Senior Engineer Dr. Paul Hutton provides a brief update on Metropolitan’s ongoing science efforts in the Delta
At the January meeting of Metropolitan’s Special Committee on the Bay Delta, Senior Engineer Dr. Paul Hutton updated committee members on Metropolitan’s ongoing science activities, focusing on the work he’s been conducting reagrding historical Delta outflow and salinity trends over the last nine decades.
Dr. Hutton said the motivation for doing this work is two-fold: “First, it’s to understand how and why Bay Delta outflow and salinity has changed over time, and I have a quote from Aristotle to support that. He said, ‘if you would understand anything, observe its beginnings and its development,’” he said. “A second motivation is to provide historical context for regulatory decision making. I have another quote, this one from C. K. Chesterson who said, ‘We can be almost certain of being wrong about the future if we are wrong about the past.’”
He then presented a chart showing the history of regulatory restrictions on the state and federal water projects. “There’s been a real focus on flow in terms of fishery protection in the Delta,” he said. “I believe that this focus has been a little too narrow, and that we need to focus on other drivers of ecosystem change. That’s really the basis of Metropolitan’s ongoing science efforts. Furthermore, I believe that this narrow focus is in part probably due to our misunderstanding of how flow and salinity in the Delta has changed over time.”
Metropolitan’s science efforts on flow-salinity and on natural flows will go towards supporting their efforts at the upcoming proceedings at the State Water Board. “This flow-salinity work will support our efforts towards the Delta smelt action on Fall X2, and the work I reported to you a year ago on natural flow goes to the State Board’s proposed unimpaired flow criteria,” Dr. Hutton said.
Dr. Hutton first gave the highlights from their findings. “First, the findings challenge the conventional wisdom on historical changes to Delta flows and salinity,” he said. “In particular, we looked at changes in the annual volumes of Delta outflow and surprisingly, we found that there is no meaningful, statistical trend over this 90 years. That’s a very surprising and counterintuitive finding, so we are looking at that very carefully. The reason it’s surprising is that we know over the last 9 decades that water use has gone up, so why is it that we’re not seeing a trend in the data?”
“While we didn’t find trends in the annual flow data, we did find meaningful trends in the seasonal or monthly data,” he said. “We saw meaningful decreasing trends in four months and meaningful increasing trends in two months. Lastly, the salinity trends that we found tracked very closely with what we found in the flow trends.”
Dr. Hutton said that they have been working to get their findings published. “It’s been experience of staff that the science advancements are more credible to regulators and to the courts if the work has been peer reviewed,” he said. “Most of our science efforts have in them a component to put together a manuscript to try to get published and over the last year, we had three papers published.”
- Nine Decades of Salinity Observations in the San Francisco Bay and Delta: Modeling and Trend Evaluations (ASCE Library)
- Reconstructing the natural hydrology of the San Francisco Bay–Delta watershed (Hydrology and Earth System Sciences Journal)
- Evapotranspiration from Natural Vegetation in the Central Valley of California: Monthly Grass Reference-Based Vegetation Coefficients and the Dual Crop Coefficient Approach (ASCE Library)
In today’s presentation, Dr. Hutton said he would be discussing mainly how flow and salinity has changed over time; he won’t go into the question of why as that work is still in progress, but he will talk about some of the potential drivers for the change.
“Potential drivers of changes to outflow include exports and imports to the basin which are a key driver,” he said. “Landscape change, another potential driver, including things like ag and urban development, levees and flood control channels, and forest management. Reservoir construction; reservoirs change the timing of flows. Groundwater interactions, another potential driver, and then of course climate change, and how that affects the timing of precipitation and temperature in the valley.”
Dr. Hutton then defined Delta outflow as the freshwater inflow from the Sacramento and San Joaquin River that enters the bay, minus any exports from the Delta and minus any in-Delta use from agriculture.
He presented a chart showing annual Delta outflow from 1922-2012. “This is the actual data over the last 9 decades and you can see this is all over the place. It’s widely variable with climate; it varies everywhere from less than 5 MAF a year to over 50 MAF a year. To visually help you make any sense of any possible trends, I’ve overlaid a 5 year average on this and you can see it all sliding back and forth.”
“We used some sophisticated statistical tools to look and see if there are any meaningful trends in this data, and the answer was in fact no trends,” he said. “Again, this is really a surprising counterintuitive finding, because we do know the demands and water use have gone up over the 9 decades, so we are looking really closely at why we are coming to this finding and try to give an explanation for this.”
When they did apply those same statistical tools to the monthly data, they did find meaningful downward trends in February, April, May, and November, and two meaningful upward trends in July and August.
Dr. Hutton then focused on the fall months and the Delta smelt Fall X2 action. “Interestingly enough, statistical tests have come up with an uncertain answer for September and October and you may ask why is that,” he said. “What this is, these months are really sensitive to what we assume about in-Delta water use, that’s by agriculture in the Delta, and we don’t really know to any high degree of accuracy of what the in-Delta water users are using, so really depending on what you’re assumptions are, the tests might give you one answer or another. So bottom line, we’re not sure about the importance or the meaningfulness about the trends in September and October. We do see a meaningful downward trend in November.”
“To give you some context with regard to these flow trends, in the winter and springtime, this is when the reservoirs are typically filling, so they are taking water out of the system,” he said. “Into the summer and early fall, the reservoirs are releasing water or adding water to the system. Then you have agriculture taking water out of the system through irrigation in the spring and summer months. So you can use those to give you a sense of what some of the driving factors might be.”
Dr. Hutton then turned to salinity, presenting a map of the Delta. “You can see more or less you have two forces that are pushing against each other – you’ve got the Delta outflow pushing salt seaward, and then you’ve got the tides bringing salts from the ocean landward, and they are working together to result in a salinity gradient in the estuary,” he explained.
Potential drivers of salinity change include flow changes, sea level change, and modifications to the channel or landscape that can have an effect on how salinity moves in the estuary, he said.
X2 is defined as the distance in kilometers from the Golden Gate Bridge where salinity is about 2 parts per thousand (ppt). “X2 is a compact way to describe salinity intrusion into the estuary,” he said. “You can see when flows are low and it’s salty, the X2 value is high – 81 km from Golden Gate. When the flows are high, the estuary is fresh, and the X2 value is low, moving closer and closer to Golden Gate. X2 is regulated by the State Board in the spring months, and it’s now regulated by the fish and wildlife biological opinion for Delta smelt in the fall months following wet and above normal years.”
They applied the same statistical tests to the salinity data and found in months November through June that there was a meaningful increase in salinity in these months, and a meaningful decrease in salinity in August and September, he said. “The Delta smelt biological opinion is founded on the statement that salinity has significantly increased over time, and what I showed you earlier about flow and what I’m showing you right here really by and large contradicts that statement, so we’re showing here that in the fall, in September, the estuary is actually fresher than it used to be, and in October, there has been no meaningful change. Only in November are we seeing that it’s meaningfully more salty,” Dr. Hutton said.
He then presented a time series of September, October, and November, noting that the red line is the 5 year average of the data which is used to get a sense of any trend. “One thing that you see in common with all of these is that you do see a downward trend in the early part of the record, followed by an increase,” Dr. Hutton said. “But when you consider this entire 9 decades of record, what the statistical analysis again is saying is that in September that we’re seeing a meaningful downward trend, again meaning it’s actually fresher in September than it used to be, and there’s no meaningful trend in October, and there is a meaningful trend in November.”
He then concluded with the next steps. “First we’re going to continue technical efforts to reconstruct historical hydrology and salinity, so today I showed you what we believe has happened over the last nine decades,” he said. “I reported to the Committee back over a year ago on natural flow; roughly we’d say maybe that would characterize what was going on prior to the Gold Rush, we’re also planning on looking at this intermediate period, what happened between the Gold Rush and 1920. We’re going to attempt to identify key drivers of change in flow and salinity. And again, we will continue to try to publish findings from this work, and this work will go to support our engagement with the State Water Board activities, both in terms of the Cal Water Fix petition for the change in point of diversion and also for the water quality control plan update. We believe that our science efforts, some of these like on flow and others we are working on, more on the fisheries side will go to support our engagement at the State Board, and then we will also be continuing to engage in other science and regulatory activities.”
“So with that … “
For more information …
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