Santa Cruz, CA Rain Fall, 2014

I don't usually post on weather, but I have been especially interested,as many people in California, in the rain fall. There has been a pretty bad drought over the past few years, and people seem alarmed. I have been concerned, too; however, as an analysis of the previous 100+ years of data for annual rain fall in Boulder Creek will show, I don't think this drought has been any worse than is usual for this area.

First of all, while 2013-2014 was bad - 22.41"; it isn't the worst on record. In 1930-1931, it was at 20.58", and in 1975-1976 it was at 20.13" (worst since 1888). I count 14 years where the rain fall was in the 20's, since 1888. They seem to be pretty evenly spread out, too.

I plotted the data, since 1888, shown below.

5 yr avg

Boulder Creek Rain Fall Since 1888, with 5 Year Moving Average

The dashed line represents a fit of the data to a line. The lighter, more erratic trace represents the actual annual rain fall. The red represents a five-year moving average, effectively smoothing out the data. Around the 20" was a very bad drought, for many years. Since 1940, however, there has been a roughly oscillating pattern of annual rain fall. It is even more apparent on the 10 year moving average, below.

10 yr avg

Boulder Creek Rain Fall Since 1888, with 10 Year Moving Average

In this plot, the 10 year moving average in yellow shows that there were peaks in rain fall just after 1940, just before 1960, around 1970, 1985, 2000. There were also corresponding dips between these peaks. If you look at where we are at now, in 2014, it shows that we are near the bottom of a dip. My prediction was (and I made it back in Summer - unfortunately I didn't blog it before these winter rains!) that this year would be a better rain year, just based on the fact that we are at the bottom of a dip. So far, it has been an outstanding year for rain fall, with Boulder Creek already at about 21" (almost more than all last year!). Again, this isn't something to be surprised by - if you look at the data from the past 100+ years, the rain fall is really all over the place, with hardly any rhyme or reason. And while it varies wildly, you will see that on average (see the trended line), the rain fall is remarkably flat over the last 100+ years.

Is there any particular reason for this? I thought it would be interesting to see if there was a correlation with the 11-year sun spot cycle. The plot below shows the annual sun spot number, with the 10 year moving average.

with sunspot

Boulder Creek Rain Fall Since 1888, with 10 Year Moving Average, and Annual Sun Spot Number

There doesn't appear to be much of a correlation between annual sun spot number and rain fall.

In conclusion, while the drought has been rather bad, it is about as bad as the worst droughts, historically. From the data shown, the rain fall patterns haven't changed at all over the past 100+ years. What has changed, however, is the amount of demand on the water supply. Therefore heightened conservation, and a change in water usage habits, is more necessary now than ever.

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