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Ecohydrologic separation of water between trees and streams in a Mediterranean climate


Water movement in upland humid watersheds from the soil surface to the stream is often described using the concept of translatory flow1,2, which assumes that water entering the soil as precipitation displaces the water that was present previously, pushing it deeper into the soil and eventually into the stream2. Within this framework, water at any soil depth is well mixed and plants extract the same water that eventually enters the stream. Here we present water-isotope data from various pools throughout a small watershed in the Cascade Mountains, Oregon, USA. Our data imply that a pool of tightly bound water that is retained in the soil and used by trees does not participate in translatory flow, mix with mobile water or enter the stream. Instead, water from initial rainfall events after rainless summers is locked into small pores with low matric potential until transpiration empties these pores during following dry summers. Winter rainfall does not displace this tightly bound water. As transpiration and stormflow are out of phase in the Mediterranean climate of our study site, two separate sets of water bodies with different isotopic characteristics exist in trees and streams. We conclude that complete mixing of water within the soil cannot be assumed for similar hydroclimatic regimes as has been done in the past3,4.

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Figure 1: Water isotopes (δ18O and δ2H) of bulk soil water, xylem water, stream and estimated annual average precipitation.
Figure 2: Hydrologic dynamics during autumn 2006.
Figure 3: Time course of the first large precipitation event in October 2006.
Figure 4: Conceptual model for water resource separation in a Mediterranean climate.

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This work was supported by the US Environmental Protection Agency. This manuscript has been subjected to the Environmental Protection Agency’s peer and administrative review, and it has been approved for publication as an EPA document. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. This research was conducted at the H. J. Andrews Experimental Forest, and we thank USDA Forest Service Pacific Northwest Research Station, Oregon State University and the Willamette National Forest for their support of this research facility. H.B. thanks the Oregon State University Institute for Water and Watersheds and the Ford Foundation for financial support. We thank G. Heinz and M. Johnson for help in field work, W. Rugh and K. Rodecap for isotopic analysis, and J. Selker, D. Williams, L. Hopp and R. Ozretich for helpful comments on an earlier draft of this manuscript.

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J.R.B. designed the research plan, obtained financial support, analysed and interpreted the data and wrote the paper. H.B. participated in data collection, analysis and interpretation, and contributed to the writing of the paper. R.C. coordinated sample collection efforts, led field crews and ensured the data quality of all field collections. J.M. advised throughout, assisted with interpretation of the results and contributed to the writing of the paper.

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Correspondence to J. Renée Brooks.

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Renée Brooks, J., Barnard, H., Coulombe, R. et al. Ecohydrologic separation of water between trees and streams in a Mediterranean climate. Nature Geosci 3, 100–104 (2010).

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