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| The Nov 22nd and Dec 14th peak flows on San Geronimo Creek were the same: 1,105 cfs. Daily average flows were 423 cfs & 356 cfs, respectively. In November it took 8.75 hrs of 2.34" of rain (1/4" per hour) to get to peak from 113 cfs; In December it took 7.25 hrs of 2.32" of rain (1/3" per hour). These calculations use rounded times to the nearest hour for rain and quarter hour for flow & add an hour of lag time between rain and flow. |
And then things got crazy.
Out at Limantour Beach at 8:51 am there was a 7.2-foot high tide. It felt like a hurricane, with blowing sand at eye level, sea spray, and foam. That day the King Tide plus winds wreaked havoc across West Marin, knocking down trees, blocking roads and causing power outages, flooding low-lying areas, and damaging or destroying all the docks in Tomales Bay. The Point Reyes Christmas Bird Count was happening that day, and many areas were inaccessible or too dangerous to visit. Twenty trees were down on Limantour Road, and people stuck all day out there and on the point were only able to return at the end of the day (including me and my son). The count dinner at the Dance Palace was by candlelight for the second time in history.
Our rain total for the 2025 water year so far (Oct-Dec) in San Geronimo Valley is just over 25 inches--average is about 15". If it doesn't rain at all in January it would still be above average on February 1st. Other wet falls included 2012 (25.01" and then it essentially stopped raining for all of 2013), 2014 (28.38" and then it got very dry in 2015), 2021 (35.72", with 17" of that in a remarkable October storm and again it got very dry in 2022). In fact, it appears to be a rule over the last 14 years that a very wet fall (with over 25" of rain) precedes a very dry rest-of-the-season. Will this year be different?
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| 25.16" of rain at the end of December 2024 is 57% of our annual average and 164% of the Oct-Dec average. |
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| 2013 water year (30" of rain) for comparison. The 2013 calendar year was the driest on record. Average is 45". |
And now for something fascinating--I divided the volume of runoff by the volume of rain (estimated for a 6,000-acre watershed) to estimate the runoff efficiency of the last three water years. I got a very surprising result. We know 2023 was a wet year--but only for rain. A lot of that rain soaked into the ground, making the runoff that year actually lower than 2024. 2024 had near-average precipitation, but more runoff than the wet year of 2023.
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| 2023 was wetter for rain and 2024 was wetter for runoff. So far, Oct-Dec of Water Year 2025 has similar runoff efficiency as 2024. |
So the interesting question this table raises is, "what happened in the Autumn of 2022?" For similar rainfall as 2023, why was the runoff efficiency less than half of the other years? Summer 2023 had a much higher baseflow than 2022. Even though there was 3/4 of an inch of rain near the end of September 2022, October of 2023 started out with more than double the baseflow of October 2022. So the antecedent conditions--groundwater levels and flows--were much wetter in 2023.
November 2023 had a storm that dropped 3" of rain in a couple of days, while November 2022 had less than 2" spread out all month with about 1" on the wettest day. I previously noted that an inch of rain is needed the day before a flood in order to get a big flood. So if we recalculate this table and subtract the first inch of each storm (because it doesn't generate much runoff), or more appropriately only sum any rainfall following an inch in a day, we get the following:
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| Rain vs. Runoff (blue) and Effective Rain vs. Runoff (red). Subtracting an inch from each 2-day total that has less than an inch of rain on a day shifts the "effective" rain totals to the left, lining up our outlier year 2021 a bit better and resulting in a better R-squared. This still doesn't capture the whole story, but is fun to play with. |
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