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 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

dwolock@usgs.gov

capel@usgs.gov