Climate change and hydropower production in the Swiss Alps: quantification of potential impacts and related modelling uncertainties

International audienceThis paper addresses two major challenges in climate change impact analysis on water resources systems: (i) incorporation of a large range of potential climate change scenarios and (ii) quantification of related modelling uncertainties. The methodology of climate change impact modelling is developed and illustrated through application to a hydropower plant in the Swiss Alps that uses the discharge of a highly glacierised catchment. The potential climate change impacts are analysed in terms of system performance for the control period (1961?1990) and for the future period (2070?2099) under a range of climate change scenarios. The system performance is simulated through a set of four model types, including the production of regional climate change scenarios based on global-mean warming scenarios, the corresponding discharge model, the model of glacier surface evolution and the hydropower management model. The modelling uncertainties inherent in each model type are characterised and quantified separately. The overall modelling uncertainty is simulated through Monte Carlo simulations of the system behaviour for the control and the future period. The results obtained for both periods lead to the conclusion that potential climate change has a statistically significant negative impact on the system performance

A conceptual glacio-hydrological model for high mountainous catchments

International audienceIn high mountainous catchments, the spatial precipitation and therefore the overall water balance is generally difficult to estimate. The present paper describes the structure and calibration of a semi-lumped conceptual glacio-hydrological model for the joint simulation of daily discharge and annual glacier mass balance that represents a better integrator of the water balance. The model has been developed for climate change impact studies and has therefore a parsimonious structure; it requires three input times series ? precipitation, temperature and potential evapotranspiration ? and has 7 parameters to calibrate. A multi-signal approach considering daily discharge and ? if available ? annual glacier mass balance has been developed for the calibration of these parameters. The model has been calibrated for three different catchments in the Swiss Alps having glaciation rates between 37% and 52%. It simulates well the observed daily discharge, the hydrological regime and some basic glaciological features, such as the annual mass balance

Current and future roles of meltwater–groundwater dynamics in a proglacial Alpine outwash plain

Glacierized alpine catchments are rapidly evolving due to glacier retreat and consequent geomorphological and ecological changes. As more terrain becomes ice-free, reworking of exposed terrain by the river as well as thawing of the top layer may lead to an increase in surface and subsurface water exchanges, leading to potential changes in water storage and release, which in turn may impact ecological, geomorphological and hydrological processes. In this study, we aim to understand the current and future hydrological functioning of a typical outwash plain in a Swiss Alpine catchment. As with many other fluvial aquifers in alpine environments, this outwash plain is located at the valley bottom, where catchment-wide water and sediment fluxes tend to gather from multiple sources, may store water and provide specific habitats for alpine ecosystems. Their dynamics are however rarely studied in post Little Ice Age proglacial zones. Based on geophysical investigations as well as year-round stream and groundwater observations, we developed a simplified physically based 3D MODFLOW model and performed an optimized automatic calibration using PEST HP. We highlight the strong interactions between the upstream river and the aquifer, with stream infiltration being the dominant process of recharge. Groundwater exfiltration occurs in the lower half of the outwash plain, balancing out the amount of river infiltration at a daily timescale. We show that hillslope contributions from rain and snowmelt have little impact on groundwater levels. We also show that the outwash plain acts as a bedrock-dammed aquifer and can maintain groundwater levels close to the surface during dry periods lasting months, even in the absence of glacier meltwater, but may in turn provide only limited baseflow to the stream. Finally, we explore how new outwash plains may form in the future in this catchment due to glacier recession and discuss from a hydrological perspective which cascading impacts the presence of multiple outwash plains may have. For this case study, we estimate the total dynamic storage of future outwash plains to be about 20 mm, and we demonstrate their limited capacity to provide more stream water than that which they infiltrate upstream, except for very low river flows (&lt;150 to 200 L s−1). Below this limit, they can provide limited baseflow on timescales of weeks, thus maintaining moisture conditions that may be beneficial for proglacial ecosystems. Their role in attenuating floods also appears limited, as less than 0.5 m3 s−1 of river water can be infiltrated. The studied outwash plain appears therefore to play an important role for alpine ecosystems but has a marginal hydrological effect on downstream river discharge.</p

Prediction of climate change impacts on Alpine discharge regimes under A2 and B2 SRES emission scenarios for two future time periods (2020-2049, 2070-2099)

The present work analyzes the climate change impacts on the runoff regimes of mountainous catchments in the Swiss Alps having current glaciation rates between 0 and 50 %. The hydrological response of 11 catchments to a given climate scenario is simulated through a conceptual, reservoir-based precipitation-runoff transformation model called GSM-SOCONT (Schaefli, 2005). For the glacierized catchments, the glacier surface corresponding to this future scenario is updated through a conceptual glacier surface evolution model. The analyzed climate change scenarios were derived from 19 climate experiments obtained within the EU research project PRUDENCE (Christensen et al. 2002). They are the results of 9 state-to-the-art Regional Climate Models (RCMs) driven by three coupled Atmosphere-Ocean General Circulation Models (AOGCMs), respectively HadCM3/HadAM3H, ECHAM4/OPYC3 and ARPEGE. The two first families of climate change scenarios correspond to changes in seasonal temperatures and precipitations simulated for the period 2070-2099 under the two green house gas emission scenarios A2 and B2 defined by the Intergovernmental Panel on Climate Change (12 experiments are available for A2 and 7 for B2). From the 19 PRUDENCE experiments 19 climate changes scenarios were additionally developed for a transient period (2020-2049) corresponding in first approximation to a global warming scenario of +1°C

Hydrological Drivers of Bedload Transport in an Alpine Watershed

Understanding and predicting bedload transport is an important element of watershed management. Yet, predictions of bedload remain uncertain by up to several order(s) of magnitude. In this contribution, we use a 5-year continuous time series of streamflow and bedload transport monitoring in a 13.4-km2 snow-dominated Alpine watershed in the Western Swiss Alps to investigate hydrological drivers of bedload transport. Following a calibration of the bedload sensors, and a quantification of the hydraulic forcing of streamflow upon bedload, a hydrological analysis is performed to identify daily flow hydrographs influenced by different hydrological drivers: rainfall, snowmelt, and combined rain and snowmelt events. We then quantify their respective contribution to bedload transport. Results emphasize the importance of combined rain and snowmelt events, for both annual bedload volumes (77% on average) and peaks in bedload transport rate. A non-negligible, but smaller, amount of bedload transport may occur during late summer and autumn storms, once the snowmelt contribution and baseflow have significantly decreased (9% of the annual volume on average). Although rainfall-driven changes in flow hydrographs are responsible for a large majority of the annual bedload volumes (86% on average), the identified melt-only events also represent a substantial contribution (14% on average). The results of this study help to improve current predictions of bedload transport through a better understanding of the bedload magnitude-frequency relationship under different hydrological conditions. We further discuss how bedload transport could evolve under a changing climate through its effects on Alpine watershed hydrology

Stream temperature prediction in ungauged basins: review of recent approaches and description of a new physics-derived statistical model

The development of stream temperature regression models at regional scales has regained some popularity over the past years. These models are used to predict stream temperature in ungauged catchments to assess the impact of human activities or climate change on riverine fauna over large spatial areas. A comprehensive literature review presented in this study shows that the temperature metrics predicted by the majority of models correspond to yearly aggregates, such as the popular annual maximum weekly mean temperature (MWMT). As a consequence, current models are often unable to predict the annual cycle of stream temperature, nor can the majority of them forecast the inter-annual variation of stream temperature. This study presents a new statistical model to estimate the monthly mean stream temperature of ungauged rivers over multiple years in an Alpine country (Switzerland). Contrary to similar models developed to date, which are mostly based on standard regression approaches, this one attempts to incorporate physical aspects into its structure. It is based on the analytical solution to a simplified version of the energy-balance equation over an entire stream network. Some terms of this solution cannot be readily evaluated at the regional scale due to the lack of appropriate data, and are therefore approximated using classical statistical techniques. This physics-inspired approach presents some advantages: (1) the main model structure is directly obtained from first principles, (2) the spatial extent over which the predictor variables are averaged naturally arises during model development, and (3) most of the regression coefficients can be interpreted from a physical point of view – their values can therefore be constrained to remain within plausible bounds. The evaluation of the model over a new freely available data set shows that the monthly mean stream temperature curve can be reproduced with a rootmean-square error (RMSE) of +/-1.3 °C, which is similar in precision to the predictions obtained with a multi-linear regression model. We illustrate through a simple example how the physical aspects contained in the model structure can be used to gain more insight into the stream temperature dynamics at regional scales

Dynamiques du stockage en eau souterraine et du régime hydrologique des bassins versants Alpins face aux changements climatiques

Le changement climatique aura un impact particulièrement fort sur le bilan hydrique des régions alpines. L'augmentation de la température entraîne un recul des glaciers, la proportion des précipitations qui tombent en forme de neige diminue, et la fonte des neiges se produit plus tôt dans l'année. Ces changements peuvent entraîner une diminution des débits des cours d’eau en été et en automne et éventuellement des pénuries d'eau régionales. La façon dont le régime hydrologique des bassins versants alpins changent dépend de la quantité d'eau de fonte et de précipitation qui est temporairement stockée sous forme d'eau souterraine. Dans une certaine mesure, le stockage souterrain de l'eau pourrait compenser la diminution du stockage sous forme de neige et de glace. Cependant, on en sait relativement peu à ce jour sur le fonctionnement hydrogéologique des zones alpines. Le projet poursuivait deux objectifs. D'une part, nous avons étudié la relation entre les caractéristiques géologiques des bassins, le stockage des eaux souterraines et l’écoulement de surface dans les conditions actuelles. D'autre part, nous avons étudié comment le changement climatique pourrait affecter la dynamique des eaux souterraines et des cours d'eau, en mettant l'accent sur les périodes d’étiages estivales. En particulier, nous avons cherché à savoir si les bassins versants comportant des réservoirs d'eaux souterraines plus importants vont subir moins de changement de leur régime hydrologique. Les recherches se sont concentrées sur les petits bassins versants des hautes Alpes dont le bilan hydrique est dominé par la neige et pour lesquels on dispose principalement de données pluriannuelles pour les cours d’eau. Les recherches sur l'influence de la géologie ont montré que les débits d’étiages augmentent avec l'augmentation de la surface des sédiments non consolidés et sont également influencés dans une moindre mesure par les roches solides à plus forte perméabilité. De vastes dépôts de roche meuble favorisent probablement l'infiltration. Ils peuvent stocker des quantités importantes d'eau et compenser partiellement les déficits saisonniers. Le stockage saisonnier des eaux souterraines dans de tels formations géologiques a été confirmé par des mesures gravimétriques et isotopiques dans un bassin versant de recherche. Nous avons étudié l'effet du changement climatique sur la dynamique des eaux souterraines et les débits des cours d’eau en couplant les modèles climatiques et les modèles hydro(géo)logiques. Pour un bassin de recherche, un modèle a été utilisé qui simule de manière couplée les processus des eaux souterraines et des eaux de surface. Pour les autres bassins versants, un modèle hydrologique conceptuel a été utilisé. Les simulations ont montré qu'en raison de la fonte des neiges plus précoce, la quantité d'eau souterraine stockée et le les débits des cours d’eaux auront tendance à être plus faibles à la fin de l'été/début de l'automne. Toutefois, la diminution relative du volume des eaux souterraines est bien moindre que pour les débits des cours d’eaux. Dans un avenir lointain, le débit des cours d’eaux minimal se déplacera de l'hiver vers les mois d'été. Toutefois, le débit minimal futur en été/automne reste supérieur au minimum actuel en hiver. En outre, les bassins versants présentant des formations géologiques étendues avec une bonne capacité de stockage de l'eau présentent un débit minimal plus élevé. Cela illustre les effets d'équilibrage des processus liés aux eaux souterraines. Le projet montre que les processus hydrogéologiques jouent un rôle important dans la régulation du bilan hydrique des bassins versants alpins, en particulier dans le contexte du changement climatique. En fonction des conditions géologiques, il est possible d'estimer comment les bassins versants pourraient réagir à une fonte des neiges plus précoce et à des périodes de sécheresse estivales. Les débits d’étiages actuels en hiver fournissent également des informations importantes sur l’importance réservoirs d'eaux souterraines dans un bassin versant et leur capabilité de stocker et libérer de l’eau sur une échelle de temps de plusieurs mois. À ce jour, cependant, il n'existe que quelques sites de surveillance dans la région alpine où la dynamique des réservoirs d'eaux souterraines peut être observée directement, ce qui a entraîné des incertitudes dans le projet actuel. En outre, les données sur les précipitations dans la région alpine présentent également de grandes incertitudes. Compte tenu des grands changements attendus, la base de données pour les zones alpines devrait être améliorée

SEHR-ECHO v1.0: A spatially explicit hydrologic response model for ecohydrologic applications

This paper presents the Spatially Explicit Hydrologic Response (SEHR) model developed at the Laboratory of Ecohydrology of the Ecole Polytechnique Fédérale de Lausanne for the simulation of hydrological processes at the catchment scale. The key concept of the model is the formulation of water transport by geomorphologic travel time distributions through gravity-driven transitions among geomorphic states: the mobilization of water (and possibly dissolved solutes) is simulated at the subcatchment scale and the resulting responses are convolved with the travel paths distribution within the river network to obtain the hydrologic response at the catchment outlet. The model thus breaks down the complexity of the hydrologic response into an explicit geomorphological combination of dominant spatial patterns of precipitation input and of hydrologic process controls. Nonstationarity and nonlinearity effects are tackled through soil moisture dynamics in the active soil layer. We present here the basic model set-up for precipitation-runoff simulation and a detailed discussion of its parameter estimation and of its performance for the Dischma River (Switzerland), a snow-dominated catchment with a small glacier cover