It is very important to understand that natural variability will not go away. Any projected change in average annual runoff will occur “on top” of ongoing natural variability. In many cases, natural variability can be quite large compared to the changes projected from global warming. Furthermore, relatively short instrumental records may not provide an adequate picture of the full range of natural climatic variability. The work of several researchers who have developed proxy records for precipitation and streamflow based on tree rings and geological evidence provides a longer-term view.
Figure 1 provides examples of such proxy records for the reconstructed streamflow of the Colorado River at Lee’s Ferry and for the Four Rivers Index in northern (Sacramento, American, Yuba, and Feather). These 20-year moving averages indicate that both regions have experienced extended periods of drought as well as periods of sustained above-average flow. While there is a very weak positive correlation between annual flows in the two regions, there is no consistent pattern of association for the longer-term fluctuations between wet and dry conditions. For example, northern California experienced an extended dry period from 1918–37, during which time the Four Rivers Index dropped to 13.55 million acre-feet (Maf) from its long-term mean of 17.4 Maf. At the same time, conditions in the Upper Colorado River were much wetter than the long term mean of 13.5 Maf. On the other hand, the most severe extended drought in the Upper Colorado River Basin occurred during the period 1579–98, when average annual flow was only 10.95 Maf. That same period was among the driest in the northern California tree-ring record (Meko et al. 2001 a,b). Paleo-environmental records also indicate that there were some “mega-droughts” in the pre-instrumental period that were far more severe than any experienced within recent history (Woodhouse and Overpeck 1998).
These records suggest that water supplies can change dramatically, and for extended periods, even without anthropogenic climate change. Where they are available, such reconstructions of past variability could be useful for examining the vulnerability of a water system to conditions outside of the range of recent experience.
Figure 1. Time series plots of 20-year running means of reconstructed flows for the Colorado River at Lee’s Ferry (lower line) and for the Four Rivers Index, northern (upper line). Data Sources: Stockton and Jacoby 1976, 2004; Meko et al. 2001a,b – available at: http://www.ncdc.noaa.gov/paleo/recons.html