A Water Year Definition that Works Everywhere

Water years have been used by water resource managers for more than a century to align hydrological phenomena with the annual precipitation cycle. The central idea is to start a new water year when specific hydrological conditions are met, so that precipitation carry-over from the preceding 12-months period is minimised. With the increase in global hydrological data and analysis, it is vital to understand first how to best determine the start of water years across the globe and to understand their effect on evaluating interannual changes. 

The only global analysis of the water year to date defines its start by the month of lowest stream discharge. While this definition works well in seasonal climates, it neglects snow dynamics, as the snowfall does not immediately contribute to discharge. In snow-dominated catchments, the lowest discharge often occurs just before the spring melt; consequently, precipitation from the previous water year significantly influences the discharge in the following water year.  

We present a new definition to be used in the global water year estimations. It utilizes the areal mean of snow water equivalent collected in upstream catchments and discharge of the target catchment to determine the starting month. This definition mirrors the dynamics of terrestrial water storage and aligns more closely with various national definitions. Applying this methodology across ERA5 Land variables and a combination of MSWEP, GLEAM, GRADES, and SWEML datasets, reveals significant differences in trends and coefficients of variation for annual hydrological fluxes when compared to the standard calendar year. Additionally, our snow-and-discharge-based definition minimizes water balance closure errors. Given these findings, we suggest that global interannual hydrological analysis should, at minimum, consider water years for a physically sound assessment.