Summary:

The aim of the present research was an attempt to compare apple tree water requirements in the vegetation period in the Bydgoszcz region (Poland) and in the Isparta region (Turkey). The paper refers to the 1984-2014 temperature and precipitation values in the Bydgoszcz and Isparta regions. To determine the reference evapotranspiration (ET0), the calculation model by Hargreaves modified by Droogers and Allen was applied. Potential evapotranspiration, identified with apple tree water requirements, was determined using the method of plant coefficients proposed by Doorenbos and Pruitt. In each of the seven months considered (April-October) higher apple tree water requirements occurred in the Isparta region. The highest apple tree water requirements were noted in July and for that month during the thirty-year period they were 167.3 mm and 286 mm, on average, in the Bydgoszcz and Isparta regions, respectively. Daily water requirements of apple trees in July were more than 9.2 mm in the Isparta region and 5.4 mm in the Bydgoszcz region. Apple tree water requirements throughout the vegetation period (April-October) were much higher (by 120 %) in the Isparta region than in the Bydgoszcz region. The highest precipitation deficits occurred in July and amounted to 95.5 mm and 269.1 mm for the Bydgoszcz and Isparta regions, respectively. The differences in the irrigation requirements for apple tree, next to water requirements differences, were affected by a different precipitation distribution in time in the regions compared. In the Isparta region higher precipitation occurred at the beginning (April, May) and at the end (October) of the vegetation period, while in the Bydgoszcz region - just opposite - in summer months (June, July, August).