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Climate Change Impacts on Winter Hydrological Regimes: Implications for Hydropower Operation
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The winter hydrological regime of freshwater bodies is strongly affected by climate forcings. With climate change in the spotlight, it becomes imperative to investigate how these systems react to changes in the forcings as a result of anthropogenic climate change. This PhD study focuses on the assessment of climate change impacts on the winter regime of rivers, lakes and reservoirs. In addition, the impacts of the simulated changes in the winter regime on hydropower operation in cold regions are also given attention. The first task that was carried out was to carry out an extensive review of the ice effects on hydropower systems in cold regions. To assess impacts on winter hydrological regimes, different methods and tools have been employed. The methods used include assessing historical data of winter regimes in rivers and lakes, use of large scale climatology indices to characterize winter regimes and use of process based/mechanistic models to assess climate change impacts on rivers, lakes and reservoirs. Historical Norwegian datasets on ice phenology (river and lake ice) were collected and investigated for trends and compared with previous investigations. Overall trends point to later freeze-up in lakes and earlier freeze-up in rivers while break-up occurred earlier in both rivers and lakes. However, at-site trends varied a lot in magnitude as well as direction of trends. Climatology indices derived from daily air temperature data were used as proxies to assess historical trends as well as future conditions under climate change. Over the period 1961-2010, there was an overall trend towards later onset and earlier outing of the winter season. The investigation also showed that winter severity has decreased over the period. Future conditions were investigated using climate change signals from two Global Climate Models dynamically downscaled by a high resolution Regional Climate Model. Accordingly, while the future ice season will be significantly shorter and less severe than the present period, mid-winter thaw events will significantly increase leading to unstable winter season. Further investigation was carried out for lake ice regimes in the Nordic region in the future climate using a one-dimensional lake thermal and ice cover model, Mylake. Synthetic/generic lakes of various depths (with morphometric characteristics derived from actual lakes) were used for the regional analysis. Re-analysis data with 25 km grid resolution was used to drive the model for the current climate, whereas dynamically downscaled regional climate model data was used for the future climate. Based on a comparison of the mean predictions in the future 30 year periods with the control (1961–1990) period, ice cover durations in the region will be shortened by 1 to 11 weeks in the 2050s, and 3 to 14 weeks in the 2080s. The expected changes are found to be very much dependent on latitude and less sensitive to the lake depths. The lake ice model was further modified by incorporating reservoir processes and applied to three hydropower reservoirs in different hydro-climatic setting in Norway. Some of the predicted effects of climate change include: a reduction in ice cover duration ranging between 15 to 44 days in 2050s and 27 to 81 days in 2080s, depending on the scenarios and hydro-climatic conditions of the reservoirs. As a consequence of this, the period of stratification is lengthened by 20 to 31 days in 2050s, and 22 to 36 days in 2080s. For assessing aspects of ice effects on hydropower operation in a future climate, we used the above-mentioned modeling results as well as results from a one-dimensional river ice model, MIKE-ICE for a case study river system. Results show that the season where ice is an issue will be shortened in the future, thereby reducing the period with a need for operational constraints and ice mitigation. On the other hand, ice will still be a factor to consider, and the predicted instability in winter conditions could create new challenges in the future climate.