Climate change impacts on the seasonality and generation processes of floods – projections and uncertainties for catchments with mixed snowmelt/rainfall regimes

Hydrol. Earth Syst. Sci., 19, 913–931, 2015 www.hydrol-earth-syst-sci.net/19/913/2015/ doi:10.5194/hess-19-913-2015 © Author(s) 2015. CC Attribution 3.0 License. Climate change impacts on the seasonality and generation processes of floods – projections and uncertainties for catchments with mixed snowmelt/rainfall regimes K. Vormoor1 , D. Lawrence2 , M. Heistermann1 , and A. Bronstert1 1 Institute of Earth and Environmental Science, University of Potsdam, Postdam, Germany 2 Norwegian Water Resources and Energy Directorate (NVE), Oslo, Norway Correspondence to: K. Vormoor () Received: 22 May 2014 – Published in Hydrol. Earth Syst. Sci. Discuss.: 13 June 2014 Revised: 14 January 2015 – Accepted: 22 January 2015 – Published: 12 February 2015 Abstract. Climate change is likely to impact the seasonality and generation processes of floods in the Nordic countries, which has direct implications for flood risk assessment, design flood estimation, and hydropower production management. Using a multi-model/multi-parameter approach to simulate daily discharge for a reference (1961–1990) and a future (2071–2099) period, we analysed the projected changes in flood seasonality and generation processes in six catchments with mixed snowmelt/rainfall regimes under the current climate in Norway. The multi-model/multi-parameter ensemble consists of (i) eight combinations of global and regional climate models, (ii) two methods for adjusting the climate model output to the catchment scale, and (iii) one conceptual hydrological model with 25 calibrated parameter sets. Results indicate that autumn/winter events become more frequent in all catchments considered, which leads to an intensification of the current autumn/winter flood regime for the coastal catchments, a reduction of the dominance of spring/summer flood regimes in a high-mountain catchment, and a possible systematic shift in the current flood regimes from spring/summer to autumn/winter in the two catchments located in northern and south-eastern Norway. The changes in flood regimes result from increasing event magnitudes or frequencies, or a combination of both during autumn and winter. Changes towards more dominant autumn/winter events correspond to an increasing relevance of rainfall as a flood generating process (FGP) which is most pronounced in those catchments with the largest shifts in flood seasonality. Here, rainfall replaces snowmelt as the dominant FGP primarily due to increasing temperature. We further analysed the ensemble components in contributing to overall uncertainty in the projected changes and found that the climate projections and the methods for downscaling or bias correction tend to be the largest contributors. The relative role of hydrological parameter uncertainty, however, is highest for those catchments showing the largest changes in flood seasonality, which confirms the lack of robustness in hydrological model parameterization for simulations under transient hydrometeorological conditions. 1 Introduction The hydrological cycle is likely to intensify due to climate change (IPCC, 2007; Seneviratne et al., 2012), and a recent study indicates that global warming has caused more intense precipitation over the last century on the global scale (Benestad, 2013). These changes will, in turn, have direct implications for flood risk. For mountainous and Nordic regions, changes in the ratio of rainfall and snowfall due to temperature rise are of special interest since they have direct implications for flood seasonality and for the dominant processes generating flood discharge. A coherent picture of observed positive annual and winter streamflow trends for the Nordic countries (Stahl et al., 2010; Wilson et al., 2010) has been linked to a pattern of generally increasing mean and extreme precipitation (Bhend and von Storch, 2007; Dyrrdal et al., 2012). Regarding flood seasonality, neither significant trends towards higher autumn floods as a result of increasing autumn rainfall, nor system- Published by Copernicus Publications on behalf of the European Geosciences Union. 914 K. Vormoor et al.: Climate change impacts on the seasonality and generation processes of floods atic trends in spring flood magnitudes are yet detected (Wilson et al., 2010). The same study found, however, a strong trend towards earlier spring floods at many stations. This is likely due to the observed increase in mean annual temperature during the last century, which has been reported to be 0.8 ◦ C, with the strongest decadal temperature rise during the spring season (Hanssen-Bauer et al., 2009). Climate projections for Norway for the end of the 21st century indicate increasing temperatures (2.3–4.6 ◦ C) and precipitation (5–30 %) with the largest temperature increase during winter in northern Norway, and the largest precipitation increase during autumn and winter on the west coast (Hanssen-Bauer et al., 2009). Extreme precipitation is also likely to increase for all seasons across the whole of Norway (Beniston et al., 2007; Hanssen-Bauer et al., 2009; Seneviratne et al., 2012), although such projections are highly uncertain (Fowler and Ekström, 2009). Changes in temperature and precipitation regimes will have direct implications for the snow regime in Norway. For mountainous areas and in northern Norway where mean winter temperature is a few degrees below 0, snow depth is observed to have increased in recent decades (Dyrrdal et al., 2013) and climate projections suggest further increases until 2050 (Hanssen-Bauer et al., 2009). In other parts of Norway snow depths are projected to decrease. Towards the end of the 21st century, a decrease in snow depths and a shorter snow season are projected for the whole of the country due to temperature rise. For the Nordic countries, several previous studies have investigated the hydrological impacts of climate change (e.g. Andréasson and Bergström, 2004; Roald, 2006; Beldring et al., 2008; Veijalainen et al., 2010; Lawrence and Hisdal, 2011; Lawrence and Haddeland, 2011). For Norway, Lawrence and Hisdal (2011) studied the changes in flood frequency in 115 Norwegian catchments and found coherent regional patterns of directional change in flood magnitudes under a future climate: the magnitudes of the 200-year flood, for example, is likely to increase in catchments in western and much of coastal Norway where flood generation is dominated by autumn/winter rainfall, while magnitudes are expected to decrease in the snowmelt-dominated catchments in inland areas and parts of northern Norway. This regional pattern reflects systematic changes in climate forcing, which lead to changes in hydrological flooding in terms of both seasonal prevalence and generation process (rainfall vs. snowmelt). There are, however, many catchments which are transitional between rainfall-dominated vs. snowmelt-dominated flood regimes, and interpretation of the likely direction of change in the magnitude of future floods is more diff (...truncated)