predictors account for an offset of up to 2 years between weather
conditions in a given year and the timing of the response of tree
growth versus water levels. The strength of a regression model is
expressed using the adjusted R2, which quantifies the explanatory
power of the regression model while accounting for lost degrees of
freedom as the number of predictors increases. The regression
models are validated using a leave-n-out method, where observations
are left out sequentially throughout the length of the streamflow
record allowing maximum use of the data for calibration and
validation. From the data withheld for validation, we compute the
reduction of error (RE), a statistic of the association between a series
of actual values and their estimates. A positive RE indicates
some predictive capacity (Fritts et al., 1990). The F value from
the regression model is a goodness-of-fit statistic. The standard
error (SE) and root-mean-square error of validation (RMSE) are
measures of the uncertainty in predicted values over the calibration
and validation periods, respectively. Regression residuals are
evaluated for autocorrelation using the Durbin–Watson test
(Ostrom, 1990).