Determination of Crop Coefficient (Kc) for Rosmarinus officinalis L., Lavandula angustifolia Mill. And Silybum marianum (L.) Gaertnas Medicinal Plants Using Water Balance Approach

Crop water requirement knowledge and precise estimation of plant evapotranspiration are essential for improving water use efficiency and efficient water management. Water balance approach utilizing lysimeters with correct measurement of inputs and outputs is an accurate method for crop water requirement estimation. This study aimed to determine crop coefficient (kc) and water requirements of three medicinal plants namely Silybum marianum (L.) Gaertn, Rosmarinus officinalis L. and Lavandula angustifolia Mill. using water balance approach in 2013–2014. Nine volumetric lysimeters were used to carry out the experiment in school of agriculture (Bajgah), Shiraz University. Actual daily evapotranspiration using water balance method was estimated using lysimeters. Reference evapotranspiration was calculated using climate data from the adjacent weather station applying FAO–Penman–Monteith model. Daily crop coefficient during the study period was estimated by dividing the actual evapotranspiration by reference evapotranspiration. According to the results the seasonal evapotranspiration of Silybum marianum estimated 372.6 mm and it’s crop coefficient was 0.3, 0.77, 1.04 and 0.85 during the initial, developing, mid and late season stages, respectively. Crop evapotranspiration of Lavender and Rosemary, obtained 1191.4 mm and 14534.6 mm, respectively. The maximum and minimum daily evapotranspiration of Rosemary obtained 8.17mm in August and 0.58mm in January. The corresponding minimum and maximum crop coefficient was 0.16 in January and 1.13 in August. The maximum and minimum daily evapotranspiration of Lavender obtained 7.82 mm in June and 0.62mm in January. Also the maximum and minimum crop coefficient of Lavender was 1.18 in June and 0.18 in January. Of three studied plants in limited time period and research conditions, Silybum marianum due to shorter growth period as a seasonally plant requires the lowest amount of water while Rosmarinus officinalis L. as a perennial plant requires the most amount of water. However, to increase the accuracy of findings it is highly recommended this experiment repeated for more seasons and in different areas.