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W05415 1130 WOODHOUSE ET AL.: UPDATED COLORADO RIVER RECONSTRUCTIONS ,- W05415 1110 1090 1070 1050 ARIZONA I I I I I I -----__1 I I I L_ IDAHO UTAH , , , , Figure 1. Location of gauges at Green River at Green River, Utah (A), Colorado River near Cisco, Utah (B), San Juan River 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 subbasins discussed are outlined by the dotted and solid lines (Green, Colorado with Yampa and Gunnison, and the San Juan basins). Tree ring chronologies used in Lees Ferry stepwise regression are circled; a thick black line indicates chronologies used in regression equations calibrated with both standard and residual chronologies, a gray line indicates chronologies used in the standard chronology calibration, and a thin black indicates chronologies used in the residual chronology calibration. expert system approaches, but the records may still include some anthropogenic signals. Water year (October- September) flow data in millions of cubic meters (MCM) were examined graphically and statistically to assess vari- ability, normality and the degree of persistence in the time series (Table 1). The water year flows are essentially normal, and all display a small amount of persistence at a lag of one year. The San Juan represents a considerably more arid region than the other two basins, as evidenced by the lower mean annual flow and higher coefficient of variation. The San Juan is also the only subbasin for which the first-order autocorrelation is not significantly greater than zero. 2.2. Tree Ring Chronology Network [6] In much of the western United States, tree ring widths can provide a proxy for gauge records because the same climatic factors, primarily precipitation and evapotranspira- tion, control both the growth of moisture-limited trees and processes related to streamflow [Meko et al., 1995]. Recent collections of new tree ring data and efforts to update older collections have produced a set of 62 moisture-sensitive tree ring chronologies in Colorado, southwestern Wyoming, and 430 northeastern Utah that span the common interval from 1600 to 1997 (Figure 1 and Supplementary Data 1 in the online data set at http://www.ncdc.noaa.gov/paleo/pubs/ woodhouse2006/woodhouse2006.html) t. Of the 62 chro- nologies, 17 are from ponderosa pine (Pinus ponderosa), 21 from Douglas fir (Pseudotsuga menziesii), 21 from pinyon pine (Pinus edulis), and three from limber pine (Pinus flexilus). Fifteen or more trees were typically sampled at each site using an increment borer and taking two cores from each tree. In the lab, cores were processed, crossdated, and measured using standard dendrochronological tech- niques [Stokes and Smiley, 1968; Swetnam et al., 1985]. All ring width series were uniformly processed using the ARSTAN program as follows [Cook, 1985]. Measured series were standardized using conservative detrending methods (negative exponentiaVstraight line fit or a cubic spline two thirds the length of the series) before using a robust weighted mean to combine all series into a single site chronology [ Cook et al., 1990]. Low-order autocorrelation in the chro- nologies that may, in part, be attributed to biological factors [Fritts, 1976] was removed, and the resulting residual chronologies were used in most of the subsequent analyses. However, the low-order autocorrelation in the gauge records was closely matched by persistence in the tree ring data. Consequently, the sensitivity to persistence in the tree ring data was tested in the Lees Ferry reconstruction by gener- ating reconstruction models using both the standard (persis- tence retained) and prewhitened (persistence removed) chronologies. Because the number of series in these chro- nologies decreases with time, chronologies in the resulting reconstruction models were assessed with regard to subs am- pIe signal strength [Wigley et al., 1984]. [7] Statistical analyses support the high quality and suitability of these chronologies for hydroclimatic recon- structions (Supplementary Data 1). The mean interseries correlation within each chronology averages 0.79, and mean sensitivity (average relative ring width difference from one ring to the next [Fritts, 1976]) averages 0.41. These statistics indicate the strong common signal between the trees that make up each chronology and the high degree of variability in ring widths from one year to the next. Both characteristics are consistent with strong tree ring sensitivity to climatic variability [Cook and Brffla, 1990]. ... 410 390 37" 2.3. Reconstruction Approaches [8] Multiple linear regression, with predictors entered forward stepwise [Weisberg, 1985], was used to generate the reconstruction models. In an automated process such as stepwise regression, increasing the size of the potential predictor pool also increases the likelihood of a meaningless predictor entering the model by chance alone [Rencher and Pun, 1980]. To assess the sensitivity of the reconstruction to the size and makeup of the predictor pools, two alternative reconstruction approaches were tested for each gauge. First, the "full pool" approach used all chronologies significantly correlated (p < 0.05) with the gauge record as potential predictors. Correlations were evaluated over the entire I Auxiliary material for this article contains upper Colorado streamflow reconstructions and three data tables and is avai lable electronically from the World Data Center for Paleoclimatology. NOAA NCDC. 325 Broadway, Boulder, CO 80303, USA (URL: http://www.ncdc.noaa.gov/paleo/pubsl woodhouse2006/woodhouse2006.html). 2 of 16