The balance of water fluxes into and out of a defined area over a defined time period, as represented broadly by the equation: precipitation = runoff + evapotranspiration + the change in soil-moisture storage. In mid-latitudes, for example, the annual budget is balanced by a high level of potential evapotranspiration and utilization of soil moisture in summer, compensated by a water surplus and recharge of soil moisture in winter when evaporation is less and precipitation is sometimes greater.
The balance of water, as represented broadly by the equation:
Moisture balance values of less than 1.0 indicate an annual excess of PET over P, a moisture deficit. Areas where values fall below 0.5 are regarded as drylands.
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