Impact of extreme climate years on relative "yield - evapotranspiration" relationships

The relative relationships "yield evapotranspiration" were used long time ago. The well known linear relationship yi = 1 ky (1 ei), where yi is relative yield, ky yield response factor and ei relative evapotranspiration was proposed. It's usually assumed that ky is constant for a given crop and climatic conditions. It was found, however, that ky for late variety of maize H 708 varied through the study years (1984-1990) in the Plovdiv region ( South Bulgaria , altitude 150 m). During the dry years it was significantly higher than in the medium and humid years. The range of ky for maize in this location was 1.12-1.90, the average value being 1.50. The climate in the Sofia region (the experimental field of Chelopechene, altitude 550 m) is comparatively more humid. The two regions approximately outlined the boundaries of the appropriate economical conditions for grain maize production. The experiments in the Sofia region were carried out in the years 1994-2000. The seven years results for mean variety maize showed that the relationships "yield evapotranspiration" and, respectively, ky varied at these climatic conditions too. The highest ky value was 1.41 for the driest year (2000) and the lowest value 1.05 for the most wet years (1995, 1999). The value of ky for average years was 1.21. The yield response factor ky is of more significance when the relative evapotranspiration is less than 0.7-0.8. Thus, the extreme or the average values of ky could be used for the corresponding climatic regions. The relationships between ky and relative yield were established without considering irrigation.