Virtual Experiments to Explore Non-Linear
Soil Moisture-Hydrology Interactions
at the Hillslope Scale
L. Hopp and J.J. McDonnell
Department of Forest Engineering, Oregon State University

Objectives• To examine the factors that control lateral subsurface flow
generation at the hillslope scale• To explore the interactions between controlling factors• To document non-linear and hysteretic behavior• To contribute to a framework for a general classification of hillslopes– E.g. Biosphere 2: help to learn about the hydrologic response ofhillslopes (key interactions, storages and flow-paths)

AGU 2007 Fall Meeting

H53H – Soil Moisture-Hydrology Interactions

Modeling approach• Virtual experiment approach: model calibration and evaluation
based on field observations• Topography and subsurface stormflow data from an existing
research hillslope were used to define the “base case scenario”• Numerical 3D FE model (Hydrus-3D) that solves the Richards
equation for water flow in variably saturated porous media

Panola Mountain Research Watershed: e.g. Freer at al. (WRR 2002), Tromp-van Meerveld (WRR 2006a and b)

AGU 2007 Fall Meeting

H53H – Soil Moisture-Hydrology Interactions

Base case scenario•

Irregular geometry; two layers representing soil and bedrock•

slope 13°•

Variable soil depth (0-1.86 m); mean 0.62 m, cv 56%•

Subsurface flow collection system (20 m wide)

Surface topography with soil layers

subsurface topography with bedrock

Original storm event

AGU 2007 Fall Meeting

H53H – Soil Moisture-Hydrology Interactions

Calibration

AGU 2007 Fall Meeting

H53H – Soil Moisture-Hydrology Interactions

Evaluation• Evaluation against tensiometer data (0.5-0.6 m below surface) model setup is capturing major internal flow behavior
AGU 2007 Fall Meeting

H53H – Soil Moisture-Hydrology Interactions

Variation of control factors
LowMediumHighvery High0.624 m0.91 m1.21 m-Difference Ks soil-bedrock

101102--

Slope angle

6.5°

13°

26°

40°

Storm size

29 mm

58 mm

87 mm

-Soil depth (mean)Ks(soil) 0.65 m h-1 – Ks(bedrock) 0.006 m h-1Ks(soil) 0.65 m h-1 – Ks(bedrock) 0.06 m h-1

AGU 2007 Fall Meeting

H53H – Soil Moisture-Hydrology Interactions

Hydrologic response characteristics• Results are evaluated with respect to characteristics of hydrologic
response––––––

Runoff coefficient
time to peak
duration of total subsurface flow (SSF)
peak discharge
variability of trench section contribution to total SSF
variability of peak discharge in each trench section

slope angle
L

M

HvH

storm size

soil depth

L

M

H

AGU 2007 Fall Meeting

difference Ks soil-bedrock

LM

H

L

L
M
L
M
L
M

H53H – Soil Moisture-Hydrology Interactions

MHL

M

H

LM

H

Runoff coefficientslope angle
LMHvH

storm size

soil depth

L

M

H

AGU 2007 Fall Meeting

difference Ks soil-bedrock

L

M

H

L

M

H

L

M

H

L
M
LM
L
M

H53H – Soil Moisture-Hydrology Interactions

L

M

H

Runoff coefficientslope angle
L

M

H

vH

storm sizeLM
H

Ks soil-bedrock

soil depth

L

M

H

L

M

H

L

M

H

L

M

H

M
M
M

slope +, input +runoff coefficientsubsurface flow?
bedrock Ks
Y

AGU 2007 Fall Meeting

H53H – Soil Moisture-Hydrology Interactions

NTime to peakslope angle
L

M

HvHtime to peakstorm sizeLMH

Ks soil-bedrock

soil depth

LM

HL

M

HL[h]M

HLM

H=0≤ 28M≤ 32≤ 36M

≤ 40

M

slope +, input +≤ 44

runoff coefficientsubsurface flow?
bedrock Ks

soil depth +, input -AGU 2007 Fall Meetingtime to peak

H53H – Soil Moisture-Hydrology Interactions

YN

Duration of subsurface flow
slope angle
L

M

HvH
duration of SSF

storm sizeLMH

Ks soil-bedrock

soil depthLM

H

L

M

H

L[days]

M

HLM

H=0
≤1

M

≤3≤5M

≤7

M

slope +, input +>7

runoff coefficientsubsurface flow?
bedrock Ks

soil depth +, input -slope +, input +AGU 2007 Fall Meetingtime to peak

duration of SSF
H53H – Soil Moisture-Hydrology Interactions

YN

Peak discharge
slope angle
L

M

HvH
peak discharge

storm size

LM
HKs soil-bedrock

soil depthLM

HL

M

H

L

3-1

[m h ]

M

HLM

H=0
≤ 0.2

M

≤ 0.4
≤ 0.6

M

≤ 0.8

M

slope +, input +≤1

runoff coefficientsubsurface flow?
bedrock Ks

soil depth +, input -slope +, input +AGU 2007 Fall Meetingtime to peak

duration of SSF
H53H – Soil Moisture-Hydrology Interactions

YN

Slope variations - Subsurface flow in 2 m sections
13°

6.5°

127%

99%

40°

26°

62%

AGU 2007 Fall Meeting

56%

H53H – Soil Moisture-Hydrology Interactions

Flow velocities and saturation
6.5°-3.000 -0.200 0.000 3.000

Pressure Head - h[m], Min=-1.492, Max=0.181

AGU 2007 Fall Meeting

H53H – Soil Moisture-Hydrology Interactions

Flow velocities and saturation
13°-3.000 -0.200 0.000 3.000

Pressure Head - h[m], Min=-1.492, Max=0.181

AGU 2007 Fall Meeting

H53H – Soil Moisture-Hydrology Interactions

Flow velocities and saturation
26°-3.000 -0.200 0.000 3.000

Pressure Head - h[m], Min=-1.492, Max=0.181

AGU 2007 Fall Meeting

H53H – Soil Moisture-Hydrology Interactions

Flow velocities and saturation40°-3.000 -0.200 0.000 3.000

Pressure Head - h[m], Min=-1.492, Max=0.181

AGU 2007 Fall Meeting

H53H – Soil Moisture-Hydrology Interactions

Controls on subsurface flow

slope +, input +runoff coefficientsubsurface flow?
bedrock Ks

soil depth +, input -time to peakslope +, input +duration of SSF

soil depth -, slope +/-, input +peak discharge

slope -, input -AGU 2007 Fall Meeting

spatial variability of SSF(total SSF, peak discharge)

H53H – Soil Moisture-Hydrology Interactions

YN

Soil depth variations: storage-discharge relationship• Slope 13 °, storm size 87 mm

ongoing work…
AGU 2007 Fall Meeting

H53H – Soil Moisture-Hydrology Interactions

Conclusions• The virtual experiment approach with HYDRUS 3D was helpful inexploring the factors that control lateral subsurface flow generation
at the hillslope scale• Bedrock permeability is a key factor for inducing lateral subsurface
flow• Soil depth leads to a dampening of the hydrologic response• Complex interaction between topography and slope controlling
subsurface saturation, flow paths, velocities and spatial variability• Ongoing work is exploring the storage-discharge relationship andhow it is affected by control factors

AGU 2007 Fall Meeting

H53H – Soil Moisture-Hydrology Interactions