Precipitation-Flow Frequency Analysis in Nordic Catchments

This study assesses precipitation and flow frequencies in two Finnish catchments, Pyhäntä and Ounasjärvi-luusua, using the Generalized Extreme Value (GEV) distribution applied to annual and monthly maxima over two 30-year periods. In Pyhäntä, annual precipitation extremes increased by 42% in the second period, while in Ounasjärvi-luusua, the increase was limited to return periods < 5 years (11%). Conversely, annual flow return levels rose by 48% in Ounasjärvi-luusua, but only for 2–13-year return periods in Pyhäntä. Monthly results showed unclear impacts of summer precipitation extremes and intensive drainage on corresponding flow extremes. A new index (θ) was introduced to describe the linear relationship between precipitation and flow return levels. θ for May–July closely matched annual θ, suggesting summer flows are influenced by snowmelt in Nordic regions. The study highlights that frequency analysis with limited data entails increasing uncertainty with higher return periods, particularly those exceeding the observational record.