Summary:

Till the late 90-ties of the XXth century the Penman method has been used widely to evaluate the reference evapotranspiration. The ongoing research verified the Penman method and convinced many scientists to look for the modification of his formula, because the calculated values of reference evapotranspiration were too high. Nowadays, the method most frequently used is that of Penman-Monteith. It enables the more accurate evaluation of the atmospheric evaporation ability, therefore it is preferred for the calculation of climatic water balance or drought references. Many modifications of Penman-Monteith method can be found in literature. The authors have analyzed the data from four various regions of Poland and have presented the differences in values of reference evapotranspiration depending on the method of calculation.