Forests and floods: Using field evidence to reconcile analysis methods

Abstract

The extent to which forests, relative to shorter vegetation, mitigate flood peak discharges remains controversial and relatively poorly researched, with only a few significant field studies. Considering the effect purely of change of vegetation cover, peak flow magnitude comparisons for paired catchments have suggested that forests do not mitigate large floods, whereas flood frequency comparisons have shown that forests mitigate frequencies over all magnitudes of flood. This study investigates the apparent inconsistency using field‐based evidence from four contrasting field programmes at scales of 0.34–3.1 km2. Repeated patterns are identified that provide strong evidence of real effects with physical explanations. Magnitude and frequency comparisons are both relevant to the impact of forests on peak discharges but address different questions. Both can show a convergence of response between forested and grassland/logged states at the highest recorded flows but the associated return periods may be quite variable and are subject to estimation uncertainty. For low to moderate events, the forested catchments have a lower peak magnitude for a given frequency than the grassland/logged catchments. Depending on antecedent soil saturation, a given storm may nevertheless generate peak discharges of the same magnitude for both catchment states but these peaks will have different return periods. The effect purely of change in vegetation cover may be modified by additional forestry interventions, such as road networks and drainage ditches which, by effectively increasing the drainage density, may increase peak flows for all event magnitudes. For all the sites, forest cover substantially reduces annual runoff.

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

“…The effects of changes in vegetated land are believed to be far more pronounced for low flows than high flows (e.g. Birkinshaw et al, 2014;Bathurst et al, 2020;Do et al, 2017;Vicente-Serrano et al, 2019), although there have been very few large-sample studies using observational records. To understand the influence of land cover changes on catchment hydrology, numerical modelling is often used to assess potential impacts and provide evidence for ambitious land cover changes related to policy before implementation.…”

Response to Referee#1

“…A number of model applications by e.g. Bathurst et al (2011Bathurst et al ( , 2020 conveyed the limitations of forestation as an intervention for reducing peak discharges of floods derived from extreme but infrequent rainfall events. There is a clear need to thoroughly evaluate whether our results on the effectiveness of natural infrastructure interventions during frequent erosion events can be extended to extreme events related to e.g.…”

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

“…The effects of changes in vegetated land are believed to be far more pronounced for low flows than high flows (e.g. Birkinshaw et al, 2014;Bathurst et al, 2020;Do et al, 2017; Vicente-Serrano et al, 2019) although there have been very few large-sample studies using observational records. To understand the influence of land cover changes on catchment hydrology, numerical modelling is often used to assess potential impacts and provide evidence for ambitious land cover changes related to policy before implementation.…”