Consideration of climate change in the Delta tunnel project

The Department of Water Resources will be holding a webinar on consideration of climate change for the Delta tunnel project on August 25, 2021.  The following discussion is from my testimony in 2016 for Part 1A of the WaterFix Water Right Change Petition hearing. It explains some key problems with the consideration of shifts in hydrology in the climate change analysis for the twin tunnels project. The question is whether the Department of Water Resources will address these deficiencies in the single tunnel project.

SHIFTS IN HYDROLOGY

These were my 2016 recommendations to the Water Board:

Recent observations and research point towards a much hotter and potentially drier future, with the potential for much greater increases in sea level rise than were previously predicted. The  most recent scientific literature and climate change modeling points toward major risks to water supply and water quality, which the model results presented by DWR and Reclamation for the WaterFix hearing do not address.

My recommendation is that the Board require that DWR and Reclamation submit modeled operations using the Q2 drier, warmer scenario for consideration in the WaterFix hearing. The Q2 scenario is the scenario with the greatest risk.

Here is an explanation of the different scenarios, adapted from my 2016 testimony.

CH2M Hill originally proposed to deal with uncertainty about regional climate scenarios by developing projections for subsets of the global climate model/ climate scenario ensemble. The ensemble was divided into 4 quartiles with projections of more warming and less warming, and drier or wetter. A Central Tendency for the ensemble was also calculated.

This would have been a reasonable approach to uncertainty about regional climate change scenarios if it was carried  through to the final WaterFix modeling. It also would have provided information on possible climate shifts. Instead, only the single “Central Tendency” projection was used for most BDCP and WaterFix modeling and model results, including the results presented for the WaterFix hearing

The Central Tendency (Q5) scenario provided no information about uncertainty in the BDCP / WaterFix projections of shifts in hydrology.  (See green line on chart below. The drier warmer Q2 scenario is shown by the red bars,  There are major reductions in runoff.)

CLIMATE SHIFTS

DWR’s planning studies for its 2010 analysis of modeling of climate change noted that there is a lack of analysis of potential drought conditions that are more extreme than have been seen in our relatively short hydrologic record. There is significant evidence to suggest that California has historically been subject to very severe droughts and that climate change could result in droughts being more common, longer, or more severe. However, most current DWR approaches rely on an 82-year historical hydrologic record (1922-2003) on which GCM-generated future climate changed-hydrologic conditions are superposed. This record is likely too short to incorporate the possibility of a low frequency, but extreme, drought.

DWR did fund a study of tree ring cores by David Meko at the University of Arizona. Meko’s study estimated the Sacramento Four River Index from tree ring cores,  back to 901 A.D. The reconstruction shows many extended periods of below average flows.

In a presentation for the 2009 Extreme Precipitation Symposium, Meko stated that

six-year droughts of the 1930s and 1980s-90s are as severe as any encountered in the tree­ ring record. For longer running means the tree-ring record contains examples of drought severity and duration without analog since the start of the 20th century. For example, mean flow is reconstructed at 73 percent of normal (1906-2008 observed mean, 23.8xl0^6 acre-feet) for the 25-year period ending in 1480.

My recommendations stated:

Given this history, I believe it is essential to consider extended drought periods in evaluating the proposed increase in water diversions by the SWP and CVP. I recommend that the Board require DWR and Reclamation to produce detailed information on water supply and water quality under the proposed change for the droughts of 1987-1992 and 1929-1934, and would recommend this analysis for all changes that involve significant increases in diversions.

References

Testimony by Deirdre Des Jardins for Pacific Coast Federation of Fishermen’s Associations Part 1A of theWaterFix Water Right Change Petition Hearing, September 2, 2016. Powerpoint presentation for oral testimony.

Khan, A. and Schwarz, A., Climate Change Characterization and Analysis in California Water Resources Planning Studies, Final Report. Department of Water Resources (Dec. 2010.) Last visited August 23, 2021.

Meko, D., “Central Valley Droughts Over Last 1,000 Years,” 2009 California Extreme Precipitation Symposium (UC Davis, June 24, 2009.)  Last visited August 23, 2021.

Treeflow website, Climate Assessment for the Southwest (CLIMAS) at the University of Arizona, and Western Water Assessment (WWA) at the University of Colorado.