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 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 - PowerPoint PPT PresentationTRANSCRIPT
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
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
Agricultural Chemicals Team study
Nebraska
Maryland
Indiana
Transport and lag time vary in different hydrologic systems
Overland flow
Tile-drain flow Ground-water discharge
Nebraska site
Overland-flow-dominated streamflow
Basin area = 1.5 km2
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
• 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
Tile-drain-dominated streamflow
Leary-Weber Ditch, central IndianaBasin area = 7 km2
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),
• 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
Maryland site: Ground-water discharge dominated streamflow
Morgan CreekBasin area = 29 km2
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%
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.
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
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
Bottom line: Hydrology matters
for managing agricultural chemicals
Nebraska
Maryland
Indiana
Overland flow
Tile-drain flow Ground-water discharge
