Basil (Ocimum basilicum L.) is one of the important aromatic plants belonging to the family Lamiaceae, which is used as an herb, spice as well as fresh vegetable.The present study was performed to determine the irrigation depth index under different management conditions with evaluating the effect of deficit irrigation, soil texture and nano fertilization on basil. The experiment was performed as a factorial based on randomized complete block design (RCBD) with 18 treatments and three replications at weather station, Ferdowsi University of Mashhad. Deficit irrigation treatments consisted of three levels of irrigation (I1=100% ETc), (I2=75% ETc), (I3=50%ETc) and three levels of nano fertilizers, containing nano fertilizer with full concentration (F1), 70% (F2), and nonusing of nano fertilizer (F3) were implemented in two light soil texture (S1) and medium soil texture (S2). The results showed that the average actual evapotranspiration estimation by REC-P55 device has been equal to 3.38 mm at the beginning of the growing period of basil, which amount has increased to 8.60 mm during basil development in the middle of growth period. The results of crop coefficient analysis showed that the maximum kc of basil (1.42) was detected in July. The results also indicated that the highest water use efficiency (WUE) was obtained in terms of fresh and dry herb yield as 2.06 and 0.37 kg/m3 in S2I3F3 treatment, respectively, while the maximum water use efficiency in terms of seed yield (0.37 kg/m3) was obtained in S1I2F2 treatment. Using the research results, according to different levels of water use of functions of yield Y(w), cost C(w), and benefit B(w) for basil based on mathematical and economic analysis of these functions, irrigation index and optimal irrigation depths were evaluated. It was found that with deficit irrigation under water restriction conditions, with the aim of maximum use of water volume unit, the optimal water consumption depth will be reduced by 20% compared to maximum irrigation mode. Also, with this amount of deficit irrigation, the maximum Rial return per cubic meter of water consumption would be as 1849 Rials.

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