“Water worlds” and nutrient management policies: recent isotope hydrology studies may have significant policy implications

18 Feb “Water worlds” and nutrient management policies: recent isotope hydrology studies may have significant policy implications

“Water worlds” and nutrient management policies: recent isotope hydrology studies may have significant policy implications

Jeffrey McDonell, a hydrologist from Oregon State University and this year’s Birdsall Dreiss Distinguished Lecturer, was recently in Alberta presenting on recent research in isotope hydrology.   It quickly became apparent how isotope hydrology has changed our understanding of hydrology and also how much policy may have to play catch up with science if some recent findings are found to broadly apply.  Isotope hydrology uses isotopes of oxygen (Oxygen-18 and deuterium specifically) to look at how water acts after rain events and how these precipitation events contribute to stream flows.

The implications of a recent study in Nature Geoscience “Ecohydrolgic separation of water between trees and streams in a Mediterranean climate” (J. Renée Brooks et al. 2009) are particularly intriguing.  (See the links below for more interesting isotope hydrology research.)  The study raises questions about the concept of translatory flow, 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 stream” (Brooks et al. at 1).  Data from a study in the Cascade Mountains of Oregon, however,

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 and 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.

The study contemplates that “soil is separated into two water worlds:  mobile water, which eventually enters the stream, and tightly bound water that is used by plants” (Brooks et al. at 4).

This parceling of water worlds between geological and ecological water has some significant implications for “biogeochemical cycling and transport of nutrients to streams” (Brooks et al. at 4).   Indeed, current nutrient management policies appear to be generally based on the assumption of a translator flow involving ecological water, whereby nutrients management services can be relied upon throughout the growing season.   The resulting dual water world, if applicable in the Alberta context, has implications for both the study of nutrient transport and the efficacy of nutrient (and contaminant) management policies.    For instance, the timing of nutrient applications may have to be significantly altered if the pore spaces that constitute the ecological water (i.e. tightly bound water) are full.

The study notes “this conceptual framework requires further testing to see if the underlying mechanisms are true, and if such separation of water resources holds for different climates and locations” (Brooks et al. at 4).   It may be that “tightly bound” bound water pore spaces in some of Alberta, in its semi-arid state, remain largely empty much of the year, in which case the implications of this research for nutrient management may be minimized. (The potential and/or extent of nutrient interaction between the water worlds is another issues that comes to mind). Nevertheless, as the Alberta Government attempts to get a greater understanding of groundwater resources and pursues more effective environmental policies, there is a need for increased and continued commitment to hydrological research in the province.  Similarly there is a need for rapid regulatory and policies responses to our increased scientific knowledge.

Recent policy paralysis around water quantity issues, particularly related to water allocation (in the South Saskatchewan River Basin) and wetlands, and cumulative water quality issues (particularly in relation to eutrophication of surface waters) reflects a failure to respond to our scientific knowledge.   If we are serious about environmental protection and adaptive management we need to start practicing what we preach.

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Other hydrological studies of interest relate to the residence time of water in watersheds — “Truncation of stream residence time:  How the use of stable isotopes has skewed our concept of streamwater age and origin” Michale K. Stewart, Uwe Morgenstern and Jeffrey J. McDonnell, ) and on the hydrological function of bedrock “Hillslop threshold response to rainfall: (1) A field based forensic approach” .

Also see Prof. Jeff Mcdonnell’s (University of Oregon) Hillslope and Watershed Hydrology Lab website at http://www.cof.orst.edu/cof/fe/watershd/index.php .

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