Areal reduction factors from high-resolution rain gauges in Austria

Abstract

Abstract. Areal Reduction Factors (ARFs) are widely used to convert point design precipitation derived from intensity–duration–frequency (IDF) curves into areal precipitation values for hydrological design. Despite their importance, ARFs remain highly uncertain, particularly for short durations and small areas, due to insufficient representation of precipitation spatial variability. In this study, ARFs are empirically derived using a fixed-area approach for the Feldbach region in southeastern Austria using the WegenerNet, a dense rain gauge network with high temporal resolution (5 minutes) for the period 2007–2022. The results confirm that ARFs generally increase with duration and decrease with area, while pronounced dependencies on season and return period are observed, particularly for sub-hourly durations. Contrary to common assumptions, an application of ARF is required even for areas as small as 1 km2, indicating that point measurements and pixel-based precipitation estimates cannot be considered equivalent. For sub-hourly durations, ARFs increase partially with the return period. Seasonal analysis reveals smaller ARFs during spring and summer, associated with convective precipitation, and higher values during winter. The relationship between ARFs and topography is found to be weak, with only negligible correlations between ARFs and station elevation.