Time resolution changes perspective on flood responses to climate warming

Most climate change impact assessments focusing on floods rely on daily resolution data. For snow-influenced catchments, these assessments typically project either decreases or no changes in flood magnitude and frequency because decreases in snowmelt can compensate for increases in extreme precipitation. Yet, sub-daily rainfall extremes will intensify more strongly than daily rainfall extremes under climate change. This suggests that a daily resolution may be insufficient for studying future flood responses to rainfall intensification. In this study, we show how moving from daily to hourly resolution data reshapes our understanding of how floods will change in a warming climate.

We find that, in more than 75% of the Alpine rivers studied, daily streamflow projections underestimate the magnitude and recurrence rate of the 100-yearly flood compared to hourly projections. While hourly projections show increases in flood magnitudes in strongly snow-influenced basins, daily projections point to decreasing flood magnitudes in the future. Under a high emission scenario, these differences in the sign of change between hourly and daily projections become significant before mid-century in more than half of the catchments. The flood seasonality signal also differs between daily and hourly projections for snow-influenced catchments: daily projections show a clearly earlier onset of floods compared to the historical period, whereas this signal is weak in hourly projections. While the daily perspective suggests a reduction in the magnitude of extreme floods due to a decrease in the magnitude of extreme snowmelt events in the future, the hourly perspective indicates that intensified sub-daily precipitation can compensate for snowmelt decline.

These results highlight that using daily instead of hourly projections may lead to wrong conclusions on changes in flooding in a warming climate in terms of the magnitude of change and, in snow-influenced catchments, even in terms of the sign of change. Using hourly resolution data is therefore good practice to guide adaptation strategies to flood response to climate warming.