Evaluating future low flows in the Mur River basin using a hybrid stochastic-deterministic modelling framework

Abstract

Climate change is expected to significantly impact hydrological processes, consequently altering the flow characteristics of rivers. The present work examines the hydrological impacts of climate change in the Austrian part of the Mur River basin, focusing specifically on future low flow conditions. The analysis is based on 26 climate change scenarios derived from 13 ÖKS15 climate projections, each of which was combined with two emission pathways. This approach was used to gain information about possible future changes in air temperature and precipitation until the year 2100. As these scenarios are still associated with significant biases, temperature and precipitation data were extracted for a reference period (1981-2010) and a future period (2071-2100) and only the (relative) changes were used further in a Delta-change-approach. These projected changes were then used to modify 10.000 years of daily synthetic meteorological data generated by a Stochastic Weather Generator (SWG), resulting in air temperature and precipitation time series representing both current and future climate conditions. Based on historical temperature, precipitation and discharge records for the Mur River and its catchment, a distributed rainfall-runoff model was set up and calibrated. This was subsequently used to simulate 10.000 years of streamflow discharges for current and future climate conditions at four selected gauges along the Mur River, using the adjusted synthetic weather data. The generated streamflow discharges at the chosen river profiles were analysed, focusing particularly on low flow characteristics and their developments under changing climate conditions. The study analyses flow duration curves (FDCs), low flow frequency curves (LFFCs) for the whole year and the vegetation period (April to September), as well as the duration of low flow spells and the number of low flow days per season, for the various climate change projections and the current climate. The results show significant variability between scenarios, reflecting the uncertainty associated with future climate developments. For the group of intermediate emission scenarios (RCP4.5), low flow discharges linked to a given return period are expected to increase during the growing season and for the whole year. Little to no change is expected for the number of low flow days, with only slightly longer low flow spells than under current conditions. For the group of high emission scenarios (RCP8.5), however, no change or even a decline in low flows may occur during the growing season, with a trend towards lower discharges at the more downstream gauges. Further, this scenario group shows a clear tendency towards more low flow days during summer and low flow periods of longer duration. While this development may not seem particularly extreme at first, when considered alongside increased temperatures and potentially higher water demand for plant growth, it could have significant implications for water resources management, hydropower production, aquatic ecosystems and other water-dependent sectors within the Mur River basin. The findings of the present work contribute to a better understanding of climate change impacts on low flows in the study catchment and may provide a basis for future adaptation and management strategies.