Impact of different parts of unpaved forest roads on runoff and sediment yield in a Mediterranean area

S CIE N CE OF T H E TOT AL E N V I RO N ME N T 4 0 7 ( 2 00 9 ) 9 3 7–9 44

a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m

w w w. e l s e v i e r. c o m / l o c a t e / s c i t o t e n v

Impact of different parts of unpaved forest roads on runoff and sediment yield in a Mediterranean area Antonio Jordán-López⁎, Lorena Martínez-Zavala, Nicolás Bellinfante MEDSoil Research Group, Dpto. de Cristalografía, Mineralogía y Química Agrícola, Facultad de Química (Universidad de Sevilla), Sevilla, Spain

AR TIC LE D ATA

ABSTR ACT

Article history:

Surface runoff and sediment production on unpaved forest roads in a humid Mediterranean

Received 3 June 2008

mountainous area has been studied using a simple portable rainfall simulator at an intensity of

Received in revised form

90 mm h− 1. Thirty six rainfall simulations were carried out on road plots: on the roadbank (12),

10 September 2008

on the sidecast fill (12), and on the roadbed (12). On the roadbanks, the steady-state runoff

Accepted 19 September 2008

coefficient was 85.9% and runoff flow appeared after 63 s on average. On the sidecast fills, the

Available online 7 November 2008

steady-state runoff coefficient was 58.6% and mean time to runoff was 48 s. Finally, on the roadbeds, the steady-state runoff coefficient was 21.5% and mean time to runoff was 41 s. The

Keywords:

highest soil loss rate was found on the roadbanks (486.7 g m− 2), mainly due to low plant cover,

Forest roads

soil texture and rock fragments. The total soil erosion on the roadbanks was 3 and 18 times

Hydrologic impact

higher than those from the roadbeds and the sidecast fills, respectively. As a consequence,

Rainfall simulation

roadbanks can be considered the main source of sediments on the studied sites, but the

Soil erosion

function of unpaved forest roads as source points for runoff generation is more important.

Southern Spain

1.

Introduction

Most efforts to reduce in situ soil erosion and off-site sedimentation in Mediterranean countries have traditionally focussed on the improvement of soil tillage and soil conservation techniques (Englisch et al., 2000; Giordano et al., 2000). Nevertheless, the recent development of forest road networks brings out the role of roads as runoff and sediment sources, which has not been sufficiently considered. It is well known that unpaved forest roads may cause many local changes to soil properties and hydrologic behavior of hillslopes, increasing soil erosion and mass movements after very strong rainstorms (Gresswell et al., 1979; Sidle et al., 1985; Larsen and Parks, 1997; Gucinski et al., 2001;) or as a consequence of the of raindrops splash and surface runoff (Froehlich, 1995; Ziegler et al., 2000a). Construction of roads constitutes the most damaging facet of forestry activities: the forest has to be cleared

© 2008 Elsevier B.V. All rights reserved.

for them and they are thus a cause of deforestation. Soil erosion is particularly important in forested areas, because natural erosion rates tend to be very low (Ramos-Scharrón and MacDonald, 2005). The transformation of natural hillslope profiles, the interception of surface and subsurface flows, and the construction of roadbanks, low plant cover, and the compaction of soil on the roadbed are the causes that may clear up these processes (Tague and Band, 2001). In addition, forest roads produce a more impermeable layer for the initiation of surface flow than do other watershed land surfaces (Reid and Dunne, 1984; Croke et al., 1999; Bubb and Croton, 2002). Overland flow from forest roads can carry sediments eroded from the road surface, extend channel systems (Montgomery, 1994; Wemple, 1994), and increase the risk of landslides (Sidle et al., 1985). Watersheds with dense road networks commonly experience increased sedimentation and peak flows. Forest roads often act as linearly connected systems, so that surface runoff flow may travel downslope towards the

⁎ Corresponding author. Dpto. de Cristalografía, Mineralogía y Química Agrícola, Facultad de Química (Universidad de Sevilla), C/Profesor García González, 1. CP: 41012-Sevilla, Spain. Tel.: +34 954556950; fax: +34 954557141. E-mail address: [email protected] (A. Jordán-López). URL: http://grupo.us.es/medsoil (A. Jordán-López, L. Martínez-Zavala, N. Bellinfante). 0048-9697/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2008.09.047

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Fig. 1 – Study area. Black triangles indicate the location of the experimental plots. stream network. Roads are therefore potentially susceptible to hydraulic erosion processes, and may contribute substantially to stream sedimentation, even during low-magnitude rainfall events (Ziegler et al., 2001). There are also other undesirable ecological consequences, as loss of forest production alongside roads, barrier effects, disturbance of breeding areas or migration routes of animal species, and compression of soil structures. These processes are of particular interest in forested areas, where natural erosion rates tend to be very low. A better understanding of road-sediment production rates is needed to guide future development and erosion control efforts. The aim of this work is to quantify runoff and soil erosion from unpaved forest roads in a Mediterranean humid area where rainfall can be very intense. In this study, rainfall si-

mulation tests were used to [1] study the hydrological and erosive response from different parts of unpaved forests roads (the roadbank, the sidecast fill, and the roadbed), [2] correlate these data to the site characteristics, and [3] compare our results with runoff and soil loss data obtained by other authors.

2.

Methods

2.1.

Study site

This study was carried out in Sierra de Aracena Natural Park (SW Spain), approximately on the coordinates 37°54′ N and 6°40′ W (Fig. 1). The study area is included in the Sierra de Aracena Natural Park, which covers approximately 186,000 ha

Fig. 2 – A view of the effects of intense rainfall induced erosion on roadbanks in the study area.

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Fig. 3 – Cross section of an unpaved road showing the three studied parts: roadbank, roadbed and sidecast fill.

of the Sierra Morena range (southern Spain). The relief has a very irregular topography, and the elevation ranges from 460 to 959 masl. The Lithological substrate is mainly composed of slates and acid igneous materials, which produces acidic and nutrient-poor soils (such as Leptosols, Regosols and Cambisols, classified according to FAO, 2006). The climate is Mediterranean-type, with cool, humid winters and warm, dry summers. The mean annual rainfall is 1100 mm. The mean air temperature ranges from 7 °C (January) to 25 °C (August). The main land use is the dehesa, a savannah-like landscape characterized by the presence of Holm oaks (Quercus rotundifolia and Quercus ilex) and cork oaks (Quercus suber) distributed intermittently (10–80 trees ha− 1) with an understorey of grassland and Mediterranean shrubs. Some other land uses are pines (Pinus pinaster, P. halepensis and P. pinea), eucalyptus (Eucalyptus globulus and E. camaldulensis), sweet chestnut (Castanea sativa) cultivations and riparian forests. Holm oaks and cork oaks in the study area are older than 30–40 years, while pines, eucalyptus and other cultivated trees show different age segments. The main economic activities are free-range pig livestock, cork extraction, hunting, mineral extractions, and eco-tourism.

2.2.

Rainfall simulation and field experiments

Unpaved forest roads ascending from 500 m to 650 m above sea level were selected for this experiment. Fig. 2 shows an example of the effects of rainfall induced erosion on roadbanks in the study area. The width of the road varies from 200 cm to 450 cm, with a gradient of 3–6%. These roads are used by touring cars and off-road vehicles, such as jeeps, and staff vehicles. The intensity of vehicle use can be relatively high at some points. Rainfall simulations were carried out to measure runoff and soil loss from the three different parts of the road (Fig. 3): [1] the roadbank (the cutslope created by excavation into the natural hillslope; it is steeper than the natural slope, its height varies between 1 m and 4 m, and it has sparse plant cover), [2] the sidecast fill (unconsolidated excavated material pushed to the slope below the road; sidecasts are generally not used as part of the road and are steeper than the natural slope), and [3] the roadbed (the unpaved surface of the road on which the vehicles travel). Twelve experiments were carried out on each part of the selected roads at the most-representative points. Topsoil and surface characteristics of roadbanks, roadbeds, and sidecast fills were described in each plot. We used a rainfall simulator similar to that described by Navas et al. (1990) and Lasanta et al. (2000). The structure, in

the shape of a truncated pyramid, is supported on metal legs. The simulator was covered with a plastic curtain to protect the experiments against the wind. The legs are telescopic so that the simulator can be leveled when placed on a sloping surface. At the top of the structure there is a single hollow-con nozzle (Lechler 460.880), which is connected through a rubber pipe to a mobile automatic pump (1.8 kg cm− 2 pressure). The water from the nozzle falls onto a squared area of 80 × 80 cm2 that is delineated by a steel frame. The frame was carefully tapped into the soil following the slope to prevent leakage and direct the runoff flow to the outlet of the plot. Before the experiments, rainfall intensity was measured by five rain gauges (10 cm in diameter) distributed uniformly over the plot. The mean rainfall intensity for the experiments was 90 mm h− 1 (standard deviation 1.86) and the duration of the simulations was 30 min to reach the steady-state. The selected rainfall intensity is supported by nearby weather stations data: some recurrence periods at nearby weather stations are 5 years/82.6 mm h− 1 (Cortegana), 5 years/102.2 mm h− 1 (Aracena), and 5 years/95.7 mm h− 1 (Alájar). We used distilled water because the chemical composition of the water may influence the soil response (Agassi et al., 1994). A gutter installed on the downstream side of the plot and covered with a plexiglas enclosure conducted the runoff to a sample collection box. For each rainfall test, we recorded the time-torunoff (TR). Generally, a steady-state was reached after 15 min.

Table 1 – Characterization of the experimental plots Roadbank

Sidecast fill

Slope% Mean 40.1a 29.2b CV 0.07 0.09 Surface rock fragment cover% 7.6b Mean 10.8a CV 0.30 0.49 Embedded rock fragment cover% 12.6b Mean 20.4a CV 0.15 0.22 Plant cover% Mean 15.8a 20.1b CV 0.14 0.48

Roadbed

ANOVA, p

10.1c 0.40

0.0000

1.9c 0.41

0.0000

3.9c 0.13

0.0000

1.7c 0.16

0.0000

Within a row means followed by the same letter are not significantly different. CV: coefficient of variation.

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Table 2 – Soil characteristics Roadbank

Sidecast fill

Roadbed

ANOVA, p

253.7b 0.37

280.7a,b 0.29

0.0158

621.6 0.03

561.5 0.05

124.7b 0.46

157.8b 0.36

0.0019

10.1b 0.19

7.3c 0.35

0.0000

−1

Sand (g kg ) Mean 330.5a CV 0.16 Silt (g kg− 1) Mean 447.5 CV 0.07 Clay (g kg− 1) Mean 222.0a CV 0.20 Organic matter (g kg− 1) Mean 14.7a CV 0.12

Parameter

Estimate

p

R

a b a b a b

39.6 29.0 6.9 9.1 35.5 12.9

0.0000 0.0000 0.0000 0.0000 0.0000 0.0001

0.889

Roadbank Sidecast fill

Within-a-row means followed by the same letter are not significantly different. CV: coefficient of variation.

So, the steady-state runoff coefficient (SSRR) could be calculated as the average value for the last seven runoff measurements for each test. The simulations were carried out in June–August 2007. Due to the summer drought, average topsoil gravimetric moisture in the three parts of the roads was 7–9%, but these small differences did not affect the simulation experiments. Plant cover, rock fragment cover on the soil surface and embedded rock fragment cover were determined using a 50 × 50 cm2 grid with cells of 0.25 cm2. The slope angle of the soil surface in each plot was determined using a clinometer.

2.3.

Table 3 – Estimated parameters of the logarithmic function describing the relationship between runoff rate and time of simulation, fitted by least squares approximation; y = a + b log10(x)

Soil analysis

Three samples of the top soil (0–10 cm deep) were collected for physical and chemical analysis 20 cm downslope from the plots, and mean values were taken as representative for each plot. The soil surface (0–20 cm depth) was sampled for physical and chemical analysis before the simulations. Soil samples 0.5 m from the plot were collected in order to determine the

Fig. 4 – Behavior of surface runoff from rainfall simulations on different parts of unpaved forest roads. Verticals bars are standard errors.

Roadbed

0.963 0.832

particle-size distribution and organic matter. Particles N2 mm were determined by wet sieving, and particles b2 mm were determined according to USDA (2004). A fraction of each sample was air-dried and sieved (0–2 mm) for soil organic carbon analysis, determined by the Walkley & Black method (MAPA, 1982). The organic matter content was calculated multiplying the organic carbon content by the Van Bemmelen factor (1.724).

2.4.

Data analysis

The effects of slope, rock fragment cover, and plant cover on soil erosion in the different parts of the road were evaluated by statistical analysis of the data. This included correlations, regression, and ANOVA. Assumptions of normality and homocedasticity were tested using the Shapiro–Wilk and Brown– Forsyth tests, respectively. Since most of the variables did not satisfy these assumptions, alternative non-parametric tests were used (Spearman rank-correlation coefficient and Kruskall–Wallis ANOVA for multiple independent samples). When ANOVA null hypothesis was rejected, post hoc pairwise comparisons (Bonferroni test) were performed to investigate differences between pairs of means. The relationship between runoff rate and time of simulation was described by a logarithmic function closely fitted by least squares approximation. All computations and graphical displays were made using STATISTICA version 6 (StatSoft, Inc., 2001).

3.

Results

3.1.

Description of the plots

The slope, surface rock fragment cover, embedded rock fragment cover and plant cover of the plots are shown in Table 1. After the ANOVA, some significant differences between the different parts of the studied roads were detected. The average slope value was 40.1%, 10.1%, and 29.2% for the roadbanks, roadbeds, and sidecast fills respectively. Rock fragments on the soil surface were relatively frequent on the roadbank (10.8%) and on the sidecast fill (7.6%), but they were very rare on the roadbed. Most of the rock fragments were prismatic (as a result of the weathering of slates) and were resting paralleled to the soil surface. Embedded rock fragment covers were generally larger than rock fragment cover on the soil surface. Embedded rock

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Table 4 – Average time to runoff, steady-state runoff rate, sediment yield and total soil loss on different parts of the studied roads Roadbank

Sidecast fill

Time to runoff (s) Mean 63a 48b CV 0.14 0.23 Steady-state runoff coefficient% 21.5b Mean 85.9a CV 0.15 0.17 Sediment yield (g L− 1) 3.2b Mean 13.7a CV 0.05 0.11 Total soil loss (g m− 2) 27.2b Mean 486.7a CV 0.17 0.26

Roadbed

Table 5 – R-Spearman and p-value between runoff rate and site characteristics Roadbank

ANOVA, p

41b 0.29

0.0003

58.6c 0.15

0.0000

c

6.2 0.11

0.0000

154.4c 0.22

0.0000

Slope Surface rock fragment cover Embedded rock fragment cover Plant cover Sand Clay Organic matter

Sidecast fill

Roadbed

R

p

R

p

R

p

0.105 −0.748

0.746 0.007⁎

−0.420 −0.573

0.175 0.051

0.077 − 0.685

0.812 0.018⁎

0.748

0.012⁎

0.587

0.045⁎

0.685

0.014⁎

−0.748 −0.689 0.786 −0.538

0.005⁎ 0.025⁎ 0.036⁎ 0.071

−0.685 −0.758 0.666 −0.734

0.014⁎ 0.048⁎ 0.032⁎ 0.007⁎

0.336 − 0.323 0.689 − 0.874

0.286 0.021⁎ 0.026⁎ 0.000⁎

Marked p-values are significant (b 0.05). SD: standard deviation.

Within-a-row means followed by the same letter are not significantly different. CV: coefficient of variation.

3.3.

fragment cover was 20.4% on the roadbanks, 12.6% on the sidecast fills and just 3.9% on the roadbeds. Plant cover was relatively high on the roadbanks and on the sidecast fills, but its value was very low on the roadbeds. Table 2 shows the results of the soil analysis. Generally, the soil texture was silt–loam, although the clay content was significantly higher on the roadbanks (222.0 g kg− 1). The ANOVA also showed some significant differences for the organic matter content. The roadbed had the lowest organic matter content (7.3 g kg− 1).

3.2.

Hydrological response

Fig. 4 shows the behavior of runoff during the 30 min rainfall simulation tests. The runoff rate increased quickly during the first 8–10 min for each part of the road. It then remained steady until the end of the simulation tests probably because of surface soil saturation with water and surface sealing. The runoff rate at every part of the road was described by a logarithmic function fitted by least squares approximation (Table 3). ANOVA analysis showed significant differences in time to runoff data between the three parts of the road (p = 0.0003), specifically the mean time to runoff was longer at the roadbank (Table 4). Time to runoff on the roadbanks ranged from 48 to 74 s. The runoff flow appeared after 23–60 s on the roadbeds and after 28–67 s on the sidecast fills. The highest mean steady-state runoff coefficient was determined for the roadbank (85.9%), with data ranging from 77% to 94.8% (Table 4). Steady-state runoff coefficients for the sidecast fill and the roadbed were 21.5% and 58.6%, respectively. Table 5 shows the correlations between runoff and the site characteristics. Correlations were significant and negative for surface rock fragment cover at the roadbank and the roadbed but not significant at the sidecast fill. Nevertheless the correlations were significant but positive for embedded rock fragment cover and clay content at the three parts of the road. Plant cover seems to be very important controlling the runoff generation only on the roadbanks (R = −0.748; p = 0.007) but organic matter content, in contrast is well correlated to runoff on the sidecast and roadbed plots.

Soil loss

The highest sediment concentration in runoff was detected on the roadbank, where mean sediment yield was 13.7 g L− 1 and total soil loss was 486.7 g m− 2 (Table 4). Data varied between 405.5 g m− 2 and 708.4 g m− 2. The mean soil loss from the sidecast and the roadbed was 27.2 g m− 2 and 154.4 g m− 2, respectively. The evolution of sediment concentration in runoff is represented in Fig. 5. Sediment concentration increased linearly at the beginning because it took time for the soil to get wet, as well as for the particles to become detached. Sediment concentration increased during the first 6–8 min of rainfall. After 10 min from the beginning of the experiment, there was a steady decrease in sediment concentration. The sediment yield increased from 9.0 g L− 1 (2 min after the rainfall started) to 25.2 g L− 1 (8 min) for the roadbank. The peak of sediment concentration was 8.6 g L− 1 for the roadbed after 8 min, and 4.3 g L− 1 for the sidecast fill after 18 min.

Fig. 5 – Sediment concentration in runoff from different parts of unpaved forest roads. Verticals bars are standard errors.

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Table 6 – R-Spearman and p-value between soil loss and site characteristics Soil loss

Slope Surface rock fragment cover Embedded rock fragment cover Plant cover Sand Clay Organic matter

Roadbank

Sidecast fill

Roadbed

R

p

R

p

R

p

−0.007 −0.084

0.983 0.795

−0.266 −0.406

0.404 0.191

0.084 −0.685

0.795 0.014⁎

0.084

0.795

0.517

0.085

0.685

0.014⁎

−0.084 0.238 −0.238 −0.14

0.795 0.457 0.457 0.665

−0.462 −0.839 0.839 −0.503

0.266 −0.783 0.783 −0.643

0.404 0.003⁎ 0.003⁎ 0.024⁎

0.131 0.001⁎ 0.001⁎ 0.095

Marked p-values are significant (b 0.05). SD: standard deviation.

Table 5 shows the R-Spearman coefficients between runoff rate and the characteristics of the plots. Surface rock fragment cover were negatively correlated to runoff rates on the roadbanks and the roadbeds, while embedded rock fragments were positively correlated to runoff on the three parts of the roads. Plant cover enhanced the infiltration rates and reduced the runoff coefficient downslope, especially on roadbanks and on the sidecast fills, but it showed no correlations on the roadbeds (where plant cover is always relatively low). The sand content and clay content of the soil have significant correlations with runoff rates on the three parts of the roads, affecting in a negative or positive manner respectively. Finally, the organic matter content contributed to reduce the runoff rate on the roadbed and on the sidecast fill. Table 6 shows the R-Spearman coefficients between soil loss and the characteristics of the plots. No significant correlations were determined between soil loss and the characteristics of the plots on the roadbanks. Some significant correlations were obtained for roadbeds and sidecast fills. On the roadbeds, soil loss was negatively correlated to the coverage of rock fragments on the soil surface, sand content and organic matter content, but positively correlated to embedded rock fragment cover and clay content. On the sidecast fills, soil loss was mainly influenced by the soil texture.

4.

The relationship between soil loss rates and steady-state runoff coefficients for each part of the roads is shown in Fig. 6. The highest runoff coefficients and soil losses were measured on the roadbanks. The steep slopes of the roadbanks can enhance both runoff generation and sediment yield (Diseker and Sheridan, 1971; Luce and Black, 1999; Arnaez et al., 2004; Jordán and Martínez-Zavala, 2008). High erosion rates induce sediment, nutrients and seed losses (Cerdà et al., 2000) which make the natural recovery very difficult even many years after construction. The soil plots on the sidecast fills show the lowest values of steady-state runoff rate and soil loss. The construction of forest roads generates loose materials that enhance infiltration and reduce surface runoff. The higher percentage of plant cover on the sidecast fills also reduces the effect of rain splash erosion. On the roadbed, generated sediments are mobilized by overland flow during rain events. On the simulation plots, the transport of sediments is characterized by an initial water flow occurring soon after runoff initiation (Ziegler et al., 2000b, 2002). The physical nature of the road surface tends to favor the movement of detached material (Ziegler et al., 2004). Rock fragment cover is also important when the plant cover is low. Rock fragments on the soil surface can facilitate infiltration and reduce runoff generation. In contrast, embedded rock fragments contribute to runoff generation, especially on the roadbanks. Surface or embedded rock fragments were scarce on the roadbed, but their effect was as important as soil texture or soil organic matter. Anyway, the compaction of the soil surface on the roadbed can partly explain the lower sediment yield. Although it was observed, soil sealing has not been quantified in this study; but it can facilitate runoff generation and on the roadbeds, avoiding the detachment of soil particles from the aggregates. The compacting of roadbed is also related to the intensity of forest road use as suggested by some authors (Reid and Dunne, 1984; Ziegler et al., 2001). Constantini et al. (1999) suggested that the enrichment of fine material in road runoff is due to the selective transport of fine particles by interrill transport which dominates on low angle slopes.

Discussion

Unpaved forest roads can modify the hydrological behavior of hillslopes. Soil loss on road embankments and roadbeds is usually distressing in many areas (Swift, 1984; Anderson and MacDonald, 1998; Cerdà, 2007). Unpaved roads can increase the hillslope-scale sediment production rates by up to four orders of magnitude relative to undisturbed conditions (Ramos-Scharrón and MacDonald, 2005). Wemple et al. (2001) described the erosion processes on forest roads, accentuating the effects of raindrop splash and surface runoff. Arnaez et al., (2004) showed that the gradient, plant cover and rock fragment cover of the roadbanks, sidecast fill areas and roadbeds had statistically significant effects on runoff and erosion. Roadbanks can be the primary source of sediments on forest roads (Arnaez et al., 2004; Jordán and Martínez-Zavala, 2008), but soil loss rates are partly determined by the vegetation cover, rock fragments and soil textural characteristics.

Fig. 6 – Relationship between the steady-state runoff coefficients and soil loss rates for each part of unpaved forest roads.

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Table 7 – Comparison of some values for rainfall simulations on different parts of roads and road types from published literature Part of the road/type of road Roadbanks

Vegetated road 10 years-old roadbanks (summer) Vegetated road 10 years-old roadbanks (winter) Non-vegetated in construction roadbanks (summer) Non-vegetated in construction roadbanks (winter) Sidecast fills

Gravel surface roads Unpaved roadbeds

Source

Plot area cm2

Selkirk and Riley (1996) Arnaez et al. (2004) Jordán and MartínezZavala (2008) Cerdà (2007)

10,000 1385 625 3941–4202

Cerdà (2007)

Sediment conc. in runoff g L− 1

31 57.8 57.8

45

2.0–2.2

6.5–9.0

3941–4202

45

1.2–1.6

32.7–36.2

Cerdà (2007)

3941–4202

45

13.5–22.0

29.3–36.1

Cerdà (2007)

3941–4202

45

11.1–15.1

53.3–67.2

Selkirk and Riley (1996) Jordán and MartínezZavala (2008) Arnaez et al. (2004) Selkirk and Riley (1996) Ziegler et al. (2001) Jordán and MartínezZavala (2008) Arnaez et al. (2004)

10,000 625

100 72

2.5 1.1

24 26.9

1385 10,000 585–31,875 625

75 100 100 72

0.9 1.8 14.3 4.4

33.6 31 84 50.7

75

0.6

46.4

1,385

Conclusions

1. The hydrological behavior of hillslopes in the studied area is highly modified by unpaved forest roads. Unpaved roads act as source points for runoff flow on hillslopes, enhancing soil erosion processes.

100 75 72

Runoff coefficient%

1.4 5.1 6.6

The values of sediment yield and runoff coefficients after the rainfall simulations have been compared to other data obtained by several authors after rainfall simulation on different parts of roads under representative rainfall intensities for each location (Table 7). Roadbanks present a high diversity of runoff coefficients and sediment yields depending on plant cover and seasonal differences. In this study, roadbanks have a similar response to bare roadbanks in winter, where sediment concentration in runoff was above 10 g L− 1 and runoff coefficient was higher than 50% under a rainfall intensity of 45 mm h− 1 (Cerdà, 2007). Roadbeds in this study have a response similar to gravel surface roads (Selkirk and Riley, 1996) and other studied unpaved roadbeds in Mediterranean environments (Arnaez et al., 2004; Jordán and Martínez-Zavala, 2008). Regarding this data, it seems reasonable to expect relatively low sediment concentrations from high intensity rainfall events in winter. Probably, unpaved roadbeds do not act as a source of sediments due to compaction of the soil surface (as shown by Arnaez et al., 2004), but play an important role as a preferential point for runoff generation. Nevertheless, the mean runoff coefficients on the studied roadbeds are relatively low when compared to other data. Finally, sidecast fills in this study show higher runoff coefficients and higher sediment yields than other studied plots in Mediterranean areas (Arnaez et al., 2004; Jordán and Martínez-Zavala, 2008). The data measured after rainfall simulations in this study are within the range of values reported under similar experimental conditions by other researchers.

5.

Rainfall intensity mm h− 1

2. Roadbanks are the main source of sediments on forest roads. Runoff coefficients are high on the studied roadbanks and sidecast fills, although the sediment concentration is much lower on the latter. 3. Plant cover, rock fragments and soil texture are important factors that condition runoff and/or soil loss on the different parts of the studied roads. 4. A key factor in runoff generation is surface protection of roadbanks and sidecast fills. As a recommendation, increasing the plant cover is necessary for reducing the intensity of runoff generation by forest roads. Another possible complementary accomplishment for reducing soil loss and runoff should be the design of proper road drainage (e.g., with the construction of drains or adequate sloping).

Acknowledgements The authors want to thank Dr. F.J. Burroughs for her suggestions about the field experiments and two anonymous reviewers for helping to improve the manuscript. Thanks also to Eva Palomo for providing infrastructure for the field work and Arturo P. Granged (University of Seville) and Félix González (EGMASA) who helped to improve the field experiments. We are particularly grateful to the members of the field crew: Alejandra Nava, Patricia Illana, Felipe González, Gastón Casas and “Pepe” for their assistance during the experiments.

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