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W05415 WOODHOUSE ET AL.: UPDATED COLORADO RIVER RECONSTRUCTIONS W05415 Table 8. Terciles and Percentiles of lO-year Moving Average Reconstmcted Flow at Lees Ferry During Periods of Contrasting Flow Anomalies in Subbasinsa Lees Flow Lees Flow Contrast Tercile Percentile Green dry/San Juan wet 1597 middle 0.546 1600 dry 0.171 1674 dry 0.283 1743 middle 0.406 1856 middle 0.475 1941 dry 0.276 1942 dry 0.320 1943 middle 0.363 Green wet/San Juan dry 1735 middle 0.463 1766 middle 0.658 1774 middle 0.518 Green dry/Colorado wet 1818 middle 0.615 1823 middle 0.489 San Juan wet/Colorado dry 1859 middle 0.499 San Juan dry/Colorado wet 1820 middle 0.610 aYear listed is last of 10. smoothed time series display a pattern of high magnitude variations in the 16th and 17th centuries and the 19th and 20th centuries, with dampened variability centered on the 18th century. The driest multidecadal period in the Lees Ferry reconstruction occurs in the late 16th century. The low-flow period at the end of the 19th century shares a similar magnitude. In this multidecadal context the 19S0s drought is also notable as the 4th lowest flow period at Lees Ferry. Generally high-flow regimes occurred across the basin in the early 17th and early 20th centuries. The most recent decades of the reconstruction were also quite wet. As in the case of the multiyear and decadal flow regimes discussed above, the magnitude of departures for these multidecadal flow regimes varies somewhat across the basin. This is particularly true for the early 1700s through the mid 1800s, which is the period when the wavelet analyses show a significant loss of multidecadal power in the basin. However, the timing and duration of multidecadal flow regimes is markedly coherent across the Upper Colorado River Basin. 4.3. Comparison with Previous Lees Ferry Reconstructions [36] Because of the central importance of the Lees Ferry record to the allocation of Colorado River water supply, it is important that the reconstruction be as accurate as possible, and that the uncertainty be appreciated. The discussion in section 3.3 dealt with uncertainty due to modeling choices: the use of standard versus residual chronologies and the decision to use individual chronologies or chronologies reduced by PCA in the regressions. Previous reconstruction efforts [Stockton and Jacoby, 1976; Hidalgo et al., 2000] not only used different modeling procedures from ours, but also a different tree ring network and a much shorter calibration period. In this section we compare our updated reconstructions, versions Lees-A and Lees-D, with recon- stmctions by Stockton and Jacoby [1976] and Hidalgo et al. [2000). We refer to these two previous reconstructions as SJl976 and HDP2000. The comparison focuses on two statistics: the long-term mean annual flow, and the most severe sustained drought as measured by the . lowest ~econ- structed 20-year moving average of flow. Lees-A IS ?ur model using regression of flow on residual chronologIes. Lees-D is our model using regression of flow on PCs of standard chronologies. Those two versions were selected for the comparison because they represent the most conserva- tive (wettest) and least conservative (driest) of the .alterna- tive reconstructions from the updated chronologIeS (see section 3.3). . [37] Time series plots of smoothed reconstructIOns (Figure 10) generally agree in timing of highs and lows, but disagree considerably on the magnitude of s?me flow anomalies. The plots for the updated reconstructIOns gen- erally show wetter conditions than the previous reconstruc- tions. HDP2000 represents the driest scenario, with greatly amplified low-flow features in the late 1500s, late 1700s and near 1900. Much less disagreement among the four reconstructions is evident in the calibration period than in the precalibration period. .. . [38] Selected calibration and reconstructIon stat.Ist.ICS for the four models are listed in Table 9. Flow statIstIcs are given in units of both billion cubic meters (BCM) and million acre-feet (MAF) to facilitate comparison with pre- vious published studies. Note that the reconstructions differ considerably in calibration period as well as in the nu~ber of tree ring chronologies on which the final reconstmctlOns depend. Agreement of the reconstructions in the calibration period ~Figure 10) is not surprising as all four models have high R values (Table 9). Perhaps the most striking dis- agreement in the models is the magnitude of the late ~ 50~s drought (the period of the lowest 20-year mean), whIch IS estimated at 11.2 BCM (9.1 MAF) by HDP2000 and 15.6 BCM (12.6 MAF) by Lees-A. The updated reconstruc- tions suggest the long-term mean annual flow is not as low as previously estimated. Our driest updated reconstmction model (Lees-D) gives a long-term mean of 17.6 BCM (14.3 MAF), which is some 0.9 BCM (0.8 MAF) higher than the original estimate by Stockton and Jacoby [1976). [39] Differences in the reconstructions are undoubtedly related to differences in the basic data and the statistical models used for reconstruction. The most obvious data difference between this and past efforts would be that 0.4 i tn c: 0 :;::; 0.2 cu 'S; Q) C 0.0 'E cu 'tJ -0.2 c: J! en -0.4 1600 1700 1900 2000 1800 Year AD Figure 9. Reconstructed upper Colorado River flows, smoothed with a 50-year spline. 11 of 16