Role of erosion processes on the morphogenesis of a semiarid badland area. Bardenas Reales (NE Spain)

• Physical–chemical properties controlled morphogenesis.
• Annual rainfall distribution controls soil loss and mass movements.
• Possible connectivity shifts can be conjectured.

Bardenas Reales badlands are located in the middle-western sector of the Ebro Depression, northern Spain. This consists of a big erosional depression with deeply dissected valley bounded by steep slopes. Labile argillaceous bedrock, scarce high intensity rainfall, and high temperatures, make erosion the dominant morphogenetic process.Several erosion processes operate in the Bardenas Reales: rilling, gullying, piping and mass movements. The relative role played by these processes on the morphogenesis is largely controlled by physico-chemical properties of the materials and climatic conditions. Precipitation distribution over the year controls the development of mass movements, whereas regolith physical–chemical properties favor piping, gullying and rilling during the rainfall maxima. Soil erodibility in this area is clearly controlled by the sodium content and the presence of swelling clays. These factors determine the distribution and appearance of piping processes. Cracking in Holocene clay deposits, devoid of swelling clays, is related to high sodium content that confers a dispersive character. Clay dispersion causes loss of cohesion favoring piping.In the rainfall maxima soil loss is significant and controlled by precipitation amount and maximum intensity. Conversely, during periods of cyclonic rain erosion is scarce and mass movements are the dominant process. In the slopes underlain by Tertiary mudstones, mud flows are frequent during the winter months. In winter time, when rains are of low intensity and evapotranspiration is limited, infiltration through cracks is high. Consequently water content in the regolith may increase until the plastic limit is reached. Under these conditions and due to the high gradient of the slopes (34°), shear strength is surpassed and the clay material is able to slide and flow.