Monday October 2, 2023

Office calendars aren’t being thrown out to make room for their 2024 replacements just yet, but climate scientists, farmers, and water managers across the state are resetting their rain gauges – October is here and a new water year has begun!

In the United States (and the majority of the world), the water year does not follow the traditional January to December calendar. Instead, water year calendars are based on the natural water cycle, and aim to encompass the precipitation that falls in autumn, as well as the winter snowpack that will move through the watershed as it melts during summer. With this in mind, the California water year begins on October 1 and concludes on the last day of September, just as the typically dry “Mediterranean” summer comes to a close. 

The United States Geological Survey (USGS), the federal agency that monitors hydrologic conditions nationwide, standardizes their reporting according to the water year. Interestingly – and perhaps confusingly to West Coast residents – other federal agencies such as the National Weather Service present most of the country’s rainfall summaries in the traditional calendar year. However, for the Western Region, they use the water year calendar. Rain and snowfall patterns are varied in different parts of the world, so the water years used for hydrologic purposes naturally vary as well. For example, in Australia, the water year begins on July 1, while the hydrological year on the African continent has varied starting dates: in North Africa, the hydrological year usually starts in September; in West Africa, it begins around March-April; and in the southern Cape region, it begins in January.

In the United States, annual hydrologic conditions have been recorded for over 100 years! The USGS released its first water-year compilation in 1911, and California has water-year data dating back to 1901. In California’s Central Valley, the Department of Water Resources classifies water years into different types (wet, above normal, below normal, dry, or critical), based on indices for the Sacramento and San Joaquin rivers that reflect the amount of runoff available in each basin. The driest year on record in the Central Valley happened in 1977, when less than a third of the average recorded runoff occurred, totaling only about 6-million-acre feet. For context, urban water use is around 10-million-acre feet per year, and agricultural water use ranges between 30-37-million-acre feet per year. In contrast, 1983 was the highest water year on record with almost 53-million-acre feet of runoff – nearly enough to fill Lake Powell and Lake Mead, the largest reservoirs in the United States. The 2023 water year was “wet,” with over 140% of the long-term average precipitation, and much of it was delivered by a series of drenching atmospheric rivers. However, on the coattails of another hot California summer, these times may feel like memories from long ago. 

Water year classifications inform water projections, current water management practices, and storage strategies. For example, if a water year is “dry,” outflows from storage reservoirs to the Bay Delta are reduced to maintain water allocations for other demands on the system. Over time, these reduced flows may not be able to support at-risk species and habitats as there will not be enough water in the system. Climate scientists can use past years’ information to predict future water conditions, and those projections can inform water policies. As climate change continues to influence the Central Valley watersheds, water year trends help managers adapt policies to mitigate impacts. Fisheries biologists pair water year classifications with population data to understand how changes in hydrological years affect fish populations. For example, low water years often correspond to reduced abundance in cold-water species like salmon and trout. With current projections estimating more dry years, prospects are bleak for these fish. However, the more we understand how fish populations are influenced by different water year types, the better equipped we are to manage these populations in our ever-changing climate.

Current projections estimate that California is in for another wet winter with a 95% chance that El Niño will continue through the winter. So, get your rain boots ready and keep your fingers crossed for another wet winter to keep California out of drought conditions!

This post was featured in our weekly e-newsletter, the Fish Report. You can subscribe to the Fish Report here.

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