# Water balance

Water balance is assessed for a given place and a given period of time by comparing water inputs and outputs in that place and during that period. The assessment also takes into account the existing supply of stocks and future appropriation of these stocks. Water inputs are brought by precipitation. Outputs are from the combination of evaporation and the transpiration of plants, called evapotranspiration. Both quantities are estimated in terms of the amount of water per surface unit, but they are generally translated into water heights, the most currently used unit being the millimetre. As these two quantities are thus physically homogeneous, they can be compared by computing either their difference (precipitation minus evaporation), or their ratio (precipitation divided by evaporation). The balance is obviously positive whenever the difference is positive or the ratio greater than one. One expression or the other is chosen according to various conveniences or constraints. Runoff from a surface unit will be taken into account in the outputs. Infiltration is considered as stocking in the form of groundwater or capillary water in the soil. Solid precipitation represents immediately constituted stocks. These are of variable duration, inter-seasonal in the case of snow covers, inter-seasonal and inter-annual in the case of glaciers, and even inter-secular in the case of polar icecaps or of large masses on very high mountains.
The study of water balances is complicated by the fact that the two commanding variables are not independent of each other. The quantity of evaporated water obviously depends on the total available quantity of water: it stops when the water volume brought by precipitation is exhausted. This has led to the introduction of the notion of potential evapotranspiration: the quantity of water that can go into the atmosphere according to its state alone, assuming that the quantity of available water is not a limiting factor. (The amount of water added to a vase of flowers in order to keep its level constant is a measure of the potential evapotranspiration, depending on the state of the atmosphere in the place where the vase is located.) It is usual, in the study of water balances, to compare precipitation, P and potential evapotranspiration, ETP, which makes it possible to distinguish different situations according to thresholds that are of special significance for a given place or period of time:
-If P < ETP, the real evaporation will be equal to P; there will be an appropriation of reserves and an absence of runoff; the period will be said to be a deficit period. -If P > ETP, the real evaporation will be equal to the ETP; there will be runoff and a building up of reserves; the period will be called surplus period.