No abstract

Abstract. Gully erosion is an important erosive process in Mediterranean basins. However, the long-term dynamics of gully networks and the variations in sediment production in gullies are not well known. Available studies are often conducted only over a few years, while many gully networks form, grow, and change in response to environmental and land use or management changes over a long period. In order to clarify the effect of these changes, it is important to analyse the evolution of the gully network with a high temporal resolution. This study aims at analysing gully morphodynamics over a long timescale in a large Mediterranean area in order to quantify gully erosion processes and their contribution to overall sediment dynamics.A gully network of 20 km 2 located in southwestern Spain has been analysed using a sequence of 10 aerial photographs in the period 1956-2013. The extension of the gully network both increased and decreased in the study period. Gully drainage density varied between 1.93 km km −2 in 1956, a minimum of 1.37 km km −2 in 1980, and a maximum of 5.40 km km −2 in 2013. The main controlling factor of gully activity appeared to be rainfall. Land use changes were found to have only a secondary effect. A new Monte Carlo-based approach was proposed to reconstruct gully erosion rates from orthophotos. Gully erosion rates were found to be relatively stable between 1956 and 2009, with a mean value of 11.2 t ha −1 yr −1 . In the period 2009-2011, characterized by severe winter rainfalls, this value increased significantly to 591 t ha −1 yr −1 . These results show that gully erosion rates are highly variable and that a simple interpolation between the starting and ending dates greatly underestimates gully contribution during certain years, such as, for example, between 2009 and 2011. This illustrates the importance of the methodology applied using a high temporal resolution of orthophotos.

Topographical threshold conditions (s ≥ k a−b), expressed by local slope (s) and drainage area (a), have been widely used to predict gully incision locations. However, little attention has gone to the variation of the thresholds over time. Rainfall variability and changing land use or vegetation cover can potentially lead to important shifts in established thresholds. In this study, we determine topographic thresholds for gullies forming under olive groves and herbaceous crops between 1956 and 2013 in a catchment in Southern Spain. For ten different time periods, we then analysed the impact of rainfall, land use and vegetation cover on the variation of these thresholds. The results show similar topographic thresholds for olive groves and herbaceous crops. However, important variations were found over time. Rainfall indexes, in particular rainy day normal, were generally best correlated. Finally, although overall no effect of land use was obtained, the results did show a significant effect of vegetation cover, but mainly in those years where rainfall was low. This seems to indicate that during years with high rainfall, topographic thresholds are primarily controlled by rainfall, while vegetation cover seems to exert a secondary control. Copyright © 2017 John Wiley & Sons, Ltd.

No abstract

Abstract. Gully erosion is an important erosive process, especially in Mediterranean basins. However, the longterm dynamics of gully networks and the variation of sediment production in gullies is not well known. Available studies are often done over a few years only, while many gully networks form, grow, and change in response to environmental and land use or management changes over a long period. In order to clarify the effect of these changes, it is important to analyze the evolution of the gully network with a high temporal resolution. This study aims at analyzing gully morphodynamics over a long time scale (1956–2013) in a large Mediterranean area in order to quantify gully erosion processes and its contribution to overall sediment dynamics. A gully network of 20 km2 located in SW Spain, has been analyzed using a sequence of 10 aerial photographs in the period 1956–2013. The extension of the gully network both increased and decreased in the study period. Gully drainage density varied between 1.93 km km−2 in 1956, with a minimum of 1.37 km km−2 in 1980 and a maximum of 5.40 km km−2 in 2013. The main controlling factor of gully activity appeared to be rainfall, while land use changes were found to have only an indirect effect. A new Monte Carlo-based approach was proposed to reconstruct gully erosion rates from orthophotos. Gully erosion rates were found to be relatively stable between 1956–2009, with a mean value of 11.2 ton ha−1 yr−1, while in the period 2009–2011, characterized by extreme winter rainfalls, this value increased significantly, to 591 ton ha−1 yr−1. These results show that gully erosion rates are highly variable and that a simple interpolation between the start and end date would highly underestimate gully contribution during certain years, such as for example between 2009–2011. This illustrates the importance of the applied methodology using a high temporal resolution of orthophotos.