Surface runoff and soil erosion on unpaved forest roads from rainfall simulation tests in northeastern Spain
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.
Section snippets
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 V-shaped 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
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.
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).
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 31.5°. The average gradients of sidecast fill and roadbed are 27.5° and 5.3°, 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
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 Arbúcies 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
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
Acknowledgments
This research has been financed by the Instituto de Estudios Riojanos (Autonomous Community of La Rioja). Comments from José M. Garcı́a-Ruiz, Lee H. MacDonald and an anonymous reviewer helped to focus and streamline the paper.
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