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FEATURE ARTICLE FROM INTERMOUNTAIN WEST CLIMATE SUMMARY, JUNE 2006 New Streamflow Reconstructions for the Upper Colorado River Basin: Placing Recent Droughts into a Centuries-Long Context By Connie A. Woodhouse 1, Eileen McKim2, and Andrea Ray3 This article is a summary of a paper published in May 2006 in Water Resources Research, "Updated streamflow reconstructions for the Upper Colorado River Basin. " The paper s authors are Connie Woodhouse of the NOAA/NCDC, who is a participant in the Western Water Assessment, Stephen Gray, formerly of USGS, now the Wyoming State Climatologist, and David Meko, of the University of Arizona and who is a participant in the Climate Assessment of the Southwest. Over the past several decades, scientists have developed reconstructions of annual streamflow for centuries prior to the streamflow gage record using data from tree rings. These recon- structions are useful for assessing a broader range of hydrologic variability than contained in the gage records (Figure la). The Colorado River at Lees Ferry was the first tree-ring based quanti- 35000 30000 25000 :2 U 20000 ~ 'i 1 5000 0 iI 1 0000 5000 0 1900 1920 1940 1960 1980 2000 Year Figure 1 a. Comparison of observed and reconstructed streamflow, Lees Ferry gauge (blue line) and Lees-A reconstruction (red line), 1906-1997 (gauge to 1995). Figures 1-3 are reproduced from WGM (2006), see the article for details. tative reconstruction of streamflow (Stockton and Jacoby, 1976). Lees Ferry is the gage that reflects flo\;vs for the Upper Colorado River basin under the 1922 Colorado Compact. The Stockton and Jacoby reconstruction of the annual flows from 1520 to 1961 contained several important findings, including that the high- est sustained flows in the entire record occurred in the first part of the 20th century, the period upon which the Colorado River Compact of 1922 was negotiated, while the most persistent and severe drought occurred in the late 16th century. In a paper published in May 2006, Woodhouse and co-au- thors (2006) updated the Stockton and Jacoby reconstruction at Lees Ferry using an expanded tree-ring net\;vork and a longer gage record for the calibration of the reconstruction. They also developed streamflo\;v reconstructions for three key gages in the Upper Colorado River basin: the Green River at Green River, Utah; Colorado near Cisco, Utah; and San Juan near Bluff, Utah (Figure Ib). The reconstructions explain 72-81 % of the vari- ance in the gage records, and results are relatively stable across several reconstruction methodologies. The new reconstruction of Lees Ferry flows suggests a slightly higher long-term mean than Stockton and Jacoby's, but confirms the earlier findings that Col- orado River allocations were based on one of the wettest periods in the past five centuries, while droughts more severe than any 20th-21st century event have occurred in the past. In addition, five-year droughts similar to the drought of 2000-2004 (in terms of average flow) appear to have occurred as many an eight times in the past five centuries. Analyses indicate similar patterns of runoff variations across the Green, Colorado main stem and San Juan sub-basins, indicating that drought tends to occur across the entire upper basin. 1130 1110 1050 1090 1070 ARIZONA 370 I I I I I I ------__1 I I I l._ _ 430 IDAHO UTAH 410 390 j Figure 1 b. Location of gages at Green River at Green River, Utah (A), Colorado River near Cisco, Utah (B), San Juan Riv- er near Bluff, Utah (C), and Lees Ferry, Arizona (D) (dots) and tree-ring chronologies (triangles). The upper Colorado River basin is outlined in a solid line, and the Green, Colorado with Yampa and Gunnison, and San Juan sub-basins are outlined by the dotted lines. 1 NOAA/National Climatic Data Center (NCDC), 2CIRES, 3NOAA/ESRL, all in Boulder, CO FEATURE ARTICLE I 2