Runoff ProcessesRunoff Processes

What happens when we go from a landscape that looks like this …

Photo credit: Vermont Land Trust

to this?

Photo credit: Stowe Mountain Resort

… or this?

Photo credit: U.S. Geological Survey

OutlineOutline

1.1. Hydrologic cycleHydrologic cycle

2.2. Water balanceWater balance

3.3. Effects of landuse change on runoffEffects of landuse change on runoff

1. Hydrologic Cycle1. Hydrologic Cycle

Hydrologic CycleHydrologic Cycle

Describes the cycling of water through the atmosphere, biosphere and lithosphere. Includes the processes of:

• Condensation

• Precipitation

• Infiltration

• Evapotranspiration

• Runoff

Condensation:Condensation:Phase change of water from gaseous to liquid state.

Precipitation:Precipitation:Flux of water from atmosphere to earth surface.

Evapotranspiration:Evapotranspiration:Flux of water from earth surface to atmosphere.

Evaporation: flux from free water surfaces

Transpiration: flux from free water surfaces

• Phase change from liquid to gas

• Energy consumed in phase change = latent heat

InfiltrationInfiltration

Influenced by:

• Soil characteristics • Land cover• Precipitation rate

Vertical movement of water into the soil profile.

RunoffRunoffFlux of water through the lithosphere to rivers,

streams, ocean.

Includes:

• Overland flow • Subsurface flow

Soil permeability (tendency to soak up water)

Rainfallintensity

low

high

low high

humid regionshumid regions

arid regionsarid regionssubsurface flow

overland flow

Adapted from: Dunne & Leopold. Water in Environmental Planning.

Geographical patterns in runoffGeographical patterns in runoff

… but how do these patterns in runoff influence landforms?

gully erosion from overland flowgully erosion from overland flow

V-shaped valleyof a humid, temperate landscape

Photo credit: Field Studies Council, UK

Badland topographyof an arid landscape

Photo credit: National Park Service

… landforms reflect dominant runoff processes

2. Water Balance2. Water Balance

Water BalanceWater BalanceDescribes the relationship among fluxes of waterDescribes the relationship among fluxes of water

Precipitation

Evapotranspiration

Runoff

Water BalanceWater Balance

Land surface

Precipitation

EvaporationTranspiration

Runoff

Input

Output

Output

Land: Precipitation - Evapotranspiration = Runoff

3. Effects of landuse change3. Effects of landuse change

What happens when we go from a landscape that looks like this …

Photo credit: Vermont Land Trust

to this?

Photo credit: Stowe Mountain Resort

Watershed 1, H.J. Andrews Experimental Forest, Oregon

Stream gauging station, H.J. Andrews Experimental Forest, Oregon

Hydrograph: plot of runoff over time

time

Run

off

A

AA

BB

time

Run

off

B

Paired watershed studies

AA

BB

Runoff - A

Run

off

- B

Paired watershed studies

AA

BB

time

Run

off

A time

Run

off

B

Paired watershed studies

Paired watershed studies

AA

BB

Peak streamflow A

Pea

k st

ream

flow

B

Before clearing

After clearing

time

Run

off

before

after

Hydrologic effects of deforestation / development

• Higher peak stream flows• Greater annual water yield• Lower base flows• Faster runoff

Hubbard Brook (HB)

LeadingRidge (LR)

Fernow (FE)

Marcell (ME)

Hubbard Brook (HB)

LeadingRidge (LR)

Fernow (FE)

Marcell (ME)

Hydrologic effects of deforestation

First year increases in water yield after forest harvestingAdapted from Hornbeck et al., 1993. Journal of Hydrology, 150:323-344

FE

HB

HB

ME

FEFE

FE

LR

FEHB

LR

FE0

50

100

150

200

250

300

350

400

0 10 20 30 40 50 60 70 80 90 100

Percent forest area cleared

Incr

ease

in w

ater

yie

ld (

mm

)

Hydrologic effects of deforestation

from: Brown et al., 2005. A review of paired catchment studies for determining changes in water yield resulting from alterations in vegetation. J Hydrology 310: 28-61. .

Would you expect clearing for ski area development to have the same effects?

#

#

1 0 1

Kilometers

N

Ranch Brook

West Branch

Watershed area (km2)

9.6 11.7

Percent (%) of watershed in ski trails impervious surfaces * exposed bedrock

0.38 0.01 0.63

11.60

2.17 3.16

Landuse State forest, Nordic skiing

Alpine skiing

* includes buildings, paved or gravel roads and parking lots

The Mt. Mansfield paired-watershed study

RanchBrook

WestBranch

Photo credit: Stowe Mountain Resort

0 1 20.5Km

West Branch watershed

Existing Development – 17% cleared Proposed Development* – 24% cleared

* with Spruce Peak expansion

0.0

1.0

2.0

3.0

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

WY 2002

dis

ch

arg

e (

mm

/hr)

Ranch BrookWest Branch

WY 20012001

Runoff from ski area and “control” watershed

Ann

ual r

unof

f (m

m)

0

400

800

1200

1600

WY01 WY02 WY03

Ranch Brook

West Branch

Clearing for ski areas increases runoff

FE

HB

HB

ME

FEFE

FE

LR

FEHB

LR

FE

MM

0

50

100

150

200

250

300

350

400

0 10 20 30 40 50 60 70 80 90 100

Percent forest area cleared

Incr

ease

in w

ater

yie

ld (

mm

)

Increase in runoff from ski area developmentis greater than from traditional forest clearing