Definition of water balance in US English:

water balance

noun

  • 1A machine for raising loads to a height, consisting of two cars or platforms with water tanks attached, connected by a chain passing over a pulley, so that a laden car with an empty tank is raised by the weight of the unladen car with a full tank.

  • 2In a living organism: the equilibrium between water intake and water loss.

  • 3In soil science and hydrology: the equilibrium between the water entering an ecosystem in the form of precipitation, and that stored in or leaving it in the form of run-off or evaporation.

Origin

Early 19th century (in an earlier sense). From water + balance.