Do small and large floods have the same drivers of change? A regional attribution analysis in Europe

Abstract

Abstract. Recent studies have shown evidence of increasing and decreasing trends for average floods and flood quantiles across Europe. Studies attributing observed changes in flood peaks to their drivers have mostly focused on the average flood behaviour, without distinguishing small and large floods. This paper proposes a new framework for attributing flood changes to potential drivers, as a function of return period (T), in a regional context. We assume flood peaks to follow a non-stationary regional Gumbel distribution, where the median flood and the 100-year growth factor are used as parameters. They are allowed to vary in time and between catchments as a function of the drivers quantified by covariates. The elasticities of floods with respect to the drivers and the contributions of the drivers to flood changes are estimated by Bayesian inference. The prior distributions of the elasticities of flood quantiles to the drivers are estimated by hydrological reasoning and from the literature. The attribution model is applied to European flood and covariate data and aims at attributing the observed flood trend patterns to specific drivers for different return periods at the regional scale. We analyse flood discharge records from 2370 hydrometric stations in Europe over the period 1960–2010. Extreme precipitation, antecedent soil moisture and snowmelt are the potential drivers of flood change considered in this study. Results show that, in northwestern Europe, extreme precipitation mainly contributes to changes in both the median (q2) and 100-year flood (q100), while the contributions of antecedent soil moisture are of secondary importance. In southern Europe, both antecedent soil moisture and extreme precipitation contribute to flood changes, and their relative importance depends on the return period. Antecedent soil moisture is the main contributor to changes in q2, while the contributions of the two drivers to changes in larger floods (T>10 years) are comparable. In eastern Europe, snowmelt drives changes in both q2 and q100.

Understanding Heavy Tails of Flood Peak Distributions

“…Another promising research line is the attempt to detect and attribute changes that differentiates between small and large floods. Bertola et al (2020Bertola et al ( , 2021 showed that the 100-year flood changed differently than the 2-year flood in some European regions and that these changes can be attributed to different drivers. Similarly, recent studies have shown that small and large floods respond differently to changes in precipitation (Brunner et al, 2021;Wasko et al, 2019Wasko et al, , 2021.…”

Nonstationary weather and water extremes: a review of methods for their detection, attribution, and management

“…7 in Wasko and Nathan, 2019). In temperate climates of northwestern Europe, extreme precipitation is an important flood driver, while in the drier climates of southern Europe, both antecedent soil moisture and extreme precipitation matter (with a greater importance of soil moisture for smaller floods; Bertola et al, 2021). Increases in precipitation intensity are more likely to cause increases in streamflow in smaller catchments (Wasko and Sharma, 2017;Wasko and Nathan, 2019).…”

“…Numerous studies showed that arterial and field drainage can affect flood peaks (increasing magnitude and reducing time to peak; e.g. Wilcock and Wilcock, 1995;Bhattarai and O'Connor, 2004). Conversely, our understanding of the impacts of drainage on low flows and drought remains poor, as does our understanding of how other components of runoff response are affected due to inadequate monitoring of subsurface hydrological processes.…”

River flooding mechanisms and their changes in Europe revealed by explainable machine learning

“…For example, by using circular statistics analysis, Berghuijs et al (2019) examined the relative importance of three flooding mechanisms based on the seasonality of floods and three potential drivers such as the largest daily precipitation, the largest daily soil moisture excess, and the largest daily snowmelt. Bertola et al (2021) attributed changes in the magnitude of flood quantiles to changes in possible drivers by using regression analysis and determined their contributions to flood changes accordingly. In contrast to these analyses conducted at catchment or coarser levels, Kemter et al (2020) and Stein et al (2020) performed event-based classifications to determine flooding mechanisms in respective regions or catchments, both using manual criteria but with different indicators and thresholds.…”

“…In recent years, numerous studies have investigated river flooding mechanisms and some of them have provided Europeanscale assessments (e.g., Berghuijs et al, 2016;Berghuijs et al, 2019;Kemter et al, 2020;Bertola et al, 2021;Stein et al, 2021). Catchment-level floods can typically be attributed to the interaction of hydro-meteorological processes, such as extreme precipitation, soil moisture excess, and snowmelt (Merz and Blöschl, 2003;Tarasova et al, 2019).…”