In the semi-humid and humid climate of the temperate zone, supplementary irrigation water requirements depend on meteorological conditions.  A mathematical model is developed  for the assessment of monthly time series of  irrigation water requirements, based on Penman's equation and calibrated on the basis of the data obtained from irrigation systems in Czechoslovakia. In the model, monthly time series of temperature, relative humidity, sunshine, wind velocity, and precipitation are used as input data.  On account of the persistence phenomena often noted in irrigation policies, the correlation between the current irrigation water requirements and those of the previous month was taken into account.  The statistical properties of irrigation water requirements have been analyzed as the basis  for the generation of a synthetic water requirement  time series.  The model can be used for long-term planning of water resources systems incorporating supplementary irrigation water use, as is shown in the case of the Labe catchment area in Czechoslovakia.