Soil loss and runoff rates on unpaved forest roads in southern Spain after simulated rainfall

Abstract

We have studied the surface runoff and sediment production on unpaved forest roads in Los Alcornocales Natural Park (southern Spain) using a simple portable rainfall simulator at an intensity of 72 mm h −1. Thirty rainfall simulations were carried out on 10 road plots: on the roadbank (10), on the sidecast fill (10), and on the roadbed (10). On the roadbanks, the runoff coefficient was 58% and runoff flow appeared after 60–90 s. On the sidecast fills, the runoff coefficient was 27% and time to runoff was 5–108 s. Finally, on the roadbeds, the runoff coefficient was 51% and time to runoff was 25–89 s. The highest soil loss rate was found on the roadbanks (106 g m −2), due to slopes, the existence of loose colluviums, and a low plant cover. The total soil loss from the roadbank was 5 and 6 times higher than those from the roadbed and the sidecast fill, respectively. The sediment concentration increased during the first 6–8 min from the beginning of the simulations and then decreased steadily due to the loss of fine soil surface particles and porosity changes. Statistical analysis shows that plant cover on roadbanks, sidecast fills, and roadbeds has a significant effect on runoff. Rock fragment cover and slopes on the roadbank and the sidecast also influence runoff significantly. The behavior of roadbanks was similar to that of other cultivated soils and abandoned sloping fields. Sidecast fills had the same response as abandoned terraces or non-vegetated road embankments. Reducing the roadbank angle by approximately 40% and increasing the plant cover to 35–40% are necessary for keeping erosion below 18 g m −2, as it was on the sidecast fills.