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W05415 WOODHOUSE ET AL.: UPDATED COLORADO RIVER RECONSTRUCTIONS W05415 Bluff; and the full Upper Colorado River at Lees Ferry (available online at http://www.ncdc.noaa.gov/paleo/pubs/ woodhouse2006/woodhouse2006.html). These reconstruc- tions span the common years IS69 to 1997, and account for more than 70% of the variance in the gauge records. On the basis of the extensive sensitivity analyses, differ- ences in predictor pools and data reduction methods had little significant impact on important features (e.g., long- term mean, runs of drought years, etc.) of the reconstruc- tions. The use of standard versus prewhitened chronologies does have some impact on the magnitude of reconstructed high and low flows, and the standard chronology models retain a degree of low-order autocorrelation similar to that in the gauge record. [4S] The Lees Ferry reconstructions presented here differ from the efforts of Stockton and Jacoby [1976] and Hidalgo et at. [2000] in suggesting a higher long-term mean for Upper Colorado River flows, and to some degree, less extreme multiyear droughts. While the choice of predictor pools and calibration data sets may factor into these differ- ences, statistical reconstruction methodology, particularly the treatment of autocorrelation, also contributes to reduced drought magnitude and an increased long-term mean. [46] Spatially, the relationships between reconstructed subbasin flows are similar to those in the gauge records, except for the San Juan reconstruction, which is somewhat more highly correlated with the other gauge reconstructions over the instrumental period. This enhanced similarity is lessened over the full reconstruction period. lt is possible that the higher correlation between the San Juan and other basins is due to the lack of tree ring chronologies actually located in the San Juan River basin. However, exploratory analyses using several recently generated tree ring chronol- ogies in the San Juan basin did not change these results (C. Woodhouse, unpublished). The reconstructions also capture the contribution of subbasin flows to total Colorado River flow at Lees Ferry. The subbasin flows together account for about 96% of upper Colorado River flow and contributions from the three basins are relatively stable over the 43 I-year common period. [47] As seen in the comparisons of the Lees Ferry and subbasin reconstructions, over the past four centuries severe multiyear and decadal-scale droughts in the upper Colorado River basin have tended to be widespread events. The most severe 5-, 10- and 20-year droughts recorded at Lees Ferry are always reflected in the subbasin gauges, although there are subregional differences in the magnitude of droughts. When the influence of subbasin conditions on Lees Ferry flow is examined, most periods of low flow in one subbasin coincide with low flows in the other subbasins. There are some exceptions, in particular when flow in the Green River is low and the San Juan flow is high. In most of these periods of contrasting drought conditions, Lees Ferry flows are average, but a few cases (e.g., the 1930s) suggest that drought in the Green River can have an overriding influence on flows at Lees Ferry, even when high flows prevail on the San Juan. Likewise multidecadal flow regimes tend to be strongly coherent across the basin. [48] Again, the magnitude of these persistent high and low-flow events varies across the basin, but the timing and duration of these regimes is consistent among the reconstructions. 6.2. Upper Colorado River Droughts and Possible Climatic Drivers [49] The coherency of many single and multiyear droughts across the reconstructions points to common drivers for high-frequency variations in regional hydro- climate. Spectral analysis of the Lees Ferry reconstruction (Figure 6) shows significant variability in a three to seven year band associated with the El Nino Southern Oscillation (EN SO) [Cayan et al., 1999]. Similar high-frequency peaks exist in the subbasin reconstructions. Examination of gauged values and ENSO indicates a good correspondence between La Nina events and low flows on the San Juan, but the relationship is less clear in the other gauges. This agrees with Cayan and Webb [1992] who found that streamflow in the southwestern part of Colorado typically shares the strong southwestern United States response to ENSO (i.e., increased winter precipitation during El Nino events), while the response is much weaker at gauges north of this region, and Hidalgo and Dracup [2003] who reported the ENSO response is much weaker in the Colorado Headwaters and Upper Green River areas. [so] Coherency between flows at multidecadal and longer timescales also suggests that remote forcing or region-wide circulation features influence lower-frequency variations in the Upper Colorado River. Although statistical associations have been demonstrated between North American drought and North Atlantic [Enfield et ai., 2001], North Pacific [Cayan et at., 1998; McCabe et al., 2004] and Indian Ocean [Hoerling and Kumar, 2003] variability, more research is needed to understand how slow changes in sea surface temperatures are tied to Upper Colorado River flow regimes. [SI] Overall, intrabasin variations in reconstructed drought magnitude, combined with spectral analyses sug- gesting variability over a broad range of timescales (i~ter- annual to multidecadal), indicate complex and pOSSIbly nonstationary linkages between the Upper Colorado River and regional to remote forcings. Independent proxy data for ocean variability (i.e., not from western North American tree rings) and modeling studies are needed to better examine the long-term relationships between Colorado River flows and potential climatic drivers. 6.3. Implications for Management [52] The recent drought has been a wake-up call for m~ny water management agencies throughout the Colorado RIver basin. This drought (2000-2004), as measured by 5-year running means of water year total flow at Lees Ferry, is a markedly severe event in the context of the tree ring reconstruction extending to 1490, and the probability is low (p < 0.10) that any S-year period since 1850 has been as dry. However, the current drought is not without precedence in the tree ring record. Average reconstructed annual flow for the period 1844-1848 was lower than the observed flow for 1999-2004. In view of reconstruction error, it is helpful to evaluate tree ring reconstructions probabilistically, and such an evaluation suggests that eight periods between IS36 and 1850 had at least a 10% probability of being as dry as 1999- 2004. In addition, longer duration droughts have occurred in the past. The Lees Ferry reconstruction contains one sequence each of six, eight, and eleven consecutive years with t10ws below the 1906-1995 average (1663- .. 14 of 16