Determination of Evapotranspiration and Crop Coefficient of Milk Thistle in Birjand Plain

Water is one of the most important factors that limit agricultural development, especially in arid and semiarid regions around the world. One of the important data for water management is the amount of water requirement of different plants. In this regard, by measuring water input and output in lysimeters and using water balance equation, crop water use is determined. In this study, to determine crop coefficients of Milk thistle, as a valuable medicinal herb, a 1-year lysimetric experiment was conducted in Faculty of Agriculture, Birjand University, during the growing season of 2018. To conduct this project, six weighing lysimeters were used. To determine potential evapotranspiration as a reference crop, grass was grown in three lysimeters and, in three other lysimeters, Milk thistle (Silybum marianum L.) was planted. Based on the results, the reference crop evapotranspiration and actual evapotranspiration of the plant were equal to, respectively, 1179.5 and 920.2 mm, during the growing period of 177 days. The length of different stages of plant growth, including the initial, development, middle, and end stages was 22, 35, 70, and 50 days, respectively. Finally, based on the FAO method, the crop coefficient (KC) curve was drawn and the average of crop coefficient at each of four stages of plant growth was determined as 0.34, 0.69, 0.93, and 0.77, respectively.