Variability of Extreme Precipitation Duration Coefficients in Chile: Implications for Hydraulic Design

An important challenge associated with climate change is the increase in the intensities of extreme precipitation events, which produce severe flooding and cause substantial damage to hydraulic infrastructure. This intensification has been particularly evident in recent decades and is attributable to increased daily precipitation and the temporal concentration of rainfall over shorter durations. This results in increased precipitation pulses and, consequently, higher rainfall intensities. In Chile, the National Water Agency (DGA) published standardized duration coefficients in the year 1993, which may no longer align with the current climatological period, potentially underestimating the design parameters for extreme storms.

Using ERA5 reanalysis data, this study investigates changes in duration coefficients for extreme storms across a large area of Chile, covering latitudes from 17°S to 43°S. Two climatological periods are defined: the first from 1964 to 1993, that represent the past climatology, and the second from 2004 to 2023 that represent the current climatology. For each period, duration coefficients are computed by identifying annual extreme precipitation events for specific durations and normalizing these by annual maximum daily precipitation totals. Representative duration coefficients are computed for each period, taking the averages and the 90% exceedance probability envelope, reflecting an adverse scenario. The obtained coefficients are compared with those published by the National Water Agency, and change factors are computed for each hourly duration, using both representative coefficients. The computations were performed for each of the 31 stations with available historical data to enable comparative analysis.

The results indicate that the average 24-hour duration coefficients closely align with the 1.1 value suggested in the national design guidelines. However, significant variance is observed across the years of analysis, with the spatial mean of the exceedance envelope reaching 1.3. In northern Chile, characterized by the Atacama Desert and the Altiplano, ERA5 systematically underestimates short-duration coefficients associated with convective storms. Regarding changes in duration coefficients between the two climatological periods, an increase of up to 20% is observed in central Chile for durations shorter than six hours.

These findings highlight the critical need to regularly update duration coefficients in the context of a changing climate to ensure robust hydraulic infrastructure design and to mitigate risks associated with underestimation in regions experiencing intensified extreme events.