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

Crop coefficient (K_c) 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 (T_p/ET_c) 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°20′N, 108°24′E). Several relationships, K_c and days after sowing (DAS), K_c and leaf area index (LAI), and T_p/ET_c and LAI, were also analyzed. The average seasonal ET_c 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 K_c 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 K_c 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 K_c occurred at about 200 and 80 DAS for winter wheat and maize, respectively. K_c and LAI showed a saturation relationship, R² = 0.861 and 0.952, respectively, for wheat and maize, and K_c exceeded 1.0 when LAI was larger than 2.5 for both crops. The T_p/ET_c and LAI also showed saturation relationship, R² = 0.968 and 0.981, respectively, for wheat and maize. T_p/ET_c 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. T_p/ET_c 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.