|Abstract or Summary
- Climate changes. That’s what climate does. It is a natural and dynamic process. The National Weather Service (NWS) recognizes on-going climate change by publishing new figures for average climate every ten years. Climate averages for precipitation, temperature, and other weather parameters are computed on a 30 year basis but only updated once per decade. From 2001-2010, the NWS average high temperature for July in Salem, OR, for example, was based on the 30 year period, 1971-2000. Now, in 2011, climate averages are reported for a new 30 year period, 1981-2010, which will remain the norm until 2021.
With all of the discussion about anthropogenic (i.e. man-made) climate change, it is easy to overlook just how variable our natural climate can be in the relatively short-term. Our climate can and does vary by significant amounts within one human lifetime and well within the design lifetime of our water infrastructure. Sometimes this fact gets lost in the noise of the climate change debate. Part of the reason is the relatively short records of our key meteorologic and hydrologic parameters.
Here’s an example. Sacramento, CA, has one of the longest rainfall records in the western US. Annual rainfall totals are available from 1850 to present. Over the 159 year record from 1850-2008, the average annual rainfall was 18.38 inches. However, the 30-year moving average rainfall varies from 20.42 inches in 1896 down to 14.51 inches in 1937 and up again to 20.47 inches by 2007. That’s 30-40% swing of 30-year average rainfall in a single lifetime. (Lifetime, not geologic time!) Most of our short records completely miss that signal. That such significant changes can occur relatively fast has major implications for water resources infrastructure design.
This paper explores and presents findings regarding rapid variation of “climate averages” in northern California and Oregon using long term rainfall records. The results suggest that not only is stationarity dead, it likely wasn’t really alive in the first place. We simply assumed it was.