Gully erosion, land use and topographical thresholds during the last 60 years in a small rangeland catchment in SW Spain

Abstract

Gully erosion plays an important role in degradation processes of Mediterranean environments. In this paper aerial orthophotographs were used for (i) analysing the evolution of a valley bottom gully and its relation with land use and vegetation cover, (ii) exploring the role of land use and vegetation cover on the coefficients of the equation S ¼ aA Àb (where S is slope at the headcut and A is drainage area), which is based on the topographical threshold concept and is commonly used to predict gully initiation. The study was carried out in a small catchment (99Á5 ha) located in the southwest of the Iberian Peninsula. Gullies and headcuts were mapped together with land use and vegetation cover using aerial photographs for the years 1945, 1956, 1989, 1998, 2002 and 2006, which had to be digitized and orthorectified in advance. The results showed an increase of the area affected by gullying from 695 m 2 in 1945-1009 m 2 in 2006, reaching a maximum of 1560 m 2 in 1956. Gullying was closely related with land use, especially with the amount of cultivated areas within the catchment and also with grazing intensity. No clear relationship was found between the evolution of the gullied area and rainfall amounts. Finally, the values of the exponent b obtained for different headcuts and different dates (close to 0Á4) were similar to those proposed by other authors for gully erosion caused by Hortonian overland flow in semiarid environments.

Hydrological dynamics in a small catchment with silvopastoral land use in SW Spain

“…Although this can be considered a moderate animal density, the effect of overgrazing can be observed in parts of the catchment, where livestock concentrates. As a consequence, soil degradation and accelerated water erosion have been reported in previous studies (Gómez Gutiérrez et al, 2009;Pulido-Fernández et al, 2016;Rubio-Delgado et al, 2017).…”

“…Both catchments have very similar environmental characteristics. A wide range of processes and subjects were investigated, such as sheetwash, the dominant soil erosion process along hillslopes (Schnabel, 1997;, gully erosion, which is found in first and second order valley bottoms (Schnabel and Gómez Amelia, 1993;Gómez Gutiérrez et al, 2009). Gully erosion was shown to be closely related with catchment hydrology (Gómez Gutiérrez et al, 2012) and extraordinary sediment losses are caused by extreme rainfall events (Schnabel et al, 1999).…”

How to Make a Barranco: Modeling Erosion and Land-Use in Mediterranean Landscapes

“…Some barrancos are old and are related to characteristics of underlying lithologies, bedrock structure, long-term climate-driven changes in vegetation cover, and regional drainage networks [1]. However, many are clearly much younger, and are incised into unconsolidated sediments or soft calcareous bedrock [2][3][4]. Many older bedrock barrancos also show evidence of recent incision [1,5,6].…”

“…Certainly, the Mediterranean region today and in the recent past is characterized by high sediment transport levels, as a result of both sheet erosion and incision [4,6]. There is also evidence of significant episodes of erosion, including incision, at various times in the historic and prehistoric past that seem coeval with changes in agropastoral land-use patterns [7,[11][12][13].…”

“…While there has been considerable study of the impacts of land-use on and hillslope and rill erosion in the Mediterranean, the relationships between land-use and gullying and barranco formation are less well understood [4,6,14]. Moreover, quantitative studies of the effects of farming and herding practices on gully incision have been largely empirical and limited to short-term processes (from single events to several decades) that are observable (e.g., [2][3][4]6,15,16]).…”