By Jay Lund and William Fleenor

Where does water exported from the Delta come from?  And where does the salt in Delta exports come from?

Water and salt exported from the Delta comes from several sources:

• Sacramento River (largest high-quality source) (Sac)
• San Joaquin River discharge (usually modest flow, but much saltier from agricultural drainage) (SJR)
• smaller eastside streams (Mokelumne River, etc.; usually small, but good quality) (East)
• Delta drainage water and precipitation (lower quality) (Delta)
• Ocean water (salty, mixed in by tidal action, if above sources are inadequate) (Martinez).

Here are some answers in two plots, routinely produced by the California Department of Water Resources (DWR), showing estimates of the mix of these sources in southern Delta water exports.  These are for State Water Project (SWP) exports so far in 2016.  (The tidally-averaged estimates are made using DWR’s DSM2 model.)

Where does your water come from?

For 2016, so far, most of the water exported by the SWP from the southern Delta is actually Sacramento River (Sac) water which has been hydraulically dragged through the Delta.  The San Joaquin River (SJR) contributes some, and the Eastern streams (East) a bit as well.  “Delta” (Delta) water is also present, as is ocean water (Martinez) just a little in January.

Sources of water exported by State Water Project at Clifton Court in southern Delta, January-May 2016

Where does your salt come from?

The salinity in SWP exports varies with both the quantity and quality of each water source.  In January, the little bit of salty ocean (Martinez) water is a disproportionate contributor of salt to urban and agricultural diversions in the southern Delta.  The fairly low Delta water quality is also seen, disproportionate from its more modest quantity of diversion.  The Sacramento River, which is 60-80% of the water diverted, only contributes a third of the salt.  Note also the extra salinity of the lower San Joaquin River.

Note also that the field data on salinity at the SWP diversion is a bit higher that what the model estimates.  So there is some model error in this case, probably mostly from errors in the ocean boundary conditions and estimated in-Delta consumptive use.

Sources of salinity in water exported by State Water Project at Clifton Court in southern Delta, January-May 2016, salinity as electrical conductivity. Dots are field data, Colored areas are model results

What does a wet year look like?

2011 was the most recent wet year, with the last quarter of 2011 shown in the figures below.  The mix of sources was very different.  The Sacramento River remained the dominant source, with much more from the San Joaquin River (with better-than-usual water quality), less Delta water, and some eastern streamflow.

Sources of water exported by State Water Project at Clifton Court in southern Delta, September-December 2011 (wet year)

Salinity was much better, until the end of the year when a bit of ocean (Martinez) water intruded.  The San Joaquin River contributed to salinity disproportionately as well.  The model fit the field data much better.

Sources of salinity in water exported by State Water Project at Clifton Court in southern Delta, September-December 2011 (wet year), salinity as electrical conductivity. Dots are field data, Colors are model results

The Delta is a complex and changing place.  Its waters also change in quality and quantity with the tides, seasons, and years.

The authors are with the Center for Watershed Sciences, University of California – Davis.

Further reading

California Department of Water Resources, Delta water fingerprinting archive, 2005-2016,  This is a rich and thought-provoking set of field observations and modeling results.

Fleenor, W., E. Hanak, J. Lund, and J. Mount, “Delta Hydrodynamics and Water Quality with Future Conditions,” Appendix C to Comparing Futures for the Sacramento-San Joaquin Delta, Public Policy Institute of California, San Francisco, CA, July 2008.

Medellin-Azuara, J., E. Hanak, R.E. Howitt, Fleenor, W.E., and J.R. Lund, “Agricultural Losses from Salinity in California’s Sacramento-San Joaquin Delta,” San Francisco Estuary and Watershed Science, Vol. 12, No. 1, 2014.