IAGLR 2012 Conference
Cornwall, ON (May 16, 2012)
Representing Ephemeral Gully Erosion
in SWAT: Implications for Modeling
Agricultural Management Practices in
the Maumee River Watershed
Todd Redder
Amanda Flynn
Joseph DePinto
Pranesh Selvendiran
Funded by USACE-Buffalo District
through sub-contract to Ecology &
Environment, Buffalo, NY
Presentation Outline
•
•
•
•
•
•
•
•
Overview of sediment issues
Tiffin River watershed overview
General SWAT modeling approach
Key sediment erosion processes
Ephemeral Gully Erosion Model (EGEM)
EGEM implementation & testing in SWAT
Summary & conclusions
Next steps
2
Toledo Harbor Sedimentation
• Excessive sedimentation in Great Lakes harbors
caused by watershed erosion (landscape, channel)
• USACE has mandate to maintain Toledo Harbor
Federal navigation channel
– Annual dredging volume: ~640,000 yd3 (70% to open lake)
– Annual cost of dredging & disposal: $5 million
Tributary Suspended Solids
Other Detroit
6% 16%
Maumee
78%
3
Great Lakes Tributary Modeling
Program - 516(e)
• Objective: “develop a tool for watershed planning
that is usable and will be used by stakeholders who
make decisions about soil conservation …”
(http://glc.org/tributary/)
• Evaluate sediment sources & benefits of
potential management actions
• Maumee watersheds studied:
– Upper Auglaize River (AnnAGNPS model – USDA)
– Blanchard River (AnnAGNPS model – LimnoTech)
– Tiffin River (SWAT model, in progress – LimnoTech)
Tiffin Watershed Project Overview
• Tiffin Watershed
Maumee River Basin (6,300 mi2)
– Headwater watershed, north side of
Maumee River → sediment and nutrient
problems
– Dominated by agriculture (> 75%)
– Leverage knowledge and data from
other model applications (e.g., Upper
Auglaize, Blanchard)
• Objectives
– Quantify sediment and nutrient loading
– Evaluate land management alternatives
to estimate potential benefit from
reduced loading
• Timeline: Summer 2011- Summer 2013
5
Sheet & Rill Erosion
• Overland flow or small concentrated flow paths
• Calculated by SWAT using “Modified Universal Soil Loss
Equation” (MUSLE) method
– Developed based on the original “Universal Soil Loss
Equation” (USLE)
– USLE estimates annual average gross sheet and rill erosion
– MUSLE is used to calculate sediment yields from drainage
basins for individual storm events based on:
•
•
•
•
•
•
•
Runoff volume
Peak flow rate
Rainfall and runoff (R factor)
Soil erodibility (K factor)
Slope length-gradient (LS factor)
Cover and management (C factor)
Erosion control practice (P factor)
6
Ephemeral Gully (EG) Erosion
• Erosion in deep, narrow channels
• Calculated by existing EG erosion models, as a function
of:
–
–
–
–
“Potential ephemeral gully” (PEG) locations
Critical shear stress of soil (τc)
Erodibility coefficient (kd)
Parameters calculated based
on soil properties
• Headcut induced when
shear stress exceeds a
threshold for given runoff
event
• Tilling “reactivates” erosion processes
7
Schematic Representation of
Ephemeral Gully Erosion
(Source: Gordon et al. 2007 (Figure 1)
8
Potential Ephemeral Gully Locations
(Blanchard River Watershed)
Approximately 1500 PEG sites
Function of:
• CTIndex (1000)
• Watershed topography
9
Maumee Watershed:
Landscape Erosion Comparison
Upper Auglaize River
Blanchard River
10
Tiffin Watershed Modeling Approach
• Soil & Water Assessment Tool (SWAT) selected:
– Well-suited and widely applied for agricultural
watersheds
– Represents hydrology, sediment erosion &
transport, as well as nutrient loading/delivery
– Can directly consider various ag management
practices (crop rotation, tillage, tile drainage, etc.)
• Landscape erosion capabilities:
– Sheet & rill erosion: estimated via “Modified
Universal Soil Loss Equation”
– Ephemeral gully erosion: not represented in existing
versions of SWAT
11
Ephemeral Gully Erosion:
Existing Studies
• Ephemeral Gully Erosion Model (EGEM)
– Originally developed in late 1980s (Woodward 1999)
– Limitations with respect to continuous watershed
modeling
• “Tillage-induced ephemeral gully erosion
model” (TI-EGEM) represents enhanced
version (Gordon et al. 2007):
– Tracks cumulative response to storm events and
associated gully erosion
– Gully channel ‘resets’ when tilling operation occurs
– Fully implemented in AnnAGNPS model
12
Ephemeral Gully Erosion:
SWAT Implementation
• Review TI-EGEM implementation in AnnAGNPS,
compare to literature
• Revise SWAT code to incorporate TI-EGEM algorithms
–
–
Sediment erosion & transport
Associated erosion/delivery of nutrients
• Compare model results to ephemeral gully field
observations from Mississippi (USACE study)
• Conduct diagnostic simulations to confirm model
behavior matches conceptual understanding
13
Model-Data Comparisons
• Gullies observed at 4 agricultural sites in central
Mississippi (Smith 1992)
• Observations made after two significant rainfall
events (3.3” and 1.2”)
Ephemeral Gully Length
Ephemeral Gully Width
14
SWAT Diagnostic
Results
Rainfall: (varies)
Slope: 2%
Gully Erosion as a
Function of Critical
Shear Stress, Rainfall,
& Slope
Test Parameters:
•
Drainage area: 5 ha
•
Tillage depth: 0.2 m
•
Manning’s n: 0.04
Rainfall: 50.8 mm
Slope: (varies)
15
Critical Shear Stress (Pa)
SWAT Diagnostic Results:
Conventional Tillage vs. “No-Till” Case
Conventional Case:
Erosion potential
“reset” by annual
tillage
No-Till Case:
Full erosion
potential met in
first year
Summary & Conclusions
• SWAT model represents best overall choice for modeling
watershed hydrology and sediment processes in Maumee basin
• Lack of ephemeral gully treatment addressed through revisions
to SWAT code
• TI-EGEM implementation produces reasonable results and is
consistent with (limited) available data for other sites
• Model predictions can/will be further constrained by sitespecific data for Tiffin watershed
• Inclusion of ephemeral gully sub-model has important
implications for assessing benefits of candidate “best
management practices” (BMPs) for cropland – e.g.:
– Effect of tillage practices on EG erosion vs. sheet & rill
– Potential benefits of grassed “waterways” at gully sites
17
Next Steps
• Complete SWAT model development for Tiffin River watershed
• Ground-truth ephemeral gully model input/output based on
field data:
– Compare potential gully sites against observed sites
– Compare model-predicted gully width/length against field observations
• Calibrate SWAT model with a reasonable distribution of
sediment erosion sources:
– Sheet & rill
– Ephemeral gully
– In-channel erosion (i.e., sediment bed, bank contributions)
• Apply model to estimate sediment reductions for various BMPs
• Formally submit enhanced SWAT code to developers for
inclusion in future versions
• Apply enhanced SWAT to other watersheds in Maumee basin
18
Questions?
• Acknowledgements:
– Funding: USACE Buffalo District
– Partners: Ecology & Environment, Inc.
– AnnAGNPS Model:
– USDA-ARS
– USDA-NRCS
• Contact Information:
Todd Redder, P.E.
LimnoTech
Ann Arbor, MI
[email protected]
Ephemeral gully in Upper Auglaize
watershed (Bingner et al. 2005)