The relationship between inflow, outflow and accumulation of water under natural conditions for a river basin or for a lake, reservoir or other water body under study. This requires data on the volume of water mass and the area of the lake, precipitation, the number of inflows and outflows, their capacity, evaporation, etc.

In some cases it is not necessary to calculate all the components of the balance in detail. For example, if water balance calculations are carried out in relation to sufficiently large water objects, condensation can be ignored due to its relatively small values. An amount of water equal to the amount of precipitation minus river runoff evaporates from the total land area of the Earth (within the long-term average annual water balance). 

For individual water bodies and for shorter periods of time, it is necessary to consider the components of moisture input and consumption in more detail when drawing up water balances, in relation to specific conditions of moisture input and consumption. For example, in the water balance of reservoirs, in addition to inflow, precipitation and evaporation, water discharge through the structures of a hydrotechnical node (hydroelectric power plants, sluices, dams), water intake from the reservoir, filtration downstream in the alignment of a hydraulic structure may be of significant importance; the volume of water contained in ice and snow that settles in shallow parts of the reservoir when it is released in winter and rises in spring when the reservoir is filled; temporary losses due to filtration of water in the reservoir banks and its return with changing water levels in the reservoir. Over the course of the year, components of the balance such as losses due to ice formation and filtration to the reservoir banks are offset by processes in the opposite direction and are therefore not reflected in the annual balance.