1. Importance of Concentrated Flow Paths in
Agricultural Watersheds in Southern Illinois
Prabisha Shrestha
Karl W. J. Williard
Jon E. Schoonover
Department of Forestry

2. Outline
• Introduction
• Objectives
• Methods
• Results
• Discussion
• Limitations & Future Research

3. Introduction- Agriculture and Soil Erosion
• Agriculture - leading source of surface
water pollution
• Agriculture - major source of sediments,
nutrients, pesticides and pathogens
• Sediment - largest contaminant of surface
water by weight and volume
• Soil erosion in agricultural fields is a
growing concern - on-site and off-site
environmental impacts
(EPA 2009)

4. Introduction-Soil Erosion
On-site Impacts
-removal of top soil
-reduction in water-holding
capacity
-reduction in soil fertility
-decrease in agricultural
yields
Off-site Impacts
-impairment of water
quality
-destruction of aquatic
habitats
-downstream flooding
-sedimentation in
watercourses and dams

5. Introduction-Concentrated Flow Paths (CFPs)
• CFPs are narrow and shallow channels
• commonly observed in agricultural fields
• intermediate size between rills and permanent gullies
• Direct conduit for sediment and nutrient delivery
Figure 1: Conceptual model of concentrated flow path formation (Pankau 2010)

6. Introduction-Importance of CFPs
• CFPs increase the connectivity in the
landscape in terms of sediment and
nutrient delivery from upland
contributing areas to streams
• CFPs are both sediment source and
transport mechanism
• Can be ten times more than non-rill and
interill areas
• Up to two orders of magnitude higher
than transport rate by raindrop splash
(Poesen et al. 2003)

7. Introduction – Importance of CFPs
• BMPs are recommended to address agricultural NPS pollution
• BMPs such as riparian buffers - increase surface roughness and
infiltration capacity
• CFP formation through riparian buffers has been shown to decrease
the sediment trapping efficiency of BMPs
Figure 2: Riparian buffer zones (Hill 2009)

8. Objectives
• Investigate the prevalence of CFPs in
agricultural watersheds at a regional
scale
• Assess the percentage of field area
drained by the CFPs
• Determine the effects of field
characteristics on occurrence of CFPs

9. Methods – Study Area
• Jackson County, IL
• Total study area = 35,599
acres
• 389 agricultural fields –
percent drainage
calculation
• 379 agricultural fields-
statistical analysis

10. Methods-Spatial Analysis
Lidar Point
Cloud data
Watershed
delineation
Digital Elevation
Models
Percent drainage
calculation

11. Elevation map with CFPs
• Elevation map from Lidar point cloud
data
• The color gradient, yellow to blue,
represents higher to lower elevation
• The orange lines represent the
concentrated flow paths determined using
flow accumulation lines

12. Watershed Delineation
35% of field drained by CFP’s 94% of field drained by CFP’s

13. Methodology-Statistical Analysis
• Multiple Regression with models selected from the Akaike’s Information Criterion
(AIC) method
• Values of independent variables were acquired from the SSURGO database
• Weighted average of the independent variables was used for the analysis
Variables Parameters
Dependent CFP area
CFP length
CFP area to length ratio
Drainage density
Independent K factor
Organic matter
Percent clay
Percent sand
Saturated hydraulic conductivity
Slope
Slope length
T factor

14. Results-CFP Characteristics
Number of CFPs
Total 1964
Average per field 5
Minimum 0
Maximum 17
CFP length (Feet)
Average 1578.7
Minimum 22.3
Maximum 27890.9

15. Results-CFP Characteristics
Soil Texture Number of
fields
Number of
CFPs
Clay loam 1 12
Fine sandy loam 8 41
Loam 4 15
Silt loam 172 908
Silty clay 166 848
Silty clay loam 26 133

16. 0
20
40
60
80
100
120
140
0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100
Numberoffields
Percent drained
Results –Percent of Field Drained by CFP’s
Statistics % drainage
Average 81.41
Minimum 35.58
Maximum 99.75

17. Results –Multiple Regression
Variables P value
Organic matter 0.0014
T factor 0.0501
Percent clay 0.0019
Model 1: Regression analysis of CFP area (p value <0.0001, Adj. R sq.=0.0104)
Variables P value
Slope 0.0137
Organic matter 0.0001
Percent sand 0.0393
Model 2: Regression analysis of CFP length (p value <0.0001, Adj. R sq.=0.0184)

18. Results –Multiple Regression
Variables P value
Saturated hydraulic conductivity 0.0011
Slope 0.0536
T factor 0.0077
Percent clay 0.0019
Model 3: Regression analysis of CFP area to length ratio (p value <0.0001, Adj. R sq.=0.0155)
Variables P value
Slope 0.0318
Slope length 0.0051
Organic matter <0.0001
Model 4: Regression analysis of Drainage density (p value <0.0001, Adj. R sq.=0.0172)

19. Results –Multiple Regression
Dependent variable Adjusted R square
CFP area 0.0104
CFP length 0.0184
Ratio between CFP area and length 0.0155
Drainage density 0.0172
Table: Adjusted R square values for all the regression model

20. Discussion
• Percent area drained by CFPs vary from 36-99%
• A small field scale study in the area showed percent drainage of 82.5-100% (Pankau
2010)
• This finding has important implications for future riparian buffer design, as currently
most buffers are static width and designed to handle sheet flow
• Investigated field characteristics showed association with CFP properties, but did not
explain much of the variation (1-1.8%) observed
• Other factors such as precipitation and surface runoff might influence CFP formation
• Field scale study along with the use of remotely sensed data could provide more
information on the association between field properties and CFPs

21. Limitations
• Remotely sensed data – measurement uncertainties
• Field scale study - required for validation of the findings and up-to-
date information
Future Research
Targeting conservation efforts
• Research relating water quality impairment to CFP occurrence would be
helpful in quantifying the impact of CFPs
BMP design
• Research utilizing both field scale measurements and remotely sensed data
would be helpful in understanding the CFP forming hydrologic processes and
changes in CFP morphology over time

22. Summary
• Lidar-derived DEMs are useful in identifying CFP’s, which are the main
conduit for nonpoint source pollution leaving agricultural fields
• CFPs are prevalent in agricultural watersheds of southern Illinois
• This result has important implications for riparian buffer design (variable width
buffer)
• Field characteristics did not explain much of the variation observed in CFP
occurrence
• Further research in understanding physical processes forming CFPs and
resulting morphological changes can help in targeting conservation efforts
such as BMPs and regulations to protect soil health and water quality

23. Acknowledgements
Immeasurable appreciation and deepest gratitude for the help and support extended
to the following persons who in one way or another have contributed in making this
study possible.
Dr. Logan Park, member of my research committee, for his precious time,
valuable comments, suggestions and positive insights;
Ryan Pankau, Soil Conservationist at ILNRCS and former graduate student, for
his support and cooperation in accessing NRCS data and information and
suggestions;
Dr. John D. Reeve, the statistician, for the shared mathematical expertise that
contributed in data analysis and interpretation;
Fellow graduate students at the Department of Forestry, for their help throughout
the academic exploration;