تعیین ضرایب گیاهی یگانه و دوگانه کتان روغنی در دشت شهرکرد

amount of water that use by evapotranspiration. Estimation of evapotranspiration (crop water requirement) is also one the most significant stages for designing different irrigation systems, programming and correction of the water resources management. Determining crop coefficient is a necessary part of estimating evapotranspiration. Crop coefficients (Kc) are properties of plants which vary in different growth stages. Potential evapotranspiration occurs if a field of the crop had an ideal unlimited water supply. The effect of both crop transpiration and soil evaporation are integrated into a single crop coefficient. The Kc coefficient incorporates crop characteristics and averaged effects of evaporation from the soil; while in dual crop coefficient two sub-coefficients (basic crop and evaporation coefficient) are considered separately. Therefore, the dual crop coefficient approach is more complicated and computationally intensive than the single approach. Due to the importance of accurate determining of crop water requirement many researches have been done for measuring single and dual crop coefficient. The aim of this research is to determine the actual evapotranspiration and single and dual crop coefficients of linseed by lysimeter, during various stages of growth in the Shahrekord area.
This research was conducted in Shahrekord University. Shahrekord is located 97 kilometers southwest of Isfahan with the geographical coordinates from 32°18'22'' to 32°21'50'' N and 50°49'22'' to 50°53'44'' E. It is topographically situated in the north of the Zagros Mountains. The annual average temperature and precipitation in Shahrekord is 11.5°C and 320 mm, respectively. This study was done in the crop year of 2014-2015. Four lysimeters were used with the length, width and depth of 190, 65 and 140 cm. respectively. After filling lysimeters with soil and before planting, samples were taken for determining the soil characteristics, in depth of 0-30 cm. One of the lysimeters was allocated to grass, for measuring the reference evapotranspiration, and three other lysimeters to the linseed. After each irrigating the drainage water was collected and then its quality and quantity was measured. The crop evapotranspiration (ETc) and crop reference evapotranspiration (ETo) were calculated by using of water balance equation. Single crop coefficient was calculated from ratio of the measured crop evapotranspiration (ETc) and measured reference evapotranspiration (ETo) and dual crop coefficient was determined using the FAO 56 method.
The results showed that the initial, development, middle, late and total growing periods of linseed are 20, 14, 22, 24 and 80 days. The total values of growing degree days (GDD) for these stages are 226.5, 226.7, 379.1, 420.7and 1253, respectively. The average value of the single crop coefficient in the initial stage is equal to 0.74, in the middle stage is 1.23 and at the end of the final stage is 0.61. The single crop coefficient was also calculated using the FAO 56 method. The results indicated that the difference of single crop coefficients in these two methods was very little in the initial and middle stages, but it increased in the final stage. The basal crop coefficient, soil evaporation coefficient and dual crop coefficients were calculated using the FAO56 method. The basal crop coefficient for linseed at different growth stages of initial growth stage, middle and end final are 0.15, 1.15 and 0.2, respectively. During the initial stage, because of low green cover, the soil evaporations is the highest and it became decrease by increasing the green cover in the next growth stages. The actual evapotranspiration value of linseed that obtained from three lysimeters was equal to 444 mm during the growing season. The evapotranspiration value of linseed that calculated by dual crop coefficient (using FAO56 method) was 445.5 mm, while this value was equal to about 391 mm for the single crop coefficient (using FAO56 method). The comparison of these values clearly indicated that the dual crop coefficient can estimate the evapotranspiration more accurate than the single crop coefficient. The results showed that evapotranspiration of linseed and grass are 444 and 429 mm, respectively and it daily variations for linseed are 1 to 10 mm and for the grass are 4.3 to 8.5 mm. The main conclusion that can be extracting from this research is that the calculating of dual crop coefficient is necessary for estimating the accurate crop water requirements.