Around 98% of the energy in Norway comes from renewable sources and from this, 96% corresponds to hydropower, which has made it a central point in Norwegian water management. The growing interest in implementing an appropriate water regulation, relicensing existent power plants and the protection of biodiversity has increased the focus on environmental management. In addition, hydropower plays a very important role the mix of renewables as a balance for the intermittency that implies the implementation of solar and wind energies.
In 1991 a new energy law was passed in Norway that allowed trading of electricity in the market. This increased the incentives to adapt production to high prices in order to maximize revenue, which was different to the original strategies of national demand coverage that was the objective before the new regulation. From this moment, markets opened up to competition and the possibility of including different parts in the commercialization of energy was generated. It has been observed that after the decreed of the environmental law, the hydropower plants have intensified their production considerably in order to satisfy the demand in the energy market. The abrupt changes in flow and stage of the rivers located downstream of the hydropower plants in order to fulfill demand peaks, are called hydropeaking and cause a lot of impacts in the majority of organisms and riverine ecosystems.
The purpose of this study is to assess the operational changes over time for part of the Norwegian hydropower system by evaluating trends on key hydrological indicators downstream of the power plant outlets. 26 stations located in rivers under the influence of hydroelectric plants (regulated) and 4 stations located in unregulated rivers were analyzed. The information obtained from these gauges included hourly discharge measurements [m3/s] and water level [m] registered since 1985 in some cases. With COSH-Tool, a general analysis of the frequency and intensity of the increasing and decreasing peaking events was made and a clear increment in the peaking events was found from 1991, right after implementing the new environmental law. Subsequently, the significance of these trends was evaluated with Mann Kendall and Sen's slope tests and it was found that around half of the stations located in the regulated rivers presented a significant positive trend, while the gauges situated in rivers without influence of hydropower plants showed almost no trends.
With this, it can be inferred that the changes in the operational strategies did generate an impact on the rivers located downstream the power plant outlets. Therefore it is necessary to evaluate the production standards of the hydroelectric plants and it is suggested to pay more attention to the frequency and intensity with which the hydroelectric plants must establish their operation, which should be done slower in order to avoid the hydropeaking phenomenon that takes place at the moment.