4.2.2. Delta Outflow Reconstructions

The annual Central Valley runoff reconstructions (long record and short record) were used as the basis for reconstructing Delta outflow volume under pre-development and contemporary conditions using Models 2 and 3 described above. The results are shown in the form of exceedance frequencies in Figure 8. Through WY 1850, the plot shows little difference between the long and short record reconstructions to estimate pre-development annual Delta outflow volumes. Annual outflow volumes for this period are approximately 35–37 BCM, 24 BCM and 11–13 BCM at the 10th, 50th and 90th percentiles, respectively. For the contemporary period beginning in WY 1912, the plot shows small differences between the long and short record reconstructions. Annual outflow volumes for this period are approximately 40–44 BCM, 24 BCM and 9–11 BCM at the 10th, 50th and 90th percentiles, respectively.

**Figure 8.** Reconstructed delta outflow exceedance frequency under pre-development (Model 2) and contemporary (Model 3) conditions, for the long and short record reconstructions.

Differences between pre-development and contemporary Delta outflow conditions reflect differences observed in the Central Valley runoff reconstructions as well as differences in water use on the valley floor and in the Delta. For example, assuming a common historical runoff sequence from WYs 1922–2003, Gross et al. [22] reported mean annual Delta outflows of 24.5 BCM and 19.4 BCM under pre-development and contemporary conditions, with the difference approximately equal to CVP and SWP exports from the south Delta, which together average approximately 6.1 BCM [22]. Our work shows similar mean

annual Delta outflow conditions (24 BCM) for the pre-development and contemporary periods. However, the contemporary period is associated with a more variable outflow regime relative to the pre-development period, with higher outflows in the low end of the exceedance frequency domain and lower outflows in the high end of the exceedance frequency domain.

Contemporary outflow model residuals associated with Equations (2) and (4) are highly correlated (R2 = 0.94) with reconstructed Central Valley runoff residuals. Residuals are computed as the difference between reconstructed values and historical values. Figure 9 shows scatter plots and regression lines for long record (WYs 1912–2008) and short record (WYs 1912–2001) relationships, with residuals shown as 5-year center weighted averages.

**Figure 9.** Relationship between reconstructed annual delta outflow residuals and reconstructed central valley runoff residuals for the contemporary period. Residuals are presented as 5-yr center weighted averages. The long record (WYs 1912–2008) and short record (WYs 1912–2001) relationships are shown in (**a**) and (**b**), respectively.
