J. Zabalza

Impact of climate and land use change on water availability and reservoir management: Scenarios in the Upper Aragón River, Spanish Pyrenees

Streamflows in a Mediterranean mountain basin in the central Spanish Pyrenees were projected under various climate and land use change scenarios. Streamflow series projected for 2021-2050 were used to simulate the management of the Yesa reservoir, which is critical to the downstream supply of irrigation and domestic water. Streamflows were simulated using the Regional Hydro-Ecologic Simulation System (RHESSys). The results show that increased forest cover in the basin could decrease annual streamflow by 16%, mainly in early spring, summer and autumn. Regional climate models (RCMs) project a trend of warming and drying in the basin for the period 2021-2050, which will cause a 13.8% decrease in annual streamflow, mainly in late spring and summer. The combined effects of forest regeneration and climate change are expected to reduce annual streamflows by 29.6%, with marked decreases affecting all months with the exception of January and February, when the decline will be moderate. Under these streamflow reduction scenarios it is expected that it will be difficult for the Yesa reservoir to meet the current water demand, based on its current storage capacity (476 hm(3)). If the current project to enlarge the reservoir to a capacity of 1059 hm(3) is completed, the potential to apply multi-annual streamflow management, which will increase the feasibility of maintaining the current water supply. However, under future climate and land cover scenarios, reservoir storage will rarely exceed half of the expected capacity, and the river flows downstream of the reservoir is projected to be dramatically reduced.

Mapping the annual evolution of snow depth in a small catchment in the Pyrenees using the long-range terrestrial laser scanning

This paper presents the methodology used to develop snow depth distribution maps for a small catchment in the Central Spanish Pyrenees covering 55 ha in a 1:10,000 scale. The Main Map was obtained using LiDAR (light detection and ranging) technology from a long-range Terrestrial Laser Scanner (TLS) in six field surveys undertaken during the 2012 winter–spring period. This technique enabled the acquisition of information at a very high resolution concerning the spatial variability of snow cover, providing snow depth information for remote areas where data acquisition is complex and hazardous. We describe the methodological steps and the quality assessment applied in developing the maps. Comparison with manual measurements confirmed the reliability of the snow depth maps, including areas located at large distances from the scanner (800 m). This method provides a promising tool for future investigations of snow dynamics in mountainous environments.

The NAO Impact on Droughts in the Mediterranean Region

This chapter shows the influence of the North Atlantic Oscillation (NAO) on droughts in the entire Mediterranean region between 1901 and 2006. The analysis has been based on identification of positive and negative NAO winters and also detection of the anomalies of drought severity by means of the Standardized Precipitation Evapotranspiration Index (SPEI). The analysis is focussed on the winter NAO. Nevertheless, given that the SPEI drought indicator can be obtained at different time-scales, the study shows how the effects of the winter NAO on droughts are propagated for the following months when long time scales are considered. In general, during the positive phases, the negative SPEI averages are recorded in Southern Europe (the Iberian Peninsula, Italy and the Balkans), areas of Turkey and northwest Africa. On the contrary, the SPEI averages are found positive in northeast Africa. The opposite configuration, but with some differences in the spatial patterns and the magnitude of the SPEI averages, is found during the negative NAO years. The findings of this study should be of great applicability in terms of developing early warning systems. The established relationships between NAO phases and drought indices seem appropriate for drought prediction over large areas of the Mediterranean basin.

Reservoir Management in the Duero Basin (Spain): Impact on River Regimes and the Response to Environmental Change

The climatic conditions of the Iberian Peninsula result in an imbalance between water availability and demand, which is largely managed through the many dams that were built during the 20th century. However, dam operations modify the natural functioning of rivers and related subsystems. In this study we investigated the effect of reservoirs on river regimes in the Duero basin, which is one of the largest river basins in Spain. This involved calculation of a modified impoundment ratio index, and assessment of the correlations between monthly inflows and outflows. Water resources in the basin have decreased markedly during the last five decades, so we also studied how patterns of management have adapted to less water availability in the region. A significant correlation was found between the level of impoundment and the alteration of river regimes by dams. The degree of regulation was highly dependent on annual inflows into the reservoir, and consequently alterations to river regimes were more intense during dry years. The basic pattern of flow regulation involved the storage of water during winter and spring in preparation for high water demand in summer, when natural flows are low. A combination of trend and cluster analyses revealed three responses of reservoir managers to decreasing inflows during the study period: (i) for several reservoirs the level of storage was reduced; (ii) for many reservoirs, particularly those for hydropower production, the storages were increased; and (iii) for the remainder the storage levels were maintained by adjusting the outflows to the decreasing inflows. The results suggest the absence of a common approach to reservoir management, and the dominance of other interests over environmental concerns, particularly in the context of hydrological change in the basin.

Influence of Winter North Atlantic Oscillation Index (NAO) on Climate and Snow Accumulation in the Mediterranean Mountains

This work analyses the influence of NAO on the interannual evolution of winter temperature, precipitation and snowpack in the Mediterranean mountains. Due to lack of snow data in many mountain areas, the occurrence of four different winter modes are used as a proxy of the amount of accumulated snow. Winter modes are defined on the basis of combined precipitation and temperature thresholds: warm and wet (WW), warm and dry (WD), cold and wet (CW), and cold and dry (CD). The study focuses on 15 relevant mountain areas located in the Mediterranean Europe, Morocco, Turkey and Lebanon. Moreover, we present the relationship between winter NAO and snow depth data in the Swiss Alps and the Spanish Pyrenees. It has been demonstrated that snowpack accumulation in a given year is closely related to the occurrence of these winter modes. However, such relationship is variable among mountain areas and also there are differences depending on elevation in a particular mountain range. Results show that occurrence of different winter modes is strongly related to the winter NAO for the majority of the mountain chains under study, although these relationships are weaker in the easternmost part of the Mediterranean basin. Moreover, it has also been proven that the snow cover response to winter NAO may differ spatially as a consequence of the different influence of winter NAO on precipitation and temperature. In Switzerland, NAO is correlated more with temperature than with precipitation. Therefore, the influence of NAO on snow is significant at the lowest elevation areas, where temperature is the main control on snowpack accumulation. On the other hand, over the Spanish Pyrenees the NAO mainly controls the interannual variability of precipitation. In this region, the highest correlation with snow is found at high elevations where the interannual variability of temperature does not significantly influences snowpack, whereas precipitation controls mainly the accumulation of snow.