| Abstract:
The amount of evapotranspiration or amount of water consumed by plant is the basis of irrigation planning. Irrigation without awareness of the actual plant requirement for water causes economic losses and reduced water use efficiency. One of the best approaches to assess plant water requirement is determination of crop coefficient using lysimeters. The aim of this study, which was conducted at Fasa University, located 140 kilometers south-east of Shiraz, was to evaluate the different growth stages of Nigella sativa L. and determine the crop coefficient of this annual species. In this study, the crop coefficient of the study species was determined by using a weighing microlysimeter (40×30 cm) and three replications in a good moisture condition. The results showed that the length of each growth stage (initial, developmental, middle and final growth stage) was calculated to be 10, 11, 41 and 23 days, respectively. The crop coefficient (Kc) of Nigella sativa in each stage was calculated to be 0. 75, 1. 02, 1. 21 and 0. 8, respectively.
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