projecting the effects of climate change on riparian

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

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


•• 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


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


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


Mea

n Pa

tch

Wid

th (m

)

Salt cedarCottonwood

0102030405060708090

nochange


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?


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


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


Deg

rees

C

CurrentHadley ModelCanadian ModelNCAR Model

0

5

10

15

20

25

30


Deg

rees

C

CurrentHadley ModelCanadian ModelNCAR Model

Warmer Temperatures are Likely


0

1

2

3

4

5

6


Season

Deg

rees

C

Projected Changes in Mean Temperature by 2060 (SRAG 2000)

Warmer Temperatures are Likely


Season

Projected Changes in Daily Rainfall by 2060 (SRAG 2000)

-1.5

-1

-0.5

0

0.5

1

1.5

2

mm

/day


-

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


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


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


•• 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

projecting the effects of climate change on riparian

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