Q&A with Kwabena Asante and Matthew Bachman on Their Presentation at FMA for the Evolution of the Sierra Snowpack – New Insights from the Central Valley Flood Protection Plan

By Quinn Fenger, Kwabena Asante, and Matthew Bachman

California’s Central Valley Flood Protection Plan (CVFPP) is updated every five years to ensure that the state’s infrastructure and communities are prepared for the changing climate. At this year’s Floodplain Management Association conference, Matthew Bachman, a water resources engineer, and Dr. Kwabena Asante, an engineering hydrologist and climate‑resilience planner, will present “Evolution of the Sierra Snowpack – New Insights from CA Central Valley Flood Protection Plan Modeling.” The session explores why the Sierra Nevada snowpack—often called California’s “frozen reservoir” because it supplies roughly 30 % of the state’s water needs—is central to flood‑risk management and how new modeling tools reveal both risks and opportunities as the climate warms. Recent winters have swung from near‑record snowpack to well‑below‑average conditions. With climate change, future storms could produce more rain and less snow. Bachman and Asante explain how the CVFPP is helping us to understand the changes and why it matters.

Q: First, I want to start with the “why” question—why should conference attendees care about your presentation on the Sierra snowpack?

A: Matthew Bachman: Every five years, the California Department of Water resources (DWR) leads an update of the CVFPP, and the 2027 update introduces substantial improvements in the modeling tools used. GEI is supporting DWR in preparing the 2027 CVFPP Update. We have increased the granularity of the climate modeling, expanded the range of possible climate futures we analyzed, and used a new Decision Scaling framework that has improved our understanding of the likelihood of these various future climates. Even people familiar with the plan will find these updates informative.

A: Kwabena Asante: At a basic level, the Sierra snowpack is California’s largest surface‑water reservoir. The state relies on the snowpack for storing water supply and for naturally regulating the timing and magnitude of runoff. With long‑term climate change, more precipitation is falling as rain rather than snow, which means the snowpack (our natural reservoir) is diminishing. As it shrinks, we see increased flood risk because more water rushes down to the valley floor in late winter and early spring. Understanding the snowpack’s dynamics is therefore critical to managing both water resources and flood risk over the long term. That’s what this presentation focuses on.

Q: For those unfamiliar with the Central Valley Flood Protection Plan, what is it and how does the snowpack fit in?

A: Asante: The Central Valley is one of the most productive agricultural regions in the country. It also supports major communities such as Sacramento and Stockton. Protecting the lives, infrastructure, and agricultural investments in this region from floods is why the CVFPP exists. The plan takes a 50‑year view but is updated every five years to provide confidence to residents and investors that long‑term projects can proceed safely. Because the Sierra snowpack supplies about 30% of the state’s water, its condition impacts both water supply and flood protection. When more rainfall occurs instead of snow, runoff arrives earlier and with greater intensity, increasing stress on levees and reservoirs.

Q: Recent winters have swung from extreme snow to extreme drought, with storms producing more rain than snow. How are your modeling approaches accounting for these swings?

A: Bachman: A major change in the 2027 CVFPP update is the range of climate futures we evaluated. In the previous (2022) iteration, we evaluated three future scenarios: high, median, and low climate change. For the 2027 update, we analyze 24 combinations of future warming and precipitation change. This gives us a more robust picture of the future and helps planners prepare for extremes.

A: Asante: Also, the state has invested heavily in monitoring snowpack conditions. There is a long record from fixed snow sensors and periodic snow surveys, where the state works with numerous cooperating agencies and utilities to physically measure snow depth and water content in the field. Recent advances include airborne snow observatories, which provide high‑resolution measurements and improved estimates in drought years when sensors might miss patchy snow. However, our work for the 2027 CVFPP focuses on long‑term changes, which may require re‑operating reservoirs and rethinking flood management. To support this longer term focus, we’ve broadened our modeling to cover a wider set of climate scenarios and more detailed hydrologic processes.

Q: Are there emerging strategies to adapt to earlier, more volatile runoff?

A: Bachman: One promising approach is Forecast‑Informed Reservoir Operations (FIRO). Historically, state and federal reservoirs follow fixed rules—release water according to a set flood curve or delivery target. FIRO uses advanced weather forecasts to inform dynamic operations. If a large storm is predicted, operators can release water ahead of time to create space for incoming runoff. This strategy helps accommodate the flashier, earlier storms we expect with warming without sacrificing water supply.

Q: What are the key findings from your updated snowpack analysis?

A: Bachman: Preliminary results for the 2077 horizon show moderate to significant decreases in snowpack. We’re seeing large reductions in April 1st snow water equivalent (traditionally regarded as the peak of snowpack accumulation season) and reductions in annual maximum snowpack across all elevations. Snow cover lasts fewer days, and the snow line moves upslope—a 2,000‑foot rise in some watersheds. These shifts have major implications for flood‑risk management and water‑supply planning.

A: Asante: We should note as well that these projections show broad trends across 24 possible future scenarios. Our primary focus is 50 years into the future though we will also analyze some intermediate projections. But the consistent message across all scenarios and timelines is that our natural reservoir is shrinking, and the timing of runoff is changing.

Q: Were there any surprises in the results?

A: Bachman: Even though we anticipated earlier, flashier storms, I was struck by how much the volume of runoff shifts to the earlier part of the year and how dramatically the spring snow‑melt peak declines. The magnitude of the change exceeded my expectations.

A: Asante: As for myself, I always assumed that warming would simply increase flood peaks, but once the snow cover is gone, we see a slight reduction in peak flow for very extreme events. That’s because there is less moisture stored to feed the runoff. We also observe that extreme events—those with return periods of 500 or 1,000 years—show a change in trend: flood peaks increase up to a point and then begin to slightly decrease. It is counterintuitive and highlights the complexity of climate impacts. Understanding these dynamics will be important for managing both flood risk and water supply.

Q: What research questions or data gaps still need to be addressed to better manage California’s snow‑dependent water and flood systems?

A: Asante: One big question is inter-annual variability—how the natural variability in weather patterns might change with climate warming. Our methods often assume that the pattern of variability stays the same while the magnitude of events change. Better understanding the timing between storms, the length of droughts and the clustering of extreme events could improve projections. Another area of future improvement is how to manage residual risk. We can’t build our way out of the most extreme flood risks because building infrastructure to withstand every possible extreme is too costly. We need to also develop early‑warning systems and improve community resilience.

A: Bachman: I agree. We are also looking at how to integrate snowpack changes into reservoir operations and floodplain policies, as well as ways to convey complex scientific information to decision makers and the public.

Q: Any closing thoughts for conference attendees?

A: Bachman: We hope people leave with a deeper understanding of how the snowpack is changing and what that means for both flood risk and water supply.

A: Asante: Know your snow! It affects your livelihood more than you might think. The Sierra snowpack is often taken for granted as a reliable “frozen reservoir,” but as climate change shifts precipitation from snow to rain, it’s crucial to pay attention and adapt. Come to the session to learn how the snowpack is changing and why it matters for flood management.