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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2009–5030

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Summary and Conclusions

Hydrologic and water-chemistry data from several environmental compartments were used to develop an understanding of water flow and solute transport in a small, irrigated agricultural catchment. An inverse end-member mixing analysis then was used to gain a fuller understanding of solute sources and transport in the catchment. Considering the variation in crops and irrigation methods both across the catchment and over time, this approach indicates that during the study period, the net effect of processes in the catchment as a whole produced similar concentrations of most major solutes as the net effect of processes in the single field from which drainage was sampled. Data from two sampling transects in the streambed provided evidence that ground-water flow from different source areas converges to discharge to the surface near the catchment outlet, and provided insight into differences between the chemistry of relatively deep ground water that discharges upward into the stream and that of shallow ground water that flows laterally into the stream.

This work indicates that combining irrigation and artificial-drainage networks may exacerbate the ecological effects of agricultural runoff by increasing direct connectivity between fields and streams and minimizing potentially mitigating effects of longer subsurface pathways such as denitrification and dilution.

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