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WATER: Severity of past droughts quantified by new streamflow reconstructions
As part of ongoing work to improve California’s drought preparedness and better adapt to climate change, the Department of Water Resources on Monday released a report examining tree-ring data to help better understand historic periods of drought.
The report helps develop long-term reconstructions of streamflow or precipitation for the Klamath, Sacramento, and San Joaquin river basins.
The report, prepared for DWR by researchers at the University of Arizona, is available at http://www.water.ca.gov/waterconditions/docs/tree_ring_report_for_web.pdf .
Funding for part of the Klamath Basin work was provided by the U.S. Bureau of Reclamation under its WaterSMART program.
Initial work on the reconstruction project began in 2010, at a time when California was just emerging from the 2007-09 drought.
Completion of the final report coincides with a new three-year drought and a Water Year 2014 that so far is one of the driest years in the historical record.
California’s roughly 100 years of observed data are, however, only a small subset of the hydrologic record that can be reconstructed by measuring tree rings and calibrating them to observed data.
The tree-ring measurements made for this project allowed development of reconstructions that begin in the year 900 for the Sacramento River and San Joaquin River systems, and in the 1500s for various sites in the Klamath Basin.
“Streamflow reconstruction from tree rings takes advantage of the great longevity and climate sensitivity of several tree species in California and Oregon,” said lead author David Meko, a University of Arizona research professor of dendrochronology. “The tree-ring patterns record unusual climate events and modes of variability that occurred before the short period of gaged streamflow.”
Drought is a recurring part of California’s climate. The report’s reconstructions show numerous periods of four or more years when streamflows were below median conditions.
In addition, the report reveals that all three river basins share common major periods of extreme low flow conditions, although the degree of severity varies from river to river.
The most severe shared periods were the 1100s (20-50 year sustained dry periods), 1570 to early 1580s (up to decades-long periods), and 1920s -1930s (up to 20-year periods).
The Sacramento and San Joaquin basins shared 1580 as the single driest year of record. The driest single year for Klamath River streamflow was 1655 (1580 was 17th driest).
Paleoclimate information such as these reconstructed streamflows captures a broader range of hydrologic variability than provided in the historical record, thereby putting our short period of observed droughts in perspective.
A repeat of the “Dustbowl Drought” of the 1920s and 1930s (our most severe historical event in terms of duration) with today’s urban and agricultural development would sorely challenge California’s infrastructure and institutional framework for water management.
That challenge would pale in comparison to the time of the Medieval Climate Anomaly, when sustained severe drought gripped much of the western United States.
Paleoclimate information is useful in helping to understand and model natural variability in the climate system that may provide clues for improving drought prediction at the seasonal time scales important for water management.
Jeanine Jones of DWR said, “Drought prediction skillful enough to use for water management decision-making remains a research challenge for the science community. Having improved climate forecasting capabilities at time scales of months to a year in advance would provide great benefit for drought preparedness.”
Looking into the future, the reconstructions also help provide context for expected impacts of climate change.
The report compares drought durations seen in the paleoclimate record with those projected by downscaled global climate change models run to simulate conditions by the end of the century.
The results indicate that the paleoclimate data may be useful for assessing future climate projections in the context of past centuries
Report co-author Connie Woodhouse, professor and interim head of the University of Arizona School of Geography and Development, said, “These tree-ring records document the range of drought characteristics, including duration, that have occurred in the past, under natural climate variability. These droughts could occur in the future, but under warmer temperatures that will further exacerbate their impacts.”
