evaluating ephemeral gully erosion
TRANSCRIPT
Evaluating Ephemeral Gully Erosion on Cultivated Fields: A Paired Watershed
Study in Central Kansas
Aleksey Sheshukov, Ronald Graber, Tim Keane, Daniel Devlin
Kansas State University
2016 SWCS Meeting in Louisville, KYJuly 24-27, 2016
What is an Ephemeral Gully (EG)?
• Ephemeral gullies are localized areas of soil erosion that form from concentrated water flow during extreme hydrologic events
• EGs can be erased annually by tillage, but will reform with additional rainfall
• EG erosion is greater than rill erosion but less than classical gully erosion
• Gully development is a threshold phenomenon: It occurs only when a threshold has been exceeded
• Factors of gully development:– Flow hydraulics– Rainfall (or snowmelt)– Topography– Pedology (soil properties) – Land use and management
2
Soil Erosion from Ephemeral Gullies• Contribution of EG erosion
– U.S.: 17 to 73% – Belgium: 44%– Portugal: 83%– China: 41 to 91%– Kansas: 40% ???
• EG erosion rates (in kg/m2/yr)– U.S.: 0.27 to 2.87
(1.1 to 11.9 t/ac/yr) – Belgium: 0.50– Portugal: 0.1 to 0.68– China: 0.43
• 24 federal reservoirs in Kansas– Five with >40% filled with sediment– John Redmond is being dredged
3
Study Area
• Two paired HUC-12 watersheds in Little Arkansas River watershed (HUC-8 11030012) – Running Turkey Creek watershed (9,130 ha)
– Dry Turkey Creek watershed (9,500 ha)
• Agricultural watersheds
>77% cropland, 7-11% grassland
• Running Turkey (476 fields)
• Dry Turkey (584 fields)
• Water quality concerns include bacteria, nutrients, sediment, and pesticides (atrazine herbicide)
4
KANSAS
Disturbed watershed vs. undisturbed watershed• Running Turkey (476 fields)
– <20 fields with terraces
– <40 grassed waterways
• Dry Turkey (584 fields) – >100 fields with terraces
– >80 grassed waterways
5
KANSAS
Running Turkey
Dry Turkey
Goal and ObjectivesGoal —Evaluate EG (and sheet-rill ) erosion fields
in two watersheds in Central Kansas
Objective 1 — Assess watersheds and erosive fields
Objective 2 — Predict the location and length of EGs
Objective 3 — BMP recommendations
6
Activities inSouthcentral Kansas
• CEAP Project in Cheney Lake watershed (2006-2010)– 990 EGs in Goose Creek watershed– 34 acres average drainage area– 1.1 tn/ac annual EG erosion rate
• CIG in Black Kettle watershed (2009-2011)– 56 fields with pre-installed BMPs on 1,040 ha– 124 fields with post-installed BMPs on 1,909 ha– 35.8% TSS yield reduction compared to no-BMP scenario– 21.9% TSS reduction compared to preinstalled BMP conditions– BMPs included riparian buffer, no-till/cons till,
terraces/contour farming, strip cropping, permanent grass
• USDA-NIFA Grant in Little Ark (2011-2015)– Surveying fields and monitoring soil erosion– Phosphorous study on sorption/desorption– EG modeling
• Little Ark WRAPS / EPA 319 Section– Active watershed advisory council, working with KDHE– Annual watershed surveys and monitoring– City of Wichita
7
Annual tillage/crop surveys
• Annual surveys: 2010-14: crop, tillage, BMP, erosion
8
No EG; Grass waterway
EG observed
Driving surveys
9
EG Erosion Risk Maps
Parameter Dry Turkey Running TurkeyArea (ha) 9,525 9,137Land under agriculture (%) 77 79Total Cropland fields 584 476Number of Cropland fields with EGs 115 109Number of EGs identified 477 700Total EG length on fields (m) 8,543 22,600
10
Watershed modeling
• SWAT model with sub-CLU LULC dataset– 10m DEM, 6-years of KARS LULC, SSURGO
– PRISM and NCDC climate data
– 7,000 fields/HRUs
– Calibration at two USGS stations
11
Sheet & Rill Erosion• SWAT model results
– Sheet-and-rill erosion rates assessed at field level
– Top 10% fields produce >70% S&R TSS yields
– Top 10% fields occupy ~24% area
• Running Turkey– Area of Med-High S&R erosion – 607 ha
– Average Med-High S&R erosion load – 4.4 th/ha
• Dry Turkey– Area of Med-High S&R erosion – 757 ha
– Average Med-High S&R erosion load – 4.2 th/ha
12
Topographic Index Models• EG development is a threshold phenomenon• Topographic threshold factors
– Upstream drainage area (A)– Local slope (S)– Slope plan curvature (PLANC)
• Used in watershed models to identify locations of ephemeral gullies– AGNPS– RUSLER-EphGEE– tRIBS-Erosion
• Normally do not include headcut advancement• Static in nature
13
Six Topographic Index models
14
1. T = SAs Slope Area Index (SA)
2. T = SAsb Slope Area Power Index (SAP)
3. T = SAPLANC Compound TI (CTI)
4. T = ln(As/S) Topographic Wetness Index (WTI)
5. T = 3.3nLS-1/2 Kinematic Wave (nLS) Model
6. T = 3.3nLτc-1S-1/2 Modified Kinematic Wave
(nLSCSS) Model
DEM Preprocessing
15
Before After
• Culverts• Terraces and waterways• Digital dams
Geospatial Model• Input data
– 3-m LiDAR DEM
– Field boundary (CLU)
– NASS CDL/KARS layer
– SSURGO soil dataset
• Processing– Culvert reconnaissance
– Multi-year aerial imagery
– DEM conditioning
• Geospatial model– ESRI’s ArcGIS
– Model Builder and Python-based
• Model flowchart
16
Gully location and length analysis
• Compared predicted and observed gully occurrence in each catchment – 2433 catchments in Running Turkey;
– 2151 catchments in Dry Turkey
1. Error matrix approach for location analysis
2. NSE and RMSE for length analysis
3. Drainage density error for headwater and mainstem gullies
17
Location and length statistics
• CTI model showed the best statistics
• 67% accuracy in gully identification
18
Drainage density analysis
• DDE = 𝐿𝑝−𝐿𝑜
𝐿𝑜, L is gully length
19
Running Turkey undisturbed
Dry Turkey disturbed
Same TIsDifferent TIs
Mainstem Headwater
Summary thresholds for CTI model
20
Running Turkeyundisturbed
Dry Turkeydisturbed
Location 1.4 1.9
Length 1÷1.2 2.1
DDE 1.4 2.9
DDE_MS 1.4 2.3
DDE_HW 1.4 3.1
• CTI model showed consistent results for undisturbed watershed• Terrain manipulation (terraces, waterways) altered index
calculation and needs to be accounted
Gully Surveying
MgmtNo-till
(>10 yrs)
Crop Sorghum
Soil Silty Clay
Loam
Slope 1÷3 %
Area 1/8 sec
No-Till
MgmtConv-till (>10 yrs)
Crop Wheat
Soil Clay
Loam
Slope 3÷7 %
Area 5.2 ha
Tilled
21
No-Till Field
22
No-Till Field
23
Tilled Field
24
Conclusions• No-till field
– Gully channels experienced fluctuations of erosion and deposition
– Mainstem gully was net depositional, but smaller headwater gullies grew in size
– Greatest rates of deposition at confluence of main channels– Statistics showed factors other than rainfall depth, intensity, etc.
can be important– Heavy rains of spring 2015 flushed most of sediment downstream
• Tilled field– Difficult to survey due to intensive tillage– Overall, gully channel was depositional, especially related to
small events– No credible statistics developed
25
BMP Design Objectives
• Factors for high EG erosion rates– Steep gully bed slopes– Low channel roughness– Loose gully banks
• BMP aims – Increase infiltration– Decrease runoff velocities– Store more water on field– Catch and store sediment
26
Potential BMPs
• Common BMPs– Leaving residue or no-till– Planting cover crops in off-season– Creating vegetated waterways– Decrease overland flow rates by terraces, and contour
farming
• Specific ephemeral gully BMPs – Increase bank stability by decreasing bank angle and
height and increasing bank vegetation– Design a gully channel that are stable part of drainage
network by catching sediment and storing in channel using bio-materials or creating mini-alluvial fans
27
