DOI: 10.25921/3w6k-2e19
title: NOAA/WDS Paleoclimatology - Meko et al. 2007 Upper Colorado River Flow Reconstruction

description: New tree-ring records of ring-width from remnant preserved wood are analyzed to extend the record of reconstructed annual flows of the Colorado River at Lee Ferry into the Medieval Climate Anomaly, when epic droughts are hypothesized from other paleoclimatic evidence to have affected various parts of western North America. The most extreme low-frequency feature of the new reconstruction, covering A.D. 762-2005, is a hydrologic drought in the mid-1100s. The drought is characterized by a decrease of more than 15% in mean annual flow averaged over 25 years, and by the absence of high annual flows over a longer period of about six decades. The drought is consistent in timing with dry conditions inferred from tree-ring data in the Great Basin and Colorado Plateau, but regional differences in intensity emphasize the importance of basin-specific paleoclimatic data in quantifying likely effects of drought on water supply.
description: Reconstructed annual flows of the Colorado River at Lee Ferry based on new tree-ring records of ring-width from remnant preserved wood. Residual site chronologies were converted to estimates of annual (water-year total) flow of the Colorado River at Lees Ferry(hereafter called "flow") by the following steps. 1) Each chronology was separately converted to an estimate of flow by a lagged regression model. For this model, the predictors were the lags -2 yr to +2 yr on the residual chronology, where the lag is relative to the water year. The predictor was the flow in year t, and the calibration period was the available overlap of the chronology with the 1906-2004 flow data. Predictors were entered stepwise into the regression, with a critical p-value of 0.05 for entry, 0.10 for removal. The lagged model was subjected to split-sample validation to ensure that a statistically significant signal for flow existed in both halves of the the available overlap period. The long-term tree-ring data were substituted into the resulting regression equation to generate a "single-site" reconstruction of flow. Repeating this step for each chronology yielded 11 separate single-site reconstructions of flow, each with time coverage depending on the coverage of the corresponding tree-ring chronology. 2) Single-site reconstructions were grouped into four different subsets depending on time overlap. The common periods of single-site reconstructions in these subsets determined the four reconstruction periods listed in Table 1 in the text. 3) For each of the four subsets of single-site reconstructions, steps 4-6 below were taken to arrive at a single flow reconstruction. This procedure is a weighting of the individual single-site reconstructions. 4) A PCA was run on the single-site reconstructions. The covariance matrix rather than correlation matrix was used in the PCA to retain the influence of the differing variances of the single-site reconstructions. 5) Any PC explaining at least 5 percent of the variance of single-site reconstructions was designated an "important" PC. The PC scores (time series) for the important PCs made up the pool of potential predictors for the next step. 6) Flow was regressed on the PC scores to arrive at a reconstruction of flow for the sub-period. As with the single-site regressions, predictors were entered stepwise into the regression, with a critical p-value of 0.05 for entry, 0.10 for removal. No lags were used in this model, as lagged effects had been handled in the single-site reconstruction. The regression model was cross-validated by successively leaving out 9 values and using the remaining observations to estimated the flow for the central year of the 9-year period. The cross-validation predictions were used to compute the cross-validation root-mean-square error, or RMSE, which is listed in Table 1 in the text. 7) For the entire period of coverage of the tree-ring reconstructions, the flow value from the subset model with the lowest RMSE was substituted as the final reconstructed value. Where reconstructions from more than one model were available, this procedure assigned the reconstruction from the most accurate model as the reconstructed flow. For periods with only one available single-site reconstruction (e.g., the earliest sub-period, or the interval A.D. 762-1181) the final reconstructed flow was necessarily provided by that model regardless of its RMSE. 8) Monte Carlo simulations, following Meko et al. (2001) were used to restore noise to the reconstructed flows and generate an ensemble of realizations of "true" flow as well as place error bars on the annual reconstruction and time averages of the annual reconstruction. Steps 9-10 below summarize the Monte Carlo analysis. 9) 1000 time series of "noise" the same length as the reconstruction were sampled from a normal distribution with mean of zero and standard deviation equal to the RMSE of the reconstruction. The time-varying RMSE (depends on which sub-period model contributed the reconstructed flow for the year) was incorporated in this sampling. In other words, the variance of the noise distribution used to generate the noise-added reconstructed flow for a year was determined by the RMSE of the corresponding reconstruction model. 10) The 1000 noise series were each added to the flow reconstruction from step 7 to get 1000 "noise-added" reconstructions. These are viewed as plausible realizations of flow given the reconstruction and its uncertainty. The 1000 noise-added reconstructions are also an ensemble from which confidence intervals for the unobserved "true" flow can be constructed (e.g., see Figure 2 in text). An 80% confidence interval for 25-year running mean flow was derived by 1) smoothing each of the 1000 noise-added reconstructions with a 25-year moving-average filter, 2) Ranking the 1000 smoothed flows in each year, and 3) identifying the 0.10 and 0.90 non-exceedance probabilities of flow from the ranked flows. See also supplementary data files: Supplementary Data 1. Tree ring chronology data and metadata (meko2007crn.txt) Supplementary Data 2. Tree ring measurements data, compressed .zip format (meko2007rwl.zip) Note: Full tree ring data series are contained here. Reconstructions were based on shorter subsets meeting minimum sample size criteria. Supplementary Data 3. Reconstruction statistics and references (meko2007stats.txt).