San Gabriel Dam
The San Gabriel Dam impounds the San Gabriel River, which has its headwaters in the San Gabriel Mountains, with peaks up to 10,000 feet. The precipitation gage at San Gabriel Dam generally reflects the upper portion of the San Gabriel River basin. The precipitation data, for the years 1938-2015, were obtained from the Los Angeles County Department of Public Works.
The reconstruction of water year streamflow at San Gabriel Dam was generated as part of a project supported by the California Department of Water Resources (CADWR). This project includes reconstructions of water year precipitation (San Gabriel Dam, Lake Arrowhead, Ojai, and Cuyamaca) and streamflow (Arroyo Seco and Santa Ana River) for southern California and the Kern River in the southern Sierras. This set of reconstructions was developed by Dave Meko, Erica Bigio, and Connie Woodhouse in 2017, based on updated and new collections of tree-ring data in California sampled for this project.
Total water year (October-September) precipitation at San Gabriel Dam was reconstructed using a two-stage regression procedure. Tree-growth at each site was first converted into an estimate of precipitation by stepwise regression of precipitation using tree-ring width indices, from the current year and lagged one year, as predictors. Squared terms on the tree-ring predictors were also included in the regression to allow for possible curvature in relationships between tree-growth and precipitation. In the second step, the gage reconstruction was generated by averaging an appropriate set of single site reconstructions. Final estimates of precipitation were interpolated from a piecewise-linear smoothed scatter plot of the observed precipitation values and the precipitation estimates averaged over the individual tree-ring sites. The procedure was repeated for subsets of tree-ring chronologies with different periods of common time coverage to build a “most-skillful” reconstruction, starting in the early 1400s, and a “longest” reconstruction, starting in the early 1100s. Details of the reconstruction method can be found here.
|Statistic||Most Skillful: Calibration||Most Skillful: Validation||Longest Model: Calibration||Longest Model: Validation|
|Explained variance (R2)||0.77||0.60|
|Reduction of Error (RE)||0.78||0.57|
|Standard Error of the Estimate||6.677 in.||9.368 in.|
|Root Mean Square Error (RMSE)||6.970 in.||9.839 in.|
Note: The statistics listed in the table represent average accuracy, while the reconstruction method yields error bars that vary in width over time -- generally wider for wet years than for dry years. The listed statistic R2, the decimal proportion of variance explained by the reconstruction in the calibration period, is computed directly from the reconstruction residuals. For an explanation of these statistics, see this document.
Figure 1. Scatter plot of observed and reconstructed San Gabriel Dam annual precipitation, 1938-2015. Note that the R2 value here is slightly different than in the table. The table R2 value is the average explained variance from the three models that make up the most skillful reconstruction. The values in the scatter plot reflects the explained variance for the two models that cover the instrumental period (more details).
Figure 2. Observed (gray) 1939-2015, and reconstructed (blue) 1900-2016, San Gabriel Dam annual precipitation. The observed mean is illustrated by the black dashed line.
Figure 3. Reconstructed annual precipitation for the San Gabriel Dam (1405-2016) is shown in blue. Observed flow is shown in gray and the long-term reconstructed mean is shown by the black dashed line.
Figure 4. The 10-year running mean (plotted on final year) of reconstructed annual precipitation for the San Gabriel Dam, 1405-2016. Reconstructed values are shown in blue and observed values are shown in gray. The long-term reconstructed mean is shown by the black dashed line.
Figure 5. Scatter plot of observed and reconstructed annual precipitation for the San Gabriel Dam, 1938-2015 (more details).
Figure 6. Observed (gray) 1938-2015, and reconstructed (blue) 1900-2016, San Gabriel Dam annual precipitation. The observed mean is illustrated by the black dashed line.
Figure 7. Reconstructed annual precipitation for the San Gabriel Dam (1126-2015) is shown in blue. Observed precipitation is shown in gray and the long-term reconstructed mean is shown by black dashed line.
Figure 8. The 10-year running mean (plotted on final year) of reconstructed San Gabriel Dam annual precipitation, 1126-2015. Reconstructed values are shown in blue and observed values are shown in gray. The long-term reconstructed mean is shown by the black dashed line.