Extreme value statistics of erosive rainfall events – a comparative assessment for agricultural production zones in Austria

In our ACRP funded project EROS-A we aim at a comprehensive analysis of the erosive energy of rainfall, estimated soil loss, and reported damage using statistical and process-based methods. In this particular study, we evaluate the return periods of erosive rainfall events at 26 meteorological stations in Austria. The main focus is on daily cumulative rainfall and the maximum 30-min rainfall intensity (I30) as these were identified as key parameters for rainfall erosivity assessment. The extreme value series were obtained using both the Annual Maxima Series (AMS) and Peak Over Threshold (POT) approach, in order to assess which of the methods will be most accurate. The assumptions of stationarity and independence of the extreme value series were carefully checked using statistical trend and independence tests and no significant deviations were found.

Generalized extreme value (GEV) and generalized Pareto (GPD) probability distributions were fitted using L-moment and maximum likelihood procedures. The GEV distribution is suited for AMS or block maxima data, whereas the GPD is suited for the POT series. For the obtained GEV and GPD models we examined extreme events with return periods of 2, 5, 10, 25, 50, and 100 years. We found that threshold selection is crucial for the POT, with diagnostic tools (such as mean residual life plots) not being fully decisive. Finally sensitivity analysis was performed where convergence of the fitted GPD to the GEV (AMS approach) helped determining robust thresholds for the GPD. The results show that the POT approach for daily cumulative precipitation is the most accurate in 69% of the cases and the AMS approach in 8% of the cases (different return periods and stations), while they have similar performance in 23% of the cases. Similar results are obtained for I30, were the success rates are 80% for the POT, 8% for the AMS and 12% for similar performance. In the next step, we will extend frequency analysis to a regional context, in order to map extreme rainfall erosivity across main agricultural production zones in Austria.