Fig. 4. Conceptual representation of changes through time in drivers of river floods. Global warming, and associated changes in temperature and precipitation, often overlaps with changes in the hydrology, such as land-cover change, construction of levees and large-scale irrigation. Global temperature data are from the National Centers for Environmental Information of NOAA. Flood occurrence data is hypothetical. The bottom box illustrates the questions pertinent to a multi-driver framework for conditional attribution.
To judge which hydrological changes should be included, it is necessary to know the hydrological history of the basin (Fig. 4). Different relevant hydrological changes have likely occurred at different times, and can be very old, as in the case of the Netherlands, where water management has a famously long history (Hoeksema, 2007). Restricting the analysis to the period coinciding with most of climate change, i.e., the last century, makes the problem more tractable, and seems more relevant from the perspective of flood risk management. In the following, we review information on land-cover change and on other human interventions.

3.2.2 Land-cover change

For flood attribution, land-cover (land-use) maps need to be available for the present and also for past periods. Ideally, to ascertain whether changes overlap with climate change, maps should cover ca. the last century. Other relevant aspects are accuracy, resolution and the number of land-cover classes that are differentiated. Fine resolution is especially useful for detailed hydrodynamic modelling in urban contexts, whereas for hydrological modelling of large basins, resolution can be coarser. The priority should be to access any local data curated by regional institutions, which will typically be finer, more accurate, and may extend further back in time. Should this not be available, potentially useful global and continental datasets are:
If different datasets have comparable merits, including multiple datasets in the modelling could enable quantifying the uncertainty relative to the land-cover. When land-cover changes affect large areas of the river basin, they should be included in the hydrological modelling step. When land-cover changes affect the urban areas adjacent to the flood, it may be appropriate to include them in the hydrodynamic modelling step. Most distributed hydrological models have internal representation of land use for processes as evapotranspiration, canopy interception, infiltration, irrigation (e.g., Horton et al., 2022); however, the associated parametrizations are not evident and are model-dependent. Another aspect that should be considered is whether the land-cover changes had implications for soil properties. Deforestation, for example, is known to cause loss of soil, especially on steep terrain. If this is the case, soil changes should be included in the modelling, either using direct available information, or recurring to assumptions.

3.2.3 Other human interventions on hydrology

Flood attribution requires information on human hydrological interventions: their key features relevant to the modelling, and the timing of their realisation. Similarly, progressive changes to the channel geometry and channel bed elevation as a result of fluvial aggradation or incision need to be assessed. As with land-cover, often the best information should be accessed in collaboration with local authorities. However, there are a few global datasets that can function as alternatives:
These datasets might still miss data for data-poor regions. It could be therefore valuable to try to assess the presence and extent of interventions indirectly, using more general datasets that have recently emerged. For example, HydroATLAS (Linke et al., 2019) contains information on hydrological, climatological, environmental and anthropogenic characteristics of river basins and segments, at 15” of resolution; the database of free-flowing rivers (Grill et al., 2019) contains information about human pressure on river segments, notably on: degree of fragmentation and of regulation by dams; on urban areas, enabling assumptions on the presence of levees and river confinement structures.
The hydrological and hydrodynamic modelling will then need to adopt methods to represent these interventions in the simulations, often by ‘burning’ them into the elevation map (Wing et al., 2019), and drawing from the large experience documented in the literature (e.g., Remo et al., 2018; Zhao et al., 2016).