[1] This paper investigates the actual extrapolation capacity of three hydrological models in differing climate conditions. We propose a general testing framework, in which we perform series of split-sample tests, testing all possible combinations of calibration-validation periods using a 10 year sliding window. This methodology, which we have called the generalized split-sample test (GSST), provides insights into the model's transposability over time under various climatic conditions. The three conceptual rainfall-runoff models yielded similar results over a set of 216 catchments in southeast Australia. First, we assessed the model's efficiency in validation using a criterion combining the root-mean-square error and bias. A relation was found between this efficiency and the changes in mean rainfall (P) but not with changes in mean potential evapotranspiration (PE) or air temperature (T). Second, we focused on average runoff volumes and found that simulation biases are greatly affected by changes in P. Calibration over a wetter (drier) climate than the validation climate leads to an overestimation (underestimation) of the mean simulated runoff. We observed different magnitudes of these models deficiencies depending on the catchment considered. Results indicate that the transfer of model parameters in time may introduce a significant level of errors in simulations, meaning increased uncertainty in the various practical applications of these models (flow simulation, forecasting, design, reservoir management, climate change impact assessments, etc.). Testing model robustness with respect to this issue should help better quantify these uncertainties.

Abstract. This paper investigates the robustness of rainfallrunoff models when their parameters are transferred in time. More specifically, we propose an approach to diagnose their ability to simulate water balance on periods with different hydroclimatic characteristics. The testing procedure consists in a series of parameter calibrations over 10 yr periods and the systematic analysis of mean flow volume errors on long records. This procedure was applied to three conceptual models of increasing structural complexity over 20 mountainous catchments in southern France. The results showed that robustness problems are common. Errors on 10 yr mean flow volume were significant for all calibration periods and model structures. Various graphical and numerical tools were used to investigate these errors and unexpectedly strong similarities were found in the temporal evolutions of these volume errors. We indeed showed that relative changes in simulated mean flow between 10 yr periods can remain similar, regardless of the calibration period or the conceptual model used. Surprisingly, using longer records for parameters optimisation or using a semi-distributed 19-parameter daily model instead of a simple 1-parameter annual formula did not provide significant improvements regarding these simulation errors on flow volumes. While the actual causes for these robustness problems can be manifold and are difficult to identify in each case, this work highlights that the transferability of water balance adjustments made during calibration can be poor, with potentially huge impacts in the case of studies in non-stationary conditions.

Abstract. Stream temperature appears to be increasing globally, but its rate remains poorly constrained due to a paucity of long-term data and difficulty in parsing effects of hydroclimate and landscape variability. Here, we address these issues using the physically based thermal model T-NET (Temperature-NETwork) coupled with the EROS semi-distributed hydrological model to reconstruct past daily stream temperature and streamflow at the scale of the entire Loire River basin in France (105 km2 with 52 278 reaches). Stream temperature increased for almost all reaches in all seasons (mean =+0.38 ∘C decade−1) over the 1963–2019 period. Increases were greatest in spring and summer, with a median increase of + 0.38 ∘C (range =+0.11 to +0.76 ∘C) and +0.44 ∘C (+0.08 to +1.02 ∘C) per decade, respectively. Rates of stream temperature increases were greater than for air temperature across seasons for the majority of reaches. Spring and summer increases were typically greatest in the southern part of the Loire basin (up to +1 ∘C decade−1) and in the largest rivers (Strahler order ≥5). Importantly, air temperature and streamflow could exert a joint influence on stream temperature trends, where the greatest stream temperature increases were accompanied by similar trends in air temperature (up to +0.71 ∘C decade−1) and the greatest decreases in streamflow (up to −16  % decade−1). Indeed, for the majority of reaches, positive stream temperature anomalies exhibited synchrony with positive anomalies in air temperature and negative anomalies in streamflow, highlighting the dual control exerted by these hydroclimatic drivers. Moreover, spring and summer stream temperature, air temperature, and streamflow time series exhibited common change points occurring in the late 1980s, suggesting a temporal coherence between changes in the hydroclimatic drivers and a rapid stream temperature response. Critically, riparian vegetation shading mitigated stream temperature increases by up to 0.16 ∘C decade−1 in smaller streams (i.e. < 30 km from the source). Our results provide strong support for basin-wide increases in stream temperature due to joint effects of rising air temperature and reduced streamflow. We suggest that some of these climate change-induced effects can be mitigated through the restoration and maintenance of riparian forests.

Résumé. -Un modèle hydrologique a été déployé sur l'ensemble du bassin du Rhône permettant de mettre en relation la météorologie de l'ensemble du bassin versant français avec la ressource en eau. L'utilisation conjointe de ce modèle hydrologique et de trois scénarios de changement climatique a permis de faire une première évaluation des tendances à attendre sur la disponibilité de la ressource en eau d'ici à 50 ans environ. Les trois scénarios de changement de climat utilisés pour ce travail indiquent tous une augmentation des tempéra-tures moyennes sur l'année entre +2 et +3 "C. Cette élévation se décline sur l'année par une élévation en moyennes mensuelles interannuelles toujours supérieures à +1 "C et pouvant atteindre ponctuellement +5 "C en fin d'été, début d'automne. Les simulations hydrologiques avec les trois scénarios climatiques utilisés convergent sur une baisse de près de -50 O/O des débits moyens mensuels en fin de période estivale. Sur les débits hivernaux, selon le scénario, cela se traduit par des évolutions pouvant aller de (< aucune >) modification à +50 % (en février) des débits moyens mensuels. Les simulations indiquent aussi une translation des régimes à influences nivales (forts débits en été) vers des régimes à influences pluviales (forts débits en hiver), conséquence de la modification du fonctionnement du manteau neigeux (moins de neige à moyenne altitude et moins longtemps). La lame d'eau annuelle devrait être légèrement diminuée.Mots-clés. -hydrologie, changement climatique, Rhône, FranceAbstract. -An hydrological model has been used on the whole of the Rhone basin linking the meteorology of the totality of French catchment with the water resource. The joint use of this hydrological model with three climate change scenarios enabled a first assessment of forthcoming trends on water resource availability by the 2050s. The three climate change scenarios used in this study al1 show an increase in the mean annual temperature from +2 to +3 OC. This increase is expressed within the year by an interannual mean monthly rise always greater than +1 "C, reaching up to +5 'C at the end of the summer, beginning of the autumn. Hydrological simulations using the three climate change scenarios al1 show a decrease of about -50% of mean monthly dis- 78F. Hendrickx charge during the end of the summer. In winter, depending on the scenario, this corresponds to changes ranging from "nil" up to +50% (in February) of mean monthly discharges. The simulations also show a change from snow-affected type flow regimes (large summer discharges) towards rain-fed type regimes (large discharges during winter), a direct consequence of the change in the snow cover (less snow at mean elevation, and during shorter periods). The runoff should decrease by a small amount.

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