Today (September 30, 2014) marks the last day of the 2013-2014 water year. Given that I have been otherwise occupied and the last water year I discussed was 2010, an update is certainly in order.
As everyone is aware, there has been a tremendous amount of discussion and a fair amount of concern about the current drought situation in California. Indeed, the November ballot contains a measure to authorize a $7.5 billion bond offering to fund “water quality, supply, treatment, and storage projects.” Without getting into the merits of the upcoming ballot measure, I wanted to offer a quick update of the precipitation situation, now that the 2014 water year is effectively in the books.
Northern Sierra 8-Station Index
For simplicity’s sake, and for direct comparison with my prior post, I will use the Northern Sierra 8-Station Annual Precipitation historical index as the basis for looking at precipitation over nearly a century. The 8 stations used for the Northern Sierra precipitation index are: Mount Shasta City, Shasta Dam, Mineral, Quincy, Brush Creek, Sierraville RS, Blue Canyon, and Pacific House. A map of the area covered is below (courtesy, the California Department of Water Resources):
Again, this does not cover the entire State by any means and the Southern Sierra region is currently undergoing an even more significant drought, but this gives us an initial starting point for understanding the context of the current drought situation. Time permitting, I will analyze other regions and post the data for comparison at a later date.
Annual Precipitation Numbers
According to the 8-Station data, the average annual precipitation for the Northern Sierra region is approximately 50 inches. The area is currently experiencing its third year in a row of below average precipitation, with 2014 (the lowest of the past three years) weighing in at slightly less that two-thirds of average. To be sure, there is a significant drought at the moment, but there is value in placing the current drought in context.
The below chart shows the annual precipitation in the Northern Sierra 8-Station region for all years covered by the index, from 1921 to date:
When placed in context, we can draw a few observations about the current drought situation.
1. The current year (2014) is the eighth lowest over the past near-century (94 years) of record keeping, placing it in the lowest 10%.
2. The prior two years (2012 and 2013) are also below average, but are somewhat closer to the norm.
3. There is tremendous variability in precipitation over time, and it is common to have a few dry years in a row, followed by a few wet years.
4. The last three years constitute a distinct dry spell. However, there are at least 2 other periods over the last century (arguably 4 or 5, depending on how we want to count them) that were just as dry.
5. There is virtually no trend in precipitation over the past century. Indeed, several of the wettest years have come in the last 3 or 4 decades, and a straight linear trend produces a slight increase in precipitation over the century. [I do not think it wise to place much stock in a slightly-increasing straight linear trend, particularly given issues with start-endpoint bias and the occasionally extreme variability from year to year, but at the very least it appears there has not been a decreasing trend over the century.]
6. If history is any guide, we should expect the recent three dry years to be followed by one or two exceptionally wet years sometime in the next two or three seasons. It remains to be seen, however, whether this expectation will hold, and we should be prepared for the possibility of a few more dry years before an uptick (see, for example, 1959-1962 and 1987-1992).
Again, this is just to help us place the current situation in context. This is not to argue that there is no drought — there is. This is not to argue that fresh water resources are not strained — by all accounts and observations they are. This is not to argue that the upcoming ballot measure is unnecessary or unwise — it may be, and personally I still need to study the issue in more depth before November arrives. This is not intended to be a comprehensive analysis of precipitation, water usage, storage, water recycling and so on. This is simply to provide an initial glimpse of meaningful historical perspective.
Furthermore, while the data over the past century do not allow us to point to any significant upward or downward trend in precipitation (notwithstanding much hand-wringing about climate change), one thing we do know for certain is that the population — and presumably, therefore, water demand — has increased tremendously over the past century. In a subsequent post we will explore the eye-opening population numbers over this same timeframe.