Crop coefficients for Calendula officinalis L. have not been determined yet, although crop coefficients of many plants have been provided by the Food and Agricultural Organization (FAO). C. officinalis L. is a medicinal plant used for manufacturing skin and beauty medication products. This study aimed to determine crop evapotranspiration (ETc) and single and dual crop coefficients (Kc) for C. officinalis L., in the southeast region of Iran with arid climate. Crop evapotranspiration was measured daily using five drainable lysimeter with a surface area of 1.55 m2. Reference ET was calculated using a drainable lysimeter. Experiments were carried out for a period of 2 years (2015 and 2016). The C. officinalis L. evapotranspiration for the result obtained from lysimeter were 1043.7 mm for 2015 and 964.15 mm for 2016 years; also the reference evapotranspiration obtained from lysimeter was 1806.15 mm for 2015 year and 1786.63 for 2016 year. Average single crop coefficients calculated during the first period were 0.71, 1.47 and 0.88 for season difference stages, and for the second period were 0.74, 1.53 and 0.98.

Keywords: C. officinalis L., lysimeter, medicinal plants, crop coefficients, evapotranspiration.