Rainfall update

Until recently I hadn't heard people's voices raised in concern about our lack of rain this winter, but it's becoming apparent that our lack of rainfall is now gathering attention. Robert Smaus writes in this week's Home section of the LA Times that, "HISTORICALLY, February is our rainiest month so it is not a busy time for gardeners. If there is little or no rain, it will be a very dry year. Though everybody loves a sunny day, gardens cannot get by on irrigation alone. They need deep soaking rains that reach tree roots and flush harmful salts (found in irrigation water) from the soil." (THE MONTHLY GARDENER BY ROBERT SMAUS, January 25, 2007.) Implicit in his writing is the common knowledge that this January (which on average is only a hair less wet than February) has been darn dry, with many days of high winds and sun, which have had an additional drying effect on everyone's garden. For those of us trying to grow natives, we rely on Mother Nature to deliver water at this time, at appropriate temperatures, to further plant growth. Watering from a hose on a sunny winter day just isn't the same as a gentle overnight rainfall.

Weather Underground predicted a 20% chance of rain this coming Saturday, now up to 30% Saturday through Wed. From a look at the sky that seems about right. It's good just to see clouds for once.

Update: 0.08" in my backyard Saturday - Sunday AM. Also, Grace has written about rainfall on her blog with the conclusion that it's not normally distributed, an assumption that I made here and here. I'll need to revisit the historical data to figure out a better statistical description of the seasonal distribution.

I'm reminded of solar flares, which were first studied in detail in the 1960s. At that time and through the late 1980s / early 1990s they were thought to be normally distributed in intensity. That's always a good starting place for scientists, but there was the glaring exception to the normal distribution: anomalously large (AL) solar events were so huge that they were outside of the normal distribution (like our high rainfall years?). At least one widely accepted model for solar radiation formalized these weird statistics with a normal distribution of low intensity flares and a separate calculation for AL flares. Even today you'll still hear people who don't want to acknowledge that their space missions have a potential solar radiation vulnerability refer to "AL flares" in a disparaging tone, as if their probability was so remote that they can be removed from all consideration. This is an unfortunate consequence of the name, I think, since as scientists gathered more data it was shown by the 1990s that a better and all-inclusive solar flare distribution was log-normal. In this model, the former AL flares merge into a smooth probability distribution - no longer anomalous. This statistical description is still accepted today and has been confirmed by more recent data. I believe it has also been connected to recent physics-based solar activity models, an area that still has much to be discovered about it, but which suggests that there is an underlying rationale for the log-normal distribution rather it being a purely phenomenological description.


  1. I have listened to talks given by climate modelers. They all seem to agree that, in the even of global warming, the type of rainfall experienced by the west will change.

    That is, the slow winter rainfall that come from the northwest and soak into the ground will become less frequent. We depend on those storms to fill our reservoirs, build our snowpack (banking moisture for the summer), and to leach salts out of the soils.

    Pineapple express and monsoonal rainfall from the south and southwest will increase in frequency. Those are the type of storms that cause flash flooding and melt away our snowpack.

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