Estimation of Crop Coefficients in Different Stages of Growth and Water Requirement of Nigella sativa L. in Arid Region of Birjand, Iran

The necessity of increasing economical productivity of agriculture and the increasing global demand for medicinal plants and considering the low water requirement of these plants have shifted the cropping pattern to their benefit. Water needs of Nigella sativa L., an important medicinal plant, was not known, therefore, we aimed to determine its water needs. (ETc) and crop coefficients (Kc) for different phenological stages in arid conditions of Birjand, in 2018. In order to conduct the research, three weighing lysimeters (with a diameter of 20 cm and a height of 16 cm) were used in three replications. Actual daily evapotranspiration was estimated by water balance in lysimeters and reference evapotranspiration was calculated based on 12 cm grass. The results showed that, in the initial stage, the rate of evapotranspiration was low, in the developmental stage it had an increasing trend, and after stabilization in the middle stage, with the onset of aging in the maturity stage of growth, the rate of evapotranspiration decreased. Actual and reference crop evapotranspiration was obtained as 670 mm and 1439 mm, respectively. The values of crop coefficients in the initial, development, middle, and final stages were obtained as 0.39, 0.53, 0.59, and 0.28, respectively. The general conclusion of the research is that by using the values of crop coefficients and water requirement, it is possible to cultivate this crop with maximum water productivity in the arid region of Birjand.