**4.1 Soil cover**

Results for plant cover for the different types of land use and vegetal covers are summarised in Table 1. The percentage of plant cover is clearly related to seasonal changes in vegetation. The lowest soil cover percentages were recorded in late summer. Conversely, the highest values were registered for the soils with the highest moisture content, peaking in the autumn and spring. These differences were mainly due to the variations registered for the density of grass cover, lichens and mosses, which dry up in the summer season and grow in the winter and spring. The existence of a marked seasonal dynamic due to the predominance of annual vegetation generated significant cycles and temporal differences, both for the protection of the soil against erosion and evapotranspiration, and for the incorporation of organic matter and, obviously, primary production.


a. Not available in this season of the year.

Table 1. Plant cover (%) per season for the different land use types/cover monitored

The results demonstrate that the highest surface cover was recorded in wood plots, with figures exceeding 90%. Old abandoned fields with recovering oaks showed a very homogeneous yearly soil cover related, in particular, to the predominance of litter cover. In 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 abandoned and the activities developed after cropland abandonment.

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, cold season.

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

## **4.2 Soil characteristics**

66 Soil Erosion Studies

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

Results for plant cover for the different types of land use and vegetal covers are summarised in Table 1. The percentage of plant cover is clearly related to seasonal changes in vegetation. The lowest soil cover percentages were recorded in late summer. Conversely, the highest values were registered for the soils with the highest moisture content, peaking in the autumn and spring. These differences were mainly due to the variations registered for the density of grass cover, lichens and mosses, which dry up in the summer season and grow in the winter and spring. The existence of a marked seasonal dynamic due to the predominance of annual vegetation generated significant cycles and temporal differences, both for the protection of the soil against erosion and evapotranspiration, and for the incorporation of organic matter and, obviously, primary

Herbs &

**Dry, hot season**  Ploughed land 2.0 3.0 0.0 5.0 Cereal crop a a a a Fallow land 3.0 20.0 10.0 33.0 Shrub land 4.0 60.0 20.0 84.0 Recovering oak 1.5 2.5 90.0 94.0 Afforested land 0 3.0 2.0 5.0 Pastureland 2.5 2.5 65.0 70.0 Mean 2.7 15.2 31.2 41.6 **Wet, cold season**  Ploughed land 9.5 2.0 0.0 11.5 Cereal crop 15.0 56.0 0.0 71.0 Fallow land 32.5 30.0 5.0 67.5 Shrub land 14.0 68.0 10.5 92.5 Recovering oak 3.5 3.0 92.0 98.5 Afforested land 13.5 1.5 0.0 15.0 Pastureland 22.5 69.0 5.0 96.5 Mean 15.8 32.8 16.1 64.6

Table 1. Plant cover (%) per season for the different land use types/cover monitored

The results demonstrate that the highest surface cover was recorded in wood plots, with figures exceeding 90%. Old abandoned fields with recovering oaks showed a very homogeneous yearly soil cover related, in particular, to the predominance of litter cover. In

shrubs Litter cover Total vegetation

cover

and reduction are obtained when the variables are highly correlated.

Lichens & mosses

**4. Results 4.1 Soil cover** 

production.

Land use types/covers

a. Not available in this season of the year.

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

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 reduce soil bulk density.

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

Soil Erosion Under Different Land Use and Cover Types in a Marginal Area of Portugal 69

Table 3 summarizes the statistical analysis for the hydrological and sedimentological parameters in the different types of land use during the dry and wet periods. The results from the preceding ANOVA demonstrate significant statistical differences (*P* <0.000) for land use and soil covers, which means that variables have an important effect on runoff and

With high intensity rainfall, the afforested and laying fallow land produce the highest runoff and sediment yield coefficient, with an annual average of 64% and 45% of the rainfall and 75 g m-2 h-1 and 43 g m-2 h-1, respectively. These results show that the soil in these plots, which has very poor plant cover and low infiltration, encourages overland flow and soil erosion. Ploughing soils for cereal crop or soil operations for forestation procedures (that involve the use of heavy machinery and deep ploughing techniques) have a direct and influential effect on soil losses, essentially increasing them. Even in the first years of tree development, especially when it is necessary to control vegetation cover to prevent forest fires and competition from other species, the soil surface remains unprotected for extended periods of the year, thus accelerating water and sediment flows. In fact, ploughing completely destroys the vegetation and litter cover, breaks up the soil structure and reduces the number of obstacles to overland flow, leading to a more efficient transport of sediments. This hydrological response is also affected by the lack of macroporosities, meaning that only a little water infiltrates into the soil matrix. The soils also offered weak resistance to

The growth of cereals results in increased soil cover, which explains the higher runoff time and lower overland flow and sediment yields. Crops protect the soil surface from splash and surface sealing. In the initial growth stage, the area covered with plants is small but as the crop matures at the end of winter and early spring it plays an increasing role in protecting the soil surface (Nunes & Coelho, 2007). The average recorded values during the wet period were twice lower than the values recorded for ploughed land without cereal crops. In fact, the effectiveness of any crop, management system or protective cover depends in particular on how much protection may be available at

Recently abandoned fields or fallow land present the highest variation in overland flow response, with values ranging from 74% to 12% of the total rainfall. The soil erosion rate varies between 68 g m-2 h-1 and 3 g m-2 h-1. A detailed analysis shows that rainfall simulations performed during the dry period present overland flow and sediment yields that are significantly higher than those which occur during the wet season. The reason for the high overland flow and therefore the effect on erosion yields may be ascribed to the low plant cover density after a long, hot, dry season, and the presence of a microcrust (2-3 mm deep) in most of the plots (Nunes, 2007). This crust considerably reduces the infiltration capacity of the topsoil and its hydraulic conductivity also tends to decrease over time (Seeger & Ries, 2001), even in sandy soils (Kidron & Yair, 2001). The increased vegetation cover and destruction of the microcrust layer in wet periods reduces the runoff percentage and enhances infiltration capacity. At the same time, it also reduces splash and sediment detachment, and therefore the erosion rates. A similar intra-annual behaviour was observed under grazing plots, however a delay in runoff, an increase in soil infiltration capacity and a reduction in soil erodibility were recorded. Soil loss from the pasture plots was lower, approximately 4 times less than that of the fallow land or

**4.3 Hydrogeomorphic response** 

penetration (Nunes et al., 2010).

different times of the year.

recently abandoned fields.

soil loss amounts regardless of the season.


organic matter within the upper layer of the soil. These changes in soil surface conditions are related to the greater contribution to organic matter provided by the leaves and roots of both the annual and perennial species of these vegetal communities.

Significance level notations are: \*\* p<0.01, \* p<0.05 level (1-tailed), ns: not significant; Means within a column followed by different letters differ at the 0.05 probability level according to Waller-Duncan test; a: not available in this season.

Degree of hydrophobocity (% of ethanol): 0- hydrofilic; ≤5- slightly hydrophobic; ≤13- strongly hydrophobic; ≤ 24- severely hydrophobic

Table 2. Physical and chemical properties of soils (mean and ± standard deviation) in different land use types/covers

The amount of water in the soil changed significantly from one season to another. On average, the soil water content in the upper 10 cm rose from 0.8% during the dry period to 18.1 % during the wet period. The highest values were detected in the top layer of the recovering oak and pasture land plots. Under very wet conditions, all the soils were hydrophilic, which agrees with the finding of Coelho et al. (2005). Under dry conditions, the shrubland and *Quercus pyrenaica* woods showed water repellency, which was more pronounced in the tree formation. This may be due to the higher levels of litter cover and organic matter in the soil that were recorded. The results for soil water repellency show a spatial discontinuity in shrubland, linked to the spatial variability of the land cover. The substances responsible for the soil's ability to repel water are related to the organic compounds derived from living or decomposing plants (Doerr et al., 2000). In fact, the relationship between these two variables and water repellency are very high (Rs= 0.782 for the percentage of soil cover with litter and Rs= 0. 674 for the percentage of organic matter content). These results are similar to those referred to in literature on the subject, which considers that there is a strong correlation between soil water repellency and organic matter and litter cover (Coelho et al., 2005; Doerr and Moody, 2004).

#### **4.3 Hydrogeomorphic response**

68 Soil Erosion Studies

organic matter within the upper layer of the soil. These changes in soil surface conditions are related to the greater contribution to organic matter provided by the leaves and roots of

> Shrub land

71.96±5.32 22.50±4.88 5.48±1.56

g cm-3 (0-10 cm) 0.85±0.13a 1.23±0.11c 1.04±0.16b 0.91±0.16ab 0.81±0.06a 1.22±0.07c \*\*

penetration, g cm-2 0.77±0.30a 2.86±1.11b 2.22±1.24b 1.88±0.99b 0.60±0.22a 3.98±0.59c \*\*

(0-10 cm) 0.55±0.27 a 0.53 ±0.31 a 1.38±0.71b 1.46±0.23b 0.84±0.22a 0.73±0.26a \*\*

Water repellency, % 0.65±0.27a 2.65±1.98a 13.25±3.45c 19.75±2.36d 1.04±1.18a 6.24±0.91b \*\* Significance level notations are: \*\* p<0.01, \* p<0.05 level (1-tailed), ns: not significant; Means within a column followed by different letters differ at the 0.05 probability level according to Waller-Duncan test;

Degree of hydrophobocity (% of ethanol): 0- hydrofilic; ≤5- slightly hydrophobic; ≤13- strongly

The amount of water in the soil changed significantly from one season to another. On average, the soil water content in the upper 10 cm rose from 0.8% during the dry period to 18.1 % during the wet period. The highest values were detected in the top layer of the recovering oak and pasture land plots. Under very wet conditions, all the soils were hydrophilic, which agrees with the finding of Coelho et al. (2005). Under dry conditions, the shrubland and *Quercus pyrenaica* woods showed water repellency, which was more pronounced in the tree formation. This may be due to the higher levels of litter cover and organic matter in the soil that were recorded. The results for soil water repellency show a spatial discontinuity in shrubland, linked to the spatial variability of the land cover. The substances responsible for the soil's ability to repel water are related to the organic compounds derived from living or decomposing plants (Doerr et al., 2000). In fact, the relationship between these two variables and water repellency are very high (Rs= 0.782 for the percentage of soil cover with litter and Rs= 0. 674 for the percentage of organic matter content). These results are similar to those referred to in literature on the subject, which considers that there is a strong correlation between soil water repellency and organic matter

Table 2. Physical and chemical properties of soils (mean and ± standard deviation) in

0.80±0.22a 13.84±3.08ab Recovering oak

69.67±4.18 25.35±4.22 6.48±1.45

3.63±1.10bc 18.13±5.48b Afforested land

71.11±3.34 24.02±33.15 4.88±0.49

3.70±1.20bc 13.64±1.25ab Pasture land

72.04±3.34 22.77±4.39 5.19±1.43

4.00±1.83c 18.00±3.91b ANOVA

ns ns ns

\*\* \*

both the annual and perennial species of these vegetal communities.

Fallow land

76.70±1.95 19.01±1.47 4.30±0.65

1.20±0.98a 14.53±3.79ab

Cereal crop

74.18±4.17 20.99±3.14 4.92±1.09

2.18±1.32ab 11.1±1.81a

and litter cover (Coelho et al., 2005; Doerr and Moody, 2004).

Texture, % (0-20 cm)

Soil bulk density,

Soil resistance to

Organic matter, %

% (0-10cm) Dry season Wet season

Soil moisture content,

a: not available in this season.

hydrophobic; ≤ 24- severely hydrophobic

different land use types/covers

Sand Silt Clay

Table 3 summarizes the statistical analysis for the hydrological and sedimentological parameters in the different types of land use during the dry and wet periods. The results from the preceding ANOVA demonstrate significant statistical differences (*P* <0.000) for land use and soil covers, which means that variables have an important effect on runoff and soil loss amounts regardless of the season.

With high intensity rainfall, the afforested and laying fallow land produce the highest runoff and sediment yield coefficient, with an annual average of 64% and 45% of the rainfall and 75 g m-2 h-1 and 43 g m-2 h-1, respectively. These results show that the soil in these plots, which has very poor plant cover and low infiltration, encourages overland flow and soil erosion. Ploughing soils for cereal crop or soil operations for forestation procedures (that involve the use of heavy machinery and deep ploughing techniques) have a direct and influential effect on soil losses, essentially increasing them. Even in the first years of tree development, especially when it is necessary to control vegetation cover to prevent forest fires and competition from other species, the soil surface remains unprotected for extended periods of the year, thus accelerating water and sediment flows. In fact, ploughing completely destroys the vegetation and litter cover, breaks up the soil structure and reduces the number of obstacles to overland flow, leading to a more efficient transport of sediments. This hydrological response is also affected by the lack of macroporosities, meaning that only a little water infiltrates into the soil matrix. The soils also offered weak resistance to penetration (Nunes et al., 2010).

The growth of cereals results in increased soil cover, which explains the higher runoff time and lower overland flow and sediment yields. Crops protect the soil surface from splash and surface sealing. In the initial growth stage, the area covered with plants is small but as the crop matures at the end of winter and early spring it plays an increasing role in protecting the soil surface (Nunes & Coelho, 2007). The average recorded values during the wet period were twice lower than the values recorded for ploughed land without cereal crops. In fact, the effectiveness of any crop, management system or protective cover depends in particular on how much protection may be available at different times of the year.

Recently abandoned fields or fallow land present the highest variation in overland flow response, with values ranging from 74% to 12% of the total rainfall. The soil erosion rate varies between 68 g m-2 h-1 and 3 g m-2 h-1. A detailed analysis shows that rainfall simulations performed during the dry period present overland flow and sediment yields that are significantly higher than those which occur during the wet season. The reason for the high overland flow and therefore the effect on erosion yields may be ascribed to the low plant cover density after a long, hot, dry season, and the presence of a microcrust (2-3 mm deep) in most of the plots (Nunes, 2007). This crust considerably reduces the infiltration capacity of the topsoil and its hydraulic conductivity also tends to decrease over time (Seeger & Ries, 2001), even in sandy soils (Kidron & Yair, 2001). The increased vegetation cover and destruction of the microcrust layer in wet periods reduces the runoff percentage and enhances infiltration capacity. At the same time, it also reduces splash and sediment detachment, and therefore the erosion rates. A similar intra-annual behaviour was observed under grazing plots, however a delay in runoff, an increase in soil infiltration capacity and a reduction in soil erodibility were recorded. Soil loss from the pasture plots was lower, approximately 4 times less than that of the fallow land or recently abandoned fields.

2001).

Soil Erosion Under Different Land Use and Cover Types in a Marginal Area of Portugal 71

flow generation, mainly due to the high litter cover, but additionally as a result of the patchy and discontinuous nature of soil water repellency at plot level, as suggested by Ferreira et al. (2005). In fact, since water repellency is a patchy soil surface phenomenon, the existence of hydrophilic patches and macropores that allow water to infiltrate the soil will considerably reduce superficial water transport. Moreover, macroporosities created by roots and soil animals allow for infiltration into the deeper layer, despite the existence of strong water repellency in the top layer of the soils. Deep and interconnected macropores may cause rapid flow to the soil without significant water recharge into the matrix (Seeger & Ries,

Shrubland presents significantly higher overland flow and erosion rates during the dry summer period than in the wet season. These results can be related to an increase in soil water repellency after a long period without rainfall. In fact, a dual trend can be described (Fig. 4): first, runoff increases slightly from the beginning and a runoff discharge peak was detected between 15-20 minutes of the experiment. This behaviour could be due to the hydrophobic character of the soil surface. After that, infiltration increases, due to a decrease in soil water repellency after wetting. The hydrophobic substances act as a cement that binds the soil mineral particles together (Coelho et al., 2005) but has a tendency to attenuate in contact with water. Similar results, with a progressive decrease in runoff, have been observed, for example by Contreras López and Solé-Benet (2003), in semi-arid Mediterranean soil (SE Spain). Jordán et al. (2008) also detected a runoff rate peak after 20 minutes in dry conditions in a Mediterranean climate. Subsequently, the overland flow declined slightly. Another reason which explains why the highest runoff and erosion amounts occur during the dry season is associated with the disappearance of the herbaceous cover, which implies a higher percentage of bare soil and an increase in soil compaction.

Fig. 4. Shrubland runoff curves in the dry period.

**4.4 Relationship between variables (statistical analysis)** 

To determine which variables have more influence on runoff and sediment loss, Spearman correlations and principal components analysis were carried out. Analysing the relationship between all the data (Table 4), significant correlation coefficients between the characteristics


n*:* number of rainfalls simulations performed under dry and wet season. *(100%)*: Percentage of rainfall simulations with runoff. \*: Calculated values based on rainfall simulations with runoff. a: Not available in this season of the year.

Table 3. Overland flow and erosion yields measured during rainfall experiments

Measurements performed on recovering *Quercus pyrenaica* revealed very low or no overland flow in both dry and wet seasons. In these plots, the infiltration capacity exceeds the intensity and quantity of rainfall simulations; both soil erosion and surface runoff are very well controlled, ensuring soil conservation and even improving some of the soil characteristics (organic matter content, porosity, exchange capacity and nitrogen content). These results also suggest that water repellency does not play an important role in overland

Time to runoff Runoff (%) Erosion (g m-2 h-1)

Wet season

> 67.70 59.83 54.00 5.30

> 41.00 20.33 8.00 12.35

> 24,00 16.90 12.10 4.53

3.70 1.80 0.00 1.78

0.00 0.00 0.00 0.00

70.00 66.20 63.20 3.49

25.90 14.50 0.00 12.15

25.65 41.44 0.000

n*:* number of rainfalls simulations performed under dry and wet season. *(100%)*: Percentage of rainfall simulations with runoff. \*: Calculated values based on rainfall simulations with runoff. a: Not available

Measurements performed on recovering *Quercus pyrenaica* revealed very low or no overland flow in both dry and wet seasons. In these plots, the infiltration capacity exceeds the intensity and quantity of rainfall simulations; both soil erosion and surface runoff are very well controlled, ensuring soil conservation and even improving some of the soil characteristics (organic matter content, porosity, exchange capacity and nitrogen content). These results also suggest that water repellency does not play an important role in overland

Aver age

45.07

20.33

30.34

4.80

0.25

63.60

23.50

26.84 23.50 0.000

Dry season

> 56.36 28.55 3.40 18.20

> > a. a. a.

67.56 21.48 2.96 22.46

> 0.90 0.38 0.00 0.33

> 0.10 0.03 0.00 0.04

145.50 97.40 65.80 33.10

> 8.10 4.10 1.30 2.82

25.32 26.08 0.000

Wet season

> 84.86 57.66 43.68 16.32

> 34.70 18.13 4.20 12.01

9.13 5.92 2.90 2.28

0.24 0.06 0.00 0.10

0.00 0.00 0.00 0.00

87.00 52.20 36.80 21.82

4.80 2.40 0.00 1.98

19.48 27.11 0.000 Aver age

43.10

18.13

13.70

0.22

0.01

74.80

3.25

21.89 33.77 0.000

season

49.00 30.30 8.00 14.25

> a. a. a.

74.00 43.83 14.00 22.54

14.00 7.83 0.00 5.02

> 2.00 0.50 0.00 0.87

71.00 61.00 47.60 8.57

41.80 32.50 21.80 8.96

29.33 24.15 0.000

Table 3. Overland flow and erosion yields measured during rainfall experiments

Land use/

Ploughed land n=5+4 (100%)

Cereal crop

Fallow land n=6+4 (100%)

Shrub land n=4+4 (62.5%)

Recovering

Afforested land n=5+4 (100%)

Pastureland n=4+4 (87.5%)

in this season of the year.

Med. S. D. ANOVA (*p-value)*

oak n=4+4 (12.5%)

n=4 (100%)

plant cover (min.)\* Dry

Max. Med. Min. S.D

Max. Med. Min. S.D

Max. Med. Min. S.D

Max. Med. Min. S.D

Max. Med. Min. S.D

Max. Med. Min. S.D

Max. Med. Min. S.D

34.00 9.30 5.15 9.36

24.30 16.37 9.20 6.57

23.05 9.55 1.00 8.23

20.00 11.23 4.30 6.00

38.00 . . .

5.00 3.58 2.10 1.31

23.00 11.00 3.50 7.00

11.40 9.10 0.000 flow generation, mainly due to the high litter cover, but additionally as a result of the patchy and discontinuous nature of soil water repellency at plot level, as suggested by Ferreira et al. (2005). In fact, since water repellency is a patchy soil surface phenomenon, the existence of hydrophilic patches and macropores that allow water to infiltrate the soil will considerably reduce superficial water transport. Moreover, macroporosities created by roots and soil animals allow for infiltration into the deeper layer, despite the existence of strong water repellency in the top layer of the soils. Deep and interconnected macropores may cause rapid flow to the soil without significant water recharge into the matrix (Seeger & Ries, 2001).

Shrubland presents significantly higher overland flow and erosion rates during the dry summer period than in the wet season. These results can be related to an increase in soil water repellency after a long period without rainfall. In fact, a dual trend can be described (Fig. 4): first, runoff increases slightly from the beginning and a runoff discharge peak was detected between 15-20 minutes of the experiment. This behaviour could be due to the hydrophobic character of the soil surface. After that, infiltration increases, due to a decrease in soil water repellency after wetting. The hydrophobic substances act as a cement that binds the soil mineral particles together (Coelho et al., 2005) but has a tendency to attenuate in contact with water. Similar results, with a progressive decrease in runoff, have been observed, for example by Contreras López and Solé-Benet (2003), in semi-arid Mediterranean soil (SE Spain). Jordán et al. (2008) also detected a runoff rate peak after 20 minutes in dry conditions in a Mediterranean climate. Subsequently, the overland flow declined slightly. Another reason which explains why the highest runoff and erosion amounts occur during the dry season is associated with the disappearance of the herbaceous cover, which implies a higher percentage of bare soil and an increase in soil compaction.

Fig. 4. Shrubland runoff curves in the dry period.

#### **4.4 Relationship between variables (statistical analysis)**

To determine which variables have more influence on runoff and sediment loss, Spearman correlations and principal components analysis were carried out. Analysing the relationship between all the data (Table 4), significant correlation coefficients between the characteristics

Soil Erosion Under Different Land Use and Cover Types in a Marginal Area of Portugal 73

The role of organic matter in stabilizing aggregates against breakdown by water seems to be evident. In all the experiments, the organic matter content was negatively related to runoff and soil erosion (Rs: -0.694 and Rs: -0.708, respectively) (Table 4). Nevertheless, a more detailed analysis shows that after a hot, dry summer the organic matter in the top layer was more closely related to runoff (Rs: -0.731). In the wettest season, this variable seems to be less important in soil runoff and erosion rates, as the relationship has a lower significance (Rs: -0.450 for runoff and Rs: -0.502 for soil erosion). During this period, others variables show a highly significant correlation with runoff and sediment yield, such as total porosity

The high negative correlation between water repellency and runoff (Rs= -0.619) and soil erosion (Rs: -0.817) during the driest season inversely corresponds to what has been found in other studies, particularly at plot level, where a clear relationship between hydrophobicity and overland flow was detected (Doerr & Moody, 2004; Doerr et al., 2003; Ferreira et al., 2000). As Doerr & Moody (2004) state, apart from the spatial variability of repellent soil itself, additional spatially variable factors may influence the hydrological effects of water repellency: the variability of macropores (root channels, animal burrows)

Although slope gradient has been identified as a very important factor affecting runoff generation and soil erosion intensity (Fox & Rorke, 2000; Morgan, 1986), our analysis shows that its influence on runoff and erosion during the two contrasting seasons is insignificant in

A positive correlation was found between eroded sediments and runoff generation in all the experiments (in dry and wet periods), despite the closer relationship obtained for the wet

The results of principal components analysis (Tables 5 and 6), which covered 77.6% of the variance in the first four axes, are determined for the first two as 53.4% of the total variability. The results therefore imply that at plot level quantitative data on surface characteristics helps to explain the processes. Axis 1 shows the contrast between the organic supply to the soil (soil cover, litter cover, soil organic matter) and erosion (surface runoff and the movement of sediment). Component 2 describes the positive plant cover relationship between herbaceous shrub cover and vegetation height. The data indicate that, regardless of the type of vegetation, the factors that offer most protection against erosion are total plant cover and soil litter. This concords with the data proposed by Elwell & Stocking (1976) and Casermeiro et al. (2004). The supply of organic carbon to the soil is also related to the role of vegetation. Soils with a higher organic carbon content appear to offer good protection against erosion. Slopes appear to be less important than plant cover in the

Component number Eigenvalue Percent of variance Cumulative

36.342 17.033 12.390 11.808 percentage

36.342 53.375 65.766 77.574

(Rs: 0.557; Rs: 0.599) and resistance to penetration (Rs= -0.444; Rs= -0.580).

will affect infiltration and water movement in repellent terrain.

season (Rs: 0.948) as opposed to dry season experiments (Rs: 0.869).

generation of surface runoff and, therefore, in sediment transport.

4.361 2.044 1.487 1.417

terms of soil erodibility.

Extraction Method: Principal Component Analysis.

Table 5. Total Variance Explained

of the plot and runoff and sediment production were found, in both the wet and dry period. The results obtained show that, annually, total plant cover is the main factor which explains runoff generation (Rs= -0.824) and the movement of sediments (Rs= -0.913). This concords with other studies (Grove & Rackham, 2001; Kosmas et al., 2000; Lattanzi & Meyer, 1974; Trimble, 1990) which found a (negative) linear relationship between the total volume of overland flow and the mass per unit of plant cover. In fact, plant cover decreases the kinetic energy of the rain that is released and dissipated to the soil surface, reducing the amount of detachment and, hence, the erosion that can occur (Greene & Hairsine, 2004). As a result, the increase in soil cover corresponds to an increase in the range of conditions under which the soil surface is stable and therefore less exposed to crust or seal. Consequently, an increase in plant cover during a wet season decreases splash and sediment detachment and improves the negative correlation coefficients (Rs: -0.836 for overland flow and Rs= -0.940 for erosion rates).

Vegetation type is also important, with litter cover offering more protection against overland flow and water erosion, especially under dry conditions. Several studies have analysed the influence of various forms of cover on the formation of crust and seal (Greene & Hairsine, 2004). This cover can be a canopy cover of vegetation or contact covers, such as litter cover, mulch or cryptogams (mosses, lichens, etc.), which form in close association with the soil surface. Vegetal litter cover absorbs some of the energy of raindrops and leads to an exponential decrease in splash erosion (Casermeiro et al., 2004).


 Significance level notations are: \*\* p<0.01, \* p<0.05 level (1-tailed), ns: not significant, a: not available in this season.

Table 4. Spearman-rho correlations between runoff and erosion and some characteristics of the soils.

of the plot and runoff and sediment production were found, in both the wet and dry period. The results obtained show that, annually, total plant cover is the main factor which explains runoff generation (Rs= -0.824) and the movement of sediments (Rs= -0.913). This concords with other studies (Grove & Rackham, 2001; Kosmas et al., 2000; Lattanzi & Meyer, 1974; Trimble, 1990) which found a (negative) linear relationship between the total volume of overland flow and the mass per unit of plant cover. In fact, plant cover decreases the kinetic energy of the rain that is released and dissipated to the soil surface, reducing the amount of detachment and, hence, the erosion that can occur (Greene & Hairsine, 2004). As a result, the increase in soil cover corresponds to an increase in the range of conditions under which the soil surface is stable and therefore less exposed to crust or seal. Consequently, an increase in plant cover during a wet season decreases splash and sediment detachment and improves the negative correlation coefficients (Rs: -0.836 for overland flow and Rs= -0.940 for erosion

Vegetation type is also important, with litter cover offering more protection against overland flow and water erosion, especially under dry conditions. Several studies have analysed the influence of various forms of cover on the formation of crust and seal (Greene & Hairsine, 2004). This cover can be a canopy cover of vegetation or contact covers, such as litter cover, mulch or cryptogams (mosses, lichens, etc.), which form in close association with the soil surface. Vegetal litter cover absorbs some of the energy of raindrops and leads

> Dry period n= 28

> > Erosion (g m-2 h-1)

Runoff (%)

Wet period n= 28

> Erosion (g m-2 h-1)

Runoff (%)

to an exponential decrease in splash erosion (Casermeiro et al., 2004).

Runoff (%)

Year (dry + wet period) n= 56

> Erosion (g m-2 h-1)

Slope (%) ns -0.280\* ns ns ns ns Total plant cover (%) -0.824\*\* -0.913\*\* -0.726\*\* -0.882\*\* -0.836\*\* -0.940\*\* Lichens + Mosses (%) ns ns ns ns ns ns Herbs+ shrubs (%) ns ns ns ns ns -0.375\* Litter cover (%) -0.675\*\* -0.782\*\* -0.681\*\* -0.795\*\* -0.711\*\* -0.795\*\*

(cm) -0.369\*\* -0.395\*\* ns -0.386\* -0.588\*\* -0.541\*\*

moisture (%) -0.360\*\* -0.307\* ns ns ns ns

penetration(g m-2) ns -0.369\*\* ns -0.393\* -0.444\* -0.580\*\* Total of porosity (%) 0.230\* 0.357\*\* ns ns 0.557\*\* 0.599\*\* Water repellency (%) ns -0.391\*\* -0.619\*\* -0.817\*\* a a Soil organic matter (%) -0.694\*\* -0.708\*\* -0.731\*\* -0.676\*\* -0.450\* -0.502\*\* Runoff (%) . 0.917\*\* . 0.869\*\* . 0.948\*\* Significance level notations are: \*\* p<0.01, \* p<0.05 level (1-tailed), ns: not significant, a: not available in

Table 4. Spearman-rho correlations between runoff and erosion and some characteristics of

rates).

Height of vegetation

Antecedent soil

Resistance to

this season.

the soils.

The role of organic matter in stabilizing aggregates against breakdown by water seems to be evident. In all the experiments, the organic matter content was negatively related to runoff and soil erosion (Rs: -0.694 and Rs: -0.708, respectively) (Table 4). Nevertheless, a more detailed analysis shows that after a hot, dry summer the organic matter in the top layer was more closely related to runoff (Rs: -0.731). In the wettest season, this variable seems to be less important in soil runoff and erosion rates, as the relationship has a lower significance (Rs: -0.450 for runoff and Rs: -0.502 for soil erosion). During this period, others variables show a highly significant correlation with runoff and sediment yield, such as total porosity (Rs: 0.557; Rs: 0.599) and resistance to penetration (Rs= -0.444; Rs= -0.580).

The high negative correlation between water repellency and runoff (Rs= -0.619) and soil erosion (Rs: -0.817) during the driest season inversely corresponds to what has been found in other studies, particularly at plot level, where a clear relationship between hydrophobicity and overland flow was detected (Doerr & Moody, 2004; Doerr et al., 2003; Ferreira et al., 2000). As Doerr & Moody (2004) state, apart from the spatial variability of repellent soil itself, additional spatially variable factors may influence the hydrological effects of water repellency: the variability of macropores (root channels, animal burrows) will affect infiltration and water movement in repellent terrain.

Although slope gradient has been identified as a very important factor affecting runoff generation and soil erosion intensity (Fox & Rorke, 2000; Morgan, 1986), our analysis shows that its influence on runoff and erosion during the two contrasting seasons is insignificant in terms of soil erodibility.

A positive correlation was found between eroded sediments and runoff generation in all the experiments (in dry and wet periods), despite the closer relationship obtained for the wet season (Rs: 0.948) as opposed to dry season experiments (Rs: 0.869).

The results of principal components analysis (Tables 5 and 6), which covered 77.6% of the variance in the first four axes, are determined for the first two as 53.4% of the total variability. The results therefore imply that at plot level quantitative data on surface characteristics helps to explain the processes. Axis 1 shows the contrast between the organic supply to the soil (soil cover, litter cover, soil organic matter) and erosion (surface runoff and the movement of sediment). Component 2 describes the positive plant cover relationship between herbaceous shrub cover and vegetation height. The data indicate that, regardless of the type of vegetation, the factors that offer most protection against erosion are total plant cover and soil litter. This concords with the data proposed by Elwell & Stocking (1976) and Casermeiro et al. (2004). The supply of organic carbon to the soil is also related to the role of vegetation. Soils with a higher organic carbon content appear to offer good protection against erosion. Slopes appear to be less important than plant cover in the generation of surface runoff and, therefore, in sediment transport.


Extraction Method: Principal Component Analysis.

Table 5. Total Variance Explained

Soil Erosion Under Different Land Use and Cover Types in a Marginal Area of Portugal 75

runoff and erosion processes. Sauer & Ries (2008) consider that only plant cover exceeding

Land use and the type of management applied to each site explain, to a large extent, the variability in annual plant cover and, therefore, the occurrence of overland flow and soil erosion processes (De Luna et al., 2000; Francia Martínez et al., 2006; Gómez et al., 2004). Annually, as it involves mobilisation of the top layer (laying fallow and afforested land), ploughed soil erodes more easily and causes great soil loss. Cereal growth, mainly when it offers a good vegetal protection for the surface of the soil, enhances infiltration and reduces erosion rates. However, the results for soil erosion were greater in comparison with recent abandonment and were one hundred times greater if compared to land cover after verylong abandonment (Table 7). These results also enable us to conclude that traditional cereal cultivation, in particular ploughing in preparation for cereal crops, is a very negative land management practice due to the high runoff and water erosion response. Organic matter content, probably the most important component of soil quality, is also strongly influenced and registered very low figures of less than 1% for arable land. A limit of 1.7 per cent of soil organic matter content is considered an indicator of the pre-desertification stage (Pardini et al., 2002). The gradual depletion of nutrients, which reduces soil fertility and creates a high level of soil degradation, are further reasons for abandoning agricultural plots in the changing cultivation process (Paniagua et al., 1999). Planting trees, according the CAP measures for afforestation of marginal fields, with deep ploughing and bare soil has resulted in very high erosion rates during both rainfall seasons, as observed in other agro-ecosystems in Mediterranean Europe (Shakesby et al., 2002; Ternan et al., 1997; Van-Camp et al., 2004).

Fig. 5. Relationship between runoff and soil erosion and average percentage of plant cover However, in large parts of marginal areas of the country farmland abandonment has enhanced plant colonisation, replacing historically highly erosive cereal fields with dense

60% can significantly reduce soil erosion in semi-arid environments.

Axis 3 shows both the expected opposition between soil porosity and resistance to soil penetration, and its positive relationship to the existence of lichens and mosses, which means less disturbance of the soil surface. Obviously, antecedent soil moisture does not favour water repellency, hence the opposition shown in Axis 4. These two components explain about 24 % of the total variance observed.


Extraction Method: Principal Component Analysis; Rotation Method: Varimax with Kaiser Normalization.

Table 6. Results of Rotated Component Matrix
