DELTA LEAD SCIENTIST REPORT: California’s rainy season is becoming shorter and sharper

At the March meeting of the Delta Stewardship Council, Dr. Laurel Larsen discussed a new research paper on the changing timing of precipitation during California’s wet season.  She also updated the Council on the Delta Science Program’s activities and announced a new outreach effort called “Office Hours.”

She began by noting that precipitation totals for this water year are now hovering at just around 50% of typical cumulative precipitation received by this time of year, and drought has become the main topic of discussion.  Especially after the last drought, there’s been a lot of interest in using climate models to look ahead to see what the future might have in store for water delivery to California.  For example, the Council’s climate change vulnerability assessment utilizes a suite of climate models to develop its projections for the future.  However, the models used to project these future scenarios also suggest that even during normal years when the wet season delivers an average amount of precipitation, that precipitation will fall during a shorter amount of time, such that the rainy season is shorter and sharper.

Dr. Larsen said this is a concern because a delayed onset to the rainy season means that vegetation has a longer amount of time to become dry.  This extended period of time over which flammable vegetation conditions develop will more regularly overlap with the offshore Santa Ana or Diablo winds that typically occur in late fall.  She pointed out that these two ingredients often mean massive wildfires occurring at a time when federal firefighting forces are usually low in capacity.

The article, A Later Onset of the Rainy Season in California, written by Jelena Luković et al. and published in the American Geophysical Union’s Geophysical Research Letters in January of this year, now confirms that these shifts in the start of the rainy season in California projected by climate models are already occurring. 

The researchers examined time series of precipitation for California over the past six decades. They found that since the 1960s, the rainy season has been delayed by nearly a whole month or about 27 days on average. Most of this loss in early season precipitation comes out of November, with smaller relative decreases in September and October.

So for all practical purposes, the rainy season now starts in earnest in December, instead of November as in former years,” said Dr. Larsen.  “Furthermore, the peak is indeed sharper, with a lack of early-season precipitation being compensated for by slight increases in winter precipitation.  According to the findings of this paper, this phenomenon is pervasive across the whole state and is due to an extension of summer-like atmospheric conditions into October and November, which steer mid-latitude storm tracks away from California into the North. Importantly, these observations are consistent with the climate models, which builds further confidence in their projections and planning, like the Delta Adapts vulnerability assessment.”

Dr. Larsen noted that these observations also have important implications for Delta science and management. With a delayed onset to the rainy season and extreme wildfires being the new normal, we need to understand the implications for water quality and quantity within the Delta.  Higher precipitation during the rainy season could potentially result in more abundant vegetation growth, further exacerbating wildfires.

She referenced a commentary on the article also published in Geophysical Research Letters by Dr. Daniel Swain, who wrote that even though former early fall precipitation would have been just 2-5% of the annual total, even that small an increment received prior to the onset of the offshore winds was historically sufficient to dampen the fire season substantially and reduce the fire fanning potential of those winds.  Dr. Swain further said that the loss of that early-season precipitation really pushes California into a new fire regime. He cited a study by Goss et al. in 2020 that showed extreme autumn fire behavior in California has more than doubled since the late 1970s, which is primarily attributable to increased vegetation flammability rather than shifts in the offshore wind patterns.

Dr. Larsen pointed out that the altered timing of the rainy season has important impacts on fish species, such as the endangered winter-run Chinook salmon.  “The migration of winter-run chinook salmon into the Delta for spawning coincides with the first major wet season flows, and with later migration resulting from later flows, we can expect altered food web dynamics, as well as altered Endangered Species Act triggered constraints on flow diversions,” she said.  “And this doesn’t even begin to broach the topic of upstream reservoir operations being based on historic precipitation patterns that do not reflect the current reality.”

These reports really underscore the important role that the Council can play in funding original research on how these new normal, in many respects, are going to shape the resources that we manage,” she continued.  “We are entering uncharted territory, and having the ability to respond rapidly to developing conditions such as drought or excessive wildfire seasons, as the ISB has also called for, is critical.”

Dr. Larsen pointed out that it’s something that scientists like Dr. Daniel Swain have talked about for a long time because the climate models that do a good job for California have been projecting that to occur. 

This study that that just came out is important because it actually shows that this isn’t something that’s going to be happening in the future,” she said.  “It’s already happening now.”

Councilmember Don Nottoli asked how this research is disseminated beyond the scientific community and to those managers and decision-makers who need this information.

Dr. Larsen said the paper was published in one of the higher impact journals within the field of Earth Sciences and accompanied by a press release that was widely distributed. “I am confident that that scientists within our partner agencies in California have seen this work. And I know this will become an important part of the scientific basis of the ongoing push for adaptive reservoir operation.”

Councilmember Daniel Zingali asked if this study also has implications for any potential conveyance projects? And does it mean that there will be less flow available to sustain a conveyance project?

There are several layers of complexity in the answer to that question,” Dr. Larsen said.  “One is that the precipitation that would feed flows into the Delta is going to be falling in a more concentrated period of time, which underscores the need for storage capacity to capture those flows. The other thing that’s happening here is that these flows and altered precipitation patterns are also coinciding with warmer temperatures. So one of the other things that has already been happening is that these precipitation totals are becoming increasingly skewed more towards rain and away from snow, which exacerbates the storage challenge.

Our storage system in California was built largely on the understanding that snowpack would be doing a lot of the storage for us, and when that’s no longer the case, it taxes our system of reservoirs. There are certainly implications for this sharper precipitation season for flows that make it into the Delta that have to pass through the filters of our reservoirs. So [it’s hard to know] how exactly those flows are going to be impacted without more comprehensive modeling, but there will be an impact. … This is something that many people, many state agencies are concerned with. And I do know that they are modeling as a way to try to anticipate these changes.”

Dr. Larsen said this study also really underscores the importance of looking beyond the boundaries of the legal Delta and considering the Delta as a system connected from the uppermost reaches of the watershed down through the San Francisco estuary.

I think that we continually need to be aware of these connections and thinking about Delta science and how it relates to management needs,” she said.


Delta Science Proposal Solicitation

The primary mechanism through which new research is funded in the Delta is the biannual proposal solicitation.  The most recent solicitation, which closed in February of this year, is jointly supported by the Council’s Delta Science Program and the US Bureau of Reclamation in partnership with the California Sea Grant.  The Council received 99 proposals for research responsive to the 2017 to 2021 Science Action Agenda, double the amount of proposals that we received during the solicitation.

The total award amount is expected to be up to $9 million, including $5.5 million from the Council and $3.5 million from the Bureau of Reclamation. An ad hoc panel is currently reviewing the proposals.  An announcement of the awarded projects is expected in June 2021.

Steelhead Trout Workshop

The workshop on monitoring steelhead populations in the San Joaquin basin workshop was held virtually on February 17-19.  At this workshop, the participants reviewed the management challenges and monitoring framework for San Joaquin Basin steelhead.  The workshop was held in response to the biological opinion on the long term operation of the Central Valley Project and the State Water Project, which calls for the development of a plan to monitor steelhead populations within the San Joaquin basin and/or San Joaquin River downstream of the confluence with the Stanislaus River.  This also includes steelhead and rainbow trout on non-project San Joaquin tributaries as well.

At this workshop, breakout sessions explored how steelhead populations have been monitored within the Central Valley and what lessons can be leveraged for the San Joaquin basin.  The breakout sessions also identified relevant stakeholder groups and the top monitoring priorities and information needs, and the current monitoring efforts and existing programs that could be modified to meet these needs.  On the final day, the breakout sessions explored what the balanced sampling framework would look like that accounts for extensive geographic heterogeneity and the different life stages that the project would need to encompass. 

Participants in the breakout sessions concluded that this is a complicated management and resource challenge, but with broad stakeholder interests,” said Dr. Larsen.  “Although the current monitoring framework that’s already in place is sophisticated, there are still many opportunities to better align and coordinate across different efforts and create consistency. We anticipate great things from this workshop.  The next steps include scoping a potential manuscript, reconvening some of the agency members that have assisted with the long-term operations .., and then moving towards drafting a San Joaquin basin monitoring plan. So stay tuned as those products are developed and come out.”

Bay-Delta Science Conference

The 11th biennial Bay-Delta Science Conference will be held virtually on April 6 through 9. The conference is jointly sponsored by the Council and the US Geological Survey and is also being held concurrently with the IEP workshop.  The conference provides a forum for sharing scientific information relevant to managing the San Francisco Bay and the Sacramento-San Joaquin Delta.  This year, the conference’s theme is ‘Building Resilience Through Diversity in Science.’  Registration is free.

Delta Lead Scientist “Office Hours”

Dr. Larsen is launching a new initiative called “Office Hours” that will formally be announced at the  Bay-Delta Science Conference, which will be held on the third Thursday of each month. They will be targeted broadly at the scientific community and designed to facilitate new connections between the Delta science program and any interested party. The office hours will take the format of a live Ask me anything forum on Instagram Live, which allows participants to type questions that will then be answered live and broadcasted.  Each week will have a different theme, but all questions will be welcome.   The first office hours session will be held on April 15.

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