Laure Guerit

Estimating regional flood discharge during Palaeocene-Eocene global warming

Among the most urgent challenges in future climate change scenarios is accurately predicting the magnitude to which precipitation extremes will intensify. Analogous changes have been reported for an episode of millennial-scale 5 °C warming, termed the Palaeocene-Eocene Thermal Maximum (PETM; 56 Ma), providing independent constraints on hydrological response to global warming. However, quantifying hydrologic extremes during geologic global warming analogs has proven difficult. Here we show that water discharge increased by at least 1.35 and potentially up to 14 times during the early phase of the PETM in northern Spain. We base these estimates on analyses of channel dimensions, sediment grain size, and palaeochannel gradients across the early PETM, which is regionally marked by an abrupt transition from overbank palaeosol deposits to conglomeratic fluvial sequences. We infer that extreme floods and channel mobility quickly denuded surrounding soil-mantled landscapes, plausibly enhanced by regional vegetation decline, and exported enormous quantities of terrigenous material towards the ocean. These results support hypotheses that extreme rainfall events and associated risks of flooding increase with global warming at similar, but potentially at much higher, magnitudes than currently predicted.

Landscape ‘stress’ and reorganization from χ-maps: Insights from experimental drainage networks in oblique collision setting: LANDSCAPE STRESS AND REORGANIZATION FROM χ-MAPS

Several recent studies have suggested that maps of flow length normalized for drainage area called chi (χ) could reveal landscapes in a transient state, which are prone to reorganizations of basin geometry, flow line topology and water divide locations. However, the potentially long timescales associated with the evolution of basin geometry make the capability of χ to predict such reorganization challenging to test in natural settings. Here, we investigate the evolution of experimental drainage networks developed on a wedge coupled to a piedmont and growing in oblique convergence. We use this experimental setting to investigate the relationships between χ maps, the imposed tectonic deformation and the drainage network evolution. As deposition can occur within channels or in the piedmont, our experimental streams deviate from purely bedrock channels for which the χ metric has been initially developed. Yet we show that the large‐scale χ pattern of the experimental drainage network is consistent with the imposed deformation field, as ∼2/3 of the observed χ gradients across water divide are oriented in the expected direction with respect to the imposed deformation. This suggests that χ maps can be used to infer the horizontal component of regional deformation in large‐scale natural mountainous fluvial landscapes. In addition, we observe that when a divide affected by a χ gradient migrates, the orientation of the gradient correctly anticipates the sense of landscape reorganization for ∼2/3 of these divides.