Water Balance of Land

the relation between the amount of precipitation that falls in a given place and the evaporability (or the air temperature, upon which the evaporability depends). When precipitation exceeds evaporability, there is a water overbalance, and part of the moisture that has fallen flows from the given place in groundwater and rivers. When evaporability exceeds precipitation, the water balance is deficient. Several indexes and coefficients have been proposed for the determination of the water balance. Among these are N. N. Ivanov’s moisture coefficient and M. I. Budyko’s aridity index. The moisture coefficient is the ratio of the amount of precipitation falling in a given period and the magnitude of the evaporability in the same period; this coefficient is expressed as a percentage. The aridity index is the ratio of the annual radiation balance of the underlying surface and the total amount of the heat required to evaporate the annual amount of precipitation on the surface.