Crop coefficient and ratio of transpiration to evapotranspiration of winter wheat and maize in a semi-humid region

Crop coefficient (Kc) is an important parameter for irrigation scheduling and water allocation. This study investigated and determined the crop coefficients and the ratio of transpiration to evapotranspiration (Tp/ETc) of winter wheat (Triticum aestivum L.) and maize (Zea Mays) based on lysimeter data for 10 years in a semi-arid region of northwest China (34°20N, 108°24E). Several relationships, Kc and days after sowing (DAS), Kc and leaf area index (LAI), and Tp/ETc and LAI, were also analyzed. The average seasonal ETc for winter wheat and maize were 443.6 and 424.0 mm, respectively, compared to their reference crop evapotranspiration (ET0), 483.9 and 407.7 mm, respectively, for their seasons. The seasonal transpiration accounted for 67.0 and 74.0% of their evapotranspiration for winter wheat and maize, respectively. The average, maximum and minimum seasonal Kc were 0.92, 1.33 and 0.42, respectively, for winter wheat, and 1.04, 1.43 and 0.45, respectively, for the maize. The relationship between Kc and DAS was fitted to a fifth-order polynomial equation with significant correlation coefficients, R2=0.960 and 0.980, respectively, for wheat and maize. The maximum Kc occurred at about 200 and 80 DAS for winter wheat and maize, respectively. Kc and LAI showed a saturation relationship, R2=0.861 and 0.952, respectively, for wheat and maize, and Kc exceeded 1.0 when LAI was larger than 2.5 for both crops. The Tp/ETc and LAI also showed saturation relationship, R2=0.968 and 0.981, respectively, for wheat and maize. Tp/ETc started from 0 at sowing, and reached to its maximum, near 0.9, at the middle growth stage or when LAI reached to about 3.0 for both crops. Tp/ETc maintained at its maximum when LAI was larger than 3.0. These results should help the precise planning and efficient management of irrigation for these crops in this region.