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Hydrologic and Erosional
Impacts of Altered Fire
Regimes and Plant Invasions
Fred Pierson - USDA - Agricultural Research Service
Northwest Watershed Research Center
Jeff Stone - USDA – Agricultural Research Service
Southwest Watershed Research Center
Problem: Change in Hydrologic
Function and Stability
Fire and related plant invasions are altering the
vegetative structure and function of desert
rangelands across the western United States.
Stable rangelands effectively capture and store
soil and water resources, safely release water,
resist alteration of these functions, and recover
these capacities post-disturbance (Pellant et al., 2005).
Hydrologic function is maintained
by site specific structural and spatial
soil and vegetation arrangements
that elicit hydrologic processes
favoring soil and water retention.
Vegetation/Hydrology Interactions
Vegetation
Canopy cover
Ground cover
Litter accumulation
Soil organic matter
Bulk density
Faunal activity
Hydrology/Erosion
Rainfall interception
Surface water storage
Higher infiltration
Lower soil erodibility
Soil/Site Characteristics
Slope/topography
Soil texture
Surface rock
Vegetation: Protection Against
Water/Wind Erosion
Site protection
Canopy cover
Ground cover
Bare soil
Hydrologic connectivity
Hydrology
Sediment detachment
Sediment transport
Concentrated flow
Flow velocity
Flow depth Percent Bare Ground
0 20 40 60 80 100Sed
imen
t C
on
cen
tra
tio
n (
g L
-1)
0
50
100
150
200
250
Flo
w V
elo
city
(m
s-1
)
0.00
0.05
0.10
0.15
0.20
0.25
0.30Sediment
Velocity
Hillslope -
Catchment
Scales
Climate: Driver for Disturbance
Precipitation and Climate
Snowfall vs. Rainfall
Frontal vs. Convective storms
Climate change?
Sed
imen
t Y
ield
(g m
-2)
0
50
100
150
200
250
00:00 05:00 10:00 15:00 20:00 25:00 30:00 35:00
Ru
noff
(m
m h
-1)
0
20
40
60
80
Time (mm:ss)
64 mm h-1
- Runoff
102 mm h-1
- Runoff 102 mm h-1
- Sediment
64 mm h-1
- Sediment
Hydrologic Response to Disturbance
Soil/site
propertiesVegetation
protection
Climate
drivers
Hydrologic
response
?
+ X =
Plot Scale:
Hydrologic Impacts of Fire
Reduced surface protection: Decreased rainfall interception and
surface water storage resulting in elevated rainsplash and sheetwash.
Unburned:
Runoff = 21 mm
Sediment = 7 g m-2
(strongly water
repellent)
Burned
Runoff = 25 mm
Sediment = 77 g m-2
(strongly water
repellent)
Shrub Coppice Interspace
Burned
Runoff = 32 mm
Sediment = 129 g m-2
(slightly water
repellent)
Unburned:
Runoff = 41 mm
Sediment = 10 g m-2
(slightly water
repellent)
Hillslope Scale:
Hydrologic Impacts of Fire
Decreased surface water storage and resistance to overland flow.
Increased spatial continuity of concentrated overland flow.
Increased concentrated flow velocity – greater erosive energy and transport capacity.
Fire effects typically greater and more persistent for sediment yield than runoff.
Percent Bare Ground
0 20 40 60 80 100
Vel
oci
ty (
m s
ec-1
)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Y = exp(0.015*X)*0.062
r2 = 0.70 n = 28
21 L min-1
Inflow
Cu
mu
lati
ve S
edim
ent
(g)
0
2000
4000
6000
8000
10000
12000
14000
16000
Year 0
Unburned
Year 2
Burned
Year 1
Burned
Year 0
Burned
Rill Erosion Rill Flow Velocity
Hillslope Scale:
Hydrologic Impacts of Fire
Hydrologic recovery depends on time required to achieve pre-fire
structural and functional plant community (surface protection).
Temporal variability may confound recovery assessments.
Percent Bare Ground
0 10 20 30 40 50 60 70 80 90 100
Sed
imen
t:R
un
off
Rati
o /
Rain
fall
(kg h
a-1
mm
-1)
0
5
10
15
20
25
30
35
Year 0 Unburned
Year 0 Burned
Year 1 Burned
Year 2 Burned
Y = 0.225 × exp(0.0514 × X)
r2 = 0.76 n = 26
Rainfall: 85 mm h-1, 1 h Plot Size: 32.5 m2
Time (min)
0 10 20 30 40 50 60
Infi
ltra
tio
n R
ate
(m
m h
-1)
30
40
50
60
70
80
90
Year 0 Burned Year 0 Unburned
Year 1 Burned Year 1 UnburnedYear 2 Burned Year 2 Unburned
Rainfall Rate: 85 mm h-1
Hillslope Scale:
Hydrologic Impacts of Fire
Plant Invasions: Annual Grasses
Sagebrush-Steppe CheatgrassSoil/site
properties
Vegetation
protection
Long-term changes?
• Extreme events
• Wind erosion
• Bulk density
• Soil organic matter
• Root structure/biomass
• Soil water repellency
• Faunal Activity
Rapid and uniform protection:
• Canopy cover
• Ground cover
• Litter accumulation
• Bare soil coverage
• Hydrologic connectivity
Altered Fire Regimes:
Annual Grasses
Shrub-Steppe Annual Grassland
Fire Return Interval =
3 to 10 yrs
Fire Return Interval =
10 to 70 yrs
Alteration of fuel structure and density with grass invasions increases the
expanse, uniformity, and frequency of wildfires.
Larger more frequent fires increase bare soil exposure over time and space
creating greater risk of large-scale flooding and soil loss from water and
wind erosion events.
Sagebrush-Steppe Juniper Woodland
Altered Fire Regimes:
Woody Plant Invasions
Soil/site
properties
Vegetation
protection
Gradual long-term increases:
• Bare soil coverage
• Hydrologic connectivity
• Runoff amount and frequency
• Surface soil loss
Gradual long-term decreases:
• Understory canopy cover
• Ground cover between trees
• Surface litter accumulation
• Bulk density
• Soil organic matter
Woody Plants:
Altered Site Stability
Juniper Woodland
Runoff (mm) Erosion (kg ha-1) Storm
Return
Period
(years)
Juniper
Woodland
Juniper
Removed
Juniper
Woodland
Juniper
Removed
2 0.1 0 7 0
4 0.5 0 34 0
10 1.1 0 70 0
50 4.1 0 269 0
100+ 13.5 1.0 1053 11
Storm: duration-depth-frequency
Rainfall = 53.3 mm h-1
Increased bare soil exposure facilitates
hydrologic connectivity of runoff areas
and amplifies loss of soil and water.
Cut Juniper Woodland
Climate
drivers
Vegetation
protection X
Woody Plants:
Altered Hydrologic Function
Hydrologic functions:
• Plant water use
• Timing and amount of stream flow
• Interception losses
• Precipitation distribution
• Soil protection – wind erosion
Scaling: Spatial and Temporal
1. Questions are asked at field, watershed, and basin scales.
2. Current understanding of hydrologic
function/stability is largely limited to
the plot and hillslope scales.
3. Generally assume greater off-site
impacts with increased size and
frequency of disturbance.
4. Long-term impacts of fire and
vegetation changes on landscape
development are unclear.
1. Scaling plot and hillslope level information to
watershed scales.
2. Long-term experimentation and analyses.
3. Probability-based analyses and predictions.
4. Hydrologic implications for plant invasions
across biomes.
5. Understanding of wind erosion is very limited.
Future: Research Needs
Less
Greater
Life
Property
Water Quality
Habitat
Site Productivity
Future: Managing Increased Risk
Increased probability of occurrence of damaging hydrologic events:
• Disturbances occurring more frequently
• Disturbances occurring over larger areas
• Potential for increased frequency of higher intensity storms
Challenge: Tools for Managers
• Simple and easy to use tools.
• Robust across variable
landscapes.
• Address multiple scales.
Need: Tools based on sound science with
supporting databases for management application.
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