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Book (peer-reviewed)

Publisher Bern University, Bern

Open Access

Abstract

This study presents a robust modelling framework developed for assessing the available water resources in a mountainous environment today and in the future. The semi‐discrete, physically‐based Penn State Integrated Hydrologic Model (PIHM) has been identified as a suitable hydrological model for this challenging task. Here we present the customization, the enhancement and the application of this on a catchment with sparse data in the past and in the present, we evaluate extensively and thoroughly its performance, its potential and its limitations. The melt modules for snow and ice were upgraded from a simple temperature‐index model to a model including the influence of global radiation. Stationary catchment attributes such as soil and land cover data were used to distribute parameters, and the latters were mainly estimated basing on literature or experiments on site. Besides the examination of the internal consistency with a multicriteria validation, evaluating snowpack, icemelt and different components of the water balance, model results have been further validated with discharge measurements. Results show a very good performance of the model over different spatial and temporal scales. Sound hydrological modeling frameworks are needed to serve and support decision making in local to regional water management, in particular in mountainous regions, as main suppliers of natural water resources. In view of climate change, increasingly complex physically based models are applied in order to guarantee the predictability for new climate or environmental boundary conditions. However, while the hydrological predictability might increase with complex models, data demands of course increases too. In the present study we investigate the challenges, the gains and the problems of an increased complexity in describing snow and precipitation patterns in data sparse regions at higher altitudes and in an environment with a highly complex topography. Particular efforts were put in improving the modelling of snow and glacier dynamics and melt including a time‐varying albedo and gravitational redistribution of snow in a hydrological model. Furthermore, an extensive meteorological measuring network was set up and exploited generating high resolution input data. We provide evidence of the limits of an enhanced description of the albedo of snow depending on the surrounding conditions, when other processes we cannot describe emerge. We also show the importance of the maintenance and extension of short‐term monitoring networks in mountain regions, demonstrating that an increased sampling of hydro‐meteorological and cryospheric data – even on the basis of one to three years – retrieves the best model performance, and therefore these are crucial for the improvement of the hydrological knowledge of the local system. With the developed modelling framework we estimated available water resources for the headwaters of the study area, and made an overall plausibility assessment using all the available data, with very good results. Overall the headwaters of the catchment provide 110 mio m3. Generally the western part of the catchment is richer in water resources than the eastern part, corresponding respectively to 80% and 20% of the total water resources. Tseuzier, the subbasin at the western boundary, represents the “water tower” of the area, supplying 70% of the total estimated water amount In this work we integrated many relevant aspects of hydrological modelling, especially pertinent to the context of climate change in alpine regions, like transferability in time and space, as well as flexibility, and proved our modelling framework to be robust over different scales. As such, it provides a solid basis for hydrological analysis of the system under changing forcing conditions, like in the case of climate change. We performed an exploratory climate sensitivity analysis, basing on the data of ten climate model chains driven by the single emission scenario A1B, post‐processed and interpolated to the MeteoSwiss weather station Montana (as provided by the CH2011 initiative). Two scenario periods in the 21st century were assessed relative to the short reference period 2007‐2012: the near future 2048‐2053 and the far future 2097‐2102. The most pronounced changes are expected in the snow cover dynamics in the headwaters of the study area, and evaporation rates in the lower parts of the study area, with changes in temperature as the main drivers. Basing on the applied scenarios, for all models, a reshaped annual cycle, with earlier rise of spring runoff, significantly reduced summer runoff, and a tendency for increased winter runoff in the first half of the 21st century, becoming increasingly pronounced towards the end of the century, and resulting in a significant change of regime: first transforming from a b‐glacio‐nival regime in a nival alpin regime as a transition before becoming, characterized by a nival de transition regime in the far future. All in all, the overall available water resources are going to be affected only to a minor extent, with mean changes in the order of magnitude of 5‐10%, however the major changes estimated for the summer runoff, reduced by about 20% in the near future, and almost 50% in the far future are expected to have major consequences for the water management strategies of the region. In particular in dry years conflicts might arise between different usages, which here are mainly irrigation, hydropower production, tourism and artificial snow making.
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