It is important to know how often an extreme precipitation amount can occur. This frequency can be derived via analysis of observed time series. With precipitation traditionally observed at single points (the rain gauges), the derived frequencies are not valid for e.g. catchments. Valid areal extreme values can be achieved by a subsequent reduction in space. This study analyses the impact of seasons, durations, frequencies and topography on reduction factors for extreme values (5 min to 1 day).