The Bren School of Environmental Science & Management
at the University of California, Santa Barbara

Presents

A RESEARCH COLLOQUIUM

"The two water worlds paradox: Isotope evidence that trees and streams
return different water pools to the hydrosphere"

Hosted by Christina Tague

Abstract

Hydrologists invoke translatory (piston-like) flow to describe water movement in upland, humid watersheds from the soil surface to the stream. Water entering as precipitation displaces old water, pushing it deeper into soil and eventually the stream. Within this paradigm, water at any soil depth is well mixed. Plants passively take up water along a water-potential gradient and preferentially extract water at high water potentials, implying that plants extract the same water that eventually enters the stream. Here I show recent work done together with Renee Brooks at EPA Corvallis and colleagues at OSU that focuses on water-isotope data from various pools collected throughout a small watershed at the HJ Andrews LTER site in western Oregon. These data imply that tightly bound water retained in this soil and used by trees does not participate in translatory flow, mix with mobile water, or enter the stream. Following rainless summers, initial fall rain events are locked into small pores with low matric potential until transpiration empties these pores during dry summers. Winter rainfall does not displace the tightly bound water. Because transpiration and stormflow are out of phase in our Mediterranean climate, two water worlds exist where trees and streams return different water to the hydrosphere. For similar hydroclimate regimes, these findings have profound implications for hydrological and biogeochemical modeling, which assume complete mixing of water within soil.

Biography

Jeffrey J. McDonnell is Richardson Chair in Watershed Science and University Distinguished Professor of Hydrology at Oregon State University. He is a Fellow of the American Geophysical Union and a recipient of the Dalton Medal from the European Geophysical Union. He has received the Gordon Warwick Award from the British
Geomorphological Research Group, the Nystrom Award from the Association of American Geographers and a DSc from the University of Canterbury. He is a Fellow of the International Water Academy and has delivered the Woo Lecture to the Canadian Geophysical Union, the Penman Lecture to the British Hydrological Society, the Boussinesq Lecture to the Dutch Boussinesq Centre and the Frontier Lecture to the American Geophysical Union. Professor McDonnell is a registered professional hydrologist with the American Institute of Hydrology and has co-authored approximately 150 journal articles on watershed hydrology, and co-edited the Elsevier textbook Isotope Tracers in Catchment Hydrology. He has served as Senior Advisory Editor of the Encyclopedia of Hydrological Sciences, published by John Wiley and Sons, and is currently editor-in-chief of the IAHS Book Series “Benchmark Papers in Hydrology.” Professor McDonnell is currently Visiting Professor at the Nanjing Hydraulic Research Institute and serves as Editorial Board member for the journals Ecohydrology, Hydrological Processes, Hydrology and Earth Systems Science, and the Geography Compass.

NOTE: Research colloquia are hosted by Bren faculty members and are generally high-level talks about research in a particular area of environmental science and management.

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