Kelly T. Redmond
Desert Research Institute, Reno, Nevada
Approximately $40 billion in damageable property lies behind the levees in the city, including the state capitol building and numerous government offices. In reaction the the apparent shift in flood risk, a long-running and spirited community debate has developed over what actions to take. Options include any or all of: increasing the capacity of Folsom Reservoir, increasing the outflow capacity of Folsom Reservoir, increasing the levee system capacity, relocating residents and businesses from parts of the floodplain and limiting future development in the floodplain, and building a new dam near Auburn on the largely unregulated North Fork. None of the options are cheap, some are controversial, and all choices will depend upon estimates of the risk of various sizes of floods. The issue thus turns back to what the climate system will, or could, bring in coming decades.
The change in mid-century is not simply a peculiarity of this one gage or basin. Precipitation gages at various elevations in and near the basin show a similar increase in both the number of wet years and in the maximum 3-day and 10-day totals within each winter. The California "8-station index" (covering most of the Sacramento River drainage) shows similar temporal behavior. Interestingly, the annual precipitation is increasing more slowly or not at all. The most recent two decades have brought an increase in both the number of wet years and the number of dry years. Thus, variability has increased. Floods on the eastern slopes of the Sierra, facing the Great Basin and caused by the same factors, also show an increase over the past several decades. Thus, there is little doubt that the flood frequency increase on the American River is real.
In addition, another change was first noted by Roos (1991), and later elaborated by Dettinger and Cayan (1995). The fraction of the annual runoff from the central Sierra that occurs in late spring has been decreasing for approximately the past 50 years. Relatively more of the annual runoff has been occurring in the winter. Winter and spring temperatures have become warmer in the central Sierra. This has been a more gradual trend rather than an abrupt transition.
The traditional approach has been to use whatever data are available from the past to form these estimates. A dilemma is encountered, however, when the recent record (in this case, the past 50 years) is decidedly different from the previous record. What portion of the past is most likely to be representative of the next half century or beyond?
One is thus led into a consideration of the possible causes of climate variability in this region on decadal time scales. Of particular interest is whether these variations range back and forth between approximate extremes, or appear as monotonic or unidirectional trends. Natural climate variability is generally considered to resemble the former, and possible human-induced changes are usually considered to resemble the latter.
Among the natural kinds of variability are the two phases of ENSO (which produce different responses in the California cool season), regimes and decadal oscillations in the north and central Pacific (e.g., Pacific Decadal Oscillation, PDO), regimes of ENSO behavior (predominance of one phase over the other, etc.), possible modulation of ENSO effects by the PDO, thermohaline circulation variations in the world ocean, and others only imagined, or dimly known or understood at this point.
Among the possible anthropogenic sources of variability are greenhouse gasses (GHG), aerosols, and land use changes around the earth. One expectation from GHG is for a more vigorous hydrologic cycle with more instances of extreme precipitation events (Karl and Knight, 1997). GHG responses are often expected to gradually increase in a unidirectional manner (although this need not necessarily be true in such a highly nonlinear system), whereas aerosols and land use changes could lead to many possible non-linear and non-local responses (e.g., Pielke et al, 1998).
The problem of whether the current flood regime will continue, or grow, or will relax back to values frequently seen during less active periods in the paleo records (e.g., Meko, 1998), is thus a central focus of attention.
No firm decisions have yet been made as of this writing. However, the issues are not unique to the American River. Webb and Betancourt (1992) have pointed out analogous issues in southern Arizona. Similar problems are likely to be encountered by other cities as they reassess the relationships with their rivers.
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