title: Fluvial sediment supply to a mega-delta reduced by shifting tropical-cyclone activity.
abstract: The world's rivers deliver 19 billion tonnes of sediment to the coastal zone annually, with a considerable fraction being sequestered in large deltas, home to over 500 million people. Most (more than 70 per cent) large deltas are under threat from a combination of rising sea levels, ground surface subsidence and anthropogenic sediment trapping, and a sustainable supply of fluvial sediment is therefore critical to prevent deltas being 'drowned' by rising relative sea levels. Here we combine suspended sediment load data from the Mekong River with hydrological model simulations to isolate the role of tropical cyclones in transmitting suspended sediment to one of the world's great deltas. We demonstrate that spatial variations in the Mekong's suspended sediment load are correlated (r = 0.765, P < 0.1) with observed variations in tropical-cyclone climatology, and that a substantial portion (32 per cent) of the suspended sediment load reaching the delta is delivered by runoff generated by rainfall associated with tropical cyclones. Furthermore, we estimate that the suspended load to the delta has declined by 52.6 ± 10.2 megatonnes over recent years (1981-2005), of which 33.0 ± 7.1 megatonnes is due to a shift in tropical-cyclone climatology. Consequently, tropical cyclones have a key role in controlling the magnitude of, and variability in, transmission of suspended sediment to the coast. It is likely that anthropogenic sediment trapping in upstream reservoirs is a dominant factor in explaining past, and anticipating future, declines in suspended sediment loads reaching the world's major deltas. However, our study shows that changes in tropical-cyclone climatology affect trends in fluvial suspended sediment loads and thus are also key to fully assessing the risk posed to vulnerable coastal systems.
authors.affiliation: Geography and Environment, University of Southampton, Southampton SO17 1BJ, UK.
authors.forename: Stephen E
authors.initials: SE
authors.lastname: Darby
authors.affiliation: Geography and Environment, University of Southampton, Southampton SO17 1BJ, UK.
authors.forename: Christopher R
authors.initials: CR
authors.lastname: Hackney
authors.affiliation: Geography and Environment, University of Southampton, Southampton SO17 1BJ, UK.
authors.forename: Julian
authors.initials: J
authors.lastname: Leyland
authors.affiliation: Water and Development Research Group, Aalto University, 02150 Espoo, Finland.
authors.forename: Matti
authors.initials: M
authors.lastname: Kummu
authors.affiliation: EIA Finland Ltd, Sinimäentie 10B, 02630 Espoo, Finland.
authors.forename: Hannu
authors.initials: H
authors.lastname: Lauri
authors.affiliation: Department of Geography, Environment and Earth Sciences, University of Hull, Hull HU6 7RX, UK.
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Parsons
authors.affiliation: Departments of Geology, Geography &GIS, Mechanical Science and Engineering and Ven Te Chow Hydrosystems Laboratory, University of Illinois, Champaign, Illinois 61820, USA.
authors.forename: James L
authors.initials: JL
authors.lastname: Best
authors.affiliation: Department of Geography, University of Exeter, Exeter EX4 4RJ, UK.
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Nicholas
authors.affiliation: Department of Geography, University of Exeter, Exeter EX4 4RJ, UK.
authors.forename: Rolf
authors.initials: R
authors.lastname: Aalto
mesh_terms: D001210:Asia, Southeastern; D057231:Climate Change; D055867:Cyclonic Storms / Q000266:history / Q000706:statistics & numerical data*; D019015:Geologic Sediments / Q000032:analysis*; D049673:History, 20th Century; D049674:History, 21st Century; D062070:Hydrology; D011891:Rain; D045483:Rivers / Q000737:chemistry*; D014329:Tropical Climate