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Transport of Agricultural Chemicals: Estimating Lag Times in Different Hydrologic Environments

Transport of Agricultural Chemicals: Estimating Lag Times in Different Hydrologic Environments David Wolock and Paul Capel U.S. Geological Survey. U.S. Department of the Interior U.S. Geological Survey. Quantify lag time for three of the Agricultural Chemicals Team study areas

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Transport of Agricultural Chemicals: Estimating Lag Times in Different Hydrologic Environments

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  1. Transport of Agricultural Chemicals: Estimating Lag Times in Different Hydrologic Environments David Wolock and Paul Capel U.S. Geological Survey U.S. Department of the Interior U.S. Geological Survey

  2. Quantify lag time for three of the Agricultural Chemicals Team study areas Lag time is the travel time of water and chemicals from the land surface to the stream Show implications of hydrology for agricultural chemical management Objectives

  3. Agricultural Chemicals Team study Overland flow Nebraska Transport and lag time vary in different hydrologic systems Tile-drain flow Ground-water discharge Indiana Maryland

  4. Overland-flow-dominated streamflow Nebraska site Basin area = 1.5 km2

  5. Overland-flow-dominated streamflow Nebraska site Streamflow (ft3/s) 3 13 18 23 28 13 8 June-July, 2003 Typical flow event lasts 1-2 hours.

  6. Assume overland flow velocity is 200 m/hr • Typical flow event lasts 1-2 hours • About 10% of the watershed has a travel time < 2 hours

  7. Tile-drain-dominated streamflow Basin area = 7 km2 Leary-Weber Ditch, central Indiana

  8. Streamflow (ft3/s), 12 13 14 15 16 17 18 19 20 June 2003 Typical flow event lasts 24-48 hours.

  9. Assume rapid subsurface flow velocity is 20 m/hr • Typical flow event lasts 24-48 hours • The entire watershed has a travel time < 48 hours

  10. Maryland site: Ground-water discharge dominated streamflow Basin area = 29 km2 Morgan Creek

  11. Ground-water discharge dominated streamflow Base-flow index = 70% Streamflow (mm/d) Precipitation (mm/d) Ground-water discharge to stream is continuous.

  12. Assume ground-water flow velocity is 0.1 m/day • Ground-water discharge to stream is continuous. • The entire watershed is a source of streamflow. Assume ground-water flow velocity is 0.1 m/day.

  13. Lag time Degradation potential Fraction of basin contributing to streamflow Age of streamflow Hours Low Small Young Hydrologic differences Nebraska: Overland flow Indiana: Tile drains Maryland: Ground water Hours Low Large Young Decades High Large Old

  14. Response to changes in chemical applications Minimum tillage effective? Buffer zones near stream effective? Quick Yes Yes Agricultural Chemical Management Implications Nebraska: Overland flow Indiana: Tile drains Maryland: Ground water Slow No No Quick No No

  15. Bottom line: Hydrology matters for managing agricultural chemicals Overland flow Nebraska Tile-drain flow Ground-water discharge Indiana Maryland

  16. dwolock@usgs.govcapel@usgs.gov

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