Suchergebnisse

36 páginas, 15 figuras, 3 tablas.-- El documento en word es la versión post-print del autor. ; This paper analyses monthly differences in drought impact on vegetation activity in a semi-arid region in the north-east of the Iberian Peninsula between 1987 and 2000. The study determines spatial differences in the effects of drought on the natural vegetation and agricultural crops by means of the joint use of vegetation indexes derived from AVHRR images, a drought index (standardized precipitation index), and Geographic Information Systems. The results show that the effect of drought on vegetation varies noticeably between areas, a pattern that is determined mainly by the location of land-cover types. The influence also varies each month and, in general, is higher during the spring and summer. Aridity and vegetation characteristics similarly account, in part, for spatial differences in the impact of drought on vegetation. In general, the most arid areas, where vegetation cover and activity are low, are those in which the interannual variability of vegetation activity is more determined by the drought occurrence. In assessing drought impact, this analysis takes into account the effects of drought on the vegetation and also considers spatial and seasonal differences. The results should be useful for the management of natural vegetation and crops and for the development of better drought mitigation strategies. ; This work has been supported by the project CGL2005-04508/BOS financed by the Spanish Comission of Science and Technology (CICYT) and FEDER, and ''Programa de grupos de investigación consolidados'' (grupo Clima, Cambio Global y Sistemas Naturales, BOA 48 of 20-04-2005), financed by the Aragón Government. Research of the author was supported by postdoctoral fellowship by the Ministerio de Educación, Cultura y Deporte (Spain). ; Peer reviewed

24 páginas, 14 figuras, 2 tablas.-- El documento en word es el pre-print del artículo. ; The differences in spatial patterns of drought over a range of time scales were analysed by the Standardized Precipitation Index (SPI). In a climatic area with a wide range of precipitation characteristics (the Iberian Peninsula), Pearson III distribution is flexible enough to calculate the drought index on different time scales. The Pearson III distribution was adapted to precipitation frequencies at time scales of 1, 3, 6, 12, 24 and 36 months. Spatial patterns of drought were analysed by Principal Component Analysis. The number of components found increased as the time scale did, which indicates great spatial complexity in drought analysis and uncertainty in drought classification, mainly at scales of 24 or 36 months, since the relationships between SPI series of observatories becomes more distant as the time scale increases. We concluded that there were no homogeneous regions with similar drought patterns that could be used for effective drought management or early warning. ; Thiswork 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) financed by the Spanish Comission of Science and Technology (CICYT) and FEDER, and "Programa de grupos de investigación consolidados" (grupo Clima, Cambio Global y Sistemas Naturales, BOA 147 of 18-12-2002), financed by the Aragón Government. Research of the authors was supported by postdoctoral fellowships by the Ministerio de Educación, Cultura y Deporte (Spain). ; Peer reviewed

18 páginas, 15 figuras, 1 tabla. ; This paper seeks to determine the impact of extreme phases of the Southern Oscillation (SO, El Niño/La Niña) on droughts in the Iberian Peninsula. For this purpose, 51 precipitation series (1910–2000) were used. A spatial classification based on monthly precipitation records was made to identify homogeneous regions and to analyze any spatial differences in the influence of these extreme phases. For each region a drought index was calculated (standardized precipitation index) at timescales of 1, 3, 6 and 12 months. El Niño and La Niña years were identified using the Southern Oscillation index (SOI), in line with studies conducted elsewhere. Mean values of the drought index were calculated each month for the four timescales during El Niño and La Niña years as well as for the year following these events. The statistical significance of any anomalies was evaluated by means of the nonparametric Wilcoxon Mann-Whitney test. The results indicate that extreme phases of the SO significantly affect the occurrence of droughts in the Iberian Peninsula. Moreover, spatial and temporal differences, depending on the timescale used, were identified. Large areas of the Iberian Peninsula are affected by significant negative values of SPI during the final months of La Niña years and the initial months of the following year. In contrast, other areas are affected by dry conditions during the first months of El Niño years as well as during the summers and autumns of the following year. The spatial differences in drought conditions during extreme phases of the SO are noticeable, and the drought signal is more consistent for La Niña than it is for El Niño years. Finally, the usefulness of these results for drought prediction and early warning systems is discussed. ; The author wants to acknowledge financial support from the following projects: BSO2002-02743 and REN2003-07453 (Financed by Ministerio de Educación, Cultura y Deporte Spain and FEDER) and ''Programa de grupos de investigación consolidados'' (grupo Clima, Cambio Global y Sistemas Naturales, BOA 147 of 18-12-2002), financed by Aragón Government. Research of the author was supported by postdoctoral fellowships by the Ministerio de Educación, Cultura y Deporte, Spain. ; Peer reviewed

9 páginas, 6 figuras, 1 tabla.-- El documento en word es la versión post-print del artículo. ; This Letter describes analysis of the trends of NDVI in the Iberian peninsula from 1982 to 2000. The results were compared with the influence of the North Atlantic Oscillation (NAO) index. Significant spatial differences emerge in vegetation trend analysis, identifying a positive trend in the northern part and stability or negative trends in the south. Such a spatial pattern is significantly related to NAO influence on vegetation, which is linked to precipitation distribution in the Iberian peninsula and greatly influenced by this atmospheric pattern. The relationships between vegetation trends and NAO are discussed in detail. ; This research was funded by the projects BSO2002-02743, REN2002-01023-CLI and REN2003-07453 (Financed by Ministerio de Ciencia y Tecnología, Spain and FEDER), and "Programa de grupos de investigación consolidados" (grupo Clima, Cambio Global y Sistemas Naturales, BOA 147 of 18-12-2002), financed by Aragón Government. Data used by the authors in this study include data produced through funding from the Earth Observing System Pathfinder Program of NASA's Mission to Planet Earth in cooperation with the National Oceanic and Atmospheric Administration. ; Peer reviewed

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

6 páginas, 4 figuras. ; The changing spatial patterns of the North Atlantic Oscillation (NAO) dipole and the changing correlations between the NAO and European precipitation were analyzed using the AOGCM CGCM3.1(T63) model provided by the Canadian Centre for Climate Modelling and Analysis (CCCma). Four output scenarios were considered: the IPCC pre-industrial (PI) scenario, the 20th century (20C) scenario, and the B1 and A2 emission scenarios for the 21st century. We detected some differences in the spatial patterns of the NAO-precipitation relationship among the four scenarios. Moreover, there were large differences in the temporal stability of the NAO pressure pattern, and also in the NAO-precipitation relationship among the four scenarios. Under increased greenhouse gas emissions the stability of the NAO-precipitation correlations is enhanced due to the lower variability in the pattern of the NAO dipole. ; This work has been supported by the research project CGL2005-04508/BOS, financed by the Spanish Commission of Science and Technology and FEDER, and ''Programa de grupos de investigación consolidados'' financed by the Aragón Government. ; Peer reviewed

24 páginas, 15 figuras. ; In this study, droughts are analyzed using the standardized precipitation index (SPI) at different time scales for all of Europe over the period 1901–2000. The SPI is calculated at different time scales (1–12 months), as are the average values that correspond to negative and positive phases of the North Atlantic Oscillation (NAO). The responses of droughts to the phases of the NAO vary spatially, but the response also depends on the month of the year and the time scale of the analysis. During the positive/negative phases, negative/positive SPI values are generally recorded in southern Europe, with the opposite pattern recorded in northern Europe. In certain regions, significant differences in the SPI are also recorded during spring, summer, and even autumn. In several regions, the magnitude of the average SPI anomalies is noticeably different for the positive and negative phases of the NAO, indicating the asymmetric response of droughts to the NAO. The unstable response of drought occurrence is also demonstrated, at different time scales, to positive and negative phases of the NAO throughout the twentieth century. During the second half of the twentieth century, there is a strengthening of the influence of the positive phases of the NAO on droughts. In contrast, the negative phases show a weaker influence on the SPI during the second half of the twentieth century. This pattern is related to changes in the wintertime sea level pressure fields associated with positive and negative phases of the NAO. ; This work has been supported by the following 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. Research of the first author was supported by postdoctoral fellowship by the Ministerio de Educación (Spain). ; Peer reviewed

27 páginas, 15 figuras, 6 tablas.-- El documento se encuentra en su versión post-print. ; This paper analyses the spatial and temporal variability of winter droughts in a semi-arid geographic gradient in Northeast Spain, from the Pyrenees in the north to the Mediterranean coastland in the south. Droughts that occurred between 1952 and 1999 were analysed by means of the Standardised Precipitation Index (SPI). The influence of the weather-type frequency and of the general North Atlantic atmospheric circulation patterns was analysed. The results indicate that winter droughts show an important spatial variability in the study area, differentiating three well-defined patterns. These correspond to the Pyrenees, the centre of the Ebro Valley, and the Mediterranean coastland. General negative trends in winter SPI have been found, which are indicative of the increase in winter drought conditions in the study area. Nevertheless, important spatial differences have also been recorded. Dominant north–south gradients in the influence of weather types are shown. Moreover, the negative trends in winter-SPI values agree with the negative trend in the frequency of the weather types prone to cause precipitation, such as the C, SW and W weather types and the increase in the frequency of A weather types. Nevertheless, in the Mediterranean coastland, the positive trend in SPI values agrees with the increase in the frequency of weather types of the east (E, SE), which are prone to cause precipitation in this area. Interannual variations in the frequency of the different weather types have been highly determined by different general atmospheric circulation patterns, mainly the North Atlantic Oscillation (NAO). Nevertheless, the correlation between the time series of weather-type frequency and the winter SPI is higher than that found between the SPI and the NAO. Thus, although the interannual NAO variability explains a high percentage of the interannual differences in the frequency of different weather types, it is not sufficient to explain the spatial and temporal variability of droughts, which respond better to atmospheric variability at more detailed (synoptic) spatial scales. ; 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), Procesos hidrológicos y erosivos en cuencas pirenaicas en relación a cambios de usos de suelo y variabilidad climática (PIRIHEROS, REN2003-08678/HID) 'Caracterización y modelización de procesos hidrológicos en cuencas aforadas para predicción en cuencas no aforadas' (CANOA, CGL 2004-04919-c02-01), all funded by the Spanish Commission of Science and Technology (CICYT) and FEDER, and 'Programa de grupos de investigación consolidados' (grupo Clima, Cambio Global y Sistemas Naturales, BOA 48 of 20-04-2005), financed by the Aragón Government. We want to thank to the National Institute of Meteorology (INM) for providing the data used in this work. Research of the second author was supported by postdoctoral fellowships by the Ministerio de Educación, Cultura y Deporte (Spain). ; Peer reviewed

11 páginas, 7 figuras, 1 tabla.-- This work is licensed under a Creative Commons License. ; At present, the Standardized Precipitation Index (SPI) is the most widely used drought index to provide good estimations about the intensity, magnitude and spatial extent of droughts. The main advantage of the SPI in comparison with other indices is the fact that the SPI enables both determination of drought conditions at different time scales and monitoring of different drought types. It is widely accepted that SPI time scales affect different sub-systems in the hydrological cycle due to the fact that the response of the different water usable sources to precipitation shortages can be very different. The long time scales of SPI are related to hydrological droughts (river flows and reservoir storages). Nevertheless, few analyses empirically verify these statements or the usefulness of the SPI time scales to monitor drought. In this paper, the SPI at different time scales is compared with surface hydrological variables in a big closed basin located in the central Spanish Pyrenees. We provide evidence about the way in which the longer (>12 months) SPI time scales may not be useful for drought quantification in this area. In general, the surface flows respond to short SPI time scales whereas the reservoir storages respond to longer time scales (7–10 months). Nevertheless, important seasonal differences can be identified in the SPI-usable water sources relationships. This suggests that it is necessary to test the drought indices and time scales in relation to their usefulness for monitoring different drought types under different environmental conditions and water demand situations. ; The authors want to acknowledge financial support from the following projects: BSO2002-02743, REN2003-07453, CGL2005-04508/BOS, PIRIHEROS (REN2003- 08678/HID) and CANOA (CGL 2004-04919-c02-01), funded by Ministerio de Ciencia y Tecnología (Spain) and UE-FEDER, and "Programa de grupos de investigación consolidados" (BOA 48 of 20-04-2005), funded by Aragon Government. Research of the first author was supported by postdoctoral fellowship by the Ministerio de Educación y Ciencia (Spain). ; Peer reviewed

11 páginas, 4 tablas, 4 figuras.-- El Pdf es la versión pre-print. ; The temporal evolution of vegetation activity on various land cover classes in the Spanish Pyrenees was analyzed. Two time series of the normalized difference vegetation index (NDVI) were used, corresponding to March (early spring) and August (the end of summer). The series were generated from Landsat TM and Landsat ETM+ images for the period 1984–2007. An increase in the NDVI in March was found for vegetated areas, and the opposite trend was found in both March and August for degraded areas (badlands and erosion risk areas). The rise in minimum temperature and the time variation of the cloud cover during the study period appears to be the most important factors explaining increased NDVI in the vegetated areas. In degraded areas, no climatic or topographic variable was associated with the negative NDVI trend, which may be related to erosion processes taking place in these regions. ; This research was supported by the projects CGL2006-11619/HID, CGL2008- 00831/BTE and CGL2008-01189/BTE (Spanish Commission of Science and Technology and FEDER), ACQWA (FP7-ENV-2007-1- 212250; VII Framework Program of the European Commission), 'Las sequías climáticas en la cuenca del Ebro y su respuesta hidrológica' ('Obra Social La Caixa' and the Aragón Government), and 'Programa de grupos de investigación consolidados' (Aragón Government). The first author was supported by a scholarship from The National Council for Science and Technology of Mexico (CONACYT). ; Peer reviewed

A new multiscalar drought dataset, the SPEIbase, made available to the scientific and management community is presented. The multiscalar character of the dataset allows finding the most appropriate time scale of interest for a specific application, so potential uses of the dataset range over a wide variety of disciplines. The dataset (in three file formats) is distributed under an open license. Further work includes updating the dataset past December 2006, depending on the data availability. ; This work has been supported by the research projects CGL2008-01189/BTE and CGL2006-11619/HID, funded by the Spanish Commission of Science and Technology and FEDER; EUROGEOSS (FP7-ENV-2008-1-226487) and ACQWA (FP7-ENV-2007-1- 212250), funded by the VII Framework Programme of the European Commission; and "Las sequías climáticas en la cuenca del Ebro y su respuesta hidrológica" and "La nieve en el Pirineo aragonés: Distribución espacial y su respuesta a las condiciones climática," funded by "Obra Social La Caixa" and the Aragón Government. ; Peer reviewed

15 páginas, 9 figuras, 4 tablas.-- El documento se encuentra en su versión post-print. ; Using a set of 16 Landsat TM and Landsat ETM+ images from 1984 to 2006, we tested whether climate trends in the last three decades differentially controlled the vegetal activity of eight Pinus halepensis forests located across a marked bioclimatic gradient. Our results show spatial differences in trends in the normalized difference vegetation index (NDVI) between 1984 and 2006, which were highly related to the spatial distribution of aridity. There was a strong correlation between the change in the NDVI values and the climatic water balance in each forest (R = 0.86, p = 0.006). A large increment of the NDVI was observed in forests located in the most humid areas, whereas those located in the most arid areas showed a slight decrease in the NDVI. We used a forest growth simulation model (GOTILWA+) to estimate the leaf area index (LAI) in each of the eight analyzed forests, and found similar results to those obtained for the NDVI. Significant correlation was found between the spatial pattern of NDVI and LAI trends between 1984 and 2006 (R = 0.82, p = 0.01). The climate evolution in the last four decades explains the observed and modeled changes in the NDVI and the LAI. In agreement with other studies, we showed that the general response of the forests to more favorable conditions (in terms of temperature increment and CO2 fertilization) is increased leaf activity and biomass. Nevertheless, in forests located in the most arid areas the positive trend observed in the potential evapotranspiration rates increased water stress, and had a negative effect on forest growth. Given the future predictions of warming and declining precipitation from global climate models for the Mediterranean region, an increase in stress conditions affecting these forests is expected, which will affect their growth and survival, mainly in the most arid areas. ; This work has been supported by the research projects CGL2005-04508/BOS, CGL2008-01189/BTE, CGL2006-11619/HID 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, ''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

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