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).