Årsmaxima

Årsmaxima, or annual maxima, is a statistical concept used in hydrology, meteorology, and environmental studies to describe the largest observed value of a variable within each calendar year. By extracting the yearly peak from a multiyear time series—such as daily rainfall, daily river discharge, or daily maximum temperature—an annual maxima series is formed. This block maxima approach provides a compact representation of extreme values and is commonly used for risk assessment and design purposes.

In practice, the calculation involves dividing the data into calendar years and selecting the maximum value within each year. The resulting AMS can then be analyzed with extreme value methods. Under the framework of extreme value theory, the distribution of annual maxima often fits a Generalized Extreme Value (GEV) distribution. Parameters of the GEV (location, scale, shape) can be estimated by maximum likelihood or Bayesian methods, enabling the derivation of return levels for specified return periods (for example, the 10-year or 100-year extreme).

Applications of årsmaxima include flood risk assessment, hydraulic design of infrastructure, and the study of extreme temperature or precipitation events. They are often contrasted with other extreme value approaches, such as the peaks-over-threshold method, which uses all values above a threshold rather than a fixed yearly block.

Data considerations are important: non-stationarity from climate change or land-use change may require detrending or non-stationary models; data gaps and measurement consistency affect estimates; the choice of annual block size imposes limitations compared with alternative approaches.