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Classification of Water Year Types

At the 2021 Bay-Delta Science Conference, Department of Water Resources (DWR) engineers discussed the results of their modeling study on classification of Central Valley water-year types that define operations of state and federal water projects.1 The study recognized the need to adjust the rules because of climate change and associated changes in human and environmental demands on water supplies. “Iterations of the model become a water system stress test under different incremental changes of climate.”

The study focused on the classification of water years for the Sacramento and San Joaquin rivers: currently, these are critically dry, dry, below normal, above normal, and wet. The study suggests there is likely to be a higher frequency of critical and below normal years, and a lower frequency of wet and above normal years, because of rising temperatures. The study suggests adjusting the year type criteria downward to increase the frequency of drier year-type designations. Sensitive parameters in the model included Delta inflows and outflows, Delta exports, and Delta salinity.

The study suggested that changing the classification criteria would generally result in higher Delta inflows and outflows, which would help reduce the salinity effects of sea-level rise due to climate change. The theory was evidently that DWR and the Bureau of Reclamation set export levels lower in drier water-year types. If DWR and Reclamation actually set lower exports and north-of-Delta deliveries in drier water years, there could actually be some benefit. But right now, most of those levels are discretionary and not enforceable criteria. And under existing rules, Delta inflow and outflow requirements also become progressively lower with drier water year types.

The logic behind supposed aquatic benefits to increasing the relative frequency of drier water year types is tortuous at best. The real outcomes would depend on the implementation of the other variables that are the legs of the water management stool: flow, storage, and deliveries. Those outcomes will be measured indirectly by such metrics as water temperature and salinity, and more directly by the quality of the fisheries produced.

The State Water Board’s update of the Bay-Delta Water Quality Control Plan proposes to severely limit reliance on water year types. On the other hand, the Bay-Delta Plan to date has not addressed the specifics of droughts and dry year sequences. Whether called water year types or something different, managing water is dependent on annual and inter-annual conditions.

One example of a different approach to watershed-specific water-year-type criteria is modification of reservoir storage requirements according to various year-type conditions. In a recent post, I suggested a sliding scale of minimum end-of-year (end of November or December) storage criteria for Folsom Reservoir on the American River (Figure 1). Similar criteria are relevant to Oroville Reservoir on the Feather River (Figure 2). Maintaining minimum reservoir storage criteria would go a long way toward protecting all beneficial uses. To date, both DWR and Reclamation have doggedly resisted such criteria.

Figure 1. Folsom Reservoir daily-average storage (acre-feet) 2000-2021. Recommended minimum storage criteria (end of November) are shown by circles: blue for high-storage years; light blue for intermediate-storage years; yellow for low-storage years. Red arrows are years that grossly exceeded such criteria.

Figure 2. Oroville Reservoir daily-average storage (acre-feet) 2012-2021. Recommended minimum storage criteria for end of November are shown by red circles.