H24D-08
Young and old water in global rivers and aquifers
Tuesday, 15 December 2015: 17:45
3024 (Moscone West)
Scott Jasechko1, James W Kirchner2, Jeffrey McDonnell3, Thomas P Gleeson4, Kevin M Befus5, Elco Luijendijk6, M. Bayani Cardenas5, Yoshihide Wada7 and Jeffrey M Welker8, (1)University of Calgary, Geography, Calgary, AB, Canada, (2)ETH Swiss Federal Institute of Technology Zurich, Zurich, Switzerland, (3)University of Saskatchewan, Saskatoon, SK, Canada, (4)University of Victoria, Victoria, BC, Canada, (5)University of Texas at Austin, Austin, TX, United States, (6)Georg-August-Universitaet Goettingen, Goettingen, Germany, (7)Utrecht University, Department of Physcial Geography, Utrecht, Netherlands, (8)University of Alaska Anchorage, Anchorage, AK, United States
Abstract:
The fate of solutes, nutrients and contaminants are regulated by the time that precipitation takes to travel through landscapes to reach surface waters and aquifers. Water samples collected from a stream or a groundwater well are a mixture of younger and older precipitation inputs. However, the global 3D distribution of younger versus older water flowing in rivers or stored in groundwater aquifers is not known, in part due to a longstanding focus on average age rather than age distributions. Here we analyze global rain, snow, groundwater and streamflow isotope contents, compiled from primary literature sources or specialist databases. Instead of calculating average water ages, we use the isotope data to partition fractions of younger versus older water in 260 rivers and 202 aquifers. For global rivers, we show that precipitation reaching the stream in less than 1.5-3 months generates a substantial fraction (~35%) of global runoff and constitutes an important component (>5%) of streamflow draining the great majority (90%) of watersheds. We also show that ~35% of global runoff is generated by a microscopic fraction (<0.01%) of global groundwater storage, meaning that biogeochemical processes taking place in these aquifer-stream connectivity hotspots will have disproportionately large impacts on stream water quality. By contrast, radiocarbon dating shows that most (>50%) groundwaters are relicts of ancient climates, having recharged their aquifers prior to the current Holocene epoch. Our study, that partitions both surface- and ground-water ages, shows that much of global streamflow is at least four orders of magnitude younger than most of global groundwater storage, highlighting that most stream water is far younger than most groundwater stored in their catchments.
