Suchergebnisse

12 páginas, 9 figuras, 3 tablas.-- El documento en word es el post-print del artículo. ; Here we analyse trends in drought magnitude in the middle Ebro valley, a semi-arid area of the Iberian Peninsula, between 1951 and 2000. A significant increase in the severity of drought was identified from 1951 to 2000, and principal components analysis revealed three general patterns of drought evolution. Trend analysis of these patterns indicated that trend is significant only in northern areas (p < 0.01). Trends in drought variability were also analysed; a positive trend was recorded between 1951 and 2000. However, the overall results show a high degree of spatial variability. We show that this variability is determined by several geographic/topographic factors, mainly the distance to the Mediterranean Sea and the Bay of Biscay, water bodies that regulate the origin and direction of air masses and flows. It should also be noted that spatial variability of drought was detected because we used a dense database. Our results indicate that at the sub-regional level, drought patterns should be studied using a large amount of empirical data, since spatial variability may be relevant. ; The authors want to acknowledge financial support from the projects BSO2002-02743 and CGL2005-04508=BOS (Financed by Ministerio de Educaci ón y Ciencia, Spain and FEDER), and ''Programa de grupos de investigaci on consolidados'' (grupo Clima, Cambio Global y Sistemas Naturales, BOA 48 of 20-04-2005), financed by Aragón Government. ; Peer reviewed

23 páginas, 16 figuras, 1 tabla.-- El documento se encuentra en su versión post-print. ; This paper analyses the role played by the North Atlantic Oscillation (NAO) in the creation of drought conditions in a semi-arid region of north-east Spain (the middle Ebro valley), from 1600 to the year 2000. The study used documents from ecclesiastical archives for the seventeenth, eighteenth and nineteenth centuries. For the twentieth century, instrumental precipitation records were used as well. A December–August drought index from 1600 to 1900 was compiled from the historical documentary sources (rogation ceremonies). The index was validated by means of precipitation records between 1858 and 1900 and independent precipitation data from 1600 reconstructed by means of dendrochronological records. Using instrumental data a drought index was also calculated (Standardized Precipitation Index, SPI) for the 1958–2000 period. We found that the NAO was important in explaining the droughts identified in the study area from documents and instrumental data. Positive values of the winter NAO index are prone to cause droughts in the middle Ebro valley. This finding has been verified since 1600 by means of two independent reconstructions of the winter NAO index. The same behaviour has been observed during the nineteenth and twentieth centuries by means of instrumental records. The climatic and geographic factors that explain the high influence of North Atlantic Oscillation on droughts in this region are discussed in depth. ; This work has been supported by the projects: CGL 2005-04508/BOS financed by the Spanish Commission of Science and Technology and FEDER, PIP176/2005 and "Programa de grupos de investigación consolidados" (BOA 48 of 20-04-2005) financed by the Aragón Government. ; Peer reviewed

23 páginas, 11 figuras, 4 tablas.-- El documento en word es la versión post-print del artículo. ; This paper analyses the role of climatic aridity on the spatial differences of vegetation activity and its inter-annual variability in a semi-arid region of the North East of the Iberian Peninsula. The vegetation activity was quantified by means of a monthly Normalized Difference Vegetation Index (NDVI) database from NOAA–AVHRR satellite images (1987–2000) at 1 km2 of spatial resolution. Coefficients of variation (CoV) from temporal NDVI series were also calculated. The greater temporal variability of NDVI was recorded in areas with low vegetation cover (steppe and dry farming lands), whereas the lowest temporal variability of NDVI was recorded in the irrigated lands and forests located in the most humid areas. There is a strong and negative relationship between the CoV and the NDVI distribution, but we recorded important differences among land uses. Using maps of annual precipitation and temperature obtained by means of a regression-based method, we studied the influence of aridity on spatial patterns of NDVI and CoV values. The results show the high influence of aridity on the spatial distribution of vegetation activity in the Middle Ebro Valley. Aridity causes a general decrease of NDVI and an increase of CoV. Nevertheless, non-linear relationships between aridity and NDVI and CoV were recorded for the whole of the study area and for the different land uses. The relationships between aridity and vegetation patterns in the semi-arid region studied are discussed in depth. ; This work has been supported by the projects: ''Caracterización espacio-temporal de las sequías en el valle medio del Ebro e identificación de sus impactos'' (BSO2002-02743), ''Variabilidad climática y dinámica forestal en ecosistemas de ecotono'' (REN2003-07453), ''El papel de las sequías en los procesos de modificación ambiental en el semiárido aragonés: evaluación mediante imágenes de satélite de alta resolución espacial y Sistemas de Información Geográfica'' (CGL2005-04508/BOS) financed by the Spanish Comission of Science and Technology (CICYT) and FEDER, and ''Programa de grupos de investigacio´ n consolidados'' (grupo Clima, Cambio Global y Sistemas Naturales, BOA 48 of 20-04-2005), financed by the Aragón Government. Research of the first author was supported by postdoctoral fellowship by the Ministerio de Educación, Cultura y Deporte (Spain). ; Peer reviewed

19 páginas, 7 figuras, 7 tablas.-- El documento se encuentra en su versión post-print. ; This paper analyses changes in plant cover in a valley of the Spanish Central Pyrenees during the second half of the 20th century. The role played by the extensification of farming activities in this process has been examined. The method used was based on evolutionary mapping of the plant cover, application of landscape indices and spatial analysis of livestock pressure. Throughout the century the stocking rate in the valley has decreased dramatically and sheep have been replaced by cattle. Most crop areas have been abandoned, and these areas have gone through a revegetation process (involving both natural vegetation and forest plantation). Nowadays the forest is the most abundant element. The study shows that both agricultural set-aside and the stocking rate decrease have led to a more natural landscape. Finally, the positive and negative effects of extensification and the changes in vegetation patterns associated with it are debated. ; This work has been supported by the projects: 'Efectos erosivos del fuego a lo largo de un gradiente climático: Aportaciones para la gestión de áreas quemadas' (REN2002- 00133/GLO) and 'Variabilidad climática y dinámica forestal en ecosistemas de ecotono' (REN2003-07453) financed by the Spanish Commission of Science and Technology (CICYT) and FEDER and 'Programa de grupos de investigación consolidados' (BOA 147 of 18-12-2002), financed by the Aragón Government. ; Peer reviewed

In this study, the spatial patterns of relative air humidity and its relation to urban, geographical and meteorological factors in the city of Zaragoza (Spain) is discussed. We created a relative humidity database by means of 32 urban transects. Data were taken on different days and with different weather types. This data set was used to map the mean spatial distribution of urban dry island (UDI). Using stepwise multiple regression analysis and Landsat ETM+ images the relationships between mean UDI and the main geographic-urban factors: topography, land cover, and surface reflectivity, have been analyzed. Different spatial patterns of UDI were determined using Principal Component Analysis (PCA) (Varimax rotation). The three components extracted accounted for 91% of the total variance. PC1 accounted for the most general patterns (similar to mean UDI); PC2 showed a shift of dry areas to the SE and PC3 a shift to NW. Using data on wind direction in Zaragoza, we have found that the displacement of dry areas to the SE (PC 2) was greater during NW winds while the shift to the NW (PC 3) was produced mainly by SE winds. ; This research has been possible thanks to the assistance from the following projects: Clima y calidad ambiental en la ciudad de Zaragoza ("Climate and Environmental Quality in Zaragoza"), financed by the Zaragoza City Council Environmental Department and the Programa de grupos de investigación consolidado ("Consolidated Research Group Programme"), financed by the Aragón Regional Government and European Fund for Regional Development (FEDER). ; Peer reviewed ; Peer Reviewed

In this study, the spatial patterns of relative air humidity and its relation to urban, geographical and meteorological factors in the city of Zaragoza (Spain) is discussed. We created a relative humidity database by means of 32 urban transects. Data were taken on different days and with different weather types. This data set was used to map the mean spatial distribution of urban dry island (UDI). Using stepwise multiple regression analysis and Landsat ETM+ images the relationships between mean UDI and the main geographic-urban factors: topography, land cover, and surface reflectivity, have been analyzed. Different spatial patterns of UDI were determined using Principal Component Analysis (PCA) (Varimax rotation). The three components extracted accounted for 91% of the total variance. PC1 accounted for the most general patterns (similar to mean UDI); PC2 showed a shift of dry areas to the SE and PC3 a shift to NW. Using data on wind direction in Zaragoza, we have found that the displacement of dry areas to the SE (PC 2) was greater during NW winds while the shift to the NW (PC 3) was produced mainly by SE winds. ; This research has been possible thanks to the assistance from the following projects: Clima y calidad ambiental en la ciudad de Zaragoza ("Climate and Environmental Quality in Zaragoza"), financed by the Zaragoza City Council Environmental Department and the Programa de grupos de investigación consolidado ("Consolidated Research Group Programme"), financed by the Aragón Regional Government and European Fund for Regional Development (FEDER). The authors would like to thank the Editor and anonymous reviewers for their valuable comments and suggestions. ; Peer reviewed

9 páginas, 9 figuras, 1 tabla. ; Spatial patterns of the urban heat island (UHI) in Zaragoza (Spain) were determined by Principal Component Analysis (VARIMAX rotation) of air temperature in the city, and mapped using GIS. The 3 components extracted accounted for 92.9% of the total variance. Principal component (PC) 1 accounted for the most general patterns of UHI, PC2 showed a shift of warm areas to the SE and PC3 a shift to the NW. A rotated component matrix was used to identify correlations between each component and daily maps. The spatial patterns indicated by PC2 and PC3 were determined by surface wind direction. The displacement of warm areas to the SE (PC2) was greater during NW winds while the shift to the NW (PC3) was produced mainly by SE winds. ; We acknowledge financial support from the following projects: BSO2002-02743, REN2003-07453, CGL2005-0408/BOS (funded by the Ministerio de Ciencia y Tecnología of Spain, and by FEDER); 'Programa de grupos de investigación consolidados', grupo Clima, Cambio Global y Sistemas Naturales, BOA 48 of 20-04-2005 (funded by the Government of Aragon); and 'Clima y calidad ambiental en la ciudad de Zaragoza' (funded by the Zaragoza City Council). Research of S.M.V.-S. was supported by a postdoctoral fellowship of the Ministerio de Educación, Cultura y Deporte of Spain). ; Peer reviewed

28 páginas,, 4 tablas, 7 figuras. The definitive version is available at: http://www3.interscience.wiley.com/journal/4735/home ; We compare different spatial interpolation techniques in mapping the monthly frequency of fogs in the Aragón region (northeast Spain). The local and spatially complex nature of the fogs makes them more difficult to map than other climatic variables such as precipitation and temperature. We found clear seasonal differences in the quality of the obtained maps. The localized nature of spring and summer fogs, mainly restricted to valley bottoms in mountainous areas, gives rise to several limitations. The modelling of fog frequency is more complex than that of other climate variables; to improve the model predictions, it is necessary to consider topographic variables that simulate the terrain structure. Moreover, the highly complex nature of the relationship between fog frequency and terrain means that simple linear models perform poorly in accounting for the role of geographic and topographic variables in determining the spatial distribution of fog frequency. The inclusion of non-linear relationships between fog frequency and terrain variables in the models following a general additive model (GAM) procedure leads to an improvement in model performance because the flexibility of GAMs enables the inclusion of non-linear relationships and the generation of response-curve shapes that detail the exact relationship between the dependent variable and predictors throughout the entire range of the variable. ; This work has been supported by the following research projects: CGL2005-04508/BOS (financed by the Spanish Commission of Science and Technology and FEDER), PIP176/2005 (financed by the Aragón Government), and "Programa de grupos de investigación consolidados" (BOA 48 of 20-04-2005), also financed by the Aragón Government. ; Peer reviewed

41 Pag., 14 Fig. The definitive version is available at: http://www.sciencedirect.com/science/journal/00221694 ; The influence of climate variation on the availability of water resources was analyzed in the headwaters of the Tagus River basin using two drought indices, the standardized precipitation index (SPI) and the standardized precipitation evapotranspiration index (SPEI). This basin is highly regulated and strategic, and contains two hyperannual reservoirs that are the origin of the water supply system for Mediterranean areas of southeast Spain. The indices confirmed that drought conditions have prevailed in the headwaters of the Tagus River since the 1970s. The responses in river discharge and reservoir storage were slightly higher when based on the SPEI rather than the SPI, which indicates that although precipitation had a major role in explaining temporal variability in the analyzed parameters, the influence of temperature was not negligible. Moreover, the greatest response in hydrological variables was evident over longer timescales of the climatic drought indices. Although the effect of climate variability on water resources was substantial during the analyzed period, we also showed a major change in hydrological–climatic relationships in regulated systems including reservoir storage and outflow. These were closely related to changes in external demand following commencement of the water transfer system to the Júcar and Segura basins after the 1980s. The marked reduction in water availability in the basin, which is related to more frequent droughts, contrasts with the amount of water transferred, which shows a clear upward trend associated with increasing water demand in the Mediterranean basin. ; This work has been supported by the research projects CGL2006-11619/HID, CGL2008-01189/BTE, and CGL2008-1083/CLI financed by the Spanish Commission of Science and Technology and FEDER, EUROGEOSS (FP7-ENV-2008-1-226487) and ACQWA (FP7-ENV-2007-1- 212250) financed by the VII Framework Programme of the European Commission, STRIVER (Strategy and methodology for Improved IWRM—An integrated interdisciplinary assessment in four twinning river basins), financed by the VI Framework Programme of the European Commission. "Las sequías climáticas en la cuenca del Ebro y su respuesta hidrológica" Financed by "Obra Social La Caixa" and the Aragón Government and "Programa de grupos de investigación consolidados" financed by the Aragón Government. ; Peer reviewed

Mountain systems within the Mediterranean region, e.g., the Pyrenees, are very sensitive to climate change. In the present study, we quantified the magnitude of extreme precipitation events and the number of days with torrential precipitation (daily precipitation ≥ 100 mm) in all the rain gauges available in the Pyrenees for the 1981-2015 period, analyzing the contribution of the synoptic scale in this type of event. The easternmost (under Mediterranean influence) and north-westernmost (under Atlantic influence) areas of the Pyrenees registered the highest number of torrential events. The heaviest events are expected in the eastern part, i.e., 400 mm day−1 for a return period of 200 years. Northerly advections over the Iberian Peninsula, which present a low zonal index, i.e., implying a stronger meridional component, give rise to torrential events over the western Pyrenees; and easterly advections favour extreme precipitation over the eastern Pyrenees. The air mass travels a long way, from the east coast of North America, bringing heavy rainfall to the western Pyrenees. In the case of the torrential events over the eastern Pyrenees, the trajectory of the air mass causing the events in these areas is very short and originates in the Mediterranean Basin. The North Atlantic Oscillation (NAO) index has no influence upon the occurrence of torrential events in the Pyrenees, but these events are closely related to certain Mediterranean teleconnections such as the Western Mediterranean Oscillation (WeMO) ; The present study was conducted within the framework of the Climatology Group of the University of Barcelona (2017 SGR 1362, Catalan Government) and the Spanish CLICES project (CGL2017-83866-C3-2-R, AEI/FEDER, UE). M.L.-C. was awarded a pre-doctoral FPU Grant (FPU2017/02166) from the Spanish Ministry of Science, Innovation and Universities. R.S.-N. and J.M.C. are partially supported by the Government of Aragón through the "Program of research groups" (group H38, "Clima, Agua, Cambio Global, y Sistemas Naturales")