The hydrological year

Hydrological year
31 October 2012
A view of a British lake in summer

The 'Hydrological New Year' occurs on 1st October (in the Northern Hemisphere) and it's the point when the hydrological cycle is in balance. After 1st October rainfall starts to fill up the water reserves in the ground, until 1st April (middle point of the hydrological year), when evaporation starts to deplete this stored water. This carries on until 1st October, when it starts to replenish, and the cycle begins again.

The hydrological year
Water UK briefing