Post on 15-Jan-2016
transcript
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