Sensitivity of soil moisture to terrain

and snow cover

What’s happening below the surface?

the ‘black box’ in hydrologic models

Discharge

Climate

(Precipitation,

Temperature,

Radiation)

Subsurface flow

& storage

Soil moisture:

model state variable for

water in the subsurface

How should we measure and model

soil moisture in steep terrain?

Catchment Hillslope Point/plot

Measurement scale

Model theory scale

Model calibration and

prediction scale

Example soil moisture measurements

Loch Vale hillslopes

Sky

Pond

Andrews Creek

Icy B

rook

The Loch

Denver

Loch

Vale

Colorado

0 0.5 1 Kilometer

contour interval = 50 meters

Transect 1

Transect 2

Mean annual precipitation 1000 mm

Snowmelt-dominated runoff

Conceptual model

Vertical infiltration of snow melt water

Lateral subsurface flow

Variable source area runoff generation

HILLSLOPE FLOW PROCESSES

Drier upslope than downslope

Local variability relates to snow and vegetation distribution

POINT MOISTURE DISTRIBUTION

Point sampling scheme

Hillslope 1: Andrews

3220

3230

3240

3250

3260

3270

3280

3290

0 100 200

Length (m)

Elevatio

n (m

)

10 m intervals

Snow depth and surface soil moisture

~weekly surveys

0

1

2

3

4

5

0 50 100 150 200

Sn

ow

de

pth

(m

)

Distance downslope (m)

4-May

14-Jun

29-Jun

0

10

20

30

40

50

60

70

0 50 100 150 200

VW

C (

%)

Distance downslope (m)

21-Jun

29-Jun

6-Jul

13-Jul

20-Jul

27-Jul

Point sampling results

Emergent groundwater, localized

Likely reinfiltrates

Emergent groundwater, flowing

Likely connected to stream

Point sampling results

• Points near channel do not have the highest soil moisture

• Upslope generally drier than downslope, but localized

wet areas midslope

• Spatial moisture distribution unrelated to snow

distribution

Drier upslope than downslope

Local variability relates to snow and vegetation distribution

MOISTURE DISTRIBUTION

New conceptual model

Vertical infiltration of melt water upslope

Shallow lateral subsurface flow

Bedrock and slope breaks force exfiltration

midslope

FLOW PROCESSES

Relates to lateral flow and microtopography

MOISTURE DISTRIBUTION

Continuous sampling Hillslope 1: Andrews

3220

3230

3240

3250

3260

3270

3280

3290

0 100 200

Length (m)

Elevatio

n (m

)

0

1

2

3

90 110 130 150 170 190 210 230

Day of Year

Sn

ow

de

pth

(m

)

snow 1

snow 3

snow 12

snow 13

20

25

30

35

40

90 110 130 150 170 190 210 230

Day of Year

VW

C (

%)

moisture 1

moisture 3

moisture 12

moisture 13

Vertical supply of moisture

from snow

Downslope export of

moisture through subsurface

Upslope source of moisture

Downslope export

surface runoff to channel

Hillslope 1: Andrews

3220

3230

3240

3250

3260

3270

3280

3290

0 100 200

Length (m)

Elevatio

n (m

)

R2 = 0.9285

0

0.1

0.2

0.3

0.4

0.5

0.6

30 32 34 36

VWC (%)

Sta

ge (

m)

30

31

32

33

34

35

36

90 110 130 150 170 190 210 230

Day of Year

VW

C (

%)

0

0.1

0.2

0.3

0.4

0.5

0.6

Sta

ge (

m)

R2 = 0.7787

0

0.1

0.2

0.3

0.4

0.5

0.6

22 27 32

VWC (%)

Sta

ge (

m)

22

24

26

28

30

32

90 110 130 150 170 190 210 230

Day of Year

VW

C (

%)

0

0.1

0.2

0.3

0.4

0.5

0.6

Sta

ge (

m)

Point measurements

vs. catchment response

Point 1,

upslope

moisture

supply

zone

Point 12,

midslope

exfiltration

zone

Sky

Pond

Andrews Creek

Icy B

rook

The Loch

Denver

Loch

Vale

Colorado

0 0.5 1 Kilometer

contour interval = 50 meters

Transect 1

Transect 2

Hillslope Measurement Summary

• Hillslope flow:

– Shallow lateral downslope flow during

snowmelt

– Exfiltration at bedrock and topographic

breaks

0

10

20

30

40

50

60

70

0 50 100 150 200

Distance downslope (m)

VW

C (

%) 21-Jun

29-Jun

6-Jul

13-Jul

20-Jul

27-Jul

R2 = 0.9285

0

0.1

0.2

0.3

0.4

0.5

0.6

30 32 34 36

VWC (%)

Sta

ge (

m)

• Point soil moisture:

– Locally variable, no apparent

relationship to snow distribution

• Catchment response

– Exfiltration Rapid pathways for melt

water to reach stream

– Neither moisture distribution nor runoff

source regions would be predicted by

most catchment-scale hydrologic models

How should we measure and model soil moisture

in steep terrain?

• Measurement networks designed to sample gradients

in flow & moisture

• Flexible model structure to incorporate evolving

understanding of subsurface flow mechanisms

Catchment Hillslope Point/plot

Measurement scale

Model theory scale

Model calibration and

prediction scale

Acknowledgements: Colorado State University,

Western Mountain Initiative. Cara Moore, Katie

Williams, Eric Richer, Jill Baron, Julie Markus, Jared

Heath & many others!

Soil moisture / Water content (θ) model state variable for water in the subsurface

Land surface model

Each cell = single value of θ 1D (vertical) exchange with

atmosphere

Distributed hydrologic model

Each cell = one or more values of θ 1D (vertical) exchange with atmosphere

2D or 3D redistribution of moisture

Hillslope 1: Andrews

3220

3230

3240

3250

3260

3270

3280

3290

0 100 200

Length (m)

Elevatio

n (m

)

NE-facing aspect

Average slope 31%

Hillslope characteristics