SMALL BASIN RESPONSE IN A MEDITERRANEAN MOUNTAINOUS ABANDONED FARMING AREA - RESEARCH DESIGN AND PRELIMINARY-RESULTS

A 36 ha basin named Cal Parisa has been studied since early 1988 in the eastern Pyrenees (Spain). This basin was selected as a representative of extensive areas on clayey bedrock formerly used for farming, nowadays almost abandoned, where badlands are of great importance as far as sediment production is concerned. As we are interested in the hydrological behaviour of terraced areas, and badland areas could strongly modify the hydrological and sediment response, the basin is devoid of badlands but has some bedrock scars and small gullies. Severe geoecological disturbances were produced in it by agricultural land use in the past: the basin was deforested, topography was terraced, and the drainage net was increased by ditching. Monitoring design is based on two measuring stations, one at the main outlet and the other in a 16 ha sub-basin, and two sampling stations at the outlets of the two main sub-basins; a weather station and four rain recorders. Preliminary data show a hydrological response fully controlled by antecedent moisture conditions, suggesting that soils have high infiltration capacities and that runoff is generated by saturation overland flow; delayed flow being less significant. Stream water samples show relatively high dissolved solids concentrations, dominated by calcium bicarbonate, and suspended sediments concentrations about ten times less than dissolved ones, as a consequence of the strong sediment conveyance discontinuities observed in the field.