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Sensitivity of Colorado Stream Flows to Climate Change. Dennis P. Lettenmaier Department of Civil and Environmental Engineering University of Washington Ninth SAHRA Annual Meeting Tucson September 23, 2009. Outline of this talk. Review of recent studies

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sensitivity of colorado stream flows to climate change
Sensitivity of Colorado Stream Flows to Climate Change

Dennis P. Lettenmaier

Department of Civil and Environmental Engineering

University of Washington

Ninth SAHRA Annual Meeting

Tucson

September 23, 2009

outline of this talk
Outline of this talk

Review of recent studies

Understanding the hydrologic sensitivities

Unanswered questions

slide3

+25%

58%

+10%

67%

Increase

62%

+5%

58%

87%

96%

+2%

62%

62%

71%

87%

-2%

75%

67%

67%

67%

-5%

100%

Decrease

-10%

-25%

Magnitude and Consistency of Model-Projected Changesin Annual Runoff by Water Resources Region, 2041-2060

Median change in annual runoff from 24 numerical experiments (color scale)

and fraction of 24 experiments producing common direction of change (insetnumerical values).

(After Milly, P.C.D., K.A. Dunne, A.V. Vecchia, Global pattern of trends in streamflow andwater availability in a changing climate, Nature, 438, 347-350, 2005.)

slide4

from Seager et al, Science, 2007

Means, replotted for Colorado River basin

time series annual average

Hydrology and water management implications

Time series Annual Average

PCM Projected Colorado R. Temperature

ctrl. avg.

hist. avg.

Period 1 2010-2039 Period 2 2040-2069Period 3 2070-2098

timeseries annual average
Timeseries Annual Average

PCM Projected Colorado R. Precipitation

hist. avg.

ctrl. avg.

Period 1 2010-2039 Period 2 2040-2069Period 3 2070-2098

annual average hydrograph
Annual Average Hydrograph

Simulated Historic (1950-1999)Period 1 (2010-2039)Control (static 1995 climate) Period 2 (2040-2069)Period 3 (2070-2098)

slide9

CRRM

  • Historic Streamflows to Validate
  • Projected Inflows to assess future performance of system
  • Basin storage aggregated into 4 storage reservoirs
    • Lake Powell and Lake Mead have 85% of basin storage
  • Monthly timestep
  • Reservoir evaporation = f(reservoir surface area, mean monthly temperature)
  • Hydropower = f(release, reservoir elevation)

Storage ReservoirsRun of River Reservoirs

slide12
Postmortem: Christensen and Lettenmaier (HESSD, 2007) – multimodel ensemble analysis with 11 IPCC AR4 models (downscaled as in C&L, 2004)
question why such a large discrepancy in projected colorado river flow changes
Question: Why such a large discrepancy in projected Colorado River flow changes?

~6=7% annual flow reduction in Christensen and Lettenmaier (2007)

10-25% by Milly et al (2005)

> 35% by Seager et al (2007)

slide18

Diagnosis

  • Wood et al (2002; 2004) downscaling method removes bias by mapping from PDF of GCM output to PDF of observations on a monthly basis
  • PDFs are estimated for each grid cell and month of the year
  • This same mapping is then applied to the future climate run.
  • The method does not attempt to preserve GCM inferred differences in precipitation.
  • There is in general no reason to assume that the GCM precipitation changes are applicable to higher spatial resolutions
slide19

CL2007 Re-runs

  • All precipitation values were rescaled so as to match GCM changes on an annual basis
  • This resulted in a change (reduction) in mean annual precipitation for 2040-2070 from 1.9% (CL2007) to 2.6% for A2 emissions scenario (closest to A1B used in M2005 and S2007)
  • The associated annual mean runoff reduction (Imperial Dam, averaged over 11 GCMs) changed from 5.9 to 10.0%
  • This is within (although at the lower end of) the range reported in M2005
  • Note that M2005 and S2007 use the A1B IPCC emissions scenario, vs A2 scenario used by CL2007
  • M2007 and S2007 use (partially) different GCM runs and procedures (M2005 count multiple ensembles from a single GCM as separate runs 
slide21
Dooge (1992; 1999):

where ΨP is elasticity of runoff with respect to precipitation

For temperature, it’s more convenient to think in terms of sensitivity (v. elasticity)

slide22

Inferred runoff elasticities wrt precipitation for major Colorado River tributaries, using method of Sankarasubramanian and Vogel (2001)

Visual courtesy Hugo Hidalgo, Scripps Institution of Oceanography

slide23

Summary of precipitation elasticities and temperatures sensitivities for Colorado River at Lees Ferry for VIC, NOAH, and SAC models

slide24

Spatial distribution of precipitation elasticities

Censored spatial distribution of annual runoff

slide25

VIC Precipitation elasticity histograms, all grid cells and 25% of grid cells producing most (~73%) of runoff

slide27

Temperature sensitivity (equal change in Tmin and Tmax) histograms, all grid cells and 25% of grid cells producing most (~73%) of runoff

slide28

Spatial distribution of temperature sensitivities (equal changes in Tmin and Tmax)

Censored spatial distribution of annual runoff

slide29

Composite seasonal water cycle, by quartile of the temperature sensitivity (equal change in Tmin and Tmax) distribution

slide30
So is there, or is there not, a dichotomy between the various estimates of mid-century Colorado River runoff changes?

Replotted from Seager et al (2007)

slide31

a) Lowest mid-century estimate (Christensen and Lettenmaier, 2007) is based on a precipitation downscaling method that yields smaller mid-century precipitation changes. Adjusting for this difference nearly doubles the projected change to around 10% by mid century – not far from Milly et al (2005), but still well below Seager et al (2007)

b) On the other hand, from Seager et al (2007), very roughly, mid-century ΔP  -18%, so for = 1.5-1.9, and temperature sensitivity  -0.05 - -0.07, and ΔT  2 oC, ΔQ  40% (vs > 50% + from GCM multimodel average)

slide32
More important, though, is the question: In the context of hydrologic sensitivities to (global) climate change, does the land surface hydrology matter, or does it just passively respond to changes in the atmospheric circulation?

i.e., in the long-term mean, VIMFC P-E Q, so do we really need to know anything about the land surface to determine the runoff sensitivity (from coupled models)?

OR is the coupled system sensitive to the spatial variability in the processes that control runoff generation (and hence ET), and in turn, are there critical controls on the hydrologic sensitivities that are not (and cannot, due to resolution constraints) be represented in current coupled models?

the answer
The answer …

… Probably lies in high resolution, coupled land-atmosphere simulations, that resolve areas producing most runoff, and their role in modulating (or exacerbating) regional scale sensitivities.