4.3.2. X2 Reconstructions

The annual Delta outflow reconstructions (long record and short record) were used as the basis for reconstructing seasonal (February–June and July–October) average X2 position under pre-development and contemporary conditions, using Models 2 and 3 described above. Figure 11 shows pre-development seasonal average X2 exceedance frequencies for each reconstruction. Little difference is observed between the pre-development long and short period reconstructions except in the 10th–20th percentile range. As shown in the top panel, February–June average X2 positions are approximately 60–63, 53 and 48 km at the 10th, 50th and 90th percentiles, respectively. X2 is further downstream (smaller values) in the spring, indicative of relatively higher flow conditions. As shown in the bottom panel, July–October average X2 positions are approximately 94–96, 86–87 and 82–83 km at the 10th, 50th and 90th percentiles, respectively. X2 is further upstream (larger values) in the summer and fall, indicative of relatively lower flow conditions.

**Figure 11.** Reconstructed seasonal X2 exceedance frequency under pre-development (Model 2) and contemporary (Model 3) conditions. February–June and July–October X2 exceedance curves are shown in (**a**) and (**b**), respectively. Exceedance curves are provided for long and short record reconstructions.

This same figure shows small differences between the reconstructions throughout the exceedance frequency domain for the contemporary period beginning in WY 1912. February–June average X2 positions are approximately 70–73, 58–59 and 50–51 km at the 10th, 50th and 90th percentiles, respectively. July–October average X2 positions are approximately 94–96, 84–86 and 76–77 km at the 10th, 50th and 90th percentiles, respectively. As under pre-development conditions, contemporary X2 is further downstream (smaller values) in the spring and further upstream (larger values) in the summer and fall.

Differences between pre-development and contemporary X2 conditions reflect differences observed in the Delta outflow reconstructions and differences in interannual pattern of water use. Differences in X2 also reflect natural and anthropogenic drivers that modified the estuary's flow–salinity regime, resulting in greater outflow requirements under contemporary conditions to repel salinity intrusion [22,26] The contemporary period is generally associated with greater spring (February–June) salinity intrusion and lesser summer–fall (July–October) salinity intrusion relative to the pre-development period. Seasonal differences between pre-development and contemporary X2 conditions are indicative of upstream reservoir operations that store water in winter and spring months and release water in summer and fall months.
