EphGEEEphemeral Gully Erosion Estimator

Seth M. Dabney and Dalmo VieiraUSDA-ARS-National Sedimentation Laboratory

RUSLE2 – Revised Universal Soil Loss Equation (version 2) Estimates Sheet and Rill Erosion not Ephemeral Gully Erosion

In 2014 - NRCS implemented the first update to RUSLE2 since 2006

RUSLE2Science Template

topographic deposition and fine sediment enrichment

RUSLER – RUSLE2 called through API, Slope length calculated as ratio of runoff leaving to runoff generated within cell

Ephemeral gully channels end RUSLE hillslopes (AH 703), Treynor Iowa W11

Watershed 11Treynor, Iowa, USA 1975 – 1991

contour-plowed corn averaging

7.6 Mg ha-1

36 Mg ha-1 y-1

16 t ac-1 y-1

RUSLER - RUSLE2 called through API for each raster; slope length calculated as ratio of runoff leaving to runoff generated within cell

Potential Gully Networks

RUSLER SetupDrainage catchments for each channel cell

Raster Channel Network EphGEE

ComputationalNetwork

Computational Channel Network

Refined network Simpler computations Better spatial distribution User-defined channel densification

factor

Water Surface Profiles

Steady, spatially varying discharges Flow properties using evolving channel geometry Standard Step Method

Derived from energy balance Accounts for backwater, varying channel roughness Channel junctions: equal water depth

EphGEE uses CREAMS conceptual model of George Foster

Gully Evolution MethodFoster & Lane Method “Non-erodible” layer

Determined by depth of last tillage operation

Shallow, wide channels

Gully Channel EvolutionFoster & Lane Method Two-phase erosion

Channel incision at ‘equilibrium width’ until non-erodible layer is reached

Channel widening at varying rate

ChannelWidth Depth to non-erodible layer

Non-erodible layer

Initial terrain elevation (thalweg)

Initial cross-sectional shapeWater level

Gully Channel EvolutionFoster & Lane Method I. Downcutting

Channel incision at constant erosion rate

ChannelWidth

Non-erodible layer

Phase I: DowncuttingChannelWidth

Non-erodible layer

Water level

)( cgp KD 5.1)/01.0( tg n

Potential detachment capacity (kg m-2 s-1) K – soil erodibility (s m-1)

Shear stress available for sediment transport (Pa))/1( cp tgDD Actual detachment: g – total sediment load; tc – transport capacity

Gully Channel EvolutionFoster & Lane Method II. Widening

Channel widening at varying rate Until shear stress at corner

equals critical shear stress

Final channel width

Non-erodible layer

Initial terrain elevation

Phase II: Widening

EphGEESoil Erodibility Critical shear stress Soil Erodibility

Function of soil clay content (percentage) and time since tillage

Relationships derived from data in Watson et al. (1986)*

* Watson, D.A., Laflen, J.M. & Franti, T.G. (1986) Estimating ephemeral gully erosion. ASAE Paper No. 86-2020

bCc ea

ta 0075.0294.0 286 1079.21028.10418.0 ttb

ceK 226.00306.0

daystifbanda 10000252.079.0

EphGEESediment Transport Finite volume mass balance

Incoming sediment load

Sediment transport capacity

Lateral sediment inflow

Potential Sediment Detachment

Outgoing sediment load

EphGEE – Ephemeral Gully Erosion EstimatorCross Section #2

Combined sheet, rill, and ephemeral gully erosion with no grassed waterway

33 Mg ha-1 y-1 delivered from

watershed

Combined sheet, rill, and ephemeral gully erosion with grassed waterway

17 Mg ha-1 y-1 delivered from

watershed

RUSLE2/EphGEE 17-year, 6.3 ha Watershed 11No Grassed Waterway

Characterization RUSLER/EphGEE

Sheet/Rill Sediment Yield

CS#1 (Mg ha-1 y-1) 36.3 46.2CS#2 (Mg ha-1 y-1) 48.5 64.5CS#3 (Mg ha-1 y-1) 39.7 53.4Outlet (Mg ha-1 y-1) 36.4 32.9

Observed (Mg ha-1 y-1)

RUSLE2/EphGEE 17-year, 6.3 ha Watershed 11Grassed Waterway

Characterization RUSLER/EphGEE

Sheet/Rill Sediment Yield

CS#1 (Mg ha-1 y-1) 36.3 46.2CS#2 (Mg ha-1 y-1) 48.5 64.5CS#3 (Mg ha-1 y-1) 39.7 21.6Outlet (Mg ha-1 y-1) 36.4 17.5

Observed (Mg ha-1 y-1) 14.6

EphGEE – Summary

C++ object-oriented program to be implemented as a web-based tool delivered by ARS and commercial providers

Potential gully locations determined through terrain analysis

Gully channel geometry evolves in response to a series of event runoff and sediment inputs

Ref: Dabney et al. 2015 J. Hydrol. Eng. 20(6):C4014009

Questions?

Headcuts advancement upslope from Potential Ephemeral Gully (PEG) mouth points identified using a Compound Topographic Index (CTI)