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.