This thesis contributes to the HiREPS model developed at the Vienna UT, Energy Economics Group - a highly resolved European power system, investment planning and supply security optimization program. In this work, we focus on South and Central Sweden's hydropower portfolio and the hydropower part of the HiREPS model. Chapter 1 gives a short introduction to Sweden's energy market. In chapters 2 to 4 we build up the knowledge basis (physics of a hydropower plant, used linear optimization method, functionality of the HiREPS hydropower model) for the main chapters, Chapter 5 and Chapter 6. In Chapter 5 we investigate possibilities to speed up the computation time of the model, involving a treatment of scaling issues, an assessment of redundant internal presolve proceedings of our used modelling language (AMPL) and a treatment of issues regarding the choice of starting values for the discharge in our implemented successive linear optimization. Through our modifications, we are able to cut off nearly 2/3 of the computation time and achieve a faster decrease of the linearization error caused by modelling non-linearity. Finally, the last chapter shows various simulation results regarding South and Central Sweden-s hydropower plants with an installed capacity equal to or greater 5MW. We include simulated production and profit data for the whole power plant complex for the years 2001 - 2011 to compare it with real data. Furthermore, we show results regarding detailed operation plans for selected power plants. To be precise, we have a close look at some plants along river Ljungan and at one of the few pumped-storage power plants called Kymmen, situated at river Rottnan.