A global database of water vapor isotopes measured with high temporal resolution infrared laser spectroscopy

The isotopic composition of water vapour provides integrated perspectives on the hydrological histories of air masses and has been widely used for tracing physical processes in hydrological and climatic studies. Over the last two decades, the infrared laser spectroscopy technique has been used to measure the isotopic composition of water vapour near the Earth's surface. Here, we have assembled a global database of high temporal resolution stable water vapour isotope ratios (delta O-18 and delta D) observed using this measurement technique. As of March 2018, the database includes data collected at 35 sites in 15 Koppen climate zones from the years 2004 to 2017. The key variables in each dataset are hourly values of delta O-18 and delta D in atmospheric water vapour. To support interpretation of the isotopologue data, synchronized time series of standard meteorological variables from in situ observations and ERA5 reanalyses are also provided. This database is intended to serve as a centralized platform allowing researchers to share their vapour isotope datasets, thus facilitating investigations that transcend disciplinary and geographic boundaries

Quelles relations entre la mer, les nappes souterraines et les eaux de surface dans le delta du Rhône ?

National audienceLes échanges d'eau sont contrôlés par un système de vannes et pertuis qui relient les étangs à la mer à travers la « Digue à la mer ». Sur la période 1994-2010, les volumes d'eau moyens échangés au niveau du pertuis de la Fourcade sont d'environ 11 millions de m 3 / an d'entrée marine et 20 millions de m 3 /an de sortie vers la mer. Malgré ce système, de grandes variations interannuelles se produisent. En effet, les flux restent dépendants des différences de niveau d'eau de part et d'autre, qui eux dépendent des facteurs environnementaux « internes » (apports d'eau par l'irrigation des rizières) et « externes » : les précipitations et le vent qui influencent le niveau d'eau des étangs, ainsi que les variations du niveau de la mer. Ces variations se produisent à court terme en fonction de la marée et des conditions météorologiques, avec notamment les phénomènes de surcotes marines, et à long terme en relation avec la remontée du niveau de la mer (1 à 7,5 mm/an-voir question 17 : « Changement climatique et surcotes marines, quel avenir pour les plages camarguaises ? »). Les relations entre les eaux souterraines et la mer Il existe une continuité des formations géologiques de part et d'autre du trait de côte permettant des échanges d'eau entre les eaux souterraines et la mer (fig. 1). Les différences de niveau entre la mer et les nappes régissent la direction et l'intensité des flux d'eau. Le bilan de ces flux se produit majoritairement en direction de la mer. Mais il existe néanmoins une intrusion saline, qui explique la salinité plus élevée des eaux de la nappe captive le long de la côte. L'augmentation progressive du niveau de la mer favorise l'intrusion de l'eau de mer dans les terres, ce qui entraînera à plus ou moins long terme une accentuation de la salinisation des eaux. L'eau partout présente en Camargue provient soit de la mer, soit des précipitations, soit de son fleuve. Depuis l'endiguement du Rhône, ce sont essentiellement les canaux d'irrigation et de drainage qui déterminent la distribution de l'eau du fleuve à travers son delta (photo 1). Les eaux continentales souterraines et en surface L'île de Camargue reçoit à la fois des eaux de pluie (470 millions de m 3 /an en moyenne) et des eaux d'irrigation issues du Rhône (296 et 362 millions de m 3 /an pour 1996 et 1993, Fondation Sansouire). En surface, le système lagunaire du Vaccarès est le réceptacle principal des flux hydriques de la zone (photo 2). Dans le sous-sol, on distingue deux types de nappes (fig. 1). Les sédiments récents du Rhône se trouvent sur une épaisseur variant de quelques mètres vers Arles jusqu'à environ 50 m vers le littoral et abritent des nappes plus ou moins connectées et de salinité très variable : douce, salée et même sursalée (jusqu'à 110 g/l, la mer étant à 38 g/l). Au-dessous, une formation de cailloutis grossiers abrite une nappe captive (séparée de la surface par une couche imperméable), dont la salinité augmente progressivement en direction de la mer depuis des eaux presque douces (≈2 g/l au nord) à des eaux de composition marine au sud. Les relations entre les eaux de surface et les eaux souterraines Les échanges d'eau entre la surface et le sous-sol sont complexes et se produisent soit par infiltration d'eau, soit par remontées artésiennes (sous pression). En fonction de l'occupation du sol, les eaux qui s'infiltrent proviennent de l'irrigation des rizières ou de la pluie dans les zones naturelles. Les remontées d'eau vers la surface se produisent quand la pression de l'eau souterraine le permet. On observe ce phénomène là où la topographie est légèrement plus basse, par exemple en bordure nord du Vaccarès où se trouve un forage artésien, mais il est le plus souvent invisible et difficile à quantifier. Ainsi, les étangs de Camargue, et de Vaccarès en particulier, reçoivent une quantité importante d'eau souterraine, estimée à environ un quart des apports de surface. Quelles relations entre la mer, les nappes souterraines et les eaux de surface dans le delta du Rhône

Seasonal evolution of the isotopic composition of atmospheric water vapour above a tropical lake: Deuterium excess and implication for water recycling

We present an analysis of the rainfall-evaporation-atmospheric moisture cycle in a semi-arid tropical zone (southwestern Madagascar) to quantify the recycling and mixing processes that occur above an endorheic lake system (Lake Ihotry) during an annual cycle. The study combines an isotope mass balance with a detailed field investigation of the lake system and a previously established daily time-step lake water balance model. The mass balance and Craig-Gordon equations are used to calculate the isotopic composition of the evaporative flux from the lake surface (δE) and to derive a daily time series of the ambient atmospheric water vapour composition above the lake (δAL) during a 8-month dry season. Calculated δAL results from a mixing between regional moisture (δAR) and locally evaporated water (δE), the latter representing 50% of δAL at the end of the dry season. The contribution of recycled moisture to on-lake precipitation during the wet season is estimated to ⩾16%. We show that, as expected, the deuterium excess is high in recycled precipitation and low in evaporated precipitation, but also that the recycled moisture in an endorheic system may have a low deuterium excess resulting from the low deuterium excess in regional precipitation. In case of a long evaporative season, the atmospheric moisture is not in isotopic equilibrium with the annual composition of precipitation because of the contribution of the recycled vapour to the local atmospheric pool. Our approach demonstrates the importance of water recycling on the atmospheric moisture cycle and precipitation in a tropical semi-arid system, and can be applied to other natural systems, enlarging the potential range of investigation of the atmospheric vapour cycle and rainfall sources in tropical lands. It may also represent a valuable complement to direct water vapour sampling, in yielding the long-term evolution of the atmospheric vapour composition with spatially averaged values and smoothed temporal variations

Analysis of the hydrological response of a tropical terminal lake, Lake Abiyata (Main Ethiopian Rift Valley) to changes in climate and human activities

Integrated dynamic water and chloride balance models with a catchment-scale hydrological model (PRMS) are used to investigate the response of a terminal tropical lake, Lake Abiyata, to climate variability and water use practices in its catchment. The hydrological model is used to investigate the response of the catchment to different climate and land-use change scenarios that are incorporated into the lake model. Lake depth-area-volume relationships were established from lake bathymetries. Missing data in the time series were filled using statistical regression techniques. Based on mean monthly data, the lake water balance model produced a good agreement between the simulated and observed levels of Lake Abiyata for the period 1968-83. From 1984 onwards the simulated lake level is overestimated with respect to the observed one, while the chloride concentration is largely underestimated. This discrepancy is attributed to human use of water from the influent rivers or directly from the lake. The simulated lake level and chloride concentration are in better agreement with observed values (r2 = 0*96) when human water use for irrigation and salt exploitation are included in the model. A comparison of the simulation with and without human consumption indicates that climate variability controls the interannual fluctuations and that the human water use affects the equilibrium of the system by strongly reducing the lake level. Sensitivity analysis based on a mean climatic year showed that, after prolonged mean climatic conditions, Lake Abiyata reacts more rapidly to an abrupt shift to wetter conditions than to dry conditions. This study shows the significant sensitivity of the level and salinity of the terminal Lake Abiyata to small changes in climate or land use, making it a very good 'recorder' of environmental changes that may occur in the catchment at different time scales

Improving memory effect correction to achieve high precision analysis of δ 17 O, δ 18 O δ 2 H, 17 O‐excess and d‐excess in water using cavity ring‐down laser spectroscopy

International audienceRationaleThe precision obtained in routine isotope analysis of water (δ17O, δ18O, δ2H, 17O‐excess and d‐excess values) by Cavity Ring‐Down Spectroscopy is usually below the instrument specifications provided by the manufacturer. This study aimed at reducing this discrepancy, with particular attention paid to mitigating the memory effect (ME).MethodsWe used a Picarro L2140i analyzer coupled with a high‐precision A0211 vaporizer and an A0325 autosampler. The magnitude and duration of the ME were estimated using 24 series of 50 successive injections of samples with contrasting compositions. Four memory correction methods were compared, and the instrument performance was evaluated over a 17‐month period of routine analysis, using two different run architectures.ResultsThe ME remains detectable after the 30th injection, implying that common corrections procedures only based on the last preceding sample need to be revised. We developed a new ME correction based on the composition of several successive samples, and designed a run architecture to minimize the magnitude of the ME. The standard deviation obtained from routine measurement of a QA water over a 7 months‐period was 0.015‰ for δ17O, 0.023‰ for δ18O, 0.078‰ for δ2H, 0.006‰ for 17O‐excess and 0.173‰ for d‐excess. In addition, we provided the first δ17O and 17O‐excess values for the GRESP certified reference material.ConclusionsThis study demonstrates the long‐term persistence of the ME, which is often overlooked in routine analysis of natural samples. As already evidenced when measuring labelled water, it calls for consideration of the compositions of several previous samples to obtain an appropriate correction, a prerequisite to achieve high precision data

Hydrological response of a catchment to climate and land use changes in Tropical Africa: case study South Central Ethiopia

A hydrological modelling at a catchment scale has been used to investigate the impact of climatic and land use change on water resources in data scarce Tropical Africa using a distributed precipitation-runoff modelling system. The model divides a catchment into homogeneous hydrological response units, providing the ability to impose changes in climate or land use spatially. Model parameters were either estimated from different existing data or by calibration against measured discharge data available over 11 years (1985-1995). The model simulation-period was divided into calibration (1986-1990) and validation (1991-1995) periods. The model provided relatively good fits between measured and simulated discharge both at a daily and monthly scales. Based on sensitivity analyses, a 10% decrease in rainfall produced a 30% reduction on the simulated hydrologic response of the catchment, while a 1.5 °C increase in air temperature would result in a decrease in the simulated discharge of about 15%. Converting the present day dominantly cultivated/grazing land in the studied river basin by woodland would decrease the discharge at the outlet by about 8%. In order to use the results of this kind of model for decision making and water resources management, the model should be tested under different environment and different scenario conditions. Rainfall measurement and stage-discharge rating curves should be given priority to improve model performance

Partitioning groundwater recharge between rainfall infiltration and irrigation return flows using stable isotopes: the Crau aquifer.

International audienceThis study reports an assessment of the water budget of the Crau aquifer (Southern France), which is poorly referenced in the literature. Anthropogenically controlled by a traditional irrigation practice, this alluvial type aquifer requires a robust quantification of the groundwater mass balance in order to establish sustainable water management in the region. In view of the high isotopic contrast between exogenous irrigation waters and local precipitations, stable isotopes of water can be used as conservative tracers to deduce their contributions to the surface recharge. Extensive groundwater sampling was performed to obtain delta 18O and delta 2H over the whole aquifer. Based on a new piezometric contour map, combined with an updated aquifer geometry, the isotopic data were implemented in a geostatistical approach to produce a conceptual equivalent homogeneous reservoir. This makes it possible to implement a parsimonious water and isotope mass-balance mixing model. The isotopic compositions of the two end-members were assessed, and the quantification of groundwater flows was then used to calculate the two recharge fluxes (natural and irrigation). Nearly at steady-state, the set of isotopic data treated by geostatistics gave a recharge by irrigation of 4.92 +/- 0.89 m(3) s(-1), i.e. 1109 +/- 202 mm yr(-1), and a natural recharge of 2.19 +/- 0.85 m(3) s(-1), i.e. 128 +/- 50 mm yr(-1). Thus, 69 +/- 9% of the surface recharge is caused by irrigation return flow. This study constitutes a straightforward and independent approach to assess groundwater surface recharges including uncertainties and will help to constrain future transient groundwater models of the Crau aquifer

Réponse hydrologique du Lac Mar Chiquita (Argentine) aux changements climatiques (approche par modélisation couplée lac - bassin versant - climat)

Les latitudes subtropicales de l Amérique du Sud sont soumises depuis le début des années 70 à un changement hydroclimatique majeur affectant les activités d une des régions les plus fortement peuplées de ce continent. Ce changement s est traduit notamment par une augmentation abrupte des niveaux du Lac Mar Chiquita, un lac qui couvre aujourd hui 6000 km2 alors que les enregistrements sédimentaires au cours du dernier millénaire mettent en évidence un lac de faible profondeur, régulièrement asséché. Afin de comprendre la relation entre les niveaux lacustres et la variabilité climatique, ce travail s est attaché à modéliser le fonctionnement hydrologique du Lac Mar Chiquita et la relation avec son bassin versant. L établissement d un modèle de lac met en évidence que les variations lacustres dans les années 70 répondent à une augmentation des débits des rivières en provenance du bassin nord du lac, suggérant une influence climatique tropicale. Le couplage du modèle de lac avec un modèle hydrologique de bassin a permis de simuler les fluctuations lacustres en relation avec le climat du bassin nord, soulignant l impact anthropique mineur dans la récente transition hydroclimatique. En couplant cette approche avec le modèle climatique LMDZ par une méthode de downscaling statistique, pour la première fois, un modèle intégré bassin-lac modélisant les fluctuations lacustres en relation avec le climat global est présenté. Cette approche originale permet d explorer la capacité des modèles globaux à représenter le climat régional. En outre, elle confirme l influence tropicale à l origine des récentes variations hydrologiques dans cette région d Amérique du Sud.AIX-MARSEILLE3-BU Sc.St Jérô (130552102) / SudocSudocFranceF

Source Water Isotope Identification Toolkit ‘SWIIT’ for detecting historical water use by forest trees

The hydrological limitations on vegetative health are particularly complex in environments where multiple sources of water are potentially available to trees in lowland riparian zones. To anticipate how subsurface hydrological partitioning may evolve in the future, the dynamics of tree water source usage and the potential impacts to overlying vegetation, we require a robust, quantitative framework that is constrained by historical data. Here, we outline a toolkit (SWIIT- ‘Source Water Isotope Identification Toolkit’) to retrospectively investigate the dynamics of tree water uptake. SWIIT utilizes tree-ring isotopes (δ18O) in combination with a biomechanistic fractionation model to predict the isotopic signature of water utilized during any particular period of growth (seasonal/annual). Through comparisons with measured δ18O in local endmember water sources, and climatic and hydrological variables, this methodology can be used to reconstruct and inform on past ecohydrological interactions. We provide a comprehensive overview of the theoretical basis behind the modelling component and assess the data requirements to constrain its predictions. The toolkit’s utility is demonstrated for several riparian field studies characterized by differences in climate, geomorphic and hydrological complexity. Whilst our examples are for riparian areas, we suggest that SWIIT can be applied to a range of forested environments where distinct isotopic endmembers exist. To aid application of SWIIT for data-poor environments, we present a set of tool groups, which can be applied over a gradient in data availability and output resolution. We emphasize that it is possible to make scientific progress in retrospective understanding water use by vegetation under varying degrees data availability