Thursday, April 22, 2010

What's Normal for Rainfall?

"You don't need a weatherman to know which way the wind blows" but you might want to check with a record keeper before you claim that rainfall totals are "far above normal" or that we've had "rainfall totals not seen in decades" as the Sentinel did today.

Wild claims aside, it's an interesting story that cites two long time local treasures, Emily Cress, who measures rainfall in Boulder Creek and Lud McCrary in Bonny Doon. Emily has recorded 63.72 inches of rain "this season" while McCrary's rain gauge has measured around 40 inches.
Ms Cress's yard got "far more" rain than Mr McCrary's did that much is for sure. But do these measurements represent "rainfall totals not seen for decades." In a word, no; unless you've been spending your winters in Phoenix.

But what's normal? Anyone who's lived in the San Lorenzo Valley or the Santa Cruz Mountains knows we get more rain than downtown Santa Cruz. Emily and Bud's records show how being on different sides of the mountains affects rainfall totals. If you live in a gulch near Bear Creek you're going to get more rain than if you're on the western side of the coastal range.

The San Lorenzo Valley Water District records rainfall at their office on Highway 9 in Boulder Creek. Their rainfall "year" is from October 1 through September 30. Last year they recorded 40.72 inches. They haven't updated their website with April's total yet, but through the end of March they had recorded 46.29 inches. It's a good guess that April's totals will add 5 to 7 inches to that which would certainly put them, unarguably, "far above" last season. But was last season "normal"?

The district has rainfall records that date to1881 but the data prior to1991 was not recorded at the exact same location. Since 91 they're reporting their own data collected from one side or the other of Highway 9 in downtown Boulder Creek. So for at least the last two decades we've got relatively comparable figures. The average rainfall totals reported at this site since1991 is 53.71 inches per season.

Is average normal?

As recently as the winter of 2006-2007 the recorded rainfall at the Highway 9 location was 27.35 inches but the two winters prior to that we had almost 64 inches each year So the average for the last five winters, before this one, is 46.67 inches. Since it's almost certain we're going to be above the recent average this year that means that in the last 6 winters we've had three above and three below average. Is that "normal".

If you been around here just a little longer you might remember that we had over 90 inches of rain in the winter of 1997-98. And you only have to go back to the winters of 81-82 and 82-83 when we had 87.17 and 111.48 inches!

The dictionary says normal is something usual or ordinary, something people expect. Usual or ordinary is not the way I would describe anything about the San Lorenzo Valley or Santa Cruz Mountains but we do expect it to rain some, so in one respect the Sentinel had it right - we're "far above normal".

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