Soil Hydrology/Preferential Flow Impacts - Mark Williams, USDA-ARS, from the 2020 Conservation Tillage and Technology Conference, held March 3-4, 2020, Ada, OH, USA.

1. Truths and Myths of Preferential
Flow to Tile Drains
Mark Williams - USDA ARS NSERL, West Lafayette, IN

2. Why do we care about preferential flow?
Tile drain

3. Who’s to blame – cracks or holes?
We don’t know enough about preferential flow
processes to debate the differences (my opinion)

4. Today’s objectives:
1. We aren’t going to debate differences between cracks and holes
2. We aren’t going to discuss nutrient transport – WATER ONLY
3. We are going to use lab, plot, and field study results to evaluate
commonly discussed characteristics of preferential flow and
related processes

5. TRUE OR FALSE
Preferential flow occurs
in most soils
Question #1

6. “The fact that preferential flow in soil is such a widespread
phenomenon suggests that a continuous network of fast flow
pathways exists in most soils, at least … within the depth of
the vadose zone that is influenced by structure-forming
processes (i.e., the depth influenced by soil wetting-drying,
biological activity, and plant rooting depth)”
Jarvis et al. 2012

7. Author Location n Tracer New water (%)
Range Average
Cey et al. (1998) Canada (Ontario) 2 EC 20 – 36 28.0
Chikhaoul et al. (2008) Canada (Quebec) 2 EC 20 – 70 45.0
Cullum (2009) U.S. (Mississippi) 8 Br- 17 – 69 33.9
Everts & Kanwar (1990) U.S. (Iowa) 2 Br- 1 – 2 1.5
Granger et al. (2010) England 2 δ2H 18 – 25 21.5
Heppell & Chapman (2008) England 4 EC 5 – 35 16.3
Klaus et al. (2013) Germany 3 Br- 12 – 13 12.5
Kumar et al. (1997) U.S. (Iowa) 22 n/a 0 – 61 13.3
Schilling & Helmers (2008) U.S. (Iowa) 2 EC, Cl- 25 – 41 33.0
Smith & Capel (2018) U.S. (Iowa) 20 EC 11 – 75 31.0
Stone & Wilson (2006) U.S. (Indiana) 2 Cl- 11 – 51 31.0
Vidon & Cuadra (2010) U.S. (Indiana) 6 EC 11 – 50 30.2
Vidon et al. (2012) U.S. (Indiana) 7 Cations 37 – 68 51.1
Williams et al. (2016) U.S. (Ohio) 14 δ18O 26 – 69 44.5
Zajicek et al. (2011) Czech Republic 6 Temperature 3 – 73 30.4
Zajicek et al. (2016) Czech Republic 18 δ18O 1 – 58 28.0
Summary 0 – 75 (Avg.) 28.2
Observed preferential flow to tile drains
from field-scale studies

8. TRUE OR FALSE
Preferential flow occurs
in most soils
TRUE

9. TRUE OR FALSE
Water transported to tile
drains in cracks or holes
is solely surface runoff
Question #2

10. Collecting undisturbed lysimeters
Undisturbed soil
12 x 12 x 12 in boxes
3/8 in plastic spacer
Silt loam (Ap)
0-8 in
Silty clay loam (Bt)
8-12 in

11. Measuring natural and artificial preferential flow paths
Matrix flow ‘Natural’ preferential flow
Preferential edge-flow
Bottom plate
Inner flow
Outer flow (1 in)

12. We saturated the boxes (10 total) from
the bottom up and let them drain to field
capacity
We applied a bromide tracer to the
surface of all of the boxes
Conducted rainfall simulation: 1 h event
(1.25 in) and collected samples every 5
min during event and up to 24 h after
the event
Experimental procedure

13. 0
5
10
15
20
25
30
35
40
0 100 200 300 400
0
5
10
15
20
25
30
35
40
0 100 200 300 400
Time (min)
Bulk soil collector
Edge collector
0
200
400
600
800
0
5
10
15
20
25
30
35
40
0 100 200 300 400
Box 3
Flowrate(mL/min)
0
200
400
600
800
0
5
10
15
20
25
30
35
40
0 100 200 300 400
Box 9
Flow rate
Bromide
X
Bulk soil collector
Edge collector

14. Bromide(mg/L)
Fraction of sample comprised of Rainfall
X
Bulk soil
Edge collector
The rapid transport (10-15 min) of high bromide concentrations
from the soil surface through 12 inches of soil…
…was not associated with rain water, but displaced soil water
0
200
400
600
800
0 0.2 0.4 0.6 0.8 1

15. TRUE OR FALSE
Water transported to tile
drains in cracks or holes
is solely surface runoff
FALSE

16. TRUE OR FALSE
Tillage decreases the
amount of preferential
flow to tile drains
Question #3

17. TD2
TD1
0 50 100
meters
Drainage area
Tile outlet
Rain gauge
Ohio, USA
UBWC
Site description
Drainage area: 4 – 5 ha
Soil type: Silt loam
Tile depth: 1.0 m
Tillage: No-till (30+ years)
Tilled field TD1 (disk)
TD2 remained no-tilled
Sampling strategy
Hourly water samples during events
2 events prior to tillage
5 events after tillage

18. 0
10
20
30
40
50
60
70
80
1 2 3 4 5 6 7
Eventwater(%)
Effect of tillage on preferential flow
Tillage did not impact the proportion of event water
(i.e., rain water) in tile discharge
Storm event
Tilled field
No-till field
Pre-tillage Post-tillage

19. 4 field sites with 3 plots in each field
- 2 fields were long-term no-till; 2 fields were rotational tillage
Three tillage/fertilizer treatments:
a. Broadcast (no-till)
b. Inject (slits 3-4 in deep, 30 in apart)
c. Till (tillage 3-4 in deep)
Rainfall simulation: 1.5 in of rain over 90 min

20. 0
10
20
30
40
50
Broadcast Inject Till
Effect of tillage on preferential flow
Eventwater(%)
Tillage did not impact the proportion of event water
(i.e., rain water) in leachate

21. Why don’t we see an effect?
Most “conservation” tillage only mixes the
soil up to 3-4 inches deep
- Mohler et al. (2006)
Dye tracing on no-till clay soils revealed
that matrix flow is generally the dominant
flow pathway within the top 3-4 inches of
the soil profile, with preferential flow
patterns observed at lower depths
- Grant et al. (2019)
36” to tile
Blue/green colors represent matrix flow
Red/orange colors represent preferential flow

22. TRUE OR FALSE
Tillage decreases the
amount of preferential
flow to tile drains
FALSE

23. TRUE OR FALSE
Preferential flow is
always greater when
field conditions are dry
Question #4

24. Creating artificial preferential flow paths
Tile drain
Soil
0.3 in diameter flow path
WHY?
- Only matrix flow through soil
- Only one preferential flow path

25. Experimental procedure
We conducted 3 sequential 1-hr.
rainfall simulations (0.75 in/hr)
Rainfall #1
15 min break
Rainfall #2
75 min break
Rainfall #3
Collected samples every 5 min during
each event and up to 24 h after the
last event

26. Effect of antecedent wetness on
preferential flow
0
0.2
0.4
0.6
0.8
1
1 2 3
DischargetoPrecipitation
Ratio(Q/P)
Rainfall
0
0.2
0.4
0.6
0.8
1
1 2 3
Increasing wetness
Rain water
Soil water
94% 76% 29%
Perspective 1:
The amount of rainfall in tile flow decreases
with increasing wetness -> 94% to 29%
Thus, preferential flow of rainfall is greater
under dry antecedent conditions
Perspective 2:
The amount of rainfall discharged relative
to total rainfall is constant -> 23% to 30%
Thus, preferential flow of rainfall is similar
across antecedent conditions
23% 30% 27%

27. Effect of antecedent wetness on
preferential flow
0
0.2
0.4
0.6
0.8
1
1 2
DischargetoPrecipitation
Ratio(Q/P)
Rainfall
Field capacity 28 days of drying
Soil conditions prior to rainfall simulation
Rain water
Soil water
Similar response for both repacked and
undisturbed soil boxes
Contributions of rain water are similar across
wetness conditions with constant rainfall intensity

28. What happens in the field?
TD2
TD1
0 50 100
meters
Drainage area
Tile outlet
Rain gauge
Ohio, USA
UBWC
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80
DischargetoPrecipitation
Ratio(Q/P)
7-d antecedent condition + rainfall
April - June
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80
Increasing wetness
x
Q/P
Qe/P
Contributions of rain water vary by
storm event due to differences in rainfall
intensity, but soil/groundwater still
accounts for the bulk of increasing flow

29. My hypothesis…
Antecedent conditions + rainfall amount control tile flow generation
(threshold-type response)
Small flow events could be either antecedent-controlled or intensity-
controlled resulting a varying rain water contributions – tile flow could be
comprised of 0% rainfall up to 100% rainfall
Large events are generally antecedent-controlled resulting less variable rain
water contributions (i.e., lower uncertainty)
Small flow events
Antecedent-controlled
Tile water is 0% rainfall
Small spring storm with
wet field conditions
Intensity-controlled
Tile water is 100% rainfall
Intense summer storm
with dry field conditions
Or anything in between
Large flow events
Antecedent-controlled
Tile water is 20-40% rainfall (a
narrow range relative to small flow
events)

30. TRUE OR FALSE
Preferential flow is
always greater when
field conditions are dry
FALSE

31. What we’ve learned today
Preferential flow occurs in most soils
Preferential flow isn’t just surface runoff
- Fast flow paths can transport runoff, soil water, and/or groundwater
Tillage does not typically decrease the amount of rainfall transport in
preferential flow paths
- Most water movement in the top 3-4 inches is matrix flow
Dry antecedent conditions don’t always lead to more preferential flow
- Increases in tile flow with increases in wetness are the largely the
result of soil water/groundwater mobilization

32. Why didn’t we talk about phosphorus?
Most studies reporting a link between phosphorus and preferential flow
are anecdotal observations
Those studies that have measured both phosphorus and preferential
flow are almost always associated with a fertilizer application
…there is very little data from lab experiments or field studies that
include both sources of water (i.e., tracer studies) and phosphorus
concentrations from soils that weren’t recently fertilized

33. Contact Information
Mark Williams
USDA-ARS NSERL
275 S. Russell St.
West Lafayette, IN 47907
mark.williams2@usda.gov