When a year starts shapes global hydrological conclusions

<title>Abstract</title> <p>January 1 is an arbitrary point to start a year for hydrological analysis. Water years offer a solution by shifting the start to a hydrologically meaningful point, but local and global definitions diverge considerably. Here we show that local water years reflect the seasonal cycles of snow accumulation, midlatitude storm tracks, and the intertropical convergence zone, producing a coherent global pattern that no existing global definition reproduces. We introduce the first observation-based global definition of the water year, derived from upstream snow water equivalent and discharge, that recovers this pattern and is applicable at a subcatchment scale. Applying it reveals that the point when a year starts systematically alters estimates of interannual variability, the spatial extent of significant changes, precipitation–flux correlations, and water-balance closure across the global land surface. The start of the water year is thus a first-order yet rarely scrutinized decision in Earth-system and water-cycle analysis.</p>