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W05415 WOODHOUSE ET AL.: UPDATED COLORADO RIVER RECONSTRUCTIONS W05415 ~ 0 75 u:: ----------~---~------ 50 Lowest bserv 1500 1550 1600 1650 1700 1750 1800 1850 1900 1950 2000 Ending Year of 5-yr Running Mean OJ E 0 c: '15 ~ !!...- ~ 0 u:: - - - - - - - - - - - - - ~ ~ - - -- - - 50 owest 0 serve 1500 1550 1600 1650 1700 1750 1800 1850 1900 1950 2000 Ending Year of 5-yr Running Mean 150 ~ OJ E 125 0 c: ~ 0 75 u:: 50 1500 1900 1950 2000 Figure 11. Current drought in long-term context from reconstructed 5-year running means of natural flow at Lees Ferry, Arizona. (a) Observed and reconstructed flow. (b) Observed flow and flow with 0.25 nonexceedance probability derived from reconstruction and its estimated error variance. (c) Observed flow and reconstructed flow with 0.10 nonexceedance probability. Flow is plotted as percentage of 1906-1995 mean of observed mean annual flow, or 18,788 million cubic meters (15.232 MAF). Reconstruction series is from model Lees-A (see text). The standard error of a 5-year mean was estimated as Sm = RMSEcv/V5, where RMSEcv is the cross-validation root- mean-square error of the annual reconstructed values. The computed standard error and the assumption that the errors are normally distributed yield confidence intervals and threshold levels of reconstructed 5-year mean flow with specific empirical nonexceedance probabilities. [42] The reconstructed (Lees-A) 5-year means for Lees Ferry along with the threshold levels of flow with 0.2S and 0.10 nonexceedance probability are plotted in Figure 11 with the observed 1999-2004 mean as a baseline ("Lowest Observed") for comparison. The S-year mean for 1999- 2004 was 12,187 MCM, or 64.9% of the 1906-1995 mean natural flow. The time series plots indicate that only one S-year period, 1844-1848, was drier than 1999-2004 (Figure 1Ia). Annual reconstructed flow during this period averaged 63% of normal. [43] A probabilistic interpretation of the reconstruction indicates, however, that several other periods have a rea- sonably large probability of being drier than 1999-2004. Two additional periods, in the early IS00s and early 1600s, have a 25% or greater chance of being as dry as 1999-2004 (Figure 11 b). Six periods in addition to the 1840s have a 10% or greater chance of being drier than 1999 - 2004 (Figure 11 c). During the signature drought of 1844-1848, the probability is 10% that the true 5-year mean flow was as low as 54.8% of normal (10,290 MCM or 8.3422 MAF). It should be emphasized that Lees-A is the most conservative (wettest) of our Lees Ferry reconstructions, and that other versions give even more frequent past occurrences of flow lower than in 1999-2004. 6. Discussion and Conclusions 6.1. Updated Reconstructions [44] An updated and expanded set of tree ring chronol- ogies has enabled the generation of high-quality water year streamflow reconstructions for four key gauges in the Upper Colorado River basin; the Green River at Green River, UT; Colorado River near Cisco; San Juan near 13 of 16