Recent gully erosion in the El Cautivo badlands (Tabernas, SE Spain)

Detailed geomorphic observations in El Cautivo badland area, in the semiarid SE of Spain, show a recent renewal of linear channel erosion following a period of aggradation. Prior to this renewal, the landscape was characterized by smooth colluvial pediments on the north-east facing aspects and relatively steep crusted microhillslopes on south-west facing aspects. First-order valleys within the badland landscape were smooth-floored and filled with silty deposits connected with pediment colluvial deposits. The conditions under which valley bottom deposition was replaced by channel incision were analyzed by a study of flow hydraulics (magnitude–frequency of eroding events vs. resistance of valley floor to water flow) and the pollen sequence in the recently cut deposits. Changes in the vegetation-carpeting the valley bottom seem to be responsible for the deposition–erosion alternation rather than changes in the magnitude–frequency of major runoff events, given the high sensitivity of flow hydraulics to changes in roughness. Present rainfall rates would not, on their own, be able to erode a well-vegetated valley bottom. Yet, the pollen sequence in the valley fills demonstrates a progressive deterioration in climatic conditions with the eventual disappearance of groups that require a constant level of soil moisture (Alnus, Corylus, Cyperaceae). An increase in yearly rainfall or a change in temporal patterns would increase infiltration leading to permanent denser vegetation on footslopes and valley floors, helped by the natural water harvesting system. This vegetation cover would reduce the flow velocity and induce the deposition of the colluvium and valley fills, whereas erosion on headslopes would have been active enough to supply the sediments that were deposited downslope or on the valley bottoms. The reactivation of linear erosion would seem, therefore, to be a consequence of the decay of this permanent vegetation, induced by an increase in the duration or severity of drought periods, which meant that the water storage capacity of the valley fillings was insufficient to support it.