Loire Research Articles

Rivers of Risk and Redemption in Gregory of Tours’ Writings

Rivers of Risk and Redemption in Gregory of Tours’ Writings

Metapopulation modelling of riparian tree species persistence in river networks under climate change

Floodplain landscapes are highly fragmented by river regulation resulting in habitat degradation and flood regime perturbation, posing risks to population persistence. Climate change is expected to pose supplementary risks in this context of fragmented landscapes, and especially for river systems adaptation management programs are developed. The association of habitat quality and quantity with the landscape dynamics and resilience to human-induced disturbances is still poorly understood in the context of species survival and colonization processes, but essential to prioritize conservation and restoration actions. We present a modelling approach that elucidates network connectivity and landscape dynamics in spatial and temporal context to identify vital corridors and conservation priorities in the Loire river and its tributaries. Alteration of flooding and flow regimes is believed to be critical to population dynamics in river ecosystems. Still, little is known of critical levels of alteration both spatially and temporally. We applied metapopulation modelling approaches for a dispersal-limited tree species, white elm; and a recruitment-limited tree species, black poplar. In different model steps the connectivity and natural dynamics of the river landscape are confronted with physical alterations (dams/dykes) to species survival and then future scenarios for climatic changes and potential adaptation measures are entered in the model and translated in population persistence over the river basin. For the two tree species we highlighted crucial network zones in relation to habitat quality and connectivity. Where the human impact model already shows currently restricted metapopulation development, climate change is projected to aggravate this persistence perspective substantially. For both species a significant drawback to the basin population is observed, with 1/3 for elm and ¼ for poplar after 25 years already. But proposed adaptation measures prove effective to even bring metapopulation strength and persistence up to a level above the current level.

Present-day palynomorph deposits in an estuarine context: The case of the Loire Estuary

Estuaries are dynamic systems that collect terrestrial, aerial, fluvial, and marine inputs, including organic microfossils, which, when fossilized and observed on palynological slides, are also referred to as palynomorphs (pollen and non-pollen palynomorphs including dinoflagellate cysts or dinocysts). To understand these organic microfossil deposit arrangements across the Loire estuary, palynomorph counts were undertaken in 31 surface sediments collected across longitudinal and perpendicular transects of the Loire active riverbed, from the upper inner estuary to the river mouth. Main results suggest a large homogeneity of the pollen content throughout the entire upstream-downstream transect, with a dominance of arboreal taxa (Pinus, Quercus, Alnus) and Poaceae. Also, perpendicular transects across the channel show a great similarity between the muddy surface layers and the underlying consolidated clay layers. This is probably due to: i) homogeneity of the landscape at a regional scale (large catchment area of the Loire River), and ii) complex hydrodynamic processes involving strong mixing of the palynological signal. Furthermore, despite scarce woodlands in the regional landscape, arboreal pollen (especially Pinus and Quercus) represents >60% of the total pollen percentages. This could be explained by several factors: i) generally higher arboreal pollen production and dispersion as compared to herbaceous taxa, ii) distant inputs from marine areas downstream and/or forested regions far upstream, and iii) differential selection or inheritance from underlying sediments. Differentiation between the outer and inner estuarine environments was furthermore possible using a ratio of terrestrial versus marine palynological indicators. Among the dinocyst assemblages (marine realm), the euryhaline species Lingulodinium machaerophorum predominates; this taxon being very sensitive to strong water column stratification. Also, total dinocyst concentration increased upstream, which may result from the tidal forcing pushing salinity upriver beneath outflowing river water, and thus signing the estuarine turbidity maximum influence within the Loire River.

Estimation of stream-aquifer exchanges at regional scale using a distributed model: Sensitivity to in-stream water level fluctuations, riverbed elevation and roughness

Several studies on stream-aquifer interactions focus on the local scale. However, the estimation of stream-aquifer exchanges for a regional river network remains challenging. This study aims at assessing the sensitivity of distributed stream-aquifer exchanges to in-stream water level fluctuations, riverbed elevation and Manning roughness coefficient.An integrated distributed surface-subsurface model is applied to the Loire river basin (117,480km2, France), where in-stream water level fluctuations are taken into account with a simplified Manning-Strickler approach. The stream-aquifer exchanges are analyzed at pluri-annual and annual scales, as well as during short-term hydrological events.The model simulates the spatio-temporal variability of in-stream water levels accurately, with Nash coefficients up to 0.96 for the Loire river. The river network mainly drains the aquifer system. The average net exchanged flow is 2·10-2m3s−1km−1, which corresponds to 12% of the averaged discharge at the outlet of the basin.The assumption of constant river stages significantly impacts the total infiltration (−70%) and exfiltration (−10%) in the basin, whereas it has a negligible influence on the average net flux. The river fluctuations increase the time variability of the stream-aquifer exchanges and may determine flow reversals during flood events and also more frequently for river stretches at equilibrium with its nearby aquifer.This study highlights the importance of accounting for river stage fluctuations in the modeling of regional hydrosystems. Moreover, a sensitivity analysis indicates that it is mandatory to develop new methodologies to better estimate the riverbed elevation at high resolution for a river network at regional scale. In a lesser extent, errors on Manning coefficient affect the timing of infiltration and exfiltration, leading to temporally localized discrepancies. However it does not affect the estimates of the global net exchanges significantly.

Spatial distribution of trace elements in the surface sediments of a major European estuary (Loire Estuary, France): Source identification and evaluation of anthropogenic contribution

Assessing the extent of metal contamination in estuarine surface sediments is hampered by the high heterogeneity of sediment characteristics, the spatial variability of trace element sources, sedimentary dynamics and geochemical processes in addition to the need of accurate reference values for deciphering natural to anthropogenic contribution. Based on 285 surface sediment samples from the Loire Estuary, the first high-resolution spatial distributions are presented for grain-size, particulate organic carbon (POC) and the eight metals/metalloids identified as priority contaminants (Cd, Zn, Pb, Cu, As, Cr, Ni, Hg) plus Ag (an urban tracer). Grain-size and/or POC are major factors controlling the spatial distribution of trace element concentrations. The V-normalized trace metal concentrations divided by the V-normalized concentrations in the basin geochemical background showed the highest Enrichment Factors for Ag and Hg (EF; up to 34 and 140, respectively). These results suggest a severe contamination in the Loire Estuary for both elements. Intra-estuarine Ag and Hg anomalies were identified by comparison between respective normalized concentrations in the Loire Estuary surface sediments and those measured in the surface sediments at the outlet of the Loire River System (watershed-derived). Anthropogenic intra-estuarine Ag and Hg stocks in the uppermost centimetre of the sediment compared with rough annual fluvial flux estimates suggest that the overall strong Enrichment Factors for Ag (EFAg) and and Hg (EFHg) in the Loire Estuary sediments are mainly due to watershed-derived inputs, highlighting the need of high temporal hydro-geochemical monitoring to establish reliable incoming fluxes. Significant correlations obtained between EFCd and EFAg, EFCu and POC and EFHg and POC revealed common behavior and/or sources. Comparison of trace element concentrations with ecotoxicological indices (Sediment Quality Guidelines) provides first standardized information on the sediment quality in the Loire Estuary. The overall mean Effect Range Median quotients (m-ERM-q) results suggested that the Loire Estuary is mainly characterized by slightly toxic sediments even if ecotoxicological impacts have been previously reported on biota.

Concentrations of organochlorine compounds (pesticides and PCBs), trace elements (Pb, Cd, Cu, and Hg), 134Cs, and 137Cs in the livers of the European otter (Lutra lutra), great cormorant (Phalacrocorax carbo), and European catfish (Silurus glanis), collected from the Loire River (France)

In this study, liver samples of the European otter (Lutra lutra), great cormorant (Phalacrocorax carbo), and European catfish (Silurus glanis) from along the Loire River (France) were analyzed for organochlorine pesticides, polychlorinated biphenyls (PCBs), trace elements (lead, cadmium, copper, and mercury), 134Cs, and 137Cs. Organochlorine compounds were found in 100 % of the samples. Residue level of PCBs in these predators was the highest followed by DDTs and endosulfan, and maximal PCB values reached 1.61, 1.96, and 0.28 mg/kg wet weight for cormorants, otters, and catfish, respectively. DDTs, endosulfan, and lindane were the abundant pesticides in the livers of these predators. Concentrations of copper, lead, and cadmium in the livers of top predators were generally low; a potentially toxic level of lead was found in only one cormorant (34.9 mg/kg dry weight). Mean mercury concentrations in cormorant, otter, and catfish livers were 5.47, 4.68, and 2.71 mg/kg dry weight, respectively, and exceeded toxic thresholds for more than 40 % of the cormorants. Levels of organochlorine compounds and trace elements in these predators are generally below known toxic thresholds, but potential combined effects are not studied and warrant more investigations.

A cost‐effective 3D electrical resistivity imaging approach applied to dike investigation

ABSTRACTAlthough DC electrical resistivity imaging is widely applied to dike investigation, either rapid 2D or high‐resolution 3D approaches fail to address actual needs. An intermediate electrical resistivity imaging approach referred to as “3D‐“ is introduced in this paper. The methodology is based on existing tools, and it offers useful and sufficiently reliable 3D images of the investigated structure within a cost‐effective and flexible procedure. The survey design, the model discretisation, and the thorough integration of a priori information are the main phases of this procedure. To demonstrate the benefits and limitations of this approach, it is applied to an existing stretch of embankment levee along the Loire River. A numerical study was carried out both on synthetic and real data to assess the 3D imaging capability of the approach and the influence of prior information on the inversion outputs. The important role of a priori information is shown to be even more essential here. The results demonstrate the efficiency and versatility of the 3D‐ approach for reliable and cost‐effective investigations of long dikes.

Circulation de géomatières sur de longues distances au Paléolithique supérieur : le cas des silex du Turonien du Sud du Bassin parisien

In the South of the Paris basin, the Lower (silex blond) and Upper (silex du Grand-Pressigny) Turonian flints were important raw materials during the Upper Paleolithic. Their exploitation is known from findings from the Berry, Touraine, Poitou and the Auvergne, and some are found in Upper Paleolithic lithic assemblages from the Paris Basin, the Rhône valley, Aquitaine and the Charente. In spite of their importance, these resources have never previously been the subject of detailed petrographic descriptions and sometimes have been confused with products of Cenozoic silicification events. We present a summary of our observations on over 700 samples from 104 different lithic source and thousands of other lithic artefacts, supported by a review of publications, geological maps and academic papers related to the topic. The massive supply of Turonian flint (especially from the Lower Turonian) forming an important structural element of Upper Paleolithic lithic assemblages is confined essentially to an area delineated by the Seuil du Poitou and the Combrailles of Creuse to the south and the Loire River to the north and the east. Its occurrence in this zone is no longer anecdotal and reveals an early relationship between several prehistoric provinces.

‘Equivalent numbers’ for species, phylogenetic or functional diversity in a nested hierarchy of multiple scales

Summary Many recent studies have searched to integrate species’ functions and phylogenies in the measurement of biodiversity. To obtain easily interpretable measures, some researchers recommended diversity indices expressed in terms of equivalent numbers of species: the number of equally likely and maximally dissimilar species needed to produce the given value of diversity. Then, biodiversity is often calculated at three scales: within communities (α diversity), among communities (β diversity) and in a region (γ diversity). These three scales are, however, insufficient to tackle the organization of biodiversity in space because, for most organisms, there is a nested hierarchy of multiple scales characterized by different patterns and processes, from the small neighbourhood to the biosphere. We developed methodologies for analysing species, functional, taxonomic or phylogenetic diversity in a hierarchy of multiple scales using equivalent numbers of species. As an example, we analysed the taxonomic and functional diversity of macroinvertebrate assemblages in the Loire River, France, at four levels: within sites (α diversity), among sites within geological regions (β1 diversity), among geological regions (β2 diversity) and at the river scale (γ diversity). The new hierarchical approaches of biodiversity revealed very low differences among sites within regions and among regions in terms of taxonomy and functional traits (size and diet), despite moderate, significant species turnover among geological regions. We compare our framework with those other authors have developed. We argue that different definitions of α, β, γ diversities are used in the literature reflecting different points of view on biodiversity. We make recommendations on how to normalize functional (or phylogenetic) dissimilarities among species to render sites and regions comparable, and discuss the pros and cons of our approach. The hierarchical approaches of biodiversity in terms of ‘equivalent numbers’ respond to current demands to obtain intuitive, easily interpretable components of biodiversity. The approaches we propose go beyond current developments by considering a hierarchy of spatial scales and unbalanced sampling design. They will provide powerful tools to detect the ecological and evolutionary processes that act differently at different scales.

Temporal and Spatial Variations of Freshwater Reservoir Ages in the Loire River Watershed

AbstractIn order to map the freshwater reservoir effect (FRE) variability of the Loire River and its tributaries, spatial and temporal carbon isotope (13C and 14C) analyses of the dissolved inorganic carbon (DIC) were conducted. Sites were selected to represent the diversity of geological settings, soil type, and land use. Results show a large spatial variability of 14C FRE ranging between 135 and 2251±30 yr, objectively correlated to DIC contents and alkalinity. Deeper investigations of the relationship between 14C activity of DIC and environmental variables show that the geological substrate is the dominant factor in the 14C reservoir effect, and far more influential than the river flow discharge.

Transferring measured discharge time series: Large‐scale comparison of Top‐kriging to geomorphology‐based inverse modeling

AbstractFew methods directly transfer streamflow measurements for continuous prediction of ungauged catchments. Top‐kriging has been used mainly to predict the statistical properties of runoff but has been shown to outperform traditional regionalization approaches of rainfall‐runoff models. We applied the Top‐kriging approach across the Loire River basin and compared predictions to a geomorphology‐based approach. Whereas Top‐kriging uses spatial correlation, the other approach has the advantage of being more physically based by using a well‐known geomorphology‐based hydrological model (WFIUH) and its inversion. Both approaches require an equal degree of calibration and provide similar performances. We also demonstrate that the Ghosh distance, which considers the nested nature of catchments, can be used efficiently to calculate weights and to identify the suitability of gauged catchments for use as donor catchments. This result is particularly relevant for catchments with Strahler orders above five, i.e., where donor catchments are more strongly nested.

Molecular data and ecological niche modelling reveal the evolutionary history of the common and Iberian moles (Talpidae) in Europe

According to mitochondrial data, the common mole, Talpa europaea, is paraphyletic. This could be explained by either an ancient introgression of mtDNA from the Iberian blind mole T. occidentalis to T. europaea, or the existence of a differentiated taxonomic entity in northern Spain that needs to be described. In this study, we combined mitochondrial (Cytb) and nuclear (HDAC2) data to investigate these two alternative hypotheses. Based on both mitochondrial and nuclear data and an extensive geographical sampling (399 sequenced individuals), we show that the populations of T. europaea from Spain and south‐western France (south of the Loire River) are phylogenetically closer to T. occidentalis than to T. europaea. The Spanish–French lineage has some morphological characters resembling more to T. occidentalis (e.g. eyes) and others resembling more to T. europaea (external measurements, mesostyle of the first upper molar). It also seems to have several distinctive dental characters, suggesting that it should be recognized as a new species. Within the three lineages, we found a marked phylogeographical pattern, with several allopatric or parapatric lineages, dating from the Pleistocene. Our genetic data combined with species distribution models support the presence of several putative glacial refugia during glacial maxima for each species.

Quantifying sediment sources in a lowland agricultural catchment pond using 137Cs activities and radiogenic 87Sr/86Sr ratios

Soil erosion often supplies high sediment loads to rivers, degrading water quality and contributing to the siltation of reservoirs and lowland river channels. These impacts are exacerbated in agricultural catchments where modifications in land management and agricultural practices were shown to accelerate sediment supply. In this study, sediment sources were identified with a novel tracing approach combining cesium (137Cs) and strontium isotopes (87Sr/86Sr) in the Louroux pond, at the outlet of a lowland cultivated catchment (24km2, Loire River basin, France) representative of drained agricultural areas of Northwestern Europe.Surface soil (n=36) and subsurface channel bank (n=17) samples were collected to characterize potential sources. Deposited sediment (n=41) was sampled across the entire surface of the pond to examine spatial variation in sediment deposits. In addition, a 1.10m sediment core was sampled in the middle of the pond to reconstruct source variations throughout time. 137Cs was used to discriminate between surface and subsurface sources, whereas 87Sr/86Sr ratios discriminated between lithological sources. A distribution modeling approach quantified the relative contribution of these sources to the sampled sediment.Results indicate that surface sources contributed to the majority of pond (μ 82%, σ 1%) and core (μ 88%, σ 2%) sediment with elevated subsurface contributions modeled near specific sites close to the banks of the Louroux pond. Contributions of the lithological sources were well mixed in surface sediment across the pond (i.e., carbonate sediment contribution, μ 48%, σ 1% and non-carbonate sediment contribution, μ 52%, σ 3%) although there were significant variations of these source contributions modeled for the sediment core between 1955 and 2013. These fluctuations reflect both the progressive implementation of land consolidation schemes in the catchment and the eutrophication of the pond.This original sediment fingerprinting study demonstrates the potential of combining radionuclide and strontium isotopic geochemistry measurements to quantify sediment sources in cultivated catchments.

Optimizing observational networks combining gliders, moored buoys and FerryBox in the Bay of Biscay and English Channel

Designing optimal observation networks in coastal oceans remains one of the major challenges towards the implementation of future efficient Integrated Ocean Observing Systems to monitor the coastal environment. In the Bay of Biscay and the English Channel, the diversity of involved processes (e.g. tidally-driven circulation, plume dynamics) requires to adapt observing systems to the specific targeted environments. Also important is the requirement for those systems to sustain coastal applications.Two observational network design experiments have been implemented for the spring season in two regions: the Loire River plume (northern part of the Bay of Biscay) and the Western English Channel. The method used to perform these experiments is based on the ArM (Array Modes) formalism using an ensemble-based approach without data assimilation.The first experiment in the Loire River plume aims to explore different possible glider endurance lines combined with a fixed mooring to monitor temperature and salinity. Main results show an expected improvement when combining glider and mooring observations. The experiment also highlights that the chosen transect (along-shore and North–South, cross-shore) does not significantly impact the efficiency of the network. Nevertheless, the classification from the method results in slightly better performances for along-shore and North–South sections.In the Western English Channel, a tidally-driven circulation system, added value of using a glider below FerryBox temperature and salinity measurements has been assessed. FerryBox systems are characterised by a high frequency sampling rate crossing the region 2 to 3 times a day. This efficient sampling, as well as the specific vertical hydrological structure (which is homogeneous in many sub-regions of the domain), explains the fact that the added value of an associated glider transect is not significant.These experiments combining existing and future observing systems, as well as numerical ensemble simulations, highlight the key issue of monitoring the whole water column in and close to river plumes (using gliders for example) and the efficiency of the surface high frequency sampling from FerryBoxes in macrotidal regions.

Anthropogenic tritium in the Loire River estuary, France

This work is carried out in the frame of a radioecological monitoring of anthropogenic tritium from upstream and downstream of several nuclear power plants along the Loire River to its estuary. This paper studies the variation of anthropogenic tritium species in the Loire River system from upstream to the mouth of the estuary. Tritiated water (HTO and HTO in sediment pore water) and organically bound tritium (OBT) forms were analysed after dedicated pre-treatments. The collected environmental samples consist in (i) surface-sediment and core samples from the river floor, (ii) surface and water column samples. A maximum 3H activity concentration of 26±3Bq·L−1 in the Loire River estuary is obtained whereas an environmental background level around 1Bq·L−1 is determined for a non influenced continental area by anthropogenic activities. The European follow-up indicator used as a screening value is 100Bq·L−1. The conservative tritium behaviour was used in order to characterize the tidal regime and river flow influences in the mixing zone of the Loire River estuary. Furthermore, OBT levels and total organically carbon (TOC) content are explored. Finally, ratios of OBT relative to HTO in sediment pore water in surface-sediment and core samples are also discussed.

River Loire levees hazard studies – CARDigues’ model principles and utilization examples on Blois levees

Along the river Loire, in order to have a homogenous method to do specific risk assessment studies, a new model named CARDigues (for Levee Breach Hazard Calculation) was developed in a partnership with DREAL Centre-Val de Loire (owner of levees), Cerema and Irstea. This model enables to approach the probability of failure on every levee sections and to integrate and cross different “stability” parameters such topography and included structures, geology and material geotechnical characteristics, hydraulic loads… and observations of visual inspections or instrumentation results considered as disorders (seepage, burrowing animals, vegetation, pipes, etc.). This model and integrated tool CARDigues enables to check for each levee section, the probability of appearance and rupture of five breaching scenarios initiated by: overflowing, internal erosion, slope instability, external erosion and uplift. It has been recently updated and has been applied on several levee systems by different contractors. The article presents the CARDigues model principles and its recent developments (version V28.00) with examples on river Loire and how it is currently used for a relevant and global levee system diagnosis and assessment. Levee reinforcement or improvement management is also a perspective of applications for this model CARDigues.

Salt and the Earliest Scandinavian Raids in France: Was there a Connection?

The island of Noirmoutier lies in the bay of Bourgneuf just south of the mouth of the River Loire, near Nantes in western France. In the early decades of the ninth century Noirmoutier and other nearby islands and coastal areas became primary targets for Scandinavian raids. What was so attractive about this small area for the Northmen? Why did they keep coming back year after year? After examining these early raids in some detail, this paper will highlight the importance of salt production and trade in this part of France in the early medieval period before suggesting the possibility that one of the objectives of the Northmen might well have been salt. The question of how the Northmen might have been able to profit from salt will also be posed and different possibilities explored.

Using recent high-frequency surveys to reconstitute 35 years of organic carbon variations in a eutrophic lowland river.

Concentrations of dissolved and particulate organic carbon (DOC and POC), total suspended solids (TSS), were measured daily, and phytoplankton pigments (chlorophyll-a and pheopigments) were measured every 3 days at three strategic stations along the eutrophic Loire River between November 2011 and November 2013 marked by a high annual and seasonal variability in hydrological regimes. This unique high-frequency dataset allowed to determine the POC origin (autochthonous or allochthonous). Some strong relationships were evidenced between POC, total pigments and TSS and were tested on a long-term database with a lower frequency (monthly data) to reconstitute unmeasured algal and detrital POC concentrations and estimate annual total organic carbon (TOC) fluxes from 1980 onwards. The results were subjected to only ≈25 % uncertainty and showed that the annual TOC fluxes at the outlet of the Loire River decreased from 520 10(3) tC year(-1) (i.e. 4.7 t km(-2) year(-1)) in the early 1990s to 150 10(3) tC year(-1) (i.e. 1.4 t km(-2) year(-1)) in 2012. Although DOC always dominates, the autochthonous POC represented 35 % of the TOC load at the basin outlet by the end of the 1980s and declined to finally represent 15 % only of the TOC. The control of phosphorus direct inputs and the invasion by Corbicula clams spp. which both occurred since the early 1990s probably highly reduced the development of phytoplankton. Consequently, the autochthonous POC contribution declined and TSS concentrations in summertime significantly decreased as well as a result of both less phytoplankton and less calcite precipitation. At the present time, at least 75 % of the POC has allochthonous origins in the upper Middle Loire but downstream, autochthonous POC dominates during summer phytoplanktonic blooms when total pigments concentrations reach up to 70 μg L(-1) (equivalent to 75 % of the total POC).

Poor oxic conditions in a large estuary reduce connectivity from marine to freshwater habitats of a diadromous fish

Connectivity in aquatic systems is often related to abundance and permeability of physical barriers, such as dams, which delay or impede movements of biota with important consequences for aquatic biodiversity. Water quality may, however, also control connectivity between essential habitats. In macrotidal estuaries, Estuarine Turbidity Maxima (ETM) have a strong impact on water quality because of the low oxygen concentration occurring as a response to the related high bacterial and low photosynthetic activities. In this study, we assess Allis shad estuarine spawning migration in 2011 and 2012 in the Loire River (France) where the ETM occurs at spring and summer. Using an acoustic telemetry array, we show that trans-estuarine migration is inhibited during hypoxic episodes in the middle part of the estuary. Shad tends to stay in downstream areas, and even at sea, where oxygen conditions are more suitable. Trans-estuarine migration occurs hastily during neap tide when the ETM decreases, both in terms of spatial extent and intensity, inducing a shift in a set of covariates including dissolved oxygen, which increases, and suspended matter, which decreases. In the context of climate warming, ETM are expected to increase with probable adverse implications for shad migration success and doubtless other diadromous populations.

Quantification of the contribution of the Beauce groundwater aquifer to the discharge of the Loire River using thermal infrared satellite imaging

Abstract. Seven Landsat thermal infrared (TIR) images taken over the period 2000–2010 were used to establish longitudinal temperature profiles of the middle Loire River where it flows above the Beauce aquifer. The groundwater discharge along the river course was quantified for each identified groundwater catchment area using a heat budget based on temperature variations of the Loire River estimated from the TIR images. The results showed that 75 % of the temperature differences, between in situ observations and TIR image-based estimations, remained within the ±1 °C interval. The main discharge area of the Beauce aquifer into the Loire River was located between river kilometers 630 and 650, where there was a temperature drop of 1–1.5 °C in the summer and a rise of 0.5 °C in winter. According to the heat budgets, groundwater discharge was higher during the winter period (13.5 m3 s−1) than during the summer period (5.3 m3 s−1). These findings are in line with the results of both a groundwater budget and a process-based distributed hydrogeological model. Groundwater input was also found to be higher during the Loire's flow recession periods.