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

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Understanding flow pathways and mechanisms that generate streamflow is important to understanding agrochemical contamination in surface waters in agricultural watersheds. Two environmental tracers, δ¹⁸O and electrical conductivity (EC), were monitored in tile drainage (draining 12 ha) and stream water (draining nested catchments of 6-5700 ha) from 2000 to 2008 in the semi-arid agricultural Missouri Flat Creek (MFC) watershed, near Pullman Washington, USA. Tile drainage and streamflow generated in the watershed were found to have baseline δ¹⁸O value of -14·7[per thousand] (VSMOW) year round. Winter precipitation accounted for 67% of total annual precipitation and was found to dominate streamflow, tile drainage, and groundwater recharge. 'Old' and 'new' water partitioning in streamflow were not identifiable using δ¹⁸O, but seasonal shifts of nitrate-corrected EC suggest that deep soil pathways primarily generated summer streamflow (mean EC 250 μS/cm) while shallow soil pathways dominated streamflow generation during winter (EC declining as low as 100 μS/cm). Using summer isotopic and EC excursions from tile drainage in larger catchment (4700-5700 ha) stream waters, summer in-stream evaporation fractions were estimated to be from 20% to 40%, with the greatest evaporation occurring from August to October. Seasonal watershed and environmental tracer dynamics in the MFC watershed appeared to be similar to those at larger watershed scales in the Palouse River basin. A 0·9[per thousand] enrichment, in shallow groundwater drained to streams (tile drainage and soil seepage), of δ¹⁸O values from 2000 to 2008 may be evidence of altered precipitation conditions due to the Pacific Decadal Oscillation (PDO) in the Inland Northwest.
Moravec, Bryan G. , Keller, C. Kent , Smith, Jeffrey L. , Allen-King, Richelle M. , Goodwin, Angela J. , Fairley, Jerry P. , Larson, Peter B.
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Hydrological processes 2010 Feb. 15, v. 24, no. 4
John Wiley & Sons, Ltd.
Journal Articles, USDA Authors, Peer-Reviewed
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