An analysis of the nature of the relationships between the productivity of maize grain and irrigation rate, the productivity of irrigation water and the additional yield, based on data from field trials, appeared in the experimental field of Agricultural Institute, Stara Zagora, Bulgaria. The data are for the period from 1998 to 2013. The field surveys were conducted with optimal water culture and under irrigated conditions. The extra yield obtained as a result of the irrigation rate has been calculated. The yields obtained from natural moisture loss range from 3360 to 7830 kg/ha. As a result of an optimal irrigation rate, yields increased almost twice. The corn grain yield limits for optimal irrigation are in the range 4570 to 13606 kg/ha. The extra yield ranges from 3360 to 7395 kg/ha. The regression analysis establishes a strong positive linear relationship between the extra yield and productivity of the irrigation rate. When analyzing the relationship between extra yield and precipitation during the vegetation period and between extra yield and precipitation in the July-August period, a low degree of correlation was established. The irrigation water productivity, which ranges from 6,72 to 37,73 kg.ha-1.mm, has been calculated over the years with different rainfall provision. The character of the dependence "extra yield-yield of irrigated water" and the coefficient of determination (R2 = 0,81) are established.