Review: From multi-scale conceptualization to a classification system for inland groundwater-dependent ecosystems

Aquifers provide water, nutrients and energy with various patterns for many aquatic and terrestrial ecosystems. Groundwater-dependent ecosystems (GDEs) are increasingly recognized for their ecological and socio-economic values. The current knowledge of the processes governing the ecohydrological functioning of inland GDEs is reviewed, in order to assess the key drivers constraining their viability. These processes occur both at the watershed and emergence scale. Recharge patterns, geomorphology, internal geometry and geochemistry of aquifers control water availability and nutritive status of groundwater. The interface structure between the groundwater system and the biocenoses may modify the groundwater features by physicochemical or biological processes, for which biocenoses need to adapt. Four major types of aquifer-GDE interface have been described: springs, surface waters, peatlands and terrestrial ecosystems. The ecological roles of groundwater are conditioned by morphological characteristics for spring GDEs, by the hyporheic zone structure for surface waters, by the organic soil structure and volume for peatland GDEs, and by water-table fluctuation and surface floods in terrestrial GDEs. Based on these considerations, an ecohydrological classification system for GDEs is proposed and applied to Central and Western-Central Europe, as a basis for modeling approaches for GDEs and as a tool for groundwater and landscape management.     

Tsunami mapping in the Gulf of Guayaquil,Ecuador, due to local seismicity

The North-Andean subduction zone generates recurrent tsunamigenic earthquakes. The seismicity is usually considered to be segmented because of different specific morphological features of the Nazca Plate driving the subduction motion. Most of the recent powerful earthquakes in the margin were located in its northern part. To the south, the region of the Gulf of Guayaquil, only (undocumented) three events in 1901, 1933 and 1953 were possibly powerful and tsunamigenic. Here we are interested in the tsunami signature due to local seismicity. Two realistic earthquake scenarios (M_w = 7 and M_w = 7.5) taking into account the hypothesized segmentation of the area are proposed. Their return period is supposed to be intra-centenary. Then, a larger magnitude unsegmented M_w = 8 scenario is computed (half-millennium return period). The interior of the Gulf of Guayaquil as well as the Santa Elena Peninsula are sheltered areas including numerous coastal infrastructures and the city of Guayaquil. It is predicted that potential flooding would occur at high tide only for both segmented and unsegmented scenarios in (1) south of Playas with however only a few centimeters of wave height and (2) Chanduy (a few meters). Both are important zones of coastal farms.     

Secondary flows induced by wind forcing in the Rhône region of freshwater influence

Secondary flows induced by the blocking effect of a river plume on a transverse upwelling are investigated in a microtidal region of freshwater influence (ROFI). A nested version of the SYMPHONIE primitive-equation free-surface model for 3-D baroclinic coastal flows has been developed for the Rhône ROFI. The main characteristics of the model are a generalized sigma coordinate system in finite differences, using a time splitting for external and internal modes and high-order numerical advection schemes for density fields in combination with an modified turbulence closure scheme. The nesting system consists of two grids forced by the high-resolution ALADIN model atmospheric data. The coarse grid of 3 km resolution for the whole Gulf of Lions allows the forcing of the Liguro-Provençal large-scale current when the fine mesh of 1-km resolution is centred on the river mouth of the Grand Rhône. Documented field experiments from the Biodypar 3 field campaign performed during March 1999 are used for validation. Numerical results, CTD profiles and a SPOT TSM visible image are in good agreement concerning the shape and structure of the river plume. Other coastal flow features can be observed from satellite imagery. Computations of realistic situations recover these main secondary structures. Complementary process-oriented runs give an explanation of how the coastal upwelling induced by an inhomogeneous offshore wind is destabilized by the combination of the river plume and along-shelf current-blocking effects. In the end, a factor-separation analysis provides evidence that the locally non-linear effects in momentum contribute to the occurrence of secondary vortices.Responsible Editor: Phil Dyke     

The role of groundwater in cliff instability: an example at Cape Blanc‐Nez (Pas‐de‐Calais,France)

Cliff retreat in northern Boulonnais is described. The distribution of the amount of retreat is related to structural and topographic factors. Both control the runoff–infiltration balance and therefore the mechanical behaviour of the rocks, which determines the modes of failure. The spatial variability of the retreat rate is explained, but predicting the temporal variability of the retreat rate, which is central to risk management, is much more difficult. Rainfall and piezometric surface data enhanced a ‘piston flow’ mechanism during November 2000. The result of its occurrence on the stability of the cliff and conditions of its recurrence are examined with a view to better understanding the rate of recession of coastal cliffs and wiser management of risk. Copyright © 2005 John Wiley & Sons, Ltd.     

Datation par la méthode U/Th d'un travertin quaternaire du Sud-Est marocain : implications paléoclimatiques pendant le Pléistocène moyen et supérieur

A large travertine outcrop south of Errachidia, southern Morocco, was studied and U/Th dated. The carbonate fraction was provided by groundwaters then, as now, from the eastern High Atlas percolating through the regional Infra-Cenomanian aquifer. There were two main periods of accumulation at ca 262 kyr BP and 20–11.5 kyr BP separated by a long discontinuity with some limited weathering and erosion and correlated in part with a period of erosion at 30–20 kyr BP further to the west. The two travertine-deposition periods suggest increased rainfall and/or cooler thermal conditions in the eastern High Atlas source regions. Massive travertine accumulation ceased at the end of the Upper Pleistocene. To cite this article: L. Boudad et al., C. R. Geoscience 335 (2003).To cite this article: L. Boudad et al., C. R. Geoscience 335 (2003).     

Estimate of recharge of a rising water table in semiarid Niger from 3H and 14C modeling

A hydrodynamic survey carried out in semiarid southwest Niger revealed an increase in the unconfined ground water reserves of approximately 10% over the last 50 years due to the clearing of native vegetation. Isotopic samplings (3H, 18O, 2H for water and 14C, 13C for the dissolved inorganic carbon) were performed on about 3500 km2 of this silty aquifer to characterize recharge. Stable isotope analyses confirmed the indirect recharge process that had already been shown by hydrodynamic surveys and suggested the tracers are exclusively of atmospheric origin. An analytical model that takes into account the long-term rise in the water table was used to interpret 3H and 14C contents in ground water. The natural, preclearing median annual renewal rate (i.e., recharge as a fraction of the saturated aquifer volume) lies between 0.04% and 0.06%. For representative characteristics of the aquifer (30 m of saturated thickness, porosity between 10% and 25%), this implies a recharge of between 1 and 5 mm/year, which is much lower than the estimates of 20 to 50 mm/year for recent years, obtained using hydrological and hydrodynamic methods and the same aquifer parameters. Our study, therefore, reveals that land clearing in semiarid Niger increased ground water recharge by about one order of magnitude.     

Interannual-decadal variability of wintertime mixed layer depths in the North Pacific detected by an ensemble of ocean syntheses

Climate Dynamics - The interannual-decadal variability of the wintertime mixed layer depths (MLDs) over the North Pacific is investigated from an empirical orthogonal function (EOF) analysis of an...