San Pedro River, Arizona, National Geographic Magazine
Projecting the Effects of Climate Projecting the Effects of Climate Change on Riparian Ecosystems in Change on Riparian Ecosystems in
the Southwest: The Upper San Pedro the Southwest: The Upper San Pedro as a Case Studyas a Case Study
Mark DixonMark Dixon11 and Juliet Strombergand Juliet Stromberg22
11University of South DakotaUniversity of South Dakota22Arizona State UniversityArizona State University
Climate and Riparian Areas Workshop, Casa Grande, ArizonaClimate and Riparian Areas Workshop, Casa Grande, ArizonaApril 11, 2007April 11, 2007
San Pedro River, Arizona, National Geographic Magazine
Yuma Clapper Rail
Photo from USFWS
Wilson’s Warbler
Photo ©Peter La Tourrette
Common Black-hawk
Photo by Cam MacDonald
Southwestern Willow Flycatcher
Photo from USGS
Western Yellow-billed Cuckoo
Photo by Robert Witzeman
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?
Human Impacts on SW Rivers
PrecipitationSnow
accumulationSnowmelt
Evapotranspiration
Infiltration & runoff
Mnt Front Recharge
Urban & AgWater demand
StreamDischarge
Projected Climate Change for SW by 2060 (SRAG 2000)
Hadley Center ModelCanadian Climate Centre ModelNCAR Regional Model
Season
0
1
2
3
4
5
6
Winter Spring Summer Fall
Season
Deg
rees
C
Change in Mean Temperature Change in Daily Precipitation
-1.5
-1
-0.5
0
0.5
1
1.5
2
Winter Spring Summer Fallm
m/d
ay-
1
-
0
1
2
3
4
5
6
Uncertainty in Future Climate
San Pedro River, Arizona, National Geographic Magazine
•• What are the potential effects of climate change What are the potential effects of climate change on riparian vegetation composition and on riparian vegetation composition and dynamics in the Southwest?dynamics in the Southwest?
•• Used the Upper San Pedro as a case studyUsed the Upper San Pedro as a case study
Research Question:Research Question:
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
MesquitebosqueCottonwood-Willow
Forest
Mesic grassland (Sacaton)
Saltcedarshrubland
Groundw
ater Decline
Populus or Salix
Tamarix
Perennial Flow
Ephemeral Flow
Intermittent Flow
Riverine marsh
Groundwater
Groundwater
Groundwater
Groundw
ater Decline
Populus or Salix
Tamarix
Perennial Flow
Ephemeral Flow
Intermittent Flow
Riverine marsh
Groundwater
Groundwater
Groundwater
Groundw
ater Decline
Populus or Salix
Tamarix
Perennial Flow
Ephemeral Flow
Intermittent Flow
Riverine marsh
Groundwater
Groundwater
Groundwater
Groundw
ater Decline
Populus or Salix
Tamarix
Perennial Flow
Ephemeral Flow
Intermittent Flow
Riverine marsh
Groundwater
Groundwater
Groundwater
Groundw
ater Decline
Populus or Salix
Tamarix
Perennial Flow
Ephemeral Flow
Intermittent Flow
Riverine marsh
Groundwater
Groundwater
Groundwater
Groundw
ater Decline
Populus or Salix
Tamarix
Perennial Flow
Ephemeral Flow
Intermittent Flow
Riverine marsh
Groundwater
Groundwater
Groundwater
Longitudinal Variation in Riparian Vegetation
San Pedro River 1890s
Turner at al. 2003The Changing Mile Revisited
San Pedro River 1962
Historic Vegetation
Change
© Photo by D. J. Huebner, U Texas, Austin
•• Regionally synchronous channel Regionally synchronous channel entrenchment (arroyo cutting) in entrenchment (arroyo cutting) in 1890s 1890s –– early 1900searly 1900s
•• Incision of 1Incision of 1--10 m on San Pedro10 m on San Pedro
•• Channel widening until ~1950sChannel widening until ~1950s
•• Channel narrowing & floodplain Channel narrowing & floodplain formation after 1950sformation after 1950s
•• Reduced rates of channel migration Reduced rates of channel migration since 1980ssince 1980s
Historic Geomorphic ChangeHistoric Geomorphic Change
Riparian Forest Expansion since 1950s1955 1996
San Pedro River, Arizona, National Geographic Magazine
What are the potential effects of climate What are the potential effects of climate change on riparian vegetation composition change on riparian vegetation composition and dynamics on the Upper San Pedro?and dynamics on the Upper San Pedro?
Research Question:Research Question:
Watershed Runoff Model (SWAT)
Channel Migration Model (MEANDER)
Stream Flow Projections
Daily Climate Land Cover
Site Conditions (Groundwater, etc.)
Projected Channel Migration
Riparian Vegetation Model
Projected Vegetation Change
Climate Change Scenarios
Watershed Runoff Model (SWAT)
Channel Migration Model (MEANDER)
Stream Flow Projections
Daily Climate Land Cover
Site Conditions (Groundwater, etc.)
Projected Channel Migration
Riparian Vegetation Model
Projected Vegetation Change
Climate Change Scenarios
Modeling Climate Change Effects• Simulate range of
transient climate change scenarios (2003-2102)
• Model effects of climate on physical processes (stream flow, channel migration)
• 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
• Modeling sites that span the range of hydro conditions along Upper San Pedro
Kolbe (Perennial)
-3.0-2.0-1.00.01.02.03.04.05.06.0
0 10 20 30 40 50 60 70 80 90 100 110
Distance W to E (m)
Elev
atio
n ab
ove
Thal
weg
(m)
Plot Elev GW Deep GW ShallowPalominas UA (Wet Intermittent)
-3.0-2.0-1.00.01.02.03.04.05.06.0
-25 0 25 50 75 100 125 150 175 200 225 250
Distance W to E (m)El
evat
ion
abov
e Th
alw
eg (m
)Plot Elev GW Deep GW ShallowContention (Dry Intermittent)
-3.0-2.0-1.00.01.02.03.04.05.06.0
0 40 80 120 160 200 240 280 320 360 400 440
Distance W to E (m)
Elev
atio
n ab
ove
Thal
weg
(m)
Plot Elev GW Max Depth GW Min Depth
Kolbe (Perennial)
-3.0-2.0-1.00.01.02.03.04.05.06.0
0 10 20 30 40 50 60 70 80 90 100 110
Distance W to E (m)
Elev
atio
n ab
ove
Thal
weg
(m)
Plot Elev GW Deep GW ShallowPalominas UA (Wet Intermittent)
-3.0-2.0-1.00.01.02.03.04.05.06.0
-25 0 25 50 75 100 125 150 175 200 225 250
Distance W to E (m)El
evat
ion
abov
e Th
alw
eg (m
)Plot Elev GW Deep GW ShallowContention (Dry Intermittent)
-3.0-2.0-1.00.01.02.03.04.05.06.0
0 40 80 120 160 200 240 280 320 360 400 440
Distance W to E (m)
Elev
atio
n ab
ove
Thal
weg
(m)
Plot Elev GW Max Depth GW Min Depth
Modeling Watershed Runoff and Streamflow
• Flow scenarios– Ran calibrated basin runoff
model (SWAT) for the upper San Pedro basin for the five climate scenarios
#
#
#
#
#
#
#
#
#
#
4
14
11
6
9
3
1
10
2
13
15
17
12
7
16
85
20680
22140
22902
23120
27445
27880
28619
29562
Upper San Pedro Sub-basins
Graphic courtesy of M. Hernandez, ARS, Tucson
0
100
200
300
400
500
600
700
2002 2012 2022 2032 2042 2052 2062 2072 2082 2092 2102
Simulation Year
Max
imum
Flo
w (m
3 /s)
no climate change
0
100
200
300
400
500
600
700
2002 2012 2022 2032 2042 2052 2062 2072 2082 2092 2102
Simulation YearM
axim
um F
low
(m3 /s
) warm dry (-50%)
0
200
400
600
800
1000
1200
1400
2002 2012 2022 2032 2042 2052 2062 2072 2082 2092 2102
Simulation Year
Max
imum
Flo
w (m
3 /s) warm very wet (+100%)
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
• Projected veg changes under the 5 climate scenarios
• Ran fine-scale model multiple times to scale results up to patch and site
age mortality
max basal area
tree density
stress mortality
~groundwater effect
max life span
DBH
diameter growth
Basal area
max growth rate
crowding effect
saplings to trees
disturb effect
groundwater depth
relative growth conditions
flood erosion
Fire
Results
Results: Hydrologic
Change• Major increase in flood
magnitude & frequency under wetter scenarios
• Near cessation of winter floods under warm dry scenario
0
100
200
300
400
500
600
700
2002 2012 2022 2032 2042 2052 2062 2072 2082 2092 2102
Simulation Year
Max
imum
Flo
w (m
3 /s)
No Climate Change
0
100
200
300
400
500
600
700
2002 2012 2022 2032 2042 2052 2062 2072 2082 2092 2102
Simulation YearM
axim
um F
low
(m3 /s
) Warm Dry (-50%)
0
200
400
600
800
1000
1200
1400
2002 2012 2022 2032 2042 2052 2062 2072 2082 2092 2102
Simulation Year
Max
imum
Flo
w (m
3 /s) Warm Very Wet (+100%)
Results: Channel Migration• Strong differences in simulated channel
migration among scenarios & sites
0
50
100
150
200
250
Her
efor
d
Kol
be
Mos
on
Palo
3
Palo
UA
Sum
mer
s
Con
tent
ion
St. D
avid
Sites
Tota
l Wid
th R
ewor
ked
(m)
No ChangeHistoricalWetVery WetWarmDry
Results: Vegetation Change• Cottonwood-willow patch
width declines under all scenarios
• Coverage of mesquite increases dramatically
04080
120160200240280
2003 2023 2043 2063 2083 2103
Year
Tota
l Pat
ch W
idth
(m)
04080
120160200
2003 2023 2043 2063 2083 2103
Year
Tota
l Pat
ch W
idt
240280
h (m
)
04080
120160200
2003 2023 2043 2063 2083 2103
Year
Tota
l Pat
ch W
idt
240280
h (m
)
WARM VERY WET
NO CHANGE
WARM DRY
CottonwoodMesquiteSalt cedarAshSacatonOther
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 Warm Dry Warm Very Wet
Cottonwood Mesquite Sacaton Other Eroded
Recruitment of New Patches
• New cottonwood & saltcedar patches form by channel migration
• But, recruitment is insufficient to balance senescence of old stands
• Greatest cottonwood decline under driest scenario, least under wettest
% Change in Total Coverage
-100%
-80%
-60%
-40%
-20%
0%
20%
nochange
warm wet very wet dry
% C
hang
e in
Wid
th
cottonwoodsaltcedar
New Recruitment
0
10
20
3040
50
60
70
nochange
warm wet very wet dry
Mea
n W
idth
of N
ewPa
tche
s (m
)
Results: Vegetation Change• Least reduction of pioneer species (cottonwood,
saltcedar) at most geomorphically dynamic sites
0.0%
50.0%
100.0%
150.0%
200.0%
250.0%H
eref
ord
Kol
be
Mos
on
Palo
3
Palo
UA
Sum
mer
s
Con
tent
ion
St. D
avid
Sites
% F
lood
plai
n R
ewor
ked
No ChangeHistoricalWetVery WetWarmDry
Results: Vegetation
Change
0102030405060708090
nochange
warm wet very wet dry
Mea
n Pa
tch
Wid
th (m
)
Salt cedarCottonwood
0102030405060708090
nochange
warm wet very wet dry
Mea
n Pa
tch
Wid
th (m
)
Wet Sites
Dry Sites
• Site hydrology influences recruitment of cottonwood vs. saltcedar
• Suggests that future groundwater conditions will have strong effect on composition
Photo by Mark Anderson, USGS
Drought Effects on San Pedro River (2005)Drought Effects on San Pedro River (2005)
Photo by Josh Ayers, USGS
Photo from AZ Daily StarGraphic 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
• 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
Acknowledgments
1955
2002
1973
1978 1996
1983
• Traced channel centerline on historic aerial photos
• Calculated area “reworked” by the river between dates
• Related to historic flows (cumulative stream power) between photo dates
Quantifying Historic Channel Migration
Functions of FloodsFunctions of FloodsWinter floods (longer duration)Winter floods (longer duration)•• Move channelsMove channels•• Stimulate cottonwoodStimulate cottonwood--willow and sycamore recruitmentwillow and sycamore recruitment•• ReRe--soak floodplains and help sustain soak floodplains and help sustain baseflowsbaseflows
Summer floods (short duration)Summer floods (short duration)•• Stimulate growth of annual and perennial plants and Stimulate growth of annual and perennial plants and
recruitment of mesquiterecruitment of mesquite•• ReRe--soak floodplain and sustain soak floodplain and sustain baseflowsbaseflows•• Move leaf litter and stimulate decompositionMove 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
-400
4080
120160200
winter spring summer autumnSeason
Prec
ipita
tion
(mm
)
Current Hadley Canadian NCAR
Projected Precipitation Changes at Tombstone by 2060 (SRAG 2000)
Projected Temperature Changes at Tombstone by 2060 (SRAG 2000)
0
5
10
15
20
25
30
winter spring summer autumnSeason
Deg
rees
C
CurrentHadley ModelCanadian ModelNCAR Model
0
5
10
15
20
25
30
winter spring summer autumnSeason
Deg
rees
C
CurrentHadley ModelCanadian ModelNCAR Model
Warmer Temperatures are Likely
Hadley Center ModelCanadian Climate Centre ModelNCAR Regional Model
0
1
2
3
4
5
6
Winter Spring Summer Fall
Season
Deg
rees
C
Projected Changes in Mean Temperature by 2060 (SRAG 2000)
Warmer Temperatures are Likely
Hadley Center ModelCanadian Climate Centre ModelNCAR Regional Model
Season
Projected Changes in Daily Rainfall by 2060 (SRAG 2000)
-1.5
-1
-0.5
0
0.5
1
1.5
2
mm
/day
Winter Spring Summer Fall
-
1
-
Changes in Rainfall are Uncertain
Riparian pioneer plants (e.g., cottonwood, willow) depend on flows to disperse seeds and establish on open, moist sediment bars.
•• Large, long duration winter/spring Large, long duration winter/spring floods move and deposit sediment, floods move and deposit sediment, preparing bare surfaces suitable for preparing bare surfaces suitable for seedling growthseedling growth
•• Slowly receding spring floods Slowly receding spring floods deposit seeds and provide moisture deposit seeds and provide moisture for growthfor growth
Cottonwood and willow establish after Cottonwood and willow establish after winter/spring floods of appropriate size winter/spring floods of appropriate size and timingand timing
0
5
10
15
20
25
30
35
Mea
n da
ily fl
ow (m
3 /s)
ON
DJ
FM
AM
JJ
AS
Month
1987
1988
1989
1990
1992
1994
Non-recruitment years
0
5
10
15
20
25
30
35
Mea
n da
ily fl
ow (m
3 /s)
ON
DJ
FM
AM
JJ
AS
Month
1987
1988
1989
1990
1992
1994
Non-recruitment years
0
5
10
15
20
25
30
35
Mea
n da
ily fl
ow (m
3 /s)
ON
DJ
FM
AM
JJ
AS
Month
1987
1988
1989
1990
1992
1994
Non-recruitment years
0
5
10
15
20
25
30
35
Mea
n da
ily fl
ow (m
3 /s)
ON
DJ
FM
AM
JJ
AS
1991
1993
1995
Recent cottonwood recruitment years
0
5
10
15
20
25
30
35
Mea
n da
ily fl
ow (m
3 /s)
ON
DJ
FM
AM
JJ
AS
1991
1993
1995
Recent cottonwood recruitment years
Effects of Cattle Removal
1984
1998
Following cattle removal in 1988…
• Expansion of riparian vegetation
• Channel narrowing & stabilization
Photos by BLM
View from Hereford Bridge
San Pedro River, Arizona, National Geographic Magazine
•• Filter, bufferFilter, buffer–– Protect aquatic resources from pollutionProtect aquatic resources from pollution
Ecological Importance of Ecological Importance of Riparian ZonesRiparian Zones
•• CorridorCorridor–– organism movements, population viabilityorganism movements, population viability
•• BiodiversityBiodiversity–– high diversity of species and habitatshigh diversity of species and habitats–– ““green ribbongreen ribbon”” across arid landscapesacross arid landscapes
Rain
Floods
Regional Aquifer
Floodplain Aquifer
Surface Flow
Pumping
ET
Riparian Vegetation
ET
Climate
Temp.
Bank recharge
Subsurface flow
RunoffInfiltration,
recharge
Stream capture
Temp.
Fire