Underestimation of DDF values obtained from paper pluviograms

Extreme rainfall triggers many hazards in river basins (e.g., floods, landslides, mud-rock-debris flows) with particularly heavy impacts in cities and urbanized areas (e.g., urban flooding, storm/sewer drainage overflows). In Chile, persistent and ubiquitous urban flooding problems led us to question the official depth-duration-frequency (DDF) estimates used in drainage design in the country (Soto, 2004). In a preliminary analysis for the city of Concepción, we found severe underestimation of short duration rainfall, increasing with return period but decreasing with duration (Soto and Meier, 2013). After considering all possible alternative explanations, we hypothesised that the strong negative bias is caused by two simplifying procedures that were applied when estimating the DDF values from the original weekly paper charts (pluviograms - Meier et al., 2013, 2015)