2011 - Cooperative Institute for Research in Environmental Sciences ...

Balaji Rajagopalan
with graduate student Kenneth Nowak
Multidecadal Variability of Colorado River
Streamflow
FUNDING: NOAA–WESTERN WATER ASSESSMENT ,
U.S. BUREAU OF RECLAMATION
The Colorado River,
with its large reservoir
storage capacity, is the
main driver of socioeconomic
growth in the
Southwest, more so since
the beginning of the 20th
century. The prolonged
drought during the recent
decade is placing enormous
stress on the waterresources
supply from the
river. This decade-long
dry spell is unprecedented
in observed history,
and the question on
water-resource managers’
minds is: How does the
Colorado River flow vary
on a multidecadal time
scale? Understanding this variability is crucial for efficient
management of water resources in the future. We performed
a systematic analysis of Colorado River streamflow
to identify its variability from observed and paleo-reconstructed
flow, and we propose an interesting hypothesis.
Wavelet spectrum of the Colorado River streamflow
at the Lees Ferry gauge shows significant power in the
8-to-16-year-period (decadal) band since about 1970. The
annual precipitation over the Upper Colorado River Basin
(UCRB, the basin that generates almost all of the flow)
also exhibits similar spectral characteristics (Figure 1). In
addition, there is strong power around the 64-year period
(low frequency) that is seen in the UCRB temperatures.
This led to an interesting indication that the decadal-scale
modulation of streamflow is from the moisture delivery
(i.e., precipitation) while the lower-frequency modulation
is due to temperature.
Spectrum of long paleo-reconstructed flow (Figure 2)
also shows these features, suggesting that the decadal and
low-frequency variability seen in the observational data are
robust. Furthermore, the power at the 8-to-16-year-period
band is modulated at around 60- to 70-year timescales.
Runoff efficiency (precipitation/flow) also has a similar
spectral signature. The series of the strength of the 8-to-16year-period
band shows this modulation, and a moving
window (about 30-year) variance of the Colorado River flow
indicates that the two vary together—i.e., increased variance
with increased strength of the decadal band, such as the
current period.
The low-frequency band seems to vary with the Atlantic
Multidecadal Oscillation (AMO) index—consistent with
other studies that link the variability of Western U.S. temperatures
with AMO.
Based on these analyses, we propose the following hy-
46 CIRES Annual Report 2011
Figure 1. Wavelet power spectrum of Lees Ferry Flow. Features of
interest: 1) decadal (active past 30 years) 2) low frequency (more
persistent).
LOW FREQUENCY VARIABILITY EPOCHS
DECADAL VARIABILITY EPOCHS
Figure 2: Wavelet Spectrum of Paleo Reconstructed Lees Ferry Flow.
pothesis for multidecadal variability of Colorado River flow:
• UCRB temperature modulates at a very low frequency
(about a 64-year period).
• The warmer phase of the modulation creates boundary
conditions for lower runoff efficiency.
• Precipitation variability is driven by Pacific forcings.
• The streamflow evolves with the precipitation but is
constrained by the runoff efficiency regime created by the
temperature blanket.
These results are part of a larger research effort to develop
consistent and robust stochastic streamflow simulations
at multiple locations on multidecadal timescales, for use in
efficient water-resources planning and management in the
Colorado River Basin.