Analysis of air/river maximum daily temperature characteristics using the peaks over threshold approach

The understanding of river temperature is very important when assessing fish habitat conditions in terms of fish growth as well as fish distribution within river systems. The quantification of extreme water temperatures is also important, especially when studying the suitability of thermal habitats during high air/water temperature events. The present study deals with the analysis of extreme air and water temperatures using the partial duration series approach, also known as the peak over threshold (POT) approach. The advantage of the POT approach is that extreme air and water temperatures can be described not only in magnitude but also in duration and degree day accumulation for each event. In the present study, 74 years of air temperature data (Miramichi meteorological station) were analysed as well as 21 years of river temperature data from the Little Southwest Miramichi River. For example, results showed that the 0.01 exceedance probability (F(x) = .99) for maximum daily air temperature event was calculated at 38.4 degrees C whereas for the same exceedance probability, the maximum daily water temperature event was slightly lower at 32.7 degrees C. The duration of events varied between 1 and 5 days for air temperature and between 1 and 7 days for water temperature. Maximum daily water temperature exceeding 30 degrees C generally occurred over a period of 3 days or more, and the degree day accumulation was higher than 6 degrees C center dot day. This study showed that the POT approach was effective in quantifying both air and water temperature extremes within the Miramichi River study area.