easily and quickly after rain but has a lower moisture-retention capacity and a lower nutrient-retention capacity. Unlike texture, there was a significant difference in bulk density (g cm−3) among land covers (pvalue <0.001). The lowest values were recorded for the cereal crop and arable land afforested with *Pinus pinaster*, as a consequence of ploughing up the top layer for cereal

Conversely, the highest values for bulk density, which correspond to the lowest porosity percentages, were registered in grazing plots and fallow land or short-term abandoned land. Soil bulk density is a more direct measure of soil compaction (Roberson, 1996) and perhaps the greatest impact of grazing consists of changes to the soil structure due to compaction (Roberson, 1996; Wood, 2001). In fact, the intense and continual pressure from moving livestock easily compacts soils, particularly when they are wet and more susceptible to compaction (Brady, 1984; Warren et al., 1986). Firestone (1995), for example, observed a 13% increase in the bulk density of grazed soils under oaks in California. Orr (1960) measured an increase of up to 20% in bulk density in the top 4 inches of grazed South Dakota steam bottom soil when compared with exclosures. Compaction is a strong direct effect of force which leads to the indirect effect of reduced infiltration and the resulting force of increased overland flow, which in turn leads to increased erosion (Trimble & Mendel, 1995). Extension of the abandonment stage and the expansion of shrub and wood communities tend to

All the soils were very low in organic matter. In the soils with cereal crops and in fallow land the organic matter content was around 0.50%. Despite the higher organic matter obtained for afforested and grazed land, there were still no significant differences between these four land uses. Soil erodibility in all land use types was expected to be high because of the sandy soil texture and low organic matter content. Vegetation restoration after abandonment, involving the development of shrub and tree cover, seems to enhance the

abandoned and the activities developed after cropland abandonment.

the percentage of soil cover during the monitored wet season was high.

cultivation, tree planting and the removal of ground cover to avoid forest fires.

cold season.

**4.2 Soil characteristics**

reduce soil bulk density.

Principal components analysis is an ordination method, used to simplify data by reducing the number of variables. The PCA procedure generates indices called principal components, which are linear combinations of the original variables. The most efficient data description and reduction are obtained when the variables are highly correlated.
