Projecting the Effects of Climate
Change on Riparian Ecosystems in
the Southwest: The Upper San Pedro
as a Case Study
Mark Dixon1 and Juliet Stromberg2
1University
of South Dakota
2Arizona State University
Climate and Riparian Areas Workshop, Casa Grande, Arizona
April 11, 2007
San Pedro River, Arizona, National Geographic Magazine
San Pedro River, Arizona, National Geographic Magazine
Southwestern Willow Flycatcher
Photo from USGS
Western Yellow-billed
Cuckoo
Common
Black-hawk
Photo by Robert Witzeman
Photo by Cam MacDonald
Yuma Clapper
Rail
Wilson’s
Warbler
Photo from USFWS
Photo ©Peter La Tourrette
Human Impacts on SW Rivers
River damming for water
storage, hydropower, flood
control
Surface water diversion
Ground water pumping from
stream and regional aquifers
Over-use by cattle
Climate Change?
Precipitation
Snow
accumulation
Snowmelt
Urban & Ag
Water demand
Mnt Front
Recharge
Evapotranspiration
Infiltration
& runoff
Stream
Discharge
Uncertainty in Future Climate
Projected Climate Change for SW by 2060 (SRAG 2000)
Change in Daily Precipitation
6
2
5
1.5
mm/day
Degrees C
Change in Mean Temperature
4
3
2
1
0.5
0
--0.5
1
-1
1
0
--1.5
Winter
Spring Summer
Fall
Winter
Season
Hadley Center Model
Canadian Climate Centre Model
NCAR Regional Model
Spring Summer
Season
Fall
Research Question:
• What are the potential effects of climate change
on riparian vegetation composition and
dynamics in the Southwest?
• Used the Upper San Pedro as a case study
San Pedro River, Arizona, National Geographic Magazine
THE UPPER SAN PEDRO (SE Arizona)
• One of few undammed, low elevation
rivers in the SW US with perennial flow
• Biodiversity hotspot ( “Last Great Places”)
• Valuable migratory bird habitat
• Growing human population & riparian
ecosystem both depend on groundwater
• Threats from groundwater overdrafts
Saltcedar
shrubland
Cottonwood-Willow
Forest
Mesic grassland
(Sacaton)
Mesquite
bosque
Longitudinal Variation in Riparian Vegetation
Perennial Flow
Groundwater
Groundwater Decline
Intermittent Flow
Groundwater
Ephemeral Flow
Groundwater
Tamarix
Populus or
Salix
Riverine
marsh
San Pedro River 1890s
Historic
Vegetation
Change
San Pedro River 1962
Turner at al. 2003
The Changing Mile Revisited
Historic Geomorphic Change
• Regionally synchronous channel
entrenchment (arroyo cutting) in
1890s – early 1900s
• Incision of 1-10 m on San Pedro
• Channel widening until ~1950s
• Channel narrowing & floodplain
formation after 1950s
• Reduced rates of channel migration
since 1980s
©
Photo by D. J. Huebner, U Texas, Austin
Riparian Forest Expansion since 1950s
1955
1996
Research Question:
What are the potential effects of climate
change on riparian vegetation composition
and dynamics on the Upper San Pedro?
San Pedro River, Arizona, National Geographic Magazine
Modeling Climate Change Effects
Climate Change
Scenarios
Daily Climate
Land Cover
Watershed Runoff
Model (SWAT)
Stream Flow Projections
Channel Migration
Model (MEANDER)
Projected Channel Migration
Riparian
Vegetation Model
Projected Vegetation Change
• Simulate range of
transient climate
change scenarios
(2003-2102)
• Model effects of climate
on physical processes
(stream flow, channel
migration)
Site Conditions
(Groundwater, etc.)
• Model response of
vegetation to changes
in physical (and biotic)
drivers
Climate Change Scenarios
(2003-2102)
•
•
•
•
•
No change over 1951-2002 conditions
Warm: Warmer (+5 º C), but no change in precipitation
Warm Wet: Warmer, 50% increase in winter precipitation
Warm Very Wet: Warmer, 100% increase in winter precip
Warm Dry: Warmer, 50% decrease in winter precipitation
• Transient scenarios, developed by modifying 1951-2002
daily weather time series
Elevation above
Thalweg (m)
Elevation above
Thalweg (m)
6.0
5.0
4.0
3.0
2.0
1.0
0.0
-1.0
-2.0
-3.0
Elevation above
Thalweg (m)
• Modeling sites that
span the range of
hydro conditions
along Upper San
Pedro
6.0
5.0
4.0
3.0
2.0
1.0
0.0
-1.0
-2.0
-3.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
-1.0
-2.0
-3.0
Kolbe (Perennial)
0
10
20
30
40
50
60
70
80
90
100
110
Distance W to E (m)
Palominas
UA
(Wet Intermittent)
Plot Elev
GW Deep
GW Shallow
-25
0
25
50
75
100
125
150
175
200
225
250
Distance W to E (m)
Contention (Dry Intermittent)
Plot Elev
0
40
80
GW Deep
GW Shallow
120 160 200 240 280 320 360 400 440
Distance W to E (m)
Plot Elev
GW Max Depth
GW Min Depth
• Flow scenarios
Maximum Flow (m 3/s)
– Ran calibrated basin runoff
model (SWAT) for the upper
San Pedro basin for the five
climate scenarios
15
20680
17
16
#
#
27445
#
14
22902 28619
#
#
23120
#
#
27880
9
13
1129562
#
22140
#
10
85
7
6
2
1
4
3
12
Maximum Flow (m 3/s)
Upper San Pedro
Sub-basins
Maximum Flow (m3/s)
Modeling Watershed Runoff and
Streamflow
700
no climate change
600
500
400
300
200
100
0
2002 2012 2022 2032 2042 2052 2062 2072 2082 2092 2102
700
Simulation Year
warm dry (-50%)
600
500
400
300
200
100
0
2002 2012 2022 2032 2042 2052 2062 2072 2082 2092 2102
1400
1200
Simulation
warm very wet
(+100%)Year
1000
800
600
400
200
0
2002 2012 2022 2032 2042 2052 2062 2072 2082 2092 2102
Simulation Year
Graphic courtesy of M. Hernandez, ARS, Tucson
Modeling Channel Dynamics
• Simulated channel migration
(2003-2102) using MEANDER
(Eric Larsen, UC-Davis)
– Calibrated against historic conditions
• Ran with simulated flows from
the 5 climate scenarios
• Assumed channel movement
creates recruitment sites for
pioneer species (cottonwood,
willow, tamarisk)
Modeling Vegetation Dynamics
• Designed fine-scale riparian
vegetation model in STELLA
• Simulated reproduction,
growth, survival of 11
dominant plant species
– Competition for light and water
– Ecological differences among
species
– Initial veg., site hydrology &
channel migration
max life span
tree density
saplings to trees
age mortality
Fire
disturb effect
stress mortality
max basal area
flood erosion
Basal area
DBH
crowding effect
diameter growth
• Projected veg changes under
the 5 climate scenarios
max growth rate
groundwater depth
• Ran fine-scale model multiple
times to scale results up to
patch and site
~
groundwater effect
relative growth conditions
Results
• Near cessation of winter
floods under warm dry
scenario
Maximum Flow (m3/s)
Maximum Flow (m3/s)
• Major increase in flood
magnitude & frequency
under wetter scenarios
Maximum Flow (m3/s)
Results:
Hydrologic
Change
700
No Climate Change
600
500
400
300
200
100
0
2002 2012 2022 2032 2042 2052 2062 2072 2082 2092 2102
700
Simulation Year
Warm Dry (-50%)
600
500
400
300
200
100
0
2002 2012 2022 2032 2042 2052 2062 2072 2082 2092 2102
1400
1200
Simulation
Warm Very Wet
(+100%) Year
1000
800
600
400
200
0
2002 2012 2022 2032 2042 2052 2062 2072 2082 2092 2102
Simulation Year
Results: Channel Migration
250
No Change
200
Historical
150
W et
Very W et
100
W arm
50
Dry
Sites
St. David
Contention
Summers
Palo UA
Palo 3
Moson
Kolbe
0
Hereford
Total Width Reworked (m)
• Strong differences in simulated channel
migration among scenarios & sites
• Coverage of mesquite
increases dramatically
Total Patch Width (m)
• Cottonwood-willow patch
width declines under all
scenarios
Total Patch Width (m)
Results:
Vegetation Change
280
240
NO CHANGE
200
160
120
80
40
0
2003
280
240
WARM DRY
Total Patch Width (m)
2043
2063
2083
2103
2083
2103
2083
2103
Year
200
160
120
80
40
0
2003
Cottonwood
Mesquite
Salt cedar
Ash
Sacaton
Other
2023
280
240
2023
2043
2063
Year
WARM VERY WET
200
160
120
80
40
0
2003
2023
2043
2063
Year
Loss of Original Cottonwood Patches
• >90% of original cottonwood patches gone by 2102
– Most established in 1960s and 1970s
– Senesce as approach 100 years old
– Patches convert to mesquite under wetter conditions,
sacaton grasslands under drier
No Change
Cottonwood
Warm Dry
Mesquite
Sacaton
Warm Very Wet
Other
Eroded
• New cottonwood &
saltcedar patches form
by channel migration
Mean Width of New
Patches (m)
Recruitment of
New Patches
New Recruitment
70
60
50
40
30
20
10
0
no
change
• Greatest cottonwood
decline under driest
scenario, least under
wettest
20%
% Change in Width
• But, recruitment is
insufficient to balance
senescence of old
stands
warm
wet
very wet
dry
% Change in Total Coverage
0%
-20%
-40%
-60%
cottonwood
saltcedar
-80%
-100%
no
change
warm
wet
very wet
dry
Results: Vegetation Change
250.0%
No Change
200.0%
Historical
150.0%
Wet
Very Wet
100.0%
Warm
50.0%
Dry
Sites
St. David
Contention
Summers
Palo UA
Palo 3
Moson
Kolbe
0.0%
Hereford
% Floodplain Reworked
• Least reduction of pioneer species (cottonwood,
saltcedar) at most geomorphically dynamic sites
• Suggests that future
groundwater conditions
will have strong effect
on composition
Mean Patch Width (m)
• Site hydrology
influences recruitment
of cottonwood vs.
saltcedar
90
80
70
60
50
40
30
20
10
0
Wet Sites
Salt cedar
Cottonwood
no
change
warm
wet
very wet
dry
Dry Sites
Mean Patch Width (m)
Results:
Vegetation
Change
90
80
70
60
50
40
30
20
10
0
no
change
warm
wet
very wet
dry
Drought Effects on San Pedro River (2005)
Photo by Josh Ayers, USGS
Photo from AZ Daily Star
Photo by Mark Anderson, USGS
Graphic from AZ Daily Star
Keys to Future Change on the
Upper San Pedro
• Successional processes
– Cottonwoods and willows are short-lived pioneer
species that require fluvial disturbance to persist
• Geomorphic legacies
– Riparian forest distribution is a legacy of the San
Pedro’s geomorphic history, not in equilibrium with
present processes
• Future geomorphic constraints
– Riparian forest coverage will depend, in part, on future
flood disturbance and geomorphic processes
• Human exploitation of groundwater
– Human water use will strongly influence future
species composition (saltcedar vs. cottonwood)
Conclusions
• Uncertainty about how climate will change
• Effects of climate change on riparian vegetation
may depend on:
– Population age structure, successional trajectories of
vegetation
– Effects on disturbance – floods, fire
– Geomorphic context, history, and trajectory
– Human uses of water resources
• Present riparian zones are a product of past events
Acknowledgments
•
Sharon Lite, Tyler Rychener (Stromberg lab, ASU)
•
Jingle Wu and Alex Buyantuyev (Landscape ecology lab, ASU)
•
Eric Larsen and Alex Fremier (UC-Davis) – Channel migration modeling
•
Mariano Hernandez & Dave Goodrich (ARS) – help with SWAT model
•
Jeff Price (CSU-Chico) and Hector Galbraith – Climate change project
•
Funding by SAHRA, Department of Defense Legacy Program, Upper
San Pedro Partnership, US EPA & American Bird Conservancy
Quantifying Historic Channel Migration
• Traced channel
centerline on historic
aerial photos
1973
1955
• Calculated area
“reworked” by the river
between dates
1978
• Related to historic
flows (cumulative
stream power) between
photo dates
1996
2002
1983
Functions of Floods
Winter floods (longer duration)
• Move channels
• Stimulate cottonwood-willow and sycamore recruitment
• Re-soak floodplains and help sustain baseflows
Summer floods (short duration)
• Stimulate growth of annual and perennial plants and
recruitment of mesquite
• Re-soak floodplain and sustain baseflows
• Move leaf litter and stimulate decomposition
Possible Effects of Climate Change
on Riparian Ecosystems
Increased temperatures
• Longer growing seasons
• Expansion of mesquite and saltcedar to higher
elevations?
• Higher evaporative demand and groundwater uptake
by phreatophytes
• Higher water use by human ecosystems?
• Greater risk of groundwater decline, channel drying,
and degradation of riparian zones?
Possible Effects of Climate Change
on Riparian Ecosystems
Increased winter precipitation
• Increased establishment of pioneer trees
(cottonwood, willow, salt cedar)
• Increased rates of river channel migration
• Increased re-soaking of floodplain soils and
floodplain aquifers
• Greater recharge of regional aquifer
• Greater plant growth during the dry season
Possible Effects of Climate Change
on Riparian Ecosystems
Decreased precipitation
• Decreased flooding and establishment of riparian
trees (but greater vulnerability to erosive floods)
• Decreased re-soaking of floodplain soils and aquifer
• Decreased recharge of regional aquifer
• Lower plant growth during the dry season
• With higher temp, greater stress on riparian
ecosystems
• Channel drying and shifts to drought tolerant species
(e.g., mesquite, saltcedar)
Changes in Rainfall are Uncertain
Precipitation (mm)
Projected Precipitation Changes at
Tombstone by 2060 (SRAG 2000)
200
160
120
80
40
0
-40
winter
spring
summer
autumn
Season
Current
Hadley
Canadian
NCAR
Warmer Temperatures are Likely
Projected Temperature Changes at
Tombstone by 2060 (SRAG 2000)
Current
Degrees C
30
Hadley Model
25
Canadian Model
20
NCAR Model
15
10
5
0
winter
spring
summer autumn
Season
Warmer Temperatures are Likely
Projected Changes in Mean
Temperature by 2060 (SRAG 2000)
6
Degrees C
5
4
3
2
1
0
Winter
Spring Summer
Fall
Season
Hadley Center Model
Canadian Climate Centre Model
NCAR Regional Model
Changes in Rainfall are Uncertain
Projected Changes in Daily Rainfall
by 2060 (SRAG 2000)
2
mm/day
1.5
1
0.5
0
--0.5
-1
1
--1.5
Winter
Spring Summer
Fall
Season
Hadley Center Model
Canadian Climate Centre Model
NCAR Regional Model
Riparian pioneer plants (e.g., cottonwood,
willow) depend on flows to disperse seeds
and establish on open, moist sediment bars.
Cottonwood and willow establish after
winter/spring floods of appropriate size
and timing
• Large, long duration winter/spring
floods move and deposit sediment,
preparing bare surfaces suitable for
seedling growth
Mean daily flow (m3/s)
Recent cottonwood recruitment years
35
30
25
1991
20
1993
15
1995
10
5
0
O
N
D
J
F
M
A
M
J
J
A
S
• Slowly receding spring floods
deposit seeds and provide moisture
for growth
Mean daily flow (m3/s)
Non-recruitment years
35
1987
30
1988
25
1989
20
15
1990
10
1992
5
0
O
N
D
J
F
M
A
M
Month
J
J
A
1994
S
Effects of Cattle Removal
View from Hereford Bridge
1984
Following cattle removal in
1988…
• Expansion of riparian
vegetation
1998
• Channel narrowing &
stabilization
Photos by BLM
Ecological Importance of
Riparian Zones
• Filter, buffer
– Protect aquatic resources from pollution
• Corridor
– organism movements, population viability
• Biodiversity
– high diversity of species and habitats
– “green ribbon” across arid landscapes
San Pedro River, Arizona, National Geographic Magazine
Temp.
Climate
ET
Fire
Rain
Runoff
Infiltration,
recharge
Floods
Riparian
Vegetation
Bank recharge
Floodplain
Aquifer
Stream
capture
Subsurface
flow
Regional
Aquifer
Pumping
ET
Surface
Flow
Temp.