California Department of Water Resources on the challenges of forecasting water supply in a warmer climate

April 12, 2022 – On April 8, DWR released the updated Bulletin 120 forecast and Water Supply Index (WSI). Bulletin 120 is a key tool for water managers across the state to understand how the melting Sierra Nevada snowpack will reach California’s streams, rivers, and eventually reservoirs. The forecast also has significant legal impacts for water rights holders across the state, especially in this third year of drought.

Very little snow remains on the ground for the California Department of Water Resources‘ fourth snow survey of the 2022 season at Phillips Station in the Sierra Nevada Mountains. April 1/DWR

The 2021-22 water year saw climate-related extremes well outside the historical norm, upending the patterns used for decades to forecast California’s water supply. Predicting the amount of snowmelt that will reach California reservoirs and be available to our communities, farms, and the environment becomes more difficult each year as previous assumptions about our climate no longer apply. DWR has been closely following climate research for decades that has warned of warming temperatures and increasing extremes and understood that these dramatic changes would be possible. During the third great drought of this century, we saw these changes become a reality and prove the need to adapt quickly.

To meet this challenge, the Department of Water Resources (DWR) is fundamentally moving away from reliance on historical models and investing heavily in improving forecast data collection and focusing on dramatic changes in the watersheds of California. More than $4 million has been provided by Governor Newsom to expand the Airborne Snow Observatory (ASO) program, which increases and modernizes the amount of data collected on the state’s snowpack and additional funding has been provided to put upgraded electronic monitoring stations across the Sierra Nevada to provide additional detail on soil moisture and atmospheric conditions. The value of these investments has already been realized this year when additional data provided by ASO flights prevented an overestimate of snow cover in the Feather River Basin and helped forecasters get a full picture of the evolution of snowfall.

The April water supply forecast, which uses data from these investments, paints a bleak picture that guarantees a third year of drought: the runoff period from April to July is expected to be only 41% of the historical average. . Snowpack was just 35 percent of normal, with the fifth-lowest snowfall on record dating back to 1950. The driest January-March period in state history culminated the Sierra Nevada snowfall a month earlier than usual on March 8. Even more extreme, the Northern Sierra snowpack likely peaked in January of this year, three months earlier. Although October rainfall this fall helped reduce dry soils and was followed by heavy snowfall in December, the record dry spell of the past three months has broken all previously seen patterns.

dwr412 April 1, 2022 snow survey mountains

This winter’s weather boost is the latest evidence of an alarming trend all Californians have witnessed over the past decade, which has seen dramatic climate shifts from dry to wet years, extreme temperatures in summer and a longer and more severe forest fire season. The lesson of the past two years is clear: future water supplies can no longer be predicted based on historical patterns that no longer apply in our rapidly changing climate.

DWR’s Snow Surveys and Water Supply Forecasting team is meeting this challenge head-on by researching and investing in tools and models that will look at the full picture, not just historical patterns. DWR’s forecasting and data collection teams are also reviewing our methodology with academic partners, including the Center for Western Water Extremes, the Central Sierra Snow Lab at UC Berkeley and UC Davis, federal agencies such as NOAA, NASA-JPL, the National Center for Atmospheric Research (NCAR), and local water management to assess the best available science for monitoring and modeling changing hydrological conditions.

As the threat of a hotter, drier climate continues to impact our water supply, DWR will continue to develop and adopt the best science and technology available to ensure Californians have a clean water supply. adequate, reliable flood control and healthy ecosystems, now and in the future. the future.
Source: CA. DWR