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W05415 W05415 WOODHOUSE ET AL.: UPDATED COLORADO RIVER RECONSTRUCTIONS . . Table 6. Percentage of Observed and Reconstructed Annual Flow at Lees Ferry Contributed by Subbasins 3 Gauge Total GRUT COCI SJBL Observed, 1906-1995 95.5 35.8 45.4 14.2 Reconstmcted, 1906-1995 95.6 35.9 45.5 14.3 Reconstmcted. 1569-1997 96.3 35.7 46.7 13.9 reconstructions are quite similar to those for the period of the gauge records (Table 5b). The greater shared variance between the San Juan reconstruction and the other recon- structions, compared to the relationships in the gauge record, may be due to an absence of tree ring chronologies located within the San Juan River basin. The result of this may be a weaker representation of local basin variability. [29] The observed flows from the three subbasins gauges, Green River, Colorado-Cisco and the San Juan account for nearly all (average of 95.5%) of the total water year flow observed at Lees Ferry from 1906 -1995 (Table 6). Over the same years, the average values of contributed flows in the reconstructions are closely matched, as expected due to the regression process. Over the full common reconstruc- tion period, 1569-1997, contributions are also very similar (Table 6). Figure 5 shows the variations in flow at the four gauges and the sum of the three subbasin flows over the full reconstruction period as 5-year nmning averages. The match between the three gauge sum and the Lees Ferry flow is good (r = 0.98, P < 0.001), though there are several periods when the sum appears to be somewhat less than Lees Ferry total flow (e.g., the 1630s and the last quarter of the 1600s, both periods of higher flows). 4. Long-Term Hydroclimatic Variability in the Upper Colorado River Basin 4.1. Frequency Characteristics of Reconstructed Flows [30] We used a multitaper method (MTM) spectral anal- ysis to examine the frequency characteristics of recon- structed flows at Lees Ferry and the three subbasins gauges [Mann and Lees, 1996]. MTM provides a robust means for isolating signal peaks from a time series that may contain both periodic and aperiodic behavior. The MTM spectrum for the Lees Ferry reconstruction (Figure 6a) shows that significant (p < 0.05) high-frequency variability in Upper Colorado River flows (2-7 years) is accompanied by a strong bidecadal peak centered around ",24 years. MTM also identifies a significant multidecadal peak around 64 years. Peaks similar to those in the two to seven year band at Lees Ferry are also present in the spectra for each subbasin (Figures 6b-6d). All of the subbasin reconstruc- tions show significant bidecadal peaks, though relative power is reduced for the Green River gauge. The recon- structions for both the Colorado-Cisco and the San Juan show strong multidecadal peaks centered on ",64 years. Cross-spectral MTM reveals significant coherency across the subbasins at lower frequencies (Figure 7). Coherency and phasing of bidecadal and multidecadal peaks is partic- ularly strong. [31] The wavelet spectra for each of these reconstructed gauge records further highlights their coherence in the frequency domain (Figure 8). Wavelet analysis also shows marked nonstationarity in the strength of these signals through time. In particular each of the wavelet spectra are characterized by multidecadal variability (30~ 70 year) in the first two centuries followed by a period from the 18th through mid-19th centuries dominated by significant energy in the decadal to bidecadal bands. Beginning in the late 19th century, however, we see a return to significant multi- decadal variability. These lower-frequency modes persist until the late 20th century, when the effects of zero padding likely reduce power in the multidecadal bands [Torrence and Compo, 1998]. 4.2. Basin-Scale Flow Variability [32] The Lees Ferry and subbasin streamflow reconstruc- tions enable an examination of the spatial characteristics of long-term drought variability in the upper Colorado River basin. We first compared 5-year, 10-year, and 20-year averages of streamflow in the Lees Ferry reconstruction with averaged flows in the three subbasins to determine the degree of drought variability across the upper Colorado River basin. In general, there is a strong tendency for extreme low flows at Lees Ferry to be matched by extreme low flows in all three of the subbasins. Of the driest 5-year 25000 a. 20000 ::!: u ::!: :i 15000 0 u::: 10000 b. 5000 c. d. 0 1600 1700 1800 1900 2000 Figure 5. Five-year running averages of reconstructed annual streamflow, 1571-1995, for Lees Ferry, Arizona (black line a), the sum of the flow for the three reconstructions (gray line a), and the three subbasins, Colorado near Cisco, Utah (line b), Green River at Green River, Utah (line c), and San Juan River near Bluff, Utah (line d). 8 of 16