The aim of this study is to determine the effect of sunflower irrigating at different rates on the yield.
The experiment was conducted during the 2006–2010 period in the experimental field of the Agricultural University – Plovdiv on alluvial-meadow soil (formerly waterlogged). The following variants were tasted: 1 – without irrigation; 2, 3 and 4 – irrigation with 50, 100 and 150% of the rate, determined by the optimal variant (pre-irrigation soil moisture 75% of FC for the 0–80 cm layer, from the beginning of the vegetation period to the period of seed filling). The results showed that optimum irrigation gives maximum yield for sunflower. That irrigation regime was performed through 2–3 applications at an irrigation rate of 80 mm and irrigation depth 160–240 mm. The yield increased by 46–97% as a result (an average of 74%). Irrigation at 50% of the irrigation rate caused yield reduction in the range of 11–18%, but given the high additional yield (as compared to the non-irrigated sunflower), an average of 51%, this irrigation regime could be used under conditions of limited water resources. CIWUE (crop irrigation water use efficiency) in this irrigation regime is higher – averagely 7.9 kg.ha-1.mm-1. An irrigation rate higher than 80 mm is not recommended because the yield does not increase. CIWUE at an irrigation rate of 150% is the lowest (3.6 kg.ha-1.mm-1). The yield-irrigation rate relationship can be presented by a quadratic equation where R2>0.9.