Forests and floods: A new paradigm sheds light on age‐old controversies

Abstract

[1] The science of forests and floods is embroiled in conflict and is in urgent need of reevaluation in light of changing climates, insect epidemics, logging, and deforestation worldwide. Here we show how an inappropriate pairing of floods by meteorological input in analysis of covariance (ANCOVA) and analysis of variance (ANOVA), statistical tests used extensively for evaluating the effects of forest harvesting on floods smaller and larger than an average event, leads to incorrect estimates of changes in flood magnitude because neither the tests nor the pairing account for changes in flood frequency. We also illustrate how ANCOVA and ANOVA, originally designed for detecting changes in means, do not account for any forest harvesting induced change in variance and its critical effects on the frequency and magnitude of larger floods. The outcomes of numerous studies, which applied ANCOVA and ANOVA inappropriately, are based on logical fallacies and have contributed to an ever widening disparity between science, public perception, and often land-management policies for decades. We demonstrate how only an approach that pairs floods by similar frequency, well established in other disciplines, can evaluate the effects of forest harvesting on the inextricably linked magnitude and frequency of floods. We call for a reevaluation of past studies and the century-old, preconceived, and indefensible paradigm that shaped our scientific perception of the relation between forests, floods, and the biophysical environment.

Detection and attribution of urbanization effect on flood extremes using nonstationary flood‐frequency models

“…Assuming that the drivers included in the models can explain a large part of the natural variability of flow peaks, the detected change in the urbanizing catchment can be attributable solely to the increasing urban cover, in particular when compared to the unchanged patterns in the high flows of the rural paired catchment. Paired catchments have been widely employed in the assessment of the effects of changes in the catchment vegetation on river flow, in particular in forest hydrology [Brown et al, 2005;Alila et al, 2009]. In this study, the effects of the changes in land use on peak flows are investigated by assessing if any changes can be identified in the observed peak flows of the paired catchments.…”

Peak flow responses and recession flow characteristics after thinning of Japanese cypress forest in a headwater catchment

“…The paired-catchment approach was applied to examine the effect of thinning on peak flow based on an assumption that rainfall and runoff responses between treatment and control catchments remained consistent during the pre-and post-thinning periods. Because the catchments were small (< 1 ha), storm events in control and treated catchment were similar in time, duration, intensity, or spatial extent (Alila et al, 2009). Additionally, the dominant internal hydrological processes were similar between the two catchments and remained consistent before and after treatments Dung et al, 2012).…”

“…Analyzing the same data set, Thomas and Megahan (1998) demonstrated that peak runoff did not increase in large basins, but peak runoff in small basins increased 2-fold when comparing pre-and postharvesting periods. Reanalysis of these data were also conducted by Beschta et al (2000) and Alila et al (2009) to try to clarify these effects.…”

Flood risk reduction and flow buffering as ecosystem services – Part 1: Theory on flow persistence, flashiness and base flow

“…Reducing local flooding risk by increased drainage increases flooding risk downstream, challenging the nested-scales management of watersheds to find an optimal spatial distribution, rather than minimisation, of flooding probabilities. Well-studied effects of forest conversion on peak flows in small upper stream catchments (Bruijnzeel, 2004;Alila et al, 2009) do not necessarily translate to flooding downstream. With most of the published studies still referring to the temperate zone, the situation in the tropics (generally in the absence of snow) is contested (Bonell and Bruijnzeel, 2005).…”

“…The statistical challenges of attribution of cause and effect in such data sets are considerable with land use/land cover effects interacting with spatially and temporally variable rainfall, geological configuration and the fact that land use is not changing in random fashion or following any pre-randomised design (Alila et al, 2009;Rudel et al, 2005). Hydrological analysis across 12 catchments in Puerto Rico by Beck et al (2013) did not find significant relationships between the change in forest cover or urban area, and change in various flow characteristics, despite indications that regrowing forests increased evapotranspiration.…”