How bad is climate change going to impact water delivery?

Kevin Richards and K.T.Shum, EBMUD - California Water and Environmental Modeling Forum Annual Meeting, 2004

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Historical hydrology Difference due to climate change scenario

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Known facts about climate change…• Average global surface temperature has increased by 0.6°C since the late 19th century.

• Evidence of reduction in Artic sea ice and continental glaciers.

• 1.0 to 2.0 mm/yr of global average sea level rise during the 20th century.

Likely reductions in California water supply…

• Reduce spring snowmelt increase need for water storage in reservoirs

• Increase urban water use, increase evaporation and evapotranspiration

• Etc.

But by how much?

Most global climate models estimate an average increase of at least 2oC over the next century under business-as-usual scenario. This could:

Some quantitative results on potential water delivery impacts in California…

• Warming (i.e. early snowmelt) Preliminary studies using CALSIM II, Bardini et.al. 2001

– 20th century diminishing snowmelt trend extended

– 2.5% reduction in total CVP/SWP exports during 1928-34 drought

Preliminary studies using EBMUDSIM, Richards 2003– Shift 28% of historical Apr-Jul runoff to Nov-Mar

– <1% reduction in Mokelumne deliveries during 1928-34 drought

• Reduced precipitation Brekke et.al. 2003

– PCM scenario w/ 13% reduction in San Joaquin River region annual runoff

– Negligible reduction in dry-year deliveries to east-side contractors; 14% reduction in dry-year deliveries to west-side contractors (Delta exporters)

Richards 2003 – 10%, 20%, 30% reduction in annual runoff

– 4%, 6%, 12% reduction in dry-year deliveries to Mokelumne water users

Not many, and by not as much. Why?

Some apparent reasons for relatively small water delivery impacts simulated…

• Use of historical hydrology Climate models project long term trends and may not account for all critical factors

influencing climate variability (year-to-year and month-to-month). Most of the studies utilize a climate change forecast superimposed on the 20th

century hydrologic sequence. This presents no new information about potential climate variability and more critical hydrological sequences.

• Smart flood control Models (e.g. EBMUDSIM) know when to begin reservoir refill according to how

much snowpack remains in the watershed. Depending on the system’s rainflood requirements, there is enough snowmelt to refill

in all but a few years of the historical record due to a 28% shift of runoff. If those years were to precede a critical drought then results could be more

substantial… The sequence of wet and dry years is important!

What are some alternate approaches?

Focus on critical factors affecting water delivery…• Climate research and modeling:

Climate variability – Quantify linkage between precipitation and major drivers (e.g. PDO, ENSO); identify factors influencing extreme events (e.g. “Pineapple Express”) and quantify probability of occurrence in each watershed.

Watershed hydrology – Quantify relationships between climate variables and hydrology variables (e.g. temperature and snowpack, precipitation and runoff) in each watershed.

• Water delivery impacts research and modeling: Reservoir operations – Quantify potential changes in hydrology-based rules and

agreements under climate changes (e.g. flood control, target storage, water use, instream flows). Develop models to relate project operations to hydrology rather than utilizing historical data and empirical formulations.

Mitigation strategies - Lund et. al., 2003 and VanRheenan et.al., 2004 have explored mitigation strategies to cope with climate change impacts. This is an important aspect of understanding the problem.

Conduct more comprehensive sensitivity analyses.

…and break the problem into smaller pieces.

Preliminary results from EBMUDSIM

Kevin Richards, EBMUD

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