What would be the best date to start the water year?

To calculate annual values for the water balance, the water year (or hydrological year) is usually used instead of the calendar year. This is done to avoid that precipitation from one year influences runoff in the following year. In snow-dominated catchments in the northern hemisphere, for example, such a carryover would occur regularly if the calendar year were used to aggregate hydrological data. To ensure that the snow melts in the year in which it fell, calculations are usually based on hydrological years that start in early fall (e.g., October 1 or November 1). In other climates, a different start of the water year is used, e.g., to ensure that it does not start in the middle of the monsoon season. Worldwide, there is a wide variation in the definition of the start date of the water year.

As the water year is used for many hydrological analyses, all annual statistics are potentially influenced by the chosen start date. The water balance for a particular year depends on how the 12-month periods is defined. Similarly, the definition of the 12-month periods is also important when calculating statistics such as annual peak flows, as it depends on whether large peaks in, for example, April and October are assigned to the same year (i.e., only one of them is considered) or to two years.

In this study, we use a modeling approach to numerically evaluate the definition of a water year and discuss how the water year would be best defined for different hydroclimatic regions. For this purpose, the runoff and storage was simulated for over 600 catchments in the USA with the HBV model. We analyzed the time series of the simulated snow, soil and groundwater storage and defined the ideal starting point of the water year from a water balance perspective as the date for which the interannual variation in total storage is the smallest. The results show that the optimal definition of the starting point of the water year varies considerably from region to region.