Determination of Plant Coefficient and Evapotranspiration of Thyme in the Standard Conditions of Karaj

To determine crop coefficient and evapotranspiration of Thymus daenensis Celak., an experiment was performed in 2016 using lysimeters (direct method) and computational method In the direct method, drainage lysimeters with underground structures located at Alborz Research Station in Karaj were used. Each of the lysimeters were 0.9 meters in diameter and 1.28 meters high. In each lysimeter, a seedling of Thymus was planted. Then, two main lysimeters were selected and their irrigation was adjusted to the field capacity. On both sides of the main lysimeters, similar lysimeters were considered as margins. During the experiment period, the input and output water of the lysimeters as well as dry weight of aerial parts, the amount of  evapotranspiration, and water use  efficiency in Thymus daenensis were measured. In order to estimate the evapotranspiration of the reference plant, the computational methods of Blaney Criddle and FAO Penman-Monteith were used. Thymus plant coefficients during plant growth, was obtained as the ratio of Thymus evapotranspiration to reference plant evapotranspiration. Based on the results, the amount of cumulative evapotranspiration of the reference plant by Blaney Criddle-FAO and Penman-Monteith-FAO were 715 and 631 mm, respectively. In lysimeters, Thymus evapotranspiration up to the beginning of flowering was 100 mm, to the beginning of seed production  455 mm, and to the end of seed production  622.13 mm. The Thymus plant coefficients in initial, crop development, mid-season and late-season stages were 0.33, 0.65, 1.20, and 0.99, respectively. Water use efficiency was estimated to be 0.4 gL-1. By determining the plant coefficient and water requirement of Thymus, in addition to helping in design of intelligent systems and mechanized irrigation, it will be possible to plan and manage irrigation during the plant growth period.