Transport of Agricultural Chemicals: Estimating Lag Times in Different Hydrologic Environments
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Transport of Agricultural Chemicals:
Estimating Lag Times in Different Hydrologic Environments
David Wolock and Paul CapelU.S. Geological Survey
U.S. Department of the InteriorU.S. Geological Survey
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Objectives• 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
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Agricultural Chemicals Team study
Nebraska
Maryland
Indiana
Transport and lag time vary in different hydrologic systems
Overland flow
Tile-drain flow Ground-water discharge
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Nebraska site
Overland-flow-dominated streamflow
Basin area = 1.5 km2
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Overland-flow-dominated streamflow
Nebraska site
Typical flow event lasts 1-2 hours.
Stre
amflo
w (
ft3 /s
)
13 18 23 28 3 8 13June-July, 2003
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• 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
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Tile-drain-dominated streamflow
Leary-Weber Ditch, central IndianaBasin area = 7 km2
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02468
1012
160 184 208 232 256 280 304 328 352 376
Stre
amflo
w (c
fs),
Ti
le-d
rain
flow
(gal
/min
) 0246810121416 Pr
ecip
itatio
n (m
m)
Streamflow Tile drain flow Precipitation
12 13 14 15 16 17 18 19 20June 2003
Typical flow event lasts 24-48 hours.
Stre
amflo
w
(ft3
/s),
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• 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
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Maryland site: Ground-water discharge dominated streamflow
Morgan CreekBasin area = 29 km2
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Ground-water discharge dominated streamflow
0
2
4
6
8
10
12
14
16
18
20
10/1/2000 12/1/2000 2/1/2001 4/1/2001 6/1/2001 8/1/2001
Dis
char
ge (m
m)
0
10
20
30
40
50
60
70
80
90
100
Prec
ipita
tion
(mm
)
DischargePrecipitation
Ground-water discharge to stream is continuous.
Stre
amflo
w (
mm
/d)
Prec
ipit
atio
n (m
m/d
)
Base-flow index = 70%
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Assume ground-water flow velocity is 0.1 m/day.
• 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.
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Hydrologic differences
• Lag time
• Degradation potential
• Fraction of basin contributing to streamflow
• Age of streamflow
Hours
Low
Small
Young
Hours
Low
Large
Young
Decades
High
Large
Old
Nebraska:Overland flow
Indiana:Tile drains
Maryland:
Ground water
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Agricultural Chemical Management Implications
• Response to changes in chemical applications
• Minimum tillage effective?
• Buffer zones near stream effective?
Quick
Yes
Yes
Quick
No
No
Slow
No
No
Nebraska:Overland flow
Indiana:Tile drains
Maryland:Ground water
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Bottom line: Hydrology matters
for managing agricultural chemicals
Nebraska
Maryland
Indiana
Overland flow
Tile-drain flow Ground-water discharge