January 6th and 16th storms compared

The weekend of January 5-6, 2019 was a very wet one, and San Geronimo Creek reached 1,000 cfs for the first time since February 2017. 2018's peak flow was 753 cfs in March.

About a week and a half later, on January 16, 2019, there was another big storm--more well-advertised than the first, and windier, but not necessarily wetter. The storms were similar in many ways, but San Geronimo Creek peaked higher during the second storm, at 1,304 cfs.

Jan 6th storm, with day.hour on the x-axis.

Jan 16th storm, using the same day.hour as the Jan 6th storm, lining up
both peak flows between hours 17 and 18 on day 2. The Jan 6th peak was
at 5:30 pm and the Jan 16th peak was at 9:15 pm.

The graphs show the different patterns of rainfall and creek flow. The January 6th peak was broader, with a longer duration at high flows (over 400 cfs), however the January 16th peak had a longer duration at about 200 cfs. The antecedent conditions were wetter on the 16th--36 cfs 16 hours before the peak, vs. only 7 cfs 16 hours before the Jan 6th peak.

How do the total volumes compare? The typical pattern in San Geronimo Valley is for the Woodacre precipitation station (to the east) to be wetter than Mt. Barnabe (to the west), and that pattern held on the 6th. It was reversed on the 16th--Barnabe was wetter than Woodacre. Although the volume of runoff during the 16 hours prior to the peak was only 3% higher on the 16th, the magnitude of the peak flow on the 16th was 23% higher than on the 6th. The table below shows well how similar the storms were. Note the 6th was wetter in Woodacre, and the 16th was wetter on Mt. Barnabe.