Surface runoff and soil erosion on unpaved forest roads from rainfall simulation tests in northeastern Spain

Catena 57 (2004) 1 – 14 www.elsevier.com/locate/catena

Surface runoff and soil erosion on unpaved forest roads from rainfall simulation tests in northeastern Spain J. Arna´ez a,*, V. Larrea b, L. Ortigosa a a

Area de Geografı´a Fı´sica, Universidad de La Rioja, Edificio Luis Vives, 26004, Logron˜o, Spain b Instituto de Estudios Riojanos, Muro de la Mata 8 26001, Logron˜o, Spain

Received 30 May 2002; received in revised form 25 August 2003; accepted 17 September 2003

Abstract The generation of surface runoff and transport of sediment were studied on unpaved forest roads in the Iberian Range (Spain). To this end, a mobile rainfall simulator was used so that information could be compared. Twenty-eight rainfall simulations were carried out on the cutslope (12), sidecast fill (6) and roadbed (10). Under low soil moisture conditions, cutslopes had runoff coefficients of 58%, and overland flow was generated in 3 min. On the sidecast fill and the roadbed, the runoff coefficients were 34% and 46%, respectively. The part of the road that showed the greatest erosion was the cutslope (161 g m 2), where mass wasting and freeze – thaw processes supply loose material to be transported by overland flow. The cutslope soil loss rates exceed those from the sidecast fill and the roadbed by 16 and 11 times, respectively. In these tests, the maximum sediment concentration was recorded in the first few minutes. The concentration reduces with time as a consequence of the exhaustion of loose surface material. Correlation coefficients and regression analysis showed that the gradient, plant cover density and stone cover of the cutslopes, fill areas and roadbeds had statistically significant effects on runoff and erosion. A comparison of these data with others obtained on different land-uses allowed us to conclude that some parts of forest roads have similar hydromorphological behaviour to abandoned fields in mountainous areas and to cereal fields. D 2003 Elsevier B.V. All rights reserved. Keywords: Rainfall simulation; Forest roads; Soil erosion; Runoff; Iberian Range; Spain

* Corresponding author. Fax: +34-941-299318. E-mail address: [email protected] (J. Arna´ez). 0341-8162/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2003.09.002

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1. Introduction Unpaved forest roads are common in mountainous areas. In Spain, the number of forest roads has increased since the 1970s, partly as a result of vast reforestation projects and the development of tourist activities (Ortigosa et al., 1990). Unpaved forest roads cause substantial local changes to soil properties and the hydrogeomorphic behaviour of hillslopes (Gucinski et al., 2001). The presence of roads increases the sediment yield in many catchments as a result of the activity of mass movements on steep embankments in response to extreme rainstorms (Swanson and Dyrness, 1975; Haigh et al., 1988; Wemple et al., 2001) or as a consequence of the direct impact of raindrops and the turbulence of runoff (Froehlich, 1995; Ziegler et al., 2000). The alteration of hillside profiles, with consequent disruption of surface and subsurface flows (Tague and Band, 2001), the construction of cutslopes with steep gradients (Luce and Black, 2001), the lack of plant cover to protect the soil, and the highly compacted surface of the roadbed largely explain the variety and intensity of erosion processes. High rates of sediment production occur after the construction of forest roads (Megaham et al., 2001), when they are used for frequent transport of logs (Reid and Dunne, 1984), or when no upkeep is carried out. From a hydrological perspective, the connection of road ditches and culverts with stream networks facilitates the movement of runoff that quickly reach channels (Wemple et al., 1996). Consequently, it may produce faster flow peaks and higher total discharges (Jones and Grant, 1996; Bowling and Lettenmaier, 2001). The objective of this paper is to quantify runoff and erosion from unpaved forest roads. To this end, we measured runoff and soil losses from the three portions of forest roads (the cutslope, sidecast fill and roadbed), and related these data to the site characteristics. Runoff and erosion rates were collected using a rainfall simulator. In spite of the disadvantages, such as problems in extrapolating the results to real conditions, rainfall simulations are widely used because of their low cost and ease of operation (Walsh et al., 1998), and also because of the possibility for study under controlled conditions (Navas et al., 1990). The results of rainfall simulation tests can be used for comparative purposes (Foster et al., 2000). In this study, the rainfall simulation tests were used to study the hydrogeomorphological behaviour of different parts of unpaved forests roads and to compare the results with runoff and soil erosion rates obtained from other land uses.

2. Study area Fieldwork was carried out in the Cameros Sierra, Iberian Range, in NE Spain (Fig. 1). This mountain range is characterized by gentle summits, moderately steep slopes and Vshaped valleys. Altitudes range between 700 and 2000 m.a.s.l. The dominant lithologies are Mesozoic quartz sandstones, sandstones and limestones. The soils are generally kastanozens, 40– 50 cm deep. They are dark brown and include a surface horizon rich in organic matter with an important percentage of stones. The Cameros Sierra receive between 600 and 1000 mm of annual precipitation. Over 60% of the precipitation falls in spring and autumn. Summer is hot and dry with

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Fig. 1. Location of the study area.

occasional convective storms. Snow lies for significant periods only in the highest parts, but even here, it melts and is redeposited several times during the winter. The mean annual temperature is 6 –11 jC, with the lowest values in January. The climate could be defined as mountain Mediterranean, somewhat continentalized (Garcı´a-Ruiz and Martı´n, 1992). The vegetation varies in response to changes in altitude, aspect, soil properties and human activity. Forest vegetation is dominated by beech (Fagus sylvatica) on high and moist hillsides and oak woods (Quercus pyrenaica) on low and sunny hillsides. However, massive deforestation has facilitated the spread of scrub. On limestone soils Genista scorpius, Thymus vulgaris and Rosmarinus officinalis prevail, whereas on sandy soils Cistus laurifolius is more common.

3. Methods Five unpaved forest roads, which ascend from 700 to 1500 m.a.s.l., were selected for the study. These roads have an average width of 5– 6 m, a gradient of 5 – 6%, and were built after the 1960s. The roads are usually used by light vehicles and the intensity of use is low: Daily traffic includes three to six private vehicles and light jeeps.

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For each road, topographic and environmental data of cutslopes, bedroads and sidecast fills were collected in segments with a length of 20 m (59 in total). Twenty-eight plots were installed on the most representative segments. Rainfall simulations were carried out to measure runoff and suspended sediment yields from three different parts of the road: 

Cutslope. This results from undercutting of the hillslope to give straight or slightly convex profiles with gradients of 30– 50j. The height of these cutslopes varies between 2 and 5 m, and they have little vegetal cover. There are significant and positive correlations between hillside gradient and the angle of the cutslope, and between the hillside gradient and the cutslope height. Number of rainfall simulation tests: 12.  Sidecast fill. This is located immediately downslope of the road and results from the accumulation of debris excavated during road building. It has straight profiles, with gradients between 20j and 35j. Plant recolonization occurs very readily on sidecast fill. Number of rainfall simulation tests: 6.  Roadbed. Unpaved road surface on which the vehicles travel. Ditches and culverts have been built to capture surface runoff along the sides of some roadbeds. Number of rainfall simulation tests: 10. The rain simulator was described by Navas et al. (1990), Lasanta et al. (2000a) and Ries et al. (2000). It has metal legs in the shape of a truncated pyramid, and the structure is covered with plastic to protect the experiments against wind. The legs are telescopic so the simulator can be kept level when placed on a slope. The nozzle at the top of the structure is at a maximum height of 3.5 m and is connected through a rubber pipe to a mobile pump. The nozzle directs water onto a area of 1385 cm2 that is delineated by a steel ring (42 cm in diameter). The ring is carefully placed into the soil to avoid leakage and direct the runoff to the outlet of the plot. Before the simulations, soil samples were collected either 1 m below or above each plot. These samples allowed us to study the textural composition (sand, silt and clay) and organic matter content of the topsoil (sampling depth from 0 to 15 cm). Percentage plant cover and percentage stone cover were measured using a grid with cells of 0.5 cm2. Other surface characteristics of plots (slope angle and aspect) were precisely described. Rainfall simulations were carried out for a duration of 30 min at an intensity of around 75 mm h 1 (C.V. 16%). Rainfall intensities were measured by three micro-raingauges (opening size: 3.5 cm in diameter) distributed over the plot. Drops of 0.80 – 2 mm in size account for 85% of the total rainfall volume (Navas et al., 1990). The experiments were carried out in the summer of 1999 when soil moisture values averaged only 4.3%. Water and sediment samples are collected every 3– 5 min, once water first flowed from the plot. At the end of the test, the depth of the wetting front was determined by digging a hole and measuring the depth of the wet sector. The data obtained from each plot included: 

time lag (the number of seconds from the beginning of the rainfall simulation until the beginning of the runoff),  runoff coefficient (the percentage of rainfall that becomes runoff),  mean runoff (ml s 1),

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Table 1 Location and characteristics of the plots Cases Cutslope Mean Standard deviation Sidecast fill Mean Standard deviation Roadbed Mean Standard deviation ANOVA, p

Altitude (m)

Gradient (j)

Stone cover (%)

Plant cover (%)

1069 192.1

31.5 9.5

25 30

5 12.3

966 147.7

27.5 7.4

45 33.3

40 32.7

19 20.8 0.2467

29 25.1 0.0135

12

6

10 1102 199.3 0.3917

5.3 1.5 0.0001

peak runoff during the test (ml s 1) and  depth of the wetting front at the end of the test (cm). 

Samples were collected to measure: mean sediment concentration (g l 1), peak sediment concentration (g l 1) and  total soil loss (g m 2).  

Correlation coefficients and regressions were used to evaluate the effects of site factors on runoff and erosion in the different parts of the forest roads. The variables considered were the gradient, aspect, stone cover and plant cover. Length was not included because the small size of the plots did not allow the influence of this factor on runoff and erosion to be evaluated. Table 2 Characteristics of the soils Descriptor and units

Cutslope

Sidecast fill

Roadbed

ANOVA, p

Sand fraction (%) Mean Standard deviation

30 19.5

34.4 4

38 23.5

0.7411

Silt fraction (%) Mean Standard deviation

48 15

48.5 3.4

47 19

0.9831

Clay fraction (%) Mean Standard deviation

22 10.5

17 5.2

15 7

0.3890

Organic matter (%) Mean Standard deviation

1.4 1.3

1.8 0.2

2.6 2.2

0.3402

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4. Results 4.1. Characteristics of the plots and soils The topography and other characteristics of the plots are listed in Table 1. The plots located on cutslope have an average gradient of 31.5j. The average gradients of sidecast fill and roadbed are 27.5j and 5.3j, respectively. The analysis of variance showed significant differences between the three parts of the roads. The plots on sidecast fill have a high percentage of stones (45%) because it was covered with the debris discarded when the road was constructed. Plant cover is greatest on sidecast fill (40%) and is dominated by scrub (C. laurifolius, T. vulgaris, G. scorpius, Rosm. officinalis, Rosa canina). Lichen, fallen leaves and herbaceous plants cover some parts of roadbed that are not crossed by vehicles. The physical characteristics of the soils are listed in Table 2. Generally, the soils were loamy. The cutslope had a higher proportion of clays (22%), but the analysis of variance showed no significant differences between the three parts of forest roads.

Fig 2. Surface runoff ( Q) from simulations on different parts of unpaved forest roads. Cutslope: y = 4.00 + 2.14log10x; r2 = 0.80; p value = 0.000. Sidecast fill: y = 3.83 + 1.66log10x; r2 = 0.72; p value = 0.000. Roadbed: y = 5.19 + 2.38log10x; r2 = 0.78; p value = 0.000.

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Table 3 Hydrological results of rainfall simulation tests Surface runoff (ml s

1

)

Peak flow (ml s 1)

Runoff coefficient (%)

Wetting front (cm)

Cutslope Mean Standard deviation

1.8 0.9

2.4 1.4

57.8 28.1

5.2 51.5

Sidecast fill Mean Standard deviation

1.1 0.3

1.5 0.5

33.6 10.5

8.5 8.2

Roadbed Mean Standard deviation ANOVA, p

1.7 0.5 0.1335

2.8 0.8 0.0774

46.4 14.2 0.0852

5.5 2.7 0.3173

4.2. Hydrological response Fig. 2 shows the evolution of runoff during simulation tests. Each point is a mean of all the simulations. Overland flow response was most rapid for cutslope, as runoff began in 172 s. On sidecast fill and roadbed, surface runoff commenced 334 and 366 s after the start of the simulated rainfall. The surface runoff curves are logarithmic. Fig. 2 shows a fast rise during the first few minutes. Shortly thereafter, it levels out as a result of surface saturation, sealing, and the reduction in capillary forces in the upper few centimetres of soil. The plots on cutslope had an average runoff coefficient of 57.8%, and in some tests, this coefficient exceeded 80% (Table 3). Average runoff coefficients were lower on roadbed (46.4%), with a maximum value of approximately 60%. Runoff peak was 2.4 ml s 1 on cutslope and 2.8 ml s 1 on roadbed. Both cutslope and roadbed showed a thin wetting front, confirming the resistance to infiltration of water. On sidecast fill, the mean runoff coefficient was 33.6%, the runoff peak was 1.5 ml s 1, and the mean depth of the wetting front was 8.5 cm. 4.3. Influence of site variables on runoff It is clear from correlation coefficients and p values that the gradient is the most sensitive of all site variables in the control of runoff, especially on the cutslope and Table 4 Correlations and p values between runoff and site attributes Variable

Cutslope

Sidecast fill

Bedroad

Gradient Aspect Stone cover Plant cover

r = + 0.76; p = 0.004 Non-significant Non-significant r = 0.60; p = 0.05

r = + 0.80; p = 0.05 Non-significant Non-significant Non-significant

Non-significant Non-significant r = + 0.75; p = 0.01 r = 0.91; p = 0.000

Only the attributes with significance levels of p z 0.5 are included.

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Table 5 Sediment yields of rainfall simulation tests Sediment concentration (g l

1

)

Peak of sediment concentration (g l

1

)

Total soil loss (g m

Cutslope Mean Standard deviation

5.1 5.1

10.4 10.9

160.7 185.3

Sidecast fill Mean Standard deviation

0.9 0.6

1.6 1.2

10.5 3.3

Roadbed Mean Standard deviation ANOVA, p

0.6 0.2 0.0089

1.5 1.9 0.0163

14.2 6 0.0154

2

)

sidecast fill (Table 4). We found a positive (r= + 0.76 and + 0.80) and significant ( p = 0.004 and 0.05) relationship between both factors. Plant cover on the cutslope also influences on runoff, with a negative correlation (r = 0.60, p = 0.05) confirming that runoff is reduced with a dense plant cover. The runoff coefficient is negatively correlated with plant cover (r = 0.91) in the bedroad. A dense plant cover on the bedroad is indicative of a null or moderate intensity of use. Stone cover is also important on the generation of runoff (r = + 0.75).

Fig 3. Suspended sediment concentration (SSC) during simulations on different parts of unpaved forest roads. Cutslope: y = 27.53 7.31log10x; r2 = 0.60; p value = 0.000. Sidecast fill: y = 2.43 0.58log10x; r2 = 0.27; p value = 0.008. Roadbed: y = 3.14 0.83log10x; r2 = 0.55; p value = 0.000.

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Table 6 Correlations and p values between soil loss rates and site attributes Variable

Cutslope

Sidecast fill

Bedroad

Gradient Aspect Stone cover Plant cover

r = + 0.65; p = 0.02 Non-significant Non-significant Non-significant

Non-significant Non-significant r = + 0.76; p = 0.07 Non-significant

r = + 0.61; p = 0.06 Non-significant Non-significant Non-significant

Only the attributes with significance level of p z 0.1 are included.

4.4. Erosive response Table 5 shows the sediment outputs. The highest rates are from cutslope, where the mean suspended sediment concentration was 5.1 g l 1 and the mean soil loss was 160.7 g m 2. The values are, however, very variable. The maximum values of soil loss are higher than 400 g m 2, showing the effects of high runoff coefficient, steep gradient and low percentage of plant cover, whereas the minimum values are less than 20 g m 2. In the other two parts (sidecast fill and roadbed), erosion was more moderate: 10.5 g m 2 in sidecast fill and 14.2 g m 2 in roadbed. The same behaviour was observed for suspended sediment concentration. In Fig. 3, the curves of suspended sediment concentration are drawn. In the beginning of the experiments, a higher sediment concentration is recorded, especially on cutslope, but it quickly decreases with time. This response is related to the high sediment availability in the first minutes. As the simulation progresses, the sediment output diminishes due to exhaustion of particles and to the protective effect of the thin laminar flow of runoff against the splash. Fig. 3 also shows the higher sediment concentrations from cutslope as in compared to sidecast fill and roadbed. 4.5. Influence of site variables on erosion The gradient is the most important variable influencing cutslope erosion. The relationship between soil loss rates and the gradient is positive and significant at 98% (Table 6). Erosion is influenced by stone cover on sidecast fill, with a calculated correlation coefficient of + 0.76 ( p = 0.07).

5. Discussion Unpaved forest roads in mountain areas modify the hydrological functioning of hillslopes, and they can make an important contribution to the sediment budget of many forested basins. Batalla et al. (1995) suggest that, of the total sediment yield evaluated in the basin of the Arbu´cies River (Spain), 10% was caused by the road system. In small Carpathian drainage basins, about 70 –80% of the suspended sediment is supplied from unmetalled roads (Froehlich, 1995). Wemple et al. (2001) described the erosion processes on forest roads, with an emphasis on rain splash and surface runoff. In the case of the Iberian Range, Arna´ez and Larrea (1994) stress the occurrence of slides and sheet wash.

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Incisions (rills and microgullies) are important on the roadbed itself, and are activated by vehicle ruts and by surface flows when these are not collected in roadside ditches. Fig. 4 compares the average runoff coefficients and soil loss rates for each part of the forest roads. Both runoff coefficients and soil losses are higher in cutslopes. These have steep slopes that determine runoff generation and sediment yield (Diseker and Sheridan, 1971; Luce and Black, 1999). In addition, cutslopes have high available sediment as a result of processes activated in other seasons. Arna´ez and Larrea (1995) demonstrated that freeze – thaw processes during the winter and spring supply loose material to the cutslope. Regu¨e´s and Gallart (1996) arrived at the same conclusion for badland areas. It is probable that this erosive activity diminishes with time as cutslopes revegetate (MacDonald et al., 2001). Along the studied forest roads, scarce plant cover still predominates (Table 1). The plots on sidecast fill show low values of surface runoff and erosion (Fig. 4). The accumulation of loose material coming from the construction of the road, or from the washing away of the fines mixed in with the sidecast fill, facilitates infiltration and reduces runoff. The high percentage of vegetal cover also reduces the amount of rainsplash erosion. However, runoff and erosion may be more active if the gradient and the stones in the soil increase. A high percentage of gravels causes small concentrations of water in the stone borders where the erosion is concentrated. The measured erosion for cutslopes and sidecast fill are consistent with rates measured using other techniques. For example, the use of erosion pins indicated higher cutslope erosion rates than on sidecast fill (Arna´ez and Larrea, 1995). These authors also demonstrated that summer was the season that records the highest erosion rates. The tests on roadbeds generally yielded high runoff values but low soil losses rates (Fig. 4). A high degree of compaction or sealing of unpaved road surfaces favour a

Fig 4. Relationship between runoff and erosion rates for each road part. Each point is the mean of all simulations.

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decrease in infiltration capacity and, in turn, an increase in runoff generation (Ziegler et al., 2000). These same effects cause a high percentage of stones. According to Poesen et al. (1990) and Descroix et al. (2001), surface runoff is increased by the presence of gravels, which, as were observed in the bedroad, are embedded into the soil surface. These gravels reduced the values of soil surface hydraulic conductivity. The compacting of roadbed may also explain the low availability of sediments, since splashing is unable to detach the particles. Rills were not formed in the simulations because of the small dimensions of the plots. However, vehicle ruts or high intensity storm can promote the development of rills and shallow gullies in the wet season. Once developed, they behave similarly to ditches in capturing and re-routing surface water and cause most of the erosion (MacDonald et al., 2001). Some authors (Reid and Dunne, 1984; Ziegler et al., 2001) suggest that there is a relationship between sediment yield and the intensity of forest road use. Fig. 5 relates runoff coefficients and erosion rates obtained from rainfall simulations in areas with different land uses. The rainfall simulator used and the rainfall intensities were very similar (Lasanta et al., 2000a,b; Cerda`, 1993, 1994; Cerda` and Garcı´a-Fayos, 1994; Arna´ez et al., 1996; Regu¨e´s and Gallart, 1996). Cutslopes of forest roads have a similar response to plots cultivated for cereal production and abandoned sloping fields (1, 2 and 3). Due to the scarce plant cover, these land uses have average erosion rates of 200 –300 g m 2 h 1 and runoff coefficients of 40 – 60%. Erosion rates on badlands (4), with very

Fig 5. Runoff and erosion from rainfall simulation tests in areas with different land uses. (1) Cutslopes of forest road. (this study) (2) Cereal plots (Lasanta et al, 2000a). (3) Abandoned sloping fields (Arna´ez et al., 1996). (4) Badlands (Cerda` and Garcı´a-Fayos, 1994). (5) Abandoned bench terraces (Lasanta et al., 2000b). (6) Sidecast fill (forest roads) (this study). (7) Roadbeds (forest roads) (this study). (8) Slopes with shrub cover (Cerda`, 1994). (9) Plot affected by a forest fire (Cerda`, 1993).

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steep slopes, dry soils and no vegetation cover, can be very high (3000 g m 2 h 1), as Cerda` and Garcia-Fayos (1994) have demonstrated. Sidecast fill and roadbeds (6 and 7) have similar runoff and erosion rates as abandoned bench terraces (5). Erosion rates for these sites vary between 20 and 30 g m 2 h 1 and runoff coefficients vary between 30% and 50%. Hillslopes with shrub cover had the lowest sediment yields.

6. Conclusion The rainfall simulation method provides limited information because of the small size of the plots and the design of the simulator. It is difficult to extrapolate data about sediment production to larger scale. Nevertheless, the results of the simulations can be used for comparative purposes. This has been the objective of the study: to compare data of runoff and erosion, especially rainsplash and sheetwash erosion in three sectors of forest roads and to relate them to other soil uses. This paper demonstrates that majority of the sediments come from the cutslopes. Runoff and soil loss rates are also active on bedroads, although this part of the road may probably contribute in a more intense way to sediment production if gullies and rills are considered. The main explaining variables of runoff and erosion were the gradient and the presence of embedded gravel which is positively related to runoff and erosion. Plant cover density is negatively related. The results suggest a need to control soil erosion from unpaved forest roads in mountain catchments. Hence, the best means of reducing sediment yields is to adjust, if possible, the design of the forest road to fit the topography, and to increase mulching or replanting on intensively disturbed cutslopes.

Acknowledgements This research has been financed by the Instituto de Estudios Riojanos (Autonomous Community of La Rioja). Comments from Jose´ M. Garcı´a-Ruiz, Lee H. MacDonald and an anonymous reviewer helped to focus and streamline the paper.

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