The Changing Gila River: Past, Present and Future David S. Gutzler

University of New Mexico gutzler@unm.edu

à How is the hydrograph on the upper Gila projected to change as climate warms up?

à Snowmelt runoff? Summer low flows?

à We'll use observations, coupled dynamical models, and simple statistical models

à Three ISC technical reports (2013, 2015, 2016)

New Mexico Water Conference October 6, 2016

AZ NM

Catron Co

Grant Co

Sierra Co

Gila gage

Average annual hydrograph: Upper Gila River

Horner and Dahm (2014) TNC Flow Needs Assessment

Feb - May Jun - Jul Aug Sep - Jan

Note! Mean >> Median flow most of the year Hydrograph ≠ Seasonal Precipitation

Mean

Median

Reconstructed Past Flows

Meko et al. (2010) treeflow.info

(Solomon gage)

Highly variable flow across decades (±30%) Flow largely correlated with precipitation

Observed Climate Variability and Change in NM

warming trend lots of variability in precipitation

NM Universities Task Force on Water Supply Vulnerabilities (2015)

Projected climate change in the 21st Century

WGI AR4 (2007)

US Mountain West is a "hot spot" for warming (already happening)

Temperature

Precipitation

Winter Summer

The big projected change: Decreasing snowpack

Large projected decrease in mean winter snowpack … especially where temperature is barely cold enough for snow

Diminished snowpack also melts earlier in the year ... hence earlier timing of snowmelt runoff peaks in seasonal hydrographs

Brown and Mote (2009)

NARCCAP Gila Basin: 21st Century change

Δ Median Snow Water

Oct Jan Apr Jul

0

Jon Lewis, UNM

Dynamical Projection Gila gage (BoR, 2011) median of

all 39 simulations

Largest decreases in Spring runoff season Little projected change in mid-winter months

or in summer months

1950 2000 2050 2100

1950 2000 2050 2100

8% reduction Dec-Jun (2021-2050) – (1951-2012)

Projected climate change (runoff season)

Climate models project continued, significant increase in temperature in 21st Century ... and a relatively modest decrease in P (modest relative to interannual/ decadal variability)

1950 2000 2050 2100

1950 2000 2050 2100

4

CMIP3 A1B

Statistical Projection: Gila gage, runoff season

Observed Temperature and Precipitation (1951-2012)

Regressed onto Observed Gila flow (1951-2012)

NM4 Jan-Apr Temperature plotted against Dec-Jun flow

NM4 Nov-Apr Precipitation plotted against Dec-Jun flow

1950 2000 2050 2100

1950 2000 2050 2100

Obs-based regression applied to projected Temp and Precip, to generate projected Gila flow

1950 2000 2050 2090

7.4% reduction Dec-Jun (2021-2050) – (1951-2012)

8% reduction Dec-Jun (2021-2050) – (1951-2012)

Dynamical projection average:

2. Future of Low Flows in the upper Gila basin

Feb - May Jun - Jul Aug Sep - Jan

Low Flow Season Hypothesis: Monthly Q = f(Qprev, Pconcurrent)

June streamflow: 39 simulations

878 max 607  

June: Decreasing flows across the distribution

95% 75%

25% 5%

50%

1951- 2001- 2000 2050

1950 2000 2050

June

str

eam

flow

[cf

s]

July streamflow: 39 simulations  

780 max 1279  

July: Little change in average or low flows; highest flows increase

Predictors of Observed Low-Flow Monthly Streamflow

0

50

100

150

200

250

0 100 200 300 400 500 600 700 800

(a) Q May vs June

June

ave

rage

flow

[cf

s]

May average flow [cfs]

0

50

100

150

200

250

0 50 100 150 200 250

(b) Q June vs July

July

ave

rage

flow

[cf

s]

June average flow [cfs]

r = 0.92 r = 0.24

0

50

100

150

200

250

0 1 2 3 4 5

(b) Q vs P July

July

ave

rage

flow

[cf

s]

July precipitation [in]

0

50

100

150

200

250

0 0.5 1 1.5 2 2.5

(a) Q vs P June

June

ave

rage

flow

[cf

s]

June precipitation [in]

r = 0.13 r = 0.60

Qprev

important for June flow Low Flow Season Hypothesis: Monthly Q = f(Qprev, Pconcurrent)

Pconcurrent important for July flow

June July

Conclusions: Climate Change & the Upper Gila River

1) Big projected temperature change (continuation of observed trend)

2) ~5-10% projected decrease in upper Gila River snowmelt runoff

(2021-2050) due to climate change

... relative to ~30% decadal variability due to natural precip fluctuations

3) Lowest flows and principal low-flow season changes projected in June Uncertain mean change in July, with higher variability

4) We can derive these results using multiple, complementary techniques à combining observations, dynamical and statistical models

5) What about other rivers in NM? …. Shaleene Chavarria's poster re URG

Thanks! UNM students: John Carilli, Jon Lewis, Justin O'Shea NM Interstate Stream Commission US Bureau of Reclamation, The Nature Conservancy