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Duvert C.,Grenoble Institute of Technology | Nord G.,Grenoble Institute of Technology | Gratiot N.,Grenoble Institute of Technology | Navratil O.,IRSTEA | And 7 more authors.
Journal of Hydrology | Year: 2012

The erosion and transport of fine-grained sediment in small mountainous catchments involve complex processes occurring at different scales. The suspended sediment yields (SSYs) delivered downstream are difficult to accurately measure and estimate because they result from the coupling of all these processes. Using high frequency discharge and suspended sediment data collected in eight small mountainous catchments (0.45-22km 2) from four distinct regions, we studied the relationships between event-based SSY and a set of other variables. In almost all the catchments, the event peak discharge (Q max) proved to be the best descriptor of SSY, and the relations were approximated by single power laws of the form SSY=αQmaxβ. The β exponents ranged between 0.9 and 1.9 across the catchments, while variability in α was much higher, with coefficients ranging between 25 and 5039. The broad distribution of α was explained by a combination of site-specific physical factors, such as the percentage of degraded areas and hillslope gradient. Further analysis of the factors responsible for data dispersion in each catchment was carried out. Seasonality had a significant influence on variability; but overall, most of the scattering in the SSY-Q max regressions was explained by the short-lasting memory effects occurring between successive events (i.e. in-channel temporary storage and remobilization of sediment; antecedent moisture conditions). The predictability of SSY-Q max models was also assessed. Simulations of SSY per event and of annual SSY were conducted by using the computed regressions and the measured Q max. Estimates of SSY per event were very uncertain. In contrast, annual SSY estimates based on the site-specific models were reasonably accurate in all the catchments, with interquartile ranges remaining in the ±50% error interval. The prediction quality of SSY-Q max relations was partly attributed to the statistical compensation that likely occurred between extreme values over a year; but it also suggests that the complex processes occurring at the event scale were smoothed at the annual scale. This SSY-Q max rating appears as a parsimonious predicting tool for roughly estimating SSY in small mountainous catchments. However, in its current form the technique needs further improvement as α and β values need to be better constrained. © 2012 Elsevier B.V. Source


Lana-Renault N.,University of La Rioja | Regues D.,CSIC - Pyrenean Institute of Ecology | Serrano P.,CSIC - Pyrenean Institute of Ecology | Latron J.,Institute Of Diagnosi Ambiental I Estudis Of Laigua Idaea
Hydrological Processes | Year: 2014

The temporal and spatial dynamics of groundwater was investigated in a small catchment in the Spanish Pyrenees, which was extensively used for agriculture in the past. Analysis of the water table fluctuations at five locations over a 6-year period demonstrated that the groundwater dynamics had a marked seasonal cycle involving a wetting-up period that commenced with the first autumn rainfall events, a saturation period during winter and spring and a drying-down period from the end of spring until the end of the summer. The length of the saturation period showed great interannual variability, which was mainly influenced by the rainfall and evapotranspiration characteristics. There was marked spatial variability in the water table, especially during the wetting-up period, which could be related to differences in slope and drainage area, geomorphology, soil properties and local topography. Areas contributing to runoff generation were identified within the catchment by field mapping of moisture conditions. Areas contributing to infiltration excess runoff were correlated with former cultivated fields affected by severe sheetwash erosion. Areas contributing to saturation excess runoff were characterized by a marked spatial dynamics associated with catchment wetness conditions. The saturation spatial pattern, which was partially related to the topographic index, was very patchy throughout the catchment, suggesting the influence of other factors associated with past agricultural activities, including changes in local topography and soil properties. The relationship between water table levels and stream flow was weak, especially during the wetting-up period, suggesting little connection between ground water and the hydrological response, at least at some locations. The results suggest that in drier and human-disturbed environments, such as sub-Mediterranean mountains, saturation patterns cannot be represented only by the general topography of the catchment. They also suggest that groundwater storage and runoff is not a succession of steady-state flow conditions, as assumed in most hydrological models. © 2013 John Wiley & Sons, Ltd. Source


Doblas-Miranda E.,CREAF | Martinez-Vilalta J.,CREAF | Martinez-Vilalta J.,University of Barcelona | Lloret F.,CREAF | And 28 more authors.
Global Ecology and Biogeography | Year: 2015

Aim: Mediterranean terrestrial ecosystems serve as reference laboratories for the investigation of global change because of their transitional climate, the high spatiotemporal variability of their environmental conditions, a rich and unique biodiversity and a wide range of socio-economic conditions. As scientific development and environmental pressures increase, it is increasingly necessary to evaluate recent progress and to challenge research priorities in the face of global change. Location: Mediterranean terrestrial ecosystems. Methods: This article revisits the research priorities proposed in a 1998 assessment. Results: A new set of research priorities is proposed: (1) to establish the role of the landscape mosaic on fire-spread; (2) to further research the combined effect of different drivers on pest expansion; (3) to address the interaction between drivers of global change and recent forest management practices; (4) to obtain more realistic information on the impacts of global change and ecosystem services; (5) to assess forest mortality events associated with climatic extremes; (6) to focus global change research on identifying and managing vulnerable areas; (7) to use the functional traits concept to study resilience after disturbance; (8) to study the relationship between genotypic and phenotypic diversity as a source of forest resilience; (9) to understand the balance between C storage and water resources; (10) to analyse the interplay between landscape-scale processes and biodiversity conservation; (11) to refine models by including interactions between drivers and socio-economic contexts; (12) to understand forest-atmosphere feedbacks; (13) to represent key mechanisms linking plant hydraulics with landscape hydrology. Main conclusions: (1) The interactive nature of different global change drivers remains poorly understood. (2) There is a critical need for the rapid development of regional- and global-scale models that are more tightly connected with large-scale experiments, data networks and management practice. (3) More attention should be directed to drought-related forest decline and the current relevance of historical land use. © 2014 John Wiley & Sons Ltd. Source


Lana-Renault N.,University Utrecht | Lana-Renault N.,University of La Rioja | Latron J.,Institute Of Diagnosi Ambiental I Estudis Of Laigua Idaea | Karssenberg D.,University Utrecht | And 3 more authors.
Journal of Hydrology | Year: 2011

The stream flow response of two neighboring catchments in the central Spanish Pyrenees was compared for 26 rainstorms covering both catchments: one catchment (2.84km2) was extensively used for agriculture in the past, and the other (0.92km2) is covered by dense natural forest. Their similarity in terms of lithology and topography enabled us to separate the effects of soil and land cover on their hydrological responses. Relative to the forested catchment, peak flows in the past agricultural catchment were always greater (566 vs. 119ls-1km-2), the response time was 2- to 3-fold faster (131 vs. 356min), and the recession limbs were 1-2 orders of magnitude shorter (7 vs. 72h). Storm flow was usually greater in the former agricultural catchment, especially for low-intermediate sized flood events; only for larger events the storm flow in the forested catchment was sometimes greater. Storm flow differences were closely related to catchment wetness conditions and showed a marked seasonal pattern, with higher values in the past agricultural catchment under dry conditions, and usually higher values in the forested catchment under wet conditions. In the past agricultural catchment, runoff was generated during the entire water year, through both surface (i.e. infiltration excess and saturation excess overland flow) and subsurface flow. We suggest that the forested catchment can be characterized by a dual (or switching) behavior controlled by soil moisture conditions, which regulates the hydrological connectivity and favors the release of large amounts of subsurface flow. Differences in soil depth and permeability, together with differences in vegetation cover, may explain the contrasting dominant runoff generation processes operating in each catchment, and consequently the differences between their hydrograph characteristics. © 2011 Elsevier B.V.. Source


Cisneros M.,University of Barcelona | Cacho I.,University of Barcelona | Frigola J.,University of Barcelona | Canals M.,University of Barcelona | And 9 more authors.
Climate of the Past | Year: 2016

This study presents the reconstructed evolution of sea surface conditions in the central-western Mediterranean Sea during the late Holocene (2700 years) from a set of multi-proxy records as measured on five short sediment cores from two sites north of Minorca (cores MINMC06 and HER-MC-MR3). Sea surface temperatures (SSTs) from alkenones and Globigerina bulloides Mgĝ€/ĝ€Ca ratios are combined with Î 18O measurements in order to reconstruct changes in the regional evaporation-precipitation (E-P) balance. We also revisit the G. bulloides Mgĝ€/ĝ€Ca-SST calibration and re-adjusted it based on a set of core-top measurements from the western Mediterranean Sea. Modern regional oceanographic data indicate that Globigerina bulloides Mgĝ€/ĝ€Ca is mainly controlled by seasonal spring SST conditions, related to the April-May primary productivity bloom in the region. In contrast, the alkenone-SST signal represents an integration of the annual signal.

The construction of a robust chronological framework in the region allows for the synchronization of the different core sites and the construction of ĝ€œstackedĝ€? proxy records in order to identify the most significant climatic variability patterns. The warmest sustained period occurred during the Roman Period (RP), which was immediately followed by a general cooling trend interrupted by several centennial-scale oscillations. We propose that this general cooling trend could be controlled by changes in the annual mean insolation. Even though some particularly warm SST intervals took place during the Medieval Climate Anomaly (MCA), the Little Ice Age (LIA) was markedly unstable, with some very cold SST events mostly during its second half. Finally, proxy records for the last centuries suggest that relatively low E-P ratios and cold SSTs dominated during negative North Atlantic Oscillation (NAO) phases, although SSTs seem to present a positive connection with the Atlantic Multidecadal Oscillation (AMO) index. © Author(s) 2016. Source

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