The aim of the present work is to establish the parameters of the relationship additional yield – irrigation depth for sunflower grown in the region of Plovdiv, using different equations of the following kind: /1/ y=ax2+bx; /2/ Y=1–(1–x)n. The source data used for the study are from a field experiment, carried out in the region of Plovdiv (Bulgaria) on alluvial-meadow soil, with the sunflower hybrid PR-64-E-83. Variants of the field experiment are: 1 – without irrigation; 2, 3 and 4 – with irrigation, applying 50%, 100% and 150% of the irrigation rate, determined by the optimal variant (pre-irrigation soil moisture 75% of FC for the 0 – 80 cm layer). If we accept that the relative irrigation depth is 1.000 in variant 4, then the relative irrigation depth in the other variants is as follows: var.1 – 0.000, var.2 – 0.333 and var.3 – 0.667. The results show that equation /1/ is most suitable from a mathematical point of view. The representative type of equation /1/ is: Y=2.743x–1.745x2 by R2=1. The variations of the calculated yields using this equation to experimental yields are from -10.2 to +6.8% (R=0.998). Equation /2/ is also very accurate from a mathematical point of view and at the same time corresponding well with the biology of the crop. The variations of the calculated yields using this equation to experimental yields are from -8.4 to +15.8% (R=0.993). The following parameters are established: n=3.37 (from 2.34 to 4.67) and R=0.997 (from 0.986 to 1.000). All established parameters of the relationship yield – irrigation depth in this paper are valid when moistening the 0–100 cm layer and 75% FC pre-irrigation soil moisture is maintained for the soil layer of 0 to 80 cm.
Parameters of the additional yield - irrigation depth relationship for sunflower grown in the region of Plovdiv