Theme for 2009:
Doughts and Floods — Past and Future
University of California, Davis
Why this theme?
This year the two precipitation extremes — droughts and floods — are explored. Speakers provide a look back and into the future. Does the past give insight into potential future droughts and floods? How might climate change impact these extremes? Droughts and floods are like bookends bracketing precipitation extremes in California. How do we think about them? Two speakers provide different perspectives on precipitation extremes, which we don't usually consider. The potential winter storm scenario developed for the U.S. Geological Survey's Multi-Hazards Demonstration Project is presented. It is one probable future flood event to consider.
Special Recognition Award
The 2009 Special Recognition Award was presented to Robert F. Collins, P.H. of the U.S. Army Corps of Engineers. See the award language, biographical information, and hear the presentation on the Symposium's Robert F. Collins award page.
Holocene Megadroughts and Megafloods in California's Central Valley
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The central valley of California is intimately connected to the Sierran and coastal mountains, where it receives winter and spring runoff and to the San Francisco Bay Estuary, which receives the combined flows of the Sacramento and San Joaquin Rivers. Climate over this broad region is variable over both space and time, with a distinct north-south gradient in total precipitation and pronounced wet and dry seasons. Historic variations in the typical climate conditions have included mild to severe droughts as well as torrential rains accompanied by flooding. An examination of paleoclimate records from throughout California, spanning several thousands of years can provide a greater understanding of the natural range of climate over California's central valley and the potential impacts of global warming on its associated ecosystems than the last century or so of instrumental records has provided.
After a postglacial warming and drying trend in California, conditions grew cooler and, in many places, wetter around 3,800-2,000 years ago. These conditions had the effect of lowering salinity in the San Francisco Bay Estuary and altering environmental conditions for local ecosystems. This period was followed by gradual drying throughout the state, a general trend that has been punctuated by recurring periods of prolonged and/or severe drought over the region (megadroughts) and by catastrophic wet periods (megafloods). A number of paleoclimate records from across the state suggest that notably stable conditions have prevailed over the instrumental period, i.e., after ca. A.D. 1850, despite occasional severe, short-term anomalies experienced during this period.
Droughts, Floods, and Forest Health
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The importance of forest ecosystems to human well-being cannot be overstated. The connection between forests and water resources is well established, but the relationships among the components are only partially understood. Drought and floods can impact forest health and the quality and quantity of forest ecosystem services. This talk explores the current and possible changes in forest ecosystems, floods, and droughts in response to predicted climate changes, and introduces some potential management options.
Central Valley Droughts Over Last 1,000 Years
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Severe hydrologic droughts can stress ecosystems, cut agricultural production and reduce available water resources for hydroelectric, industrial, municipal, and other uses. Consequently, it is important to understand the natural variability of drought occurrence at various time scales, including those longer than amenable to study with relatively short gaged precipitation and streamflow records. The millennial timescale of drought variability in the Central Valley, California is addressed in this presentation. Available tree-ring reconstructions of annual flows of the Sacramento and San Joaquin rivers are summed and the two-river sum analyzed for information on hydrologic drought on annual to multi-decadal timescales. Droughts are tabulated by using moving averages over a reconstruction period from 942-1977 to place the post-1905 flows in a long-term context.
With the exception of a remarkable low-flow year in 1580, results show the period of gaged flows is fairly representative of worst-case single-year drought in the long-term record. For running means as long as six years the instrumental period likewise is a reasonable snapshot of long-term extremes: 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, 23.8x106 acre-feet) for the 25-year period ending in 1480. This is about 10 percent lower than any reconstructed 25-year mean in the 20th century. Tree rings indicate the 1400s drought was characterized by two pulses of severe drought imbedded in a longer period of 33 consecutive years with no annual flows exceeding 110 percent of normal.
A multi-decadal drought in the mid-1100s coincides with dry conditions identified by tree-ring studies in the Upper Colorado River Basin. Two of the most prominent high-flow peaks in 25-year running means are consistent with termination dates of previously identified low stands of Mono Lake. Planners in water resources, ecology, and other fields should consider the possibility of recurrence of drought patterns similar to those in the tree-ring record. It should be emphasized that tree-ring reconstructions are imperfect recorders of precipitation and runoff, and that future collections of tree-ring data, especially in northern California, could lead to improved accuracy of reconstruction and revision of the drought history.
Future California Droughts in a Climate Change World
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An often-cited impact of climate change is that floods and droughts will get worse in the coming century. Such an assertion leads to the question: "What will future droughts in California look like over the next century?" In an effort to explore this topic, characteristics of 20th century droughts in California are described followed by a quick look at paleodroughts. After reviewing climate change impacts relevant to California drought, the presentation explores how the aforementioned drought characteristics might change.
Risks and Reservoir Operations: Droughts vs. Floods & People vs. Power
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Droughts and floods cause immense hardship for an area's residents. The water infrastructure of California not only moves water from where it is plentiful to where it is rare and/or needed, it protects against very wet years/very big storms and the critically dry years, especially ones in sequence. Reservoir operators use risk management to meet water supply needs and/or maximize power generation across the range of water conditions from drought to floods. Forecasting seasonal snowmelt runoff using both historical monthly statistical models and 5- and 10-day hydrologic simulation models guides decisions about holding or releasing water. Power operations seek to maximize revenue from storage, and failing to fill when dry years occur is acceptable. Water supply operations seek to keep storage high under all water year types, but when excess water is available at very low risk to water supply, generation occurs. Challenges to power and water supply operations exist due to global warming, increased environmental flows, whitewater recreation demands, and population and demand growth.
American River Floods: Historic to Theoretical
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[Abstract not available]
A Winter Storm Scenario for USGS Multi-Hazards Demonstration Project
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In 2008, the USGS Multi-Hazards Demonstration Project (MHDP) brought together over 300 experts to create "ShakeOut" the most comprehensive earthquake scenario and the largest earthquake drill ever.
The MHDP is now preparing for its next major public project, "ARkStorm," a scenario to address massive West Coast storms analogous to those that severely impacted California in December 1861 and January 1862. The MHDP has assembled experts from National Oceanic and Atmospheric Administration (NOAA), U.S. Geological Survey (USGS), Scripps Institution of Oceanography (Scripps), the State of California, and many other organizations to design a large, but scientifically plausible, hypothetical storm. The storm will originate near the equator, resulting in an Atmospheric River (AR) of moisture that will grow large, gain speed, and slam the U.S. West Coast with intense winds and rains for a prolonged period of several weeks. The task of ARkStorm is to elevate the visibility of the very real threats to human life, property, and ecosystems posed by extreme winter storms on the U.S. West Coast. The ARkStorm scenario will provide emergency responders, resource managers, and the public a reality check on what is historically possible.
To help prepare, experts will examine in detail the possibility, cost, and consequences of floods, landslides, coastal erosion, and inundation; debris flows; environmental consequences like pollution and extirpation of endangered species; and physical damage possibilities like bridge scour, road closures, dam failure, property loss, and levee system collapse. Consideration will be given to catastrophic disruption to the water supply to California; the resulting impacts on ground water pumping, seawater intrusion, water supply degradation, and land subsidence; and a detailed examination of climatic change forces that could exacerbate the problems.
Climate Change, Atmospheric Rivers, and Future California Floods
Historically, the most dangerous storms in California have been warm wet storms that strike in winter, producing intense rains over large areas and unleashing many of the State's largest floods. The most commonly recognized of these storms have been described as "pineapple express" storms because of the way that they appear (in weather satellite imagery) to draw warm, moist air from the tropics near Hawaii northeastward into California. Recent studies, though, have shown that pineapple express storms are just one version of a common feature of midlatitude weather, called "atmospheric rivers" (ARs). We now know that, globally, about 90% of all the water vapor transported towards the poles across the midlatitudes is transported within the narrow, intense filamentary bands of moist air that form these ARs. Because AR storms are increasingly understood to have been the source of most of the largest floods in California, an evaluation of the future of floods under climate change must attempt to project the future frequencies and intensities of ARs.
Using a locally-based strategy for detecting AR-type storms along the California coast, developed at the NOAA Earth System Research Lab, climate simulations from seven global-climate models (GCMs) were analyzed to compare frequencies and magnitudes of AR storms arriving in California under simulated historical and climate-changed conditions.
First, numbers of AR episodes in the climate models and in the observational record were compared to find that, although on average most of the models generate more ARs than observed, the general distribution of AR days per winter were not so different as to preclude evaluations of the projected changes.
Next, in comparing historical to future climate simulations, changes in AR storms in the models were found to occur mostly at the extremes: Years with many AR storms become more frequent in most of the climate-change projections, but the average number of such storms per year are not projected to change much.
Similarly, although the average intensity of the storms is not projected to increase much in most models, occasional much-stronger-than-historical-range storm intensities are projected to occur under the warming scenarios.
The simulated AR storms also warm along with the winter-mean temperatures in the seven models.
Together these findings suggest that California flood risks from the warm-wet, atmospheric-river storms may increase beyond those that we have known historically, mostly in the form of occasional more-extreme-than-historical storm seasons.