THE INFLUENCE OF PHYSICOCHEMICAL PROPERTIES ON EROSION PROCESSES IN BADLAND AREAS, EBRO BASIN, NE-SPAIN

Studies carried out in two experimental sites on recently incised Holo cene valley-fill sediments, and on a slope of exposed Tertiary clay in dicate that erosion processes and rates are strongly related to the ch emical characteristics and physico-chemical behaviour of alkaline soil s. Overland flow is the prevailing processes in non-dispersive materia ls producing erosion rates of 8-20 mm/year on the slope and 6-9 mm/yea r on the valley fill. The highest rill density is associated with both a high sodium content and a high clay dispersion index, yielding eros ion rates of 14-22 mm/year on the slope and 5-8 mm/year on the valley fill. Piping is the dominant erosion process under high clay dispersiv ity conditions, creating a positive feedback of pipe collapse producin g rill incision, which increases erosion in interrill areas. Temporal variations in erosion rates in the nondispersive horizons are strongly related to total rainfall whereas a more random distribution was foun d in the dispersive-day horizons. In dispersive horizons clay is flush ed away even during low intensity precipitation. In non-dispersive hor izons, detachment and transport of clay aggregates is more selective a nd depends directly on the shear stress and stream power of the runoff . For the total period of measurement, similar erosion rates in rill a nd interrill areas indicate a short term equilibrium between the main components of the badland system.