The analysis of the exchanges of vapour and heat above a small water table, situated in the middle of a more or less dry region, leads them to be expressed in function of the thermic and hydric changes of the environment, by which the measured, screened data are influenced. This analysis allows us,—among other results,—to observe the variation from simple to the double of the evaporation. This has come to evidence already in cases when the evaporating surface diminishes.