Flood frequency analysis in Nordic condition over the past decades: Cases from Finland

Climate change and anthropogenic activities have always affected the hydrological condition of watersheds. The uniqueness of Nordic watersheds characteristics (systems of lakes and rivers dominant by cold climate) and land cover (drained and pristine forests and peatlands) results in different river regimes in these regions compared to the other parts of the world. Long extreme cold winters usually freeze the river and lakes deeply to some depth, while, during short Nordic summers, the river flows can be influenced by forest and forestry activities, especially drainage systems. In addition, the changing climate is another driver that impacts river flows, especially extreme hydrological events (floods and droughts). This study investigates the long-term flood frequency alteration in two snowmelt and rainfall-dominant seasons for several headwaters in Finland as a Nordic region. The long-term daily discharge, rainfall, snow depth, and temperature data for selected watersheds were analyzed. The monthly and annual changes in mean, maximum, and minimum of discharge and rainfall and their trends were assessed to detect the rain and snowmelt-dominated seasons. Then the flood frequencies are estimated using  EV (Extreme Value) method for both seasons in different periods. Investigating such changes provides a broad view of the current and long-term situation of the river systems, which can help for long-term water resources planning and hydrosystem developments.