Think the Fresno region’s drought is the worst ever? Tree data will make you think again

Tim Sheehan   Mon, June 6, 2022

The San Joaquin River watersed is amid its third consecutive year of below-normal precipitation. But three dry years are merely a drop in the bucket (pardon the pun) compared to a historical record that, thanks to climate reconstructions using tree rings, stretches back more than 1,100 years in the region.

Scientists estimate that since the year 900 – when California was inhabited only by Native American tribes and centuries before it became a land of agricultural bounty – the region now known as the central San Joaquin Valley has endured 35 periods of sustained drought lasting at least four years, according to research studies available on the California Natural Resources Agency’s website.

Researchers base their calculations on precise measurements of yearly growth rings of trees in the watershed that indicate that the river’s historic median annual flow is just under 1.7 million acre-feet. That figure is the threshold for labeling a year as a wet or dry year.

Seven of those sustained drought periods have come just since 1900, and the average length of those prolonged dry spells over the past 122 years is about 5.2 years.

The longest drought period reflected through dendrochronology – the science of tree rings and what they can reveal about weather, climate and fires – was 12 years from 1450 through 1461.

That doesn’t count periods in which a string of dry years was punctuated by a single year of above-average precipitation. The 1450-1461 drought, for example, was followed by a single wet year in 1462 before the Valley was hit by four more consecutive dry years from 1463 through 1466.

“The widths of annual growth rings in trees in many locations closely track variations in moisture. Because of this, tree-ring widths can be an excellent proxy for variations in moisture measured though streamflow, precipitation, and drought indices,” according to the 2018 state Natural Resources Agency guidebook by researchers who studied tree rings in several California river watersheds including the San Joaquin River.

“Thus sequences of wide and narrow rings document wet and dry years for times prior to the recording of precipitation and streamflow through modern gages,” the report said.

“Records of precipitation and streamflow from gages are typically less than 100 years long. While this may seem like a long interval of time, these records capture only a limited number of extreme events, such as droughts,” the authors added.

Similar patterns are revealed by tree rings studied from sites across the system of tributaries – the Merced, Stanislaus and Tuolumne rivers – that feed into the San Joaquin as it flows north to the Delta.

An acre-foot is the volume of water it takes to cover one acre of land under one foot of water. That’s about 326,000 gallons. To think of it another way, an American football field including the end zones is about 1.32 acres, so submerging the turf to a depth of 12 inches would require 1.32 acre-feet, or more than 430,000 gallons.

The following interactive charts show 1,122 years of fluctuations in the estimated annual flow of water in the San Joaquin River, using tree ring analyses conducted for the state Department of Water Resources through 1945, and calculated flows since 1945 based on observations and estimates by the Department of Water Resources and the San Joaquin River Restoration Project.

Estimates for those post-World War II years calculate the river’s natural flow as though unimpeded by Friant Dam, which was built in between 1939 and 1942 northeast of Fresno, and other water diversion projects.

Each of the first five charts covers a 200-year span of time: 900-1100, 1101-1300, 1301-1500, 1501-1700, and 1701-1900. The sixth chart covers 1901 through 2021. Each marker on the charts indicates the estimated flow of the San Joaquin River in that year, in acre-feet.

Millerton Lake, formed by the San Joaquin River behind Friant Dam northeast of Fresno, is shown at full capacity in this 2019 file photo, the most recent year in which the river’s natural flow, unimpeded by the dam, would have been above a 1,100-year historic median.

A cutaway piece of a 1,000-plus year-old giant sequoia tree from Kings Canyon National Park in eastern Fresno County has arrows on its rings to show where fire burned the tree about every 12 years on average. The width of annual tree rings an also show the abundance or lack of water in a given year.