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'}DD \0 \;JC,Ditbl> tV:--~ c}r ..~ WATER RESOURCES RESEARCH,VOL. 42, W05415, doi:10. 1029/2005WR004455, 2006 .~ ';91o~ S Updated streamflow reconstructions for the Upper .~ Colorado River Basin Connie A. Woodhouse, 1 Stephen T. Gray,2 and David M. Mek03 Received 21 July 2005; revised 9 November 2005; accepted 19 January 2006: published 1] May 2006. [I] Updated proxy reconstmctions of water year (October-September) streamflow for four ~ey gauges in the Upper Colorado River Basin were generated using an expanded tree nn~ network and longer calibration records than in previous efforts. Reconstmcted gauges mclude the Green River at Green River Utah. Colorado near Cisco Utah. San J~an near Bluff, Utah; and Colorado at Le~s Fe~, Arizona. The reco~struct{ons explam 72-81 % of the variance in the gauge records, and results are robust across several recons~m.cti~n approaches. Time series plots as well as results of cross-spectral analysIs mdIcate strong spatial coherence in runoff variations across the subbasins. The Lees Ferry reconstruction suggests a higher long-tenn mean than previous reconstructions but strongly supports earlier findings that Colorado River allocations were based on one of the wettest periods in the past 5 centuries and that droughts more severe than any 20th to 21 st century event occurred in the past. Cit~tion: Woodhouse, C. A., S. T. Gray, and D. M. Meko (2006), Updated streamflow reconstructions for the Upper Colorado River Baslll, Water Resow: Res., 42, W05415, doi:10. 1029/2005WR004455. 1. Introduction [~] The Colorado River, perhaps the most important regIonal source of surface water supply in the western United. States, was the subject of the first tree ring based effort aimed at the quantitative reconstruction of streamflow records [Stockton and Jacoby, 1976]. The reconstruction of annual flows at Lees Ferry, which reflects conditions in the entire Upper Colorado River basin (Figure 1), contained several noteworthy features. The highest sustained flows in the entire record, 1520 to 1961, occurred in the early decades of the 20th century, a period that coincides with the negotiation of the 1922 Colorado River Compact and the resulting allocation of Colorado River flows. In effect, water that was not likely to be in the river on a consistent basis was divided among the basin states. In addition, the most persistent and severe drought occurred in the late 16th century, with flows during this period much lower than for any event in the 20th century. [3] Two decades later, this landmark reconstruction was the basis for a series of studies that investigated the hydrologic, social, and economic impacts of a severe sustained drought in the Colorado River basin [Young, 1995]. These studies indicated that under the current Law of the River (the set of legal compacts and regulations that gove~ the Colorado River), a drought like the 16th century event m Stockton and Jacoby's record would greatly chal- lenge the capacity of the Colorado River to meet water supply needs, and have significant impacts on Compact obligations. ~National Climatic Data Center, NOAA. Boulder, Colorado, USA. 3 Desert Laboratory, U.s. Geological SUlVey, Tucson, Arizona, USA. Laboratory of Tree-Rmg Research, University of Arizona, Tucson. Arizona, USA. Copyright 2006 by the American Geophysical Union. 0043-] 397 /0612005WR00445 5$09 .00 [4] Severe drought conditions in the Colorado River basin, coupled with a large increase in water use over the past two decades, have recently resulted in water demands that have outstripped natural inflows [Fulp, 2005]. More- over, new water projects, additional management concerns such as endangered species, and large increases in popula- tion have altered the potential impacts of drought. These conditions have reinvigorated interest in reconstructions of Colorado River flow. Stockton and Jacoby's [1976] original Lees Ferry reconstruction ended in 1961, which has made it difficult to assess recent droughts in a long-term context. In addition, reconstruction methods have evolved greatly in recent decades. Hidalgo et al. [2000] have shown that features of the Stockton and Jacoby reconstruction, includ- ing relative drought severity and duration, are sensitive to modeling methodology. Thirty additional years of gauge data, new and updated tree ring collections, and improved methodologies now enable a longer and more robust recon- struction of Colorado River streamflow. The purpose of this paper is to describe and analyze a recently generated set of updated streamflow reconstructions for Lees Ferry and other key gauges in the Upper Colorado River Basin. 2. Data and Methods for Reconstructions 2.1. Streamflow Data [5] We selected four gauges in the Upper Colorado River basin for reconstruction: the Green River at Green River Utah; Colorado River near Cisco, Utah; San Juan River nea; Bluff, Utah; and Colorado River at Lees Ferry, Arizona. The selected gauges represent flows in the three major subbasins as well as the total flow of the Upper Colorado Basin (Figure 1). The U.S. Bureau of Reclamation provided estimates of natural flows for these locations that span the years 1906 to 1995 (1. Prairie, personal communication, 2005). These flow values have been adjusted to account for human impacts through a combination of statistical and W05415 1 of 16