Papers by Laurent Longuevergne
Comptes Rendus. Géoscience, 2021
Éric Beucler et al.
This document is proposed as a supplement of "Combining passive-and active-DTS methods to locate ... more This document is proposed as a supplement of "Combining passive-and active-DTS methods to locate and quantify groundwater discharge into streams". It contains three parts related to the data interpretation. The first one details the interpretation of punctual vertical temperature profiles (VTP) using the FLUX-BOT model. Associated results are compared with passive-and active-DTS measurements in the section 3.3 of the main manuscript which allows validating fluxes estimates. The second one presents a detailed example of the use of the FLUX-BOT model to interpret passive-DTS measurements. Lastly, raw results of active-DTS measurements as well as the processing of these data (sorting and quality check) are presented. The main manuscript focuses on the results of fluxes estimates.
Geosciences, 2019
Experimental characterization of thermal transport in fractured media through thermal tracer test... more Experimental characterization of thermal transport in fractured media through thermal tracer tests is crucial for environmental and industrial applications such as the prediction of geothermal system efficiency. However, such experiments have been poorly achieved in fractured rock due to the low permeability and complexity of these media. We have thus little knowledge about the effect of flow configuration on thermal recovery during thermal tracer tests in such systems. We present here the experimental set up and results of several single-well thermal tracer tests for different flow configurations, from fully convergent to perfect dipole, achieved in a fractured crystalline rock aquifer at the experimental site of Plœmeur (H+ observatory network). The monitoring of temperature using Fiber-Optic Distributed Temperature Sensing (FO-DTS) associated with appropriate data processing allowed to properly highlight the heat inflow in the borehole and to estimate temperature breakthroughs fo...
Vadose Zone Journal, 2018
This paper presents the French Critical Zone initiative, called OZCAR (Observatoires de la Zone C... more This paper presents the French Critical Zone initiative, called OZCAR (Observatoires de la Zone Critique-Application et Recherche-Critical Zone Observatories-Application and Research), a National Research Infrastructure (RI). OZCAR-RI is a network of instrumented sites, organized in 21 pre-existing research observatories, or observation services, and monitoring over the long term, different compartments of the zone situated between "the rock and the sky", the Earth's skin or Critical Zone (CZ). These observatories are regionally-based and all have their individual initial scientific questions, monitoring strategies, databases and modeling activities. The diversity of OZCAR-RI observatories and sites is well representative of the heterogeneity of the Critical Zone and of the scientific communities studying it. Despite this diversity, all OZCAR-RI sites share a main overarching scientific question, which is: how to monitor, understand and predict ("earthcast") the fluxes of water, solutes, gases and sediments of the Earth's near surface and how they will change in response to the "new climatic regime" (climate change, land use and land cover changes). We describe in this paper a vision for OZCAR strategic development in the next decade, aiming at designing an open infrastructure, building a national CZ community able to share a common and systemic representation of CZ dynamics, and educating a new generation of scientists more apt to tackle the wicked problem of the Anthropocene. We propose to articulate OZCAR around the following main points: i) a set of common scientific questions and cross-cutting scientific activities using the wealth of OZCAR-RI observatories along gradients and the diverse disciplines, ii) an ambitious instrumental development program, iii) a better interaction between data and models as a way of integrating the different time and spatial scales as well as fostering dialogue between communities. At the international level, OZCAR-RI aimed at strengthening the CZ community by providing a model of organization for pre-existing observatories and by widening the range of CZ instrumented sites. Embedded into the international CZ initiative, OZCAR is one of the French mirrors of the European eLTER-ESFRI (European Strategy Forum on Research Infrastructure) project.
Proceedings of the National Academy of Sciences of the United States of America, Jan 6, 2018
Assessing reliability of global models is critical because of increasing reliance on these models... more Assessing reliability of global models is critical because of increasing reliance on these models to address past and projected future climate and human stresses on global water resources. Here, we evaluate model reliability based on a comprehensive comparison of decadal trends (2002-2014) in land water storage from seven global models (WGHM, PCR-GLOBWB, GLDAS NOAH, MOSAIC, VIC, CLM, and CLSM) to trends from three Gravity Recovery and Climate Experiment (GRACE) satellite solutions in 186 river basins (∼60% of global land area). Medians of modeled basin water storage trends greatly underestimate GRACE-derived large decreasing (≤-0.5 km/y) and increasing (≥0.5 km/y) trends. Decreasing trends from GRACE are mostly related to human use (irrigation) and climate variations, whereas increasing trends reflect climate variations. For example, in the Amazon, GRACE estimates a large increasing trend of ∼43 km/y, whereas most models estimate decreasing trends (-71 to 11 km/y). Land water storag...
Precipitation minus evapotranspiration, the flux of water between the atmosphere and the Earth’s ... more Precipitation minus evapotranspiration, the flux of water between the atmosphere and the Earth’s surface, provides important information regarding the interaction of the atmosphere with the land surface. It links atmospheric and terrestrial water budgets and, thus, realizes an important boundary condition for both climate modeling and hydrological studies. Yet, due to a general lack of unbiased measurements, the atmospheric-terrestrial flux of water is poorly constrained by direct observations and rather, usually, reconstructed from data-assimilating atmospheric reanalyses. Via the terrestrial water budget equation, water storage derived from products of the Gravity Recovery and Climate Experiment (GRACE) mission combined with runoff data, can be used to assess the realism of atmospheric-terrestrial flux of water in atmospheric models. A number of studies have applied this method to global reanalysis products, with good results only for large river basins. In this study, we first assess the closure of the terrestrial water budget over a number of European river basins from the new release 5 GRACE products, after careful postprocessing and in combination with GRDC and BfG discharge data, and from precipitation minus evapotranspiration obtained from the operational analysis of the regional high-resolution NWP models COSMO-DE and -EU, a new COSMO-based reanalysis for the European CORDEX domain, the global reanalyses ERA-INTERIM and MERRA, as well as few observation-based data sets (E-OBS, GPCC, upscaled FLUXNET observations from Jung et al., 2010). This allows us to identify biases of up to 20 mm/month in the different data products, at different spatial scales down to the Oder catchment (110.000 km2). Among the atmospheric (re-) analyses, we find COSMO-EU atmosphere-terrestrial flux of water almost unbiased over Central Europe. Finally, we assess bias-corrected flux and reconstructed multi-sensor water storage variations.
Water Resources Research, 2015
Increasing interest in use of GRACE satellites and a variety of new products to monitor changes i... more Increasing interest in use of GRACE satellites and a variety of new products to monitor changes in total water storage (TWS) underscores the need to assess the reliability of output from different products. The objective of this study was to assess skills and uncertainties of different approaches for processing GRACE data to restore signal losses caused by spatial filtering based on analysis of 1 3 1 grid-scale data and in 60 river basins globally. Results indicate that scaling factors from six LSMs, including GLDAS-1 four models (Noah2.7, Mosaic, VIC, and CLM 2.0), CLM 4.0, and WGHM, are similar over most of humid, subhumid, and high-latitude regions but can differ by up to 100% over arid and semiarid basins and areas with intensive irrigation. Temporal variability in scaling factors is generally minor at the basin scale except in arid and semiarid regions, but can be appreciable at the 1 3 1 grid scale. Large differences in TWS anomalies from three processing approaches (scaling factor, additive, and multiplicative corrections) were found in arid and semiarid regions, areas with intensive irrigation, and relatively small basins (e.g., 200,000 km 2). Furthermore, TWS anomaly products from gridded data with CLM4.0 scaling factors and the additive correction approach more closely agree with WGHM output than the multiplicative correction approach. This comprehensive evaluation of GRACE processing approaches should provide valuable guidance on applicability of different processing approaches with different climate settings and varying levels of irrigation.
Global hydrological models contribute to the understanding and quantification of the global water... more Global hydrological models contribute to the understanding and quantification of the global water cycle. However, large uncertainties persist on the one hand due to the simplified representation of hydrological processes for a global scale analysis and, on the other hand due to input data uncertainties, e.g. climate forcing and data to the anthropogenic alteration of the water cycle. The time-variable solutions of the Gravity Recovery And Climate Experiment (GRACE) mission provide an independent observation of water storage change with global coverage, which can be used to improve global hydrological models. For this purpose, an ensemble Kalman filter approach is applied to assimilate GRACE total water storage (TWS) change grids into the WaterGAP Global Hydrology Model (WGHM). In contrast to existing studies, our approach involves the full error information of the GRACE solutions, i.e. spatial correlations, as well as the full spatial resolution of the data. To guarantee a realistic estimation of the uncertainties introduced by the climate forcing data, their covariance matrices are determined from an ensemble of forcing fields from different state-of-the-art climate inputs. Finally, the errors are propagated to the assimilated water storage outputs. Here, the results are presented with respect to the precipitation input.
Texas was subjected to the most extreme one-year drought on record in 2011, which had a tremendou... more Texas was subjected to the most extreme one-year drought on record in 2011, which had a tremendous impact on water resources statewide. This study aims to quantify evapotranspiration (ET) from land surface models (LSMs), remote sensing, and GRACE during the drought. Uncertainties in ET output from four LSMs, i.e., Noah, Mosaic, VIC, and SAC in NLDAS-2, two remote sensing-based products, i.e., MODIS and AVHRR, and GRACE-derived ET as a residual in the water budget (ET = P - R - ΔTWS) based on precipitation (P) from PRISM, monitored runoff (R), and total water storage (TWS) change from GRACE satellites were quantified using the three corner hat method that does not require a priori knowledge of the true value of ET. Water budgets were calculated using the traditional flux approach and a new storage approach in combination with the different ET products and GRACE TWS. The analyses were conducted using data from three river basins (humid - arid) primarily in Texas as case studies. Remote sensing-based ET shows markedly higher magnitudes during drought but significantly lower magnitudes at other times, particularly during wet periods than land surface model-based ET. Overestimation of ET during drought would result in overestimation of soil moisture depletion and much longer projected times for drought recovery. Uncertainties in ET are lowest in LSM ET (~5 mm/month), moderate in remote sensing MODIS- or AVHRR-based ET (10 - 15 mm/month), and highest in GRACE-based ET (20 - 30 mm/month). Uncertainties in total water storage changes from the water budget approach (ΔTWS = P-R-ET) are about half of uncertainties in GRACE-derived TWS changes for each of the basins. Future ET estimation should consider a hybrid approach that integrates LSM and satellite-based products to constrain uncertainties.
Journal of Hydrometeorology, 2014
Precipitation minus evapotranspiration, the net flux of water between the atmosphere and Earth’s ... more Precipitation minus evapotranspiration, the net flux of water between the atmosphere and Earth’s surface, links atmospheric and terrestrial water budgets and thus represents an important boundary condition for both climate modeling and hydrological studies. However, the atmospheric–terrestrial flux is poorly constrained by direct observations because of a lack of unbiased measurements. Thus, it is usually reconstructed from atmospheric reanalyses. Via the terrestrial water budget equation, water storage estimates from the Gravity Recovery and Climate Experiment (GRACE) combined with measured river discharge can be used to assess the realism of the atmospheric–terrestrial flux in models. In this contribution, the closure of the terrestrial water budget is assessed over a number of European river basins using the recently reprocessed GRACE release 05 data, together with precipitation and evapotranspiration from the operational analyses of high-resolution, limited-area NWP models [Cons...
Surveys in Geophysics, 2014
West-African countries have been exposed to changes in rainfall patterns over the last decades, i... more West-African countries have been exposed to changes in rainfall patterns over the last decades, including a significant negative trend. This causes adverse effects on water resources of the region, for instance, reduced freshwater availability. Assessing and predicting large-scale total water storage (TWS) variations is necessary for West Africa, due to its environmental, social, and economical impacts. Hydrological models, however, may perform poorly over West Africa due to data scarcity. This study describes a new statistical, data-driven approach for predicting West African TWS changes from (past) gravity data obtained from the Gravity Recovery and Climate Experiment (GRACE), and (concurrent) rainfall data from the Tropical Rainfall Measuring Mission (TRMM) and sea surface temperature (SST) data over the Atlantic, Pacific, and Indian Oceans. The proposed method, therefore, capitalizes on the availability of remotely sensed observations for predicting monthly TWS, a quantity which is hard to observe in the field but important for measuring regional energy balance, as well as for agricultural, and water resource management. Major teleconnections within these data sets were identified using independent component analysis (ICA) and linked via low-degree autoregressive models to build a predictive framework. After a learning phase of 72 months, our approach predicted TWS from rainfall and SST data alone that fitted to the observed GRACE-TWS better than that from a global hydrological model. Our results indicated a fit of 79% and 67% for the first year prediction of the two dominant annual and inter-annual modes of TWS variations. This fit reduces to 62% and 57% for the second year of projection. The proposed approach, therefore, represents strong potential to predict the TWS over West Africa up to two years. It also has the potential to bridge the present GRACE data gaps of one month about each 162 days as well as a-hopefully-limited gap between GRACE and the GRACE follow-on mission over West Africa. The presented method could
Water Resources Research, 2012
[1] Satellite monitoring of changes in terrestrial water storage provides invaluable information ... more [1] Satellite monitoring of changes in terrestrial water storage provides invaluable information regarding the basin-scale dynamics of hydrological systems where ground-based records are limited. In the Bengal Basin of Bangladesh, we test the ability of satellite measurements under the Gravity Recovery and Climate Experiment (GRACE) to trace both the seasonality and trend in groundwater storage associated with intensive groundwater abstraction for dry-season irrigation and wet-season (monsoonal) recharge. We show that GRACE (CSR, GRGS) datasets of recent (2003 to 2007) groundwater storage changes (ΔGWS) correlate well (r=0.77 to 0.93, p-value <0.0001) with in situ borehole records from a network of 236 monitoring stations and account for 44% of the total variation in terrestrial water storage (ΔTWS); highest correlation (r=0.93, p-value <0.0001) and lowest root mean square error (<4 cm) are realized using a spherical harmonic product of CSR. Changes in surface water storage estimated from a network of 298 river gauging stations and soilmoisture derived from Land Surface Models explain 22% and 33% of ΔTWS respectively. 2 Groundwater depletion estimated from borehole hydrographs (−0.52±0.30 km 3 /yr) is within the range of satellite-derived estimates (−0.44 to −2.04 km /yr) that result from uncertainty associated with the simulation of soil moisture (CLM, NOAH, VIC) and GRACE signalprocessing techniques. Recent (2003 to 2007) estimates of groundwater depletion are substantially greater than long-term (1985 to 2007) mean (−0.21±0.03 km 3 /yr) and are explained primarily by substantial increases in groundwater abstraction for the dry-season irrigation and public water supplies over the last two decades.
Water Resources Research, 2014
Proliferation of evapotranspiration (ET) products warrants comparison of these products. The stud... more Proliferation of evapotranspiration (ET) products warrants comparison of these products. The study objective was to assess uncertainty in ET output from four land surface models (LSMs), Noah, Mosaic, VIC, and SAC in NLDAS-2, two remote sensing-based products, MODIS and AVHRR, and GRACE-inferred ET from a water budget with precipitation from PRISM, monitored runoff, and total water storage change (TWSC) from GRACE satellites. The three cornered hat method, which does not require a priori knowledge of the true ET value, was used to estimate ET uncertainties. In addition, TWSC or total water storage anomaly (TWSA) from GRACE was compared with water budget estimates of TWSC from a flux-based approach or TWSA from a storage-based approach. The analyses were conducted using data from three regions (humid-arid) in the South Central United States as case studies. Uncertainties in ET are lowest in LSM ET (5 mm/mo), moderate in MODIS or AVHRR-based ET (10-15 mm/mo), and highest in GRACEinferred ET (20-30 mm/month). There is a trade-off between spatial resolution and uncertainty, with lower uncertainty in the coarser-resolution LSM ET (14 km) relative to higher uncertainty in the finer-resolution (1-8 km) RS ET. Root-mean-square (RMS) of uncertainties in water budget estimates of TWSC is about half of RMS of uncertainties in GRACE-derived TWSC for each of the regions. Future ET estimation should consider a hybrid approach that integrates strengths of LSMs and satellite-based products to constrain uncertainties.
Water Resources Research, 2007
We investigate the ability of combining the Karhunen-Loève transform (KLT) with the kriging metho... more We investigate the ability of combining the Karhunen-Loève transform (KLT) with the kriging method to extract regional information from a set of point measurements. This method was applied to a set of 195 piezometric head time series over a period of 17 years from observation wells distributed within the French and German area of the Rhine valley alluvial groundwater body. Piezometric head time series are analyzed with KLT in order to highlight characteristic temporal signals, classified from the most energetic (global) to the least energetic (local) signals. The first five signals amount to 80% of the global variance of the system and are inferred to represent different hydrological contributions (exchanges with rivers and rainfall), but they also represent a significant anthropogenic component. Kriging is then used to regionalize the signals and to build a reconstruction model of the behavior of the whole aquifer containing only filtered information coming from identified source signals.
Water Resources Research, 2010
The Gravity Recovery and Climate Experiment (GRACE) satellites provide observations of water stor... more The Gravity Recovery and Climate Experiment (GRACE) satellites provide observations of water storage variation at regional scales. However, when focusing on a region of interest, limited spatial resolution and noise contamination can cause estimation bias and spatial leakage, problems that are exacerbated as the region of interest approaches the GRACE resolution limit of a few hundred km. Reliable estimates of water storage variations in small basins require compromises between competing needs for noise suppression and spatial resolution. The objective of this study was to quantitatively investigate processing methods and their impacts on bias, leakage, GRACE noise reduction, and estimated total error, allowing solution of the trade-offs. Among the methods tested is a recently developed concentration algorithm called spatiospectral localization, which optimizes the basin shape description, taking into account limited spatial resolution. This method is particularly suited to retrieval of basin-scale water storage variations and is effective for small basins. To increase confidence in derived methods, water storage variations were calculated for both CSR (Center for Space Research) and GRGS (Groupe de Recherche de Géodésie Spatiale) GRACE products, which employ different processing strategies. The processing techniques were tested on the intensively monitored High Plains Aquifer (450,000 km 2 area), where application of the appropriate optimal processing method allowed retrieval of water storage variations over a portion of the aquifer as small as ∼200,000 km 2 .
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Papers by Laurent Longuevergne