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W05415 WOODHOUSE ET AL: UPDATED COLORADO RIVER RECONSTRUCTIONS W05415 r, Table 2. Regression Statistics for Reconstmction Modelsa Gauge Predictors in Pool Number of StepslNumber of Predictors R2 F level RE RMSE Full Pool .. GRfUT 28 9/7 0,72 30,0 0.66 1149.2 CO/Cisco 38 5/5 0.77 54.0 0.73 1248.7 SJiBlufT 24 4/4 0.73 56.7 0.70 589.7 Limited Pool GR/UT [8 9/7 0.72 30.0 0.66 1149.2 CO/Cisco 32 4/4 0.73 57.7 0.69 1330.4 SJ/BlutT 8 3/3 0.67 58.8 0.64 640.6 Lees Fer/J,b Lees-A (res) 31 7!7 0.81 48.7 0.76 2579.1 Lees-B (std) 30 7/7 0.84 61.2 0.81 2337.1 Lees-C (res,PCA) 3 III 0.72 226.9 0.71 2861.3 Lees-D (std,PCA) 4 1/1 0.77 294.7 0.76 2599.5 aSubbasin models based on full pool and watershed-limited pool of potential predictors and statistics for four alternative modeling choices for Lees Ferry record are given. Validation statistics RE and RMSE are based on cross validation. bSee Table 3 for detinitions of models. root nor log 1 0 transformation of the flow record offered more than marginal improvement for that gauge, the deci- sion was made to use the untransformed flows. The stepwise validation results indicated that strict adherence to the F enter and F remove criteria did not result in any obvious overfitting of the models. The numbers of steps and predictors chronologies for the four gauges are listed in Table 2. Regression coefficients are given in Supple- mentary Data 2 (http://www.ncdc.noaa.gov/paleo/pubs/ woodhouse2006/woodhouse2006.html). Linear neural net- works using the suite of predictors included in the regression equations yielded explained variance values that were the same as those from the regression approaches. An example of the comparison between a gauge record and a reconstruction is shown for Lees Feny in Figure 2. 3.2. Sensitivity of the Reconstruction Models to Predictor Pool [\4] The predictor pool sensitivity tests apply only to the gauges on the Colorado-Cisco and San Juan, as the same predictors were selected from both pools for the Green River gauge. Limiting the pool by watershed boundary reduced the number of potential predictors from 38 to 32 chronologies for the Colorado-Cisco and from 24 to 8 chronologies for the San Juan (Supplementary Data 2). Stepwise regression for the Colorado-Cisco and the San Juan gauge yielded two and three predictors, respectively, in the full pool regression equation that were not in the limited pool equation. In these cases, as expected, the explained variance is reduced in the watershed-limited models. To address reconstruction sensitivity, reconstructions based on full pool and limited pools of predictors were compared with attention to critical precalibration periods, such as the well-known drought in the late 16th century [Stockton and Jacoby, 1976; Gray et al., 2004; Stahle et al., 2000]. A comparison of reconstructions for the San Juan and Colorado-Cisco from the two different models indicates only slight differences, particularly during periods of drought (Figure 3). On consideration of calibration/validation accuracy, the relative insensitivity of reconstructions to predictor pool reduction, and ability to reproduce statistical features of the observed record, we decided to adopt the full pool predictor subsets for the final subbasin reconstructions and analysis. 35000 30000 25000 ::E (.) 20000 ::E ~ 15000 i! 1 0000 5000 0 1900 1920 1940 1960 Year 1980 2000 Figure 2. Comparison of observed and reconstructed streamflow, Lees Feny gauge (blue line) and Lees-A reconstruction (red line), 1906-1997 (gauge to 1995). 4 of 16