by Jay Lund
So far, October and November 2019 has been the driest (or almost the driest) beginning of any recorded water year with almost zero precipitation. (The 2020 water year began October 1, 2019 – so you might have missed a New Year’s party already.)
Should we worry about a drought yet?
Yes, this is California, where droughts and flood can happen in any year, and sometimes in the same year. Water managers should always worry about drought (and floods) at all times in all years, especially during the November-March wet season.
No, not especially anyway, because we are still early in the 2019 wet season, most precipitation falls later in the water year, and there is not strong correlation between October-November precipitation and total water year precipitation.
The California Department of Water Resources does a great job assembling current and historical data on water conditions, updated daily on the California Data Exchange Center website. It is a wonderful data playground.
Plotting historical 1921-2018 northern Sierra precipitation for October+November against water year total precipitation gives the following scatter-plot, correlation, and statistics.
From this plot, statistically, every inch lost in October+November precipitation averages 1.4 inches less total water year precipitation for the northern Sierras. Since October+November averages 9.3 inches of precipitation historically, if it does not rain for the rest of November (a storm is in the forecast), then we would have on average 13 inches less than average northern Sierra precipitation this year. (Average historical northern Sierra precipitation is 50 inches.) Although this is not good news, it is not doom. Given the high variability of California’s climate and poor correlation between monthly precipitations, anything could happen.
It is comforting that California’s reservoirs are relatively full today, which provides something of a buffer against dry years and shorter droughts. And the last few wetter years also have refilled groundwater some from the last drought.
The these recent dry months lengthened this year’s fire season (if anyone had not noticed). With a warming climate and more houses in fire-prone areas, any fire season extension due to dry weather becomes increasingly threatening.
Will California have a drought? Yes.
Is the next drought beginning this year? Perhaps, but probably not. Still, water managers should (and usually do) prepare for drought, even if recent months had been wet – because it will become dry eventually.
I am looking forward to rain (and hopefully snow). A good storm is forecast for later this week, so we get at least some precipitation in November, slightly improving odds of a wetter year.
Jay Lund is a professor of civil and environmental engineering and director of the Center for Watershed Sciences at UC Davis.
Lund, J. (2015), “The California Drought of 2015: January,” CaliforniaWaterBlog.com, January 5, 2015.
California Weather Blog, http://www.weatherwest.com/
L’Heureux, M., “Seeing Red Across the North Pacific Ocean”, October 23, 2019: https://www.climate.gov/news-features/blogs/enso/seeing-red-across-north-pacific-ocean A recent discussion on Pacific blobs affecting drought in California.