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The terms ∆ soil and ∆ snow can be ignored when evaluating big catchments on a yearly basis under a Mediterranean climate with dry summers. It is assumed that the soil and snow water storage are almost completely exhausted by the end of the hydrological year. The term ∆ aquifers can also be neglected when the global water balance is not greatly affected by subterranean circulation. Equation 1 then becomes: R = PET Eq. 2 Thus, the main variables that control water yield in a basin are precipitation, interception and evapotranspiration. These factors are closely related to changes in climate and land-use/plant cover. Many studies have focused on the former despite works on land use by Shachori et al. (1) and Hewlett (2). Nevertheless, a number of recent reports on basins consider the hydrological effects of felling and clearcutting (see 3-8), reforestation (9-11), and forest fires (12). All of these effects are more difficult to assess as the size of the basin increases, due to the large variability of rainfall and basin characteristics (13). The effects of climate and land use should be clearly separated when managing and forecasting discharge. This is especially true in Mediterranean areas which are affected by rapid economic changes and are extremely dependent on mountain water resources. This paper deals with the role of climate evolution and land-use changes on the evolution of water resources in the Central Spanish Pyrenees. It also provides a good example of the challenges involved in managing water. THE STUDY AREA The study area covered 7430 km 2 in the Central Spanish Pyrenees between the Esca and Noguera Ribagorzana rivers (Fig. 1). Altitudes range from 500 to more than 3000 m a.s.l., with a very contrasted relief (14). The climate is influenced by the Atlantic and Mediterranean and the effect of macrorelief on precipitation and temperature. The average annual precipitation in the Central Ebro Depression is approximately 300 mm. The average annual temperature ranges from 13-15°C. Annual precipitation in the mountains exceeds 600 mm and sometimes 2000 Plans to increase the amount of irrigated land in Mediterranean countries should consider how changes in climate and land-use affect water resources. In this study, both precipitation and temperature were used to analyze regional trends in discharge in the basins of the Central Spanish Pyrenees since the mid-20th century. Annual variations in the relationship between precipitation and discharge suggested that discharge was relatively lower in the second half of the study period, coinciding with major changes in land use. On a monthly scale, precipitation increased significantly in October, April, and July, and decreased in March, and temperature increased in January and February and decreased in April. Nevertheless, discharge has decreased significantly in most months in the past 50 years. Land-use and plant-cover changes are the only nonclimatic factor that can explain the loss of around 30% of the average annual discharge.

Abstract. Unconfined debris flows (i.e. not in incised channels) are one of the most active geomorphic processes in mountainous areas. Since they can threaten settlements and infrastructure, statistical and physically based procedures have been developed to assess the potential for landslide erosion. In this study, information on debris flow characteristics was obtained in the field to define the debris flow runout distance and to establish relationships between debris flow parameters. Such relationships are needed for building models which allow us to improve the spatial prediction of debris flow hazards. In general, unconfined debris flows triggered in the Flysch Sector of the Central Spanish Pyrenees are of the same order of magnitude as others reported in the literature. The deposition of sediment started at 17.8 • , and the runout distance represented 60% of the difference in height between the head of the landslide and the point at which deposition started. The runout distance was relatively well correlated with the volume of sediment.

Sedimentological, geomorphological and palynological study of three stratified screes from two environments in the Central Spanish Pyrenees allow reconstruction of climate and vegetational change since the Last Glacial Maximum (LGM). In both localities (at 700 and 1000 m a.s.l.) the deposits show a similar stratigraphy: i) a basal stratified scree, in one case fossilized by speleothems; ii) a paleosol, with abundant charcoal and evidence of human activity; and iii) a massive coarse scree. The stratified scree developed during the Glacial Maximum and Late Glacial. A humid period during the early Holocene was characterized by speleothem formation and carbonate cementation in some screes. Forest fires around 3300 BP were widespread in the Pyrenees and were probably related to human impact. Recent scree reactivation due to an increase in clast supply is limited to cold stages during the late Holocene. Resumé L'étude sédimentologique, géomorphologique et palynologique de troix dépôts de pente stratifiés localisés dans deux environments des Pyrénées centrales espagnoles permet de reconstituer des changements du climat et de la végétation depuis le Dernier Maximum Glaciaire. Dans les deux sites les dépôts présentent la même stratigraphie: i) un éboulis stratifié basal parfois fossilisé par des spéleothèmes; ii) un paléosol, avec de nombreux charbons de bois et des preuves d'activités humaines; et iii) un éboulis grossier massif. Les éboulis stratifiés se sont développés pendant le Dernier Maximum Glaciaire et le Tardiglaciaire. Une période humide au début de l'Holocène est caracterisé par la formation de spéleothèmes et de cimentations carbonatées dans quelques éboulis. Les feux de forêt autour de 3300 ans BP semblent être très repandus dans les Pyrenées et ils sont en rapport, probablement, avec l'impact humain. La dernière réactivation de l'éboulisation due à l'accroissement de la fourniture de débris est limitée aux épisodes froids de la fin de l'Holocène.

This article was prepared with the support of the following research projects: "Debris fall assessment in mountain catchments for local end-users-DAMO-CLES" (EVG1-1999-00027P), financed by the European Commission, and "Assessment of sediment sources and runoff generation areas in relation to land-use changes-HIDROESCALA" (REN2000-1709-C04-01/GLO), financed by CICYT.