Long-Term Effects of Wildfire on
Vegetation and Water Balance
in Yosemite’s Illilouette Creek Basin
Gabrielle Boisramé, Scott Stephens, Sally Thompson, Brandon
Collins, Miguel Naranjo, Julia Cavalli, and Melissa Ferriter
University of California at Berkeley
October 8, 2015
Fire in Western Mountain Watersheds
Dense, homogeneous forest
Infrequent, large, severe fires,
hydrology effects well studied.
Frequent, mixed severity fires,
virtually no previous work.
Outline
• Rare example of natural
fire regime (suppression
ended 1973).
• What are frequent fire’s
effects on vegetation
cover and water
availability?
• Flow data from gage on
Merced River.
• Remote sensing.
• Measured surface soil
moisture.
• Spatial modelling.
Seeing the Forest Without the Trees
Diverse post-fire vegetation and drought-resistant wetlands.
Seeing the Forest Without the Trees
Seeing the Forest Without the Trees
Precip.
P
E
T
P
Storage
E
Infil.
T
Storage
Changing Streamflow Characteristics
Since fire suppression ended…
– Runoff ratio increased very slightly
– Duration of spring snowmelt longer
– Flood frequency not significantly changed
Increases could be due to
climate, but fires not
causing negative effects.
Vegetation Change
1969, during fire
suppression
2012, after nearly 40 years
of burning
Source: Yosemite archives
Source: NAIP
Vegetation Change
1969
10km
2012
10km
Vegetation Change
80%
Percent of Vegetated Area
70%
1969
1987
2012
60%
50%
40%
30%
20%
10%
0%
Mixed Conifer Sparse Grassland
Shrub
Dense Grassland
Soil Moisture Data
Dependence on Vegetation
Distribution of VWC Under Conifers
Likelihood
Likelihood
Distribution of Water Content, Wetland Veg.
Water Content (%)
Water Content (%)
Extrapolating Soil Moisture
• Model soil moisture using statistical model
trained on:
– Vegetation
– Topography
– Fire History
Soil Moisture Maps
Weather held constant, vegetation and fire histories change
1969
1997
2014
New Weather Stations
Weather Station Findings
• Meadow site coldest on average
• 87% interception in closed canopy
• Only meadow site showed soil moisture
response to late summer rain (likely
interception in shrubs).
• Different from Yosemite Valley
– 4X more rain
– Colder by ~9°C.
RHESSys Model
Regional Hydro-Ecologic Simulation System
Tague and Band (2004)
Preliminary RHESSYs Model Results
mm
Evaporation+Transpiration
Date
• If whole watershed burned:
– 5% higher summer low flows
– Higher peak flows
– 4% Deeper snowpack on average.
– Slight average saturation deficit decrease (up to
70% at patch scale)
Spatial Differences in RHESSys
Less Storage
More Storage
Rock
Change in Saturation After Burning
Spatial Differences in RHESSys
April 28, 1968
Spatial Variability in the Field
How do veg. and water balance
change?
It Depends.
Conclusions
• Veg. is greatest predictor of soil moisture, but
topo and fire history also involved.
• Modeled moisture maps suggest greater storage
following fires (on average).
• Microscale water balance differences observable
in different veg. types.
• Perennially saturated areas present, even in
drought year.
• Spatially variable responses to fire.
Next Steps
• Study predictors of vegetation transitions.
• Analyze Landsat products for summer
moisture resilience.
• Put actual fire data into RHESSys model.
– Test mechanism of streamflow increase and soil
moisture distribution.
• Calculate water balance using weather station
data, including snowpack (coming soon).
Thank You:
- Professors Sally Thompson, Scott Stephens,
Naomi Tague, Mark Stacey, and others.
- All my Thompson lab labmates
- Summer interns and volunteers
- Sigma Xi Grants in Aid of Research
- Joint Fire Sciences
- Yosemite NP Wilderness Managers
May/June 2014 Data
100
VWC
VWC
100
50
0
0
0.5
Longitude
0
1
50
0
0.5
Elevation
VWC
VWC
0
0.5
Slope
1
0
0.5
RiverDist
1
100
VWC
1
0
0.5
Aspect
1
0
0.5
TWI
1
100
50
50
0
0.5
Latitude
50
0
1
100
0
0
100
VWC
VWC
100
0
50
50
0
0
1
2
3
Times Burned
4
1
40
20
0
VWC
0
0
1
2
Severity
0
Shrub, n=16,44,164,46,0
1
2
Times Burned
3
40
20
Other Cons, n=0,43,95,2,0
1
2
Times Burned
3
1
2
3
Times Burned
VWC
0
Wetland, n=1,0,7,40,89
40
20
3
Severity
0
2
3
Severity
Shrub, n=16,0,30,72,152
Other Cons, n=0,0,0,88,52
15
10
5
0
VWC
VWC
0
4
0
40
20
3
Severity
60
0
2
15
10
5
0
2
20
0
6
4
2
0
3
Meadow, n=0,0,11,59,0
40
2
3
Times Burned
VWC
5
Wetland, n=1,47,28,61,0
VWC
VWC
Meadow, n=0,0,34,36,0
2
3
Times Burned
10
Sparse, n=0,0,45,65,60
VWC
0
Dry Conifers, n=15,64,166,150,0
VWC
5
6
4
2
0
VWC
10
Sparse, n=0,6,132,32,0
VWC
VWC
Dry Conifers, n=15,65,256,34,25
4
40
20
0
0
2
3
Severity
4
3
4
Severity
Maximum Soil Moisture
Mix of long-term trends
in Landsat Indices
Fire
Fire
Wetness Index
Wet Meadows
Dry Meadows
1988
Time
2001
Next step: Change over summer