California Reservoir Annual Forecast Tool (CRAFT) Update

Issued in November, CRAFT offers an early-season projection of water year inflow, giving a valuable head start in planning. Come February, DWR issues monthly Bulletin 120 (B-120) forecasts, providing inflow projections based on observed precipitation, snowpack, and hydrologic conditions. Because the B-120 provides a total WY inflow projection, it serves as a useful comparison to our CRAFT forecast. However, like any forecast, the B-120 is not infallible—historical cases, such as its significant underestimation of Oroville’s inflow just a few years ago, highlight that it too carries uncertainty.

The following plot illustrates how the CRAFT forecast (orange bars) compares to March’s B-120 forecast (blue bars) for a selection of reservoirs, along with the observed inflows through March 24, 2025 (green dots).
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Key Insights:

• CRAFT’s Early Advantage: Issued four months ahead of the B-120, the CRAFT forecast provides water managers with a crucial early look at seasonal inflow expectations. The fact that its projections overlap with B-120 in six of the nine reservoirs reinforces its value as a reliable long-range planning tool. Having this foresight before winter precipitation patterns fully emerge allows for more proactive operational and allocation decisions.
• Gradients in Precipitation: The B-120 forecast incorporates more recent precipitation and snowpack data, allowing it to capture regional gradients in observed precipitation. This is where the CRAFT forecast, which is based on CAP’s static forecast range of 90-110% of normal, shows some differences. As we’ve discussed previously, our ability to identify precipitation gradients is already in development and successfully captured this year’s regional differences. However, because this capability is still being tested, it has not yet been incorporated into our operational suite. As a result, the CRAFT forecast tends to project lower inflows than the B-120 in northern reservoirs (leftmost on plot), aligns well with B-120 projections in central regions, and is generally higher than B-120 in southern reservoirs (rightmost on plot).‍
• Looking Ahead: The discrepancies between the CRAFT and B-120 forecasts, especially in northern and southern reservoirs, underscore the importance of the work we are doing to expand our forecasting. As we continue to develop and integrate regional precipitation gradients into our models, we expect future CRAFT forecasts will become even more aligned with localized conditions across the state.

February and Beyond

The Northern Sierra, San Joaquin, and Tulare Basin precipitation indices for Water Year (WY) 2024-25, along with two analog years, are shown below. The pink box highlights the month of February. A few key observations:

• Since February 1, precipitation totals across all three regions have seen significant gains. The 8-station Northern Sierra index has nearly doubled, while the 6-station San Joaquin and 5-station Tulare Basin indices have more than doubled over this period.
• In the top right corner of each plot, the “Percent of average to this date” value represents how much precipitation has fallen compared to the average accumulation by this point. This is not the percentage of total water year precipitation.  A clear north-to-south gradient is evident in the traces.
  
  • Northern Sierra: 116% of average
  • San Joaquin: 73% of average
  • Tulare Basin: 91% of average
• If we average these 3 “Percent of average to this date” values, they combine to 93.3%, on track to fall within the CAP forecast range of 90–110% for the water year as a whole. This suggests that, while regional variations exist, the seasonal forecast has provided a highly actionable early estimate of total WY precipitation.
• The analog years continue to be a strong match for the current water year. Their trajectories bracket WY 2024-25, providing a useful range for comparison and highlighting their value as reference points for the water year.
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