Trends in soil frost formation in a warming maritime climate and the impacts on urban flood risk

Abstract

The most severe urban flooding in cold maritime climates is due to the co-action of long-duration rainfall, snowmelt, and soil frost. Increasing winter air temperature due to climate change is projected to change the magnitude and frequency of rain-on-snow (RoS) events and increase the number of freeze–thaw cycles and midwinter snowmelt. While daily rainfall records are readily available, less is known about the infiltration and frost formation within urban soils. Thus, there is uncertainty on how warming winter conditions may affect urban flood risk and the climate resilience of cities. The aim of this study was, therefore, to assess soil frost formation in the past 70 years in the maritime city of Reykjavík, Iceland (64° N, 21° W), and its co-action with runoff generation, and the potential implications on urban flood risk. To that end, the daily thermal and hydraulic conditions of the soil were simulated using the Simultaneous Heat and Water (SHAW) model dating back to 1949, calibrated based on hourly observations from 2007. Model simulations indicated that the minimum soil temperature at 10 cm depth has been warming at a rate of 0.015 °C/year in the past seven decades. Climate warming is also noted in a steady decline in frost depth and the duration of soil frost each winter (p-value < 0.05). However, the freezing season has shortened so that the timing of maximum frost coincides more with the timing of maximum RoS and snowmelt events. Furthermore, RoS events during frost have the capacity to produce larger runoff volumes than rainfall or snowmelt alone based on the joint frequency analysis of winter events using the copula method. This, combined with increasing volume during RoS events, suggests that winter floods may intensify in the next decades, which urges re-thinking urban stormwater management in cold climates.