**7. Case Study: Forecasts of EC Exceedances during Spring 2021**

During February 2021, an opportunity arose to compare the forecasting capability of both models in real-time during a time period where the trend in the 30 day running average EC at two of the three SJR compliance monitoring stations suggested potential future exceedance of EC objectives. California is in the second year of a severe drought and water shortages in the State's reservoirs have resulted in severe curtailment of surface deliveries to some farmers. Federal contractors with junior water rights in the SJR Basin, south of the Delta, may receive no surface water deliveries at all during the 2021 irrigation season. The central premise of the real-time salinity management program remains that coordinated actions on the part of stakeholders can optimize the use of SJR assimilative capacity preventing violations of water quality objectives.

The real-time water quality management program was initiated during a time when Vernalis was the only compliance monitoring station for salinity on the SJR. During 2020, two additional water quality stations were added for salinity management in the lower SJR—Reach 83. This action, that was subsequently introduced as an amendment to the Basin Water Quality Control Plan, ostensibly places limits on the degradation of water quality (EC) of riparian diversions into the Patterson and West Stanislaus Irrigation Districts. Although it is unclear what enforcement actions might follow non-compliance with the new 1550 μS/cm salinity objective for Reach 83, the current WARMF model and the USBR's Regression model were extended to supply 14 day forecasts of EC and salt load assimilative capacity at these stations. The basin Plan amendment provided some compliance relief for various sequences of wet, dry and critically dry years where the 30 day running average EC limit was raised using a weighting schema. Unfortunately, the formula does not provide any means to avoid the EC objective for the current water year.

The USBR's obligation under a Management Agency Agreement (MAA) signed with the CRWQCB (the State regulator) is to meet the 30 day rolling average EC objectives at the Vernalis, Crows Landing and Maze Road Bridge, the current compliance monitoring sites for EC. These objectives are ostensibly to provide suitable water quality for riparian agricultural diversions along the mainstem of the SJR and in the Delta. The premise was that stakeholders would help to sustain water quality improvements in the SJR with the help of the USBR-funded cyberinfrastructure by scheduling drainage salt loads from westside sources to coincide with dilution flows generated from east-side sources so as not to exceed the salt load assimilative capacity of the SJR, estimated at each of these stations.

In late February 2021, as watershed inflow to the SJR subsided after a series of rainfall events, both the WARMF and Regression forecasting models suggested a slowly increasing trend in the daily and 30 day running average EC (Figures 17 and 18) that might exceed the various compliance monitoring station EC objectives at Crows Landing and Maze Road (EC 30 day running average objectives of 1550 μS/cm) and at Vernalis which was transitioning from the winter 30 day running average EC objective of 1000 uS/cm to the irrigation season objective of 700 μS/cm. Note that the irrigation season objective applies after April 30 (when 30 days have elapsed). The weekly WARMF model forecast (green background) suggested on 22 February 2021 that the 30 day running average EC threshold of 1550 μS/cm at Crows Landing could be exceeded on 6 March 2021 (Figure 18a) whereas the Vernalis site still showed salt load assimilative capacity (Figure 17a). The USBR had been making

regular adjustments of New Melones reservoir releases to maintain compliance with EC objectives at Vernalis as required under the MAA. The Regression model (blue background) that was run on the same Monday February 22 (Figure 18b) suggested an occurrence of the same exceedance event although the date of the exceedance was predicted one day earlier. In order to lower the 30 day running average EC at Crows Landing, west-side return flows upstream of Crows Landing would need to fall below the 1550 μS/cm criterion.

WARMF and Regression model forecasts made on April 26 were much closer in their predictions (Figure 18c,d) and neither suggested that 30 day running average would drop below the zero line—indicating continuing exceedance and lack of SJR SLAC (Figure 18e,f). The forecasts made by the models on 1 June 2021 show that the daily mean EC dropped below the objective on 19 May 2021 and continued to drive the 30 day rolling average downward until it dropped below the 1550 μS/cm objective and transitioned into positive territory on 28 May 2021 (Figure 19a,b).

**Figure 17.** Comparison of daily WARMF and Regression model forecasts for EC at the Crows Landing compliance monitoring station on 22 February 2021 (**a,b**); 26 April 2021 (**c,d,e,f**); and 1 June 2021 (**g,h**). Graphs (**e,f**) show the 30-day running average EC forecast on 26 April 2021 relative to the the 30-day running average EC compliance objective. Conversion of flow in cfs to m3/s: 100 cfs = 2.83 m3/s.

**Figure 18.** Comparison of daily WARMF and Regression model forecasts for EC at the Crows Land-ing compliance monitoring station on 22 February 2021 (**a,b**); 26 April 2021 (**c,d,e,f**); and 1 June 2021 (**g,h**). Graphs (**e,f**) show the 30-day running average EC forecast on 26 April 2021 relative to the the 30-day running average EC compliance objective. Conversion of flow in cfs to m3/s: 100 cfs = 2.83 m3/s.

This transition is also shown in Figure 19. Figure 19c, which was produced on 1 June 2021, correctly predicted the transition to positive salt load assimilative capacity on 26 May 2021. This plot also shows the proportion of the salt load contributed by the

combination of Mud and Salt Slough relative to the total salt load measured at the Crows Landing compliance monitoring station. At this time of year, the majority of the salt load in these Sloughs is seasonal wetland drainage, which typically has an EC in excess of 1500 μS/cm.

**Figure 19.** Comparison of daily WARMF and Regression model forecasts for EC at the Crows Landing compliance monitoring station on 1 June 2021. Figures (**a**,**b**) show the 30 day running average EC and forecast for 1 June 2021. Figure (**c**) shows the SLAC at the Crows landing station. By early May wetland drainage no longer dominates Mud and Salt Sloughs and daily SLAC in the river increases. The 30 day running average SLAC crosses the zero line around 28 May 2021. Breaks in the plot are the result of temporary EC sensor malfunction at the Crows Landing station. Conversion of flow in cfs to m3/s: 100 cfs = 2.83 m3/s.
