Wet water years had more shared timing than dry water years across seven California divisions in 1901-2025

We compared the shared timing of locally dry and wet water years in seven California climate divisions. We used fixed NOAA monthly precipitation data for water years 1901-2025 and local 20% tails. Mean pair correlation was 0.395 for dry years and 0.576 for wet years. The dry-minus-wet difference was -0.181, with a 95% five-year block interval of -0.324 to -0.057. One- and ten-year blocks retained negative intervals. Separate early and late periods, 10% tails, calendar years and a post hoc November-April check had uncertain differences. The result is specific to the full-record water-year measure. It does not prove a physical cause, greater wet damage, or a difference in water supply. The package supplies exact source rows, hashes, code, all bootstrap draws, a figure and independent numerical checks.

California · precipitation · water year · spatial dependence · dry and wet years · block bootstrap