With a third and extreme year of drought, how will future hydrologic conditions compare to current ones? There are two strands of thought suggesting a challenging future:

• the 20^th Century was an unusually wet period
• climate change will confront water managers with more severe drought conditions in the future

For the Sacramento River watershed in California, there is unexpected good news. While California must confront variability in hydrologic conditions, Department of Water Resources’ recently released tree-ring data suggesting that current severe drought conditions do not signal that California will be facing more severe hydrologic conditions in the future.

Sacramento River Tree Ring Study

The California Department of Water Resources (“DWR”) recently released a draft Final Report Klamath/San Joaquin/Sacramento Hydroclimatic Reconstructions from Tree Rings, http://journalofwater.com/jow/tree-ring-studies-show-drought-is-recurring-event-in-california/.   I cracked open the study expecting to find data at least supporting the first proposition above—the 20^th century was an unusually wet period.  This expectation was based on my experience with the Colorado River.

Boy, was I wrong. The annual unimpaired runoff of the Sacramento River Four River Index averaged 18.3 million acre-feet from the year 900 through 2012 — the period that the DWR study reconstructed unimpaired runoff from tree rings (see figure).  Reflecting the fickleness of Mother Nature, there is a wide variability in annual flows.  What surprised me was that the 100-year moving average has been stable for almost the seven plus centuries of data. It is difficult to find any footprint of climate change in this chart.

Tree Rings versus DWR Estimates

How do the estimates of unimpaired runoff from DWR compare with tree ring studies? Very well (see chart).  The correlation (measure of the tendency of the two series to move together) is 0.86—in other words, annual variations in the unimpaired runoff estimated by the DWR Tree Ring study explains 75% of the annual variation in the unimpaired runoff estimated by DWR (and vice versa).  While the series move together closely, they are not identical.  Interestingly, the DWR Tree Ring study estimated unimpaired runoff for the Sacramento River at 23.5 million acre-feet in 2012 (last year estimated by study), while DWR’s estimate was only 11.8 million acre-feet.

What This All Means for California’s Water Supplies?

Tree-ring studies provide valuable information about hydrologic conditions from a longer-term perspective than other sources of hydrologic data. The tree ring data correlates well with DWR data on the unimpaired runoff of the Sacramento River Four River Index.  The longer-term perspective and data from tree-rings provides valuable insights about the nature of hydrologic risk confronting California:

• Long-term trends in unimpaired runoff of the Sacramento River Four River Index is stable—there is no obvious footprint from climate change
• While consecutive runs of more than 4 years are remote, if there has been a consecutive run of three or four years of below-threshold years, then there is a significant risk that hydrologic conditions are signaling a few more years of below-threshold conditions.

California is in the third consecutive year of below normal years. Based on tree-ring data, there is a significant risk that there may be a few more years of severe hydrologic conditions ahead.  The implications for management of water storage are straight-forward.  Taking water from storage today to avoid shortages now may become a source of regret in a couple of years.