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Title: Oxygen-18 dynamics in precipitation and streamflow in a semi-arid agricultural watershed, Eastern Washington

Author
item MORAVEC, B. - WASHINGTON STATE UNIV
item KELLER, C. - WASHINGTON STATE UNIV
item Smith, Jeffrey
item GOODWIN, A. - WASHINGTON STATE UNIV
item FAIRLEY, J - UNIVERSITY OF IDAHO
item ALLEN-KING, R. - UNIVERSITY OF BUFFALO

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/25/2009
Publication Date: 12/8/2009
Citation: Moravec, B.G., Keller, C.K., Smith, J.L., Goodwin, A.J., Fairley, J., Allen-King, R.M. 2009. Oxygen-18 dynamics in precipitation and streamflow in a semi-arid agricultural watershed, Eastern Washington. Hydrological Processes. 24:446-460 (2010). DOI: 10.1002/hyp.7515

Interpretive Summary: Stable isotopes of water (2H and oxygen-18) have been useful tools in watershed studies over the past 40 years, and have helped hydrologists gain insight into soil and surface water dynamics within watersheds. By combining stable isotope hydrology with water geochemistry, a better understanding of the hydrologic processes that shape intra-watershed dynamics can be achieved. Using a chemical measurement (electrical conductivity-EC) we found that seasonal shifts of nitrate-corrected EC suggest that shallow soil pathways contributed significantly to streamflow generation during winter, while deep soil pathways primarily generated summer streamflow. This allowed us to calculate the efficiency of the area to retain yearly precipitation which ranged from 40 to 70%. We also found that the greatest water evaporation from the area occurred from August to October. Seasonal watershed dynamics and streamflow generation in the watershed may represent these processes at larger watershed scales in the Palouse River basin. Hydrologists now have a better understanding of the importance of seasonal watershed dynamics and streamflow generation that govern hydrologic processes in watersheds.

Technical Abstract: Understanding flow pathways and mechanisms that generate streamflow is important to understanding agrochemical contamination in surface waters in agricultural watersheds. Two environmental tracers, oxygen-18 and electrical conductivity (EC), were monitored in tile drainage (draining 12 ha) and stream water (draining 660 – 5700 ha) from 2000 to 2008 in the semi-arid agricultural Missouri Flat Creek watershed, near Pullman Washington, USA. Tile drainage and streamflow generated in the watershed were found to have baseline oxygen-18 of –14.7‰ year round (which only changed during episodic events). Winter precipitation accounted for 67% of total annual precipitation and was found to be the primary contribution to streamflow and tile drainage, while summer precipitation did not contribute appreciably. “Old” and “new” water partitioning in streamflow was not identifiable using oxygen-18, but seasonal shifts of nitrate-corrected EC suggest that shallow soil pathways contributed significantly to streamflow generation during winter (mean EC 200 'S/cm), while deep soil pathways primarily generated summer streamflow (mean EC 250 'S/cm). Using summer isotopic and EC excursions of large-catchment stream waters from tile drainage, summer in-stream evaporation fractions were estimated to be from 20% to 40%, with the highest evaporation occurring from August to October. Seasonal watershed dynamics and streamflow generation in the MFC watershed may represent these processes at larger watershed scales in the Palouse River basin. A steady 0.9‰ enrichment in soil water oxygen-18 from 2000 to 2008 may be evidence of the hydrologic influence of the Pacific Decadal Oscillation in the Inland Northwest.