Fig. 3. Convergence and divergence in the sign of significant trends in extremes of precipitation and of discharge, for period 1979-2020. This is calculated for annual maximum cumulative values of precipitation and of discharge during consecutive periods of 1-day (panel A), 2-day (B) and 20-day (C). Colours show where precipitation and discharge extremes both increase (green), both decrease (orange), where precipitation increases while discharge decreases (blue), and where precipitation decreases while discharge increases (purple). Precipitation data are from ERA5 (Hersbach et al., 2020), and discharge data are from GloFAS (Harrigan et al., 2020). Trend significance is assessed with the Mann-Kendall test.

2.1.1 Mechanisms of river flood generation

The discrepancies in trends are due to non-linear relations between precipitation and flooding, generated by multi-step mediating processes. In a first step, part of precipitation is converted into runoff. In a second step, runoff accumulates into river discharge. In a final step, a portion of discharge can be converted into flood water during an extreme event when river bankfull conditions are exceeded. The first two steps are mediated by processes related to hydrology, like evapotranspiration and infiltration. These are determined by the characteristics of the land surface: slope, soil, vegetation, land-cover, and river network. The second and third steps are mediated by hydrodynamic processes, determined by the hydraulic characteristics of the river channel and of the floodplain.
The spatial and temporal pattern in which these processes play out is essential in determining their outcome. A key determinant is the state of the relevant components of the water cycle at the time of the precipitation event: the antecedent conditions. Key antecedents are: the amount of snow priorly accumulated in the mountainous part of the basin and the timing of its thawing (Berghuijs et al., 2016; Huntingford et al., 2014; Musselman et al., 2018); the level of moisture of the upper parts of the soil (Neri et al., 2019; Tramblay et al., 2019; Wasko & Nathan, 2019); for large-scale basins and events, the level of groundwater. In geographies where these phenomena are seasonal, flood occurrence will typically have strong seasonality (Rottler et al., 2021). For example, the same precipitation event can more likely result in flooding during springtime than summer (Schaller et al., 2014), due to the higher infiltration capacity of summer soils, which contain lower moisture due to higher temperatures and evapotranspiration. More recently, attention is raised to drought as an aggravating antecedent factor for floods (Rashid & Wahl, 2022), whereby soil permeability is reduced by protracted dry conditions (Alaoui et al., 2018). An example of this phenomenon unfolded in spring 2023, over vast parts of Northern Italy (NASA_Earth_Observatory, 2023).

2.2 Hydrological change has occurred

Hydrological changes reduce flood hazard, or increase it; some changes still will reduce it at one location while increasing it at another. The key problem for flood attribution is that often these changes have taken place during the period of climate change. As such, hydrological changes can amplify or counterbalance the effect of climate change on flood occurrence, and failing to take them into account vitiates the attribution.
Effects of land-cover change on river discharge and flood are difficult to predict (Kirchner et al., 2020). Changes in land-cover can be natural or anthropogenic; in the latter case they are called land-use change. Observations show that deforestation in 56 developing countries increased flood occurrence during the last decades (Bradshaw et al., 2007). Similarly, Anderson et al. (2022) show that urbanisation and re-forestation have respectively increased and decreased extreme streamflow, in the context of 729 U.S.A. catchments. Similar indications emerge from many modelling studies (e.g., Du et al., 2012).
Other types of human intervention on hydrology have taken place over a large part of the world’s rivers (Grill et al., 2019), altering hydrological and hydraulic properties relevant to flooding. Key interventions are: dam construction and management, river bed encroaching, levees and dikes, channelling and water expansion areas, civil structures like roads, bridges and drainage networks, irrigation and groundwater abstraction, and other flood management measures. While most of these interventions are explicitly meant to have a local hydrological effect, e.g., building a levee to reduce local flood hazard, some have unintended hydrological effects, e.g.: irrigation lowers the water table in the soil; river training may increase flood hazard further downstream (Munoz et al., 2018; Vorogushyn & Merz, 2013).