Time variable effectiveness and cost-benefits of different nature-based solution types and design for drought and flood management

The ecohydrology of rewilding: A pressing need for evidence in the restoration of upland Atlantic salmon streams

“…A lack of data on local hydrological conditions usually mean modelling is also needed to predict potential impacts (Blöschl et al, 2013). However, the uncertainty associated with modelling may be greater than the change associated with the impact, making the outcome of any intervention difficult to predict (Fennell et al, 2023). In addition, the scale of a particular change relative to a particular catchment or river basin will strongly influence the likely impact; the cumulative effects of multiple interventions are also difficult to assess (e.g., Golden & Hoghooghi, 2018).…”

Nature-based solutions for floods AND droughts AND biodiversity: Do we have sufficient proof of their functioning?

“…Also, Iacob et al (2014) note that there are clear benefits in increasing woodland cover related to peak flow reduction, biodiversity and carbon storage, there may also occur disbenefits such as reduced food production and synchronisation of peaks from different sub-catchments within an overarching catchment. Fennell et al (2023) report for a Scottish catchment of 1 km 2 that tree planting reduced low flows and recharged groundwater, whereas 'Runoff Attenuation Features'being soft engineered measures designed to store and attenuate rapid runoff by controlling the discharge rate, that is, 'leakinesss'had a positive but smaller effect on these two processes. Both types of NbS did attenuate high and medium flows but design and influence of antecedent wetness impacted the effect.…”

“…There are many processes related to the functioning of NbS in relation to the propagation of floods and droughts on a landscape scale that are not well described or need additional attention during the planning, design and implementation phase of an NbS. For example, rainfall interception (and evaporation) for different types of vegetation and canopy structures (Muzylo et al, 2009), seasonality and pre-event effects (Fennell et al, 2023), the impact of climatic zone and local conditions (Burek et al, 2012), the role of soil carbon content (Fennell et al, 2022), and linkage between the unsaturated zone and the deeper ground water (Szabó et al, 2023) all play a role in defining the overall functioning of a NbS (Kumar et al, 2021a). In addition, the linkage with the impact of selected measures for floods and droughts on changes in water quality and biodiversity must be improved using easy-to-translate hydrological boundary condition indicators (e.g., minimum flow velocities for rheophilic fish communities and desired ground-water levels in summer seasons for groundwater dependent on vegetation communities).…”

“…For example, a landscape approach can support the combination of options of water storage during the wet season (such as wetlands restoration or natural aquifer recharge) with measures to reduce water demand during dry seasons (such as landscape restoration, agriculture conservation or mulching). In this, the linkage between groundwater and surface water processes is to be well-understood (Fennell et al, 2023;Szabó et al, 2023) wetlands and floodplains can help reduce peak flows and increase baseflows and groundwater levels, thereby reducing risks of both floods and droughts. Yet understanding the geological features and soil characteristics can be of utmost importance to establish the infiltration capacity of, for example, retention ponds and the ease with which they drain after a rain event.…”

“…However, in some cases measures that are taken to increase sponge functioning and reduce droughts may actually increase flood peaks due to the reduced storage capacity or reduced discharge rate of areas or the changed timing of discharge peaks in relation to other tributaries in rivers. Therefore, effects of measures on droughts and flood peaks of different sizes (and biodiversity) cannot be analysed in isolation (Fennell et al, 2023) To improve the sponge functioning in catchments, surface water retention and groundwater infiltration solutions are suggested that rely on preserving, restoring or enhancing natural ecosystems and their functioning. These solutions are often referred to as 'naturebased solutions' (NbS, following the definition of UNEA 2022) 1 .…”