Factors controlling the erosion rates in a semi-arid zone (Bardenas Reales, NE Spain)

Bardenas Reales is an erosive depression 415 km2 in area located in the central-western part of the Ebro Depression. Tertiary sediments crop out in the margin of the basin whereas Quaternary sediments, Holocene in age, occur only in its centre. These Holocene sediments (clays and silts) are the result of the erosion and weathering of the Tertiary clays and are made up of 3 different stratigraphic units. Erosion is clearly the dominant process in the centre of the depression, generating gullies up to 10 km in long and 8 m deep. In order to analyse and quantify the erosion rates of the studied area, erosion was measured on two Soil Erosion Plots with volumetric and instrumental techniques that have been supplied for the last 12 years. The results show that the estimated erosion rates depend extrinsically on the used method and intrinsically on the time of the year in which they are measured. The most accurate values are those obtained by means of collector devices. The values obtained by the microtopographic profile gauge are overestimated, while the results of the measurement with erosion pins are clearly lower. The study area is located in a semi-arid environment, with two pluviometric maximums, in spring and autumn, associated with convective storms. In summer and winter, rainfall is related to Atlantic cyclonal fronts. Soil loss is significant during the pluviometric maximums when rainfall quantity and intensity are higher. During cyclonal periods, in contrast, water erosion is null or scarce and soil loss is related to mudflows. In addition, the erosion rates of each Holocene unit are different. The C2 Holocene unit has undergone greater erosion because of its physico-chemical properties. All the stratigraphic units have similar characteristics regarding Tertiary materials. Since 1993 the average erosion rate measured by the collector device on the Tertiary materials is 32 Mg ha− 1 year− 1 and 77 Mg ha− 1 year− 1 in the Holocene.