Calculation of crop coefficient of winter wheat at elongation-heading stages

An experiment was conducted to investigate the relationship between reference crop evapotranspiration(ET₀) and actual crop evapotanspiration(ET), and relationship among crop coefficient(Kc), leaf area index (LAI) and plant height(H) at winter wheat elongation-heading stages during 2000～2002 winter wheat season at Yucheng Comprehensive Experimental Station and 2002～2004 season at Tongzhou Experimental Base. Results show that ET measured using lysimeters was lower than or near to ET₀ computed using FAO 56PM method at the beginning of wheat elongation-heading stages and then increased higher than ET₀ at the end of the stage. Crop coefficient, the ratio of ET to ET₀, increased with LAI and H. The mathematical relationships between Kc and LAI and H were developed using datasets in2003 and 2004. ET values in 2001 and 2002 were estimated using above relationship incorporated with ET₀. The estimated ET values computed using both LAI and H methods agreed well with the measured except when ET was less than 3 mm/d, which might be because of higher soil evaporation at the beginning of the stageunder small LAI level. The estimated ET computed using both methods showed that there was no significant difference between LAI method and H method, andtherefore H method could be used to estimate crop evapotranspiration at wheat elongation-heading stages in the North China Plain.