Hydropower is the most important power production resource in Norway, and the country has approximately half of Europe’s reservoir capacity. Ca 85 TWh of storage capacity is distributed on more than 1000 reservoirs and in a number of cascade-coupled river systems. Long term optimisation of the use of the water resources is a complex problem that need to consider for example the complexity of the topology as well as the large seasonal and annual variation in inflow to the system. In the future, the flexibility of the hydropower system will be even more useful since the flexibility can be used to balance the variability in the increasing wind and solar power production. The stochastic optimisation model EMPSW can be used for long-term planning of hydrothermal system, for example the Scandinavian system . EMPSW simulates the northern European power supply market with detailed description of hydropower and individual water values for all reservoirs in Scandinavia. The operation of each individual hydroelectric reservoir is based on the result of formal stochastic optimisation in which all the relevant physical attributes of the market are represented. Decisions for each week are determined by solving weekly linear programming problems considering uncertainty in weather and exogenous market prices. The overall scheduling problem is obtained by solving a sequence of weekly decision problems spanning a chosen period of time. Comparisons are made with a widely used long-term hydro-thermal scheduling model, the EMPS model, which is based on aggregation-disaggregation techniques . The results indicate that the model is well suited to evaluate the flexibility of hydropower in systems with a high share of intermittent renewable generation.
: B. Mo, O. M. Hansen, L. E. Schäffer. “Methods of aggregation and disaggregation – project results”. 2020.
 O. Wolfgang, A. Haugstad, B. Mo, A. Gjelsvik, I. Wangensteen, and G. Doorman. “Hydro reservoir handling in Norway before and after deregulation”, Energy, vol. 34, no. 10, pp. 1642-1651, 2009.