This paper was peer-reviewed for scientific content. Pages 223-228. In: D.E. Stott, R.H. Mohtar and G.C. Steinhardt (eds). 2001. Sustaining the Global Farm. Selected papers from the 10th International Soil Conservation Organization Meeting held May 24-29, 1999 at Purdue University and the USDA-ARS National Soil Erosion Research Laboratory. Monitoring Water and Sediment Yield In Mediterranean Mountainous Watersheds: Preliminary Results J. Rius, R. Batalla* and R.M. Poch *J. Rius, R. Batalla, and R.M. Poch, Centre Tecnològic Forestal de Catalunya, Universitat de Lleida, Dept. Environment and Soil Sciences, UdL, Av. Rovira Roure 177, 25198 Lleida, Spain. *Corresponding author: Rbatalla@macs.udl.es ABSTRACT Water availability is one of the main objectives in the management of water resources in Mediterranean areas. Urban development and the European Union policy have led, in rural areas, to the abandonment of agricultural land since the fifties, which has brought about an increase of forestland. In order to assess the influence of watershed characteristics and management on water resources of a large pre-Pyrenean reservoir, three nested basins (2.6 km 2 , 65.2 km 2 and 222.5 km 2 ) are being monitored for water and sediment budgets. Mainland use is forest (Pinus sylvestris) and the management system is mostly selective cuttings. Monitoring scheme consists of continuous recording of discharge in the three basins, and sampling of water and suspended sediment in the two smaller ones. Rainfall is measured by means a series of rain gauges distributed in the watersheds. Besides, subsurface water flow along selected slopes of the smallest watershed is estimated by continuous recording of water content through a network of reflectometers and tensiometers. The working hypothesis is that, being subsurface flow the main source of water for rivers in subhumid environments, forest canopy ensures a more constant water supply than agricultural lands, in spite of larger water consumption by evapotranspiration. In this sense, forest management would not affect the water availability as long as the physical characteristics of the soils would be preserved. The use of these results for watershed and reservoir management and modeling needs the definition of land units within the watershed, where transfer functions for the physical properties of the soils at different scales can be applied in order to take into account soil variability. Furthermore, variable sediment sources, especially erosion processes in the hillslopes and sediment transport in the channel network, are investigated in order to build up an integrated sediment budget for the whole area. INTRODUCTION Evaluation of the water resources is essential for land use planning in semiarid areas, as the Ebro valley in NE Spain. Water is supplied through large reservoirs fed with waters draining the Pyrenean mountain range. Land use in this area has evolved during the last half century from a mosaic pattern of pastures, crops and forest to an increase of forest and range areas due to depopulation and urban development. This trend has been reinforced by the European Union policy of setting aside agricultural land. Some effects of these changes in the soil and water resources are well known, as the increase of interception losses, better water quality or increase of infiltration capacity. Nevertheless, the existing quantitative data for the whole region, representative of subhumid Mediterranean watersheds is scarce and does not allow one to draw definitive conclusions. With the aim to assess these aspects of water resources in mountainous Mediterranean drainage basins, an investigation was set in the Odèn-Port del Comte Ranges in the Pyrenees, namely the Ribera Salada watershed (Fig 1), where effects of logging on runoff and erosion are being analyzed in an area of Pinus uncinata and Pinus sylvestris. Forest is managed through selective cuttings, but the exploitation degree is not very intense and the average density of the canopy is rather high. Two aspects will be particularly analyzed: 1) the relation between main hydrological fluxes (soil moisture, rainfall-runoff relation, river response) at several temporal scales, from individual events up to annual water budget), as basis for the quantitative evaluation of water resources and, 2) the relation between discharge, sediment transport and sediment yield, as important variables for the assessment of water quality. Investigation is funded by the CICYT-Spanish Ministry of Education and Science- and is being developed in collaboration with the Forestry Institute of Catalonia and the Ebro Water Authorities. MATERIAL AND METHODS Catchment characteristics The substrate of the catchment consists of massive conglomerates and calcareous sandstones merging to calcareous siltstones to the south. The relief is tabular, with slopes often higher than 20%. The whole watershed has a south aspect and its altitude interval is comprised between 660 m and 2383 m asl. Soils are shallow, calcareous and stony. They are classified as Lithic and Typic Ustorthents and Udorthents, depending on the soil moisture regime and the presence of a lithic contact (SSS 1999). The annual rainfall ranges from 600 mm to 1200 mm in the highest points.