Assessing the Effect of Climate Oscillations and Land-use Changes on Streamflow in the Central Spanish Pyrenees

Abstract

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.

Spatial explicit assessment of rural land abandonment in the Mediterranean area

“…They attributed these changes to a decline in agricultural and pastoral activities. Begueria et al (2003) reported a negative trend in discharge in the central Pyrenean rivers coinciding with major changes in land management (land was abandoned and progressively covered by dense vegetation). A trend of progressive and intense reduction in run-off was confirmed by other experimental studies in the Pyrenees (Garcìa-Ruiz et al, 1995, 1996Molinillo et al, 1997).…”

Temporal distribution of suspended sediment transport in a humid Mediterranean badland area: The Araguás catchment, Central Pyrenees

“…This landscape resulted from strong human pressures in recent centuries, including deforestation, frequent wildfires, overgrazing, and the cultivation of steep slopes; these factors had a particularly strong effect upon the lower and mid-level parts of the mountain (Lasanta et al, 2006). Most of these activities were abandoned during the middle of the twentieth century, resulting in important hydrological and geomorphic changes in the Central Pyrenees (Beguería et al, 2003).…”

Channel changes and floodplain management in the meandering middle Ebro River, Spain

“…Similar hydrological, geomorphological and ecological consequences have been assessed in other fluvial systems. Such modifications have also been studied on the tributaries of the Ebro (García-Ruiz et al, 2001;Beguería et al, 2003;Batalla, 2003;Batalla et al, 2004;Acín, 2004;Granado, 2004;Frutos et al, 2004;Ollero and Martín-Vide, 2005). Most of the cases studied dealt with transformation of braided channels into meandering ones and problems due to incision, human invasion processes on the riparian corridors, and significant reductions of the channel mobility over the last decades.…”