California wet-season monthly precipitation concentration shows no clear change between 1961-1990 and 1996-2025

We tested monthly precipitation concentration in seven California climate divisions with a fixed NOAA nClimDiv snapshot. The primary window was October-April. Mean concentration increased from 0.23124 in water years 1961-1990 to 0.24278 in 1996-2025. The change was 0.01154, with a 95% five-year block interval of -0.00500 to 0.02764. The test thus gave no clear evidence of a within-wet-season increase. An exact second-moment split gave point changes of 0.00755 for the mean seasonal cycle and 0.00398 for variation between years. Both intervals included zero. Amount adjustment, other reference periods, other block lengths, and omission of the desert division retained primary uncertainty. A secondary full water-year check gave a change of 0.01425, with an interval of 0.00091 to 0.02684. Annual concentration therefore cannot establish a within-wet-season increase. The study adds a reproducible month-window test and decomposition to prior seasonal-share work. One monthly gridded product, short periods, regional dependence, finite-sample effects, and conditional block intervals limit inference. This study does not measure daily extremes, usable water supply, or climate causes. Exact source rows, hashes, code, primary resamples, an independent numerical audit, and a complete repeat are included.

California · precipitation · monthly concentration · wet season · variance decomposition · nClimDiv · block bootstrap · null result