Changes in and influencing factors of crop coefficient of winter wheat during the past 40 years on the Taihang Piedmont Plain

The crop coefficient (K_c) is defined as actual evapotranspiration (ET) under sufficient water supply divided by the reference crop ET (ET₀), which can be calculated using meteorological factors. The K_c is used as a basic parameter to calculate the crop water requirements. The accurate determination of K_c plays an important role in optimizing irrigation management. The K_c changes with crop growth and environmental conditions. The purpose of this study was to assess how K_c varied with crop production and weather conditions by using a long-term field experiment of field management measures of winter wheat. The actual ET of winter wheat under sufficient irrigation and ET₀ derived from daily meteorological parameters at Luancheng Agro-ecosystem Experimental Station of the Chinese Academy of Sciences from 1980 to 2020 were used to calculate the seasonal K_c. Additionally, the dominant factors affecting the K_c of winter wheat under the current production conditions were identified from experimental data of three recent years (2017–2020). The results showed that for winter wheat with sufficient water supply from 1980 to 2020, the average ET and ET₀ were 434.7 mm and 550.8 mm, respectively. The ET₀ was relatively stable, and the ET increased by 17.6%. The average K_c was 0.80 during the past four decades, with an average value of 0.76 in 1980–1990, 0.80 in 1991–2000, 0.81 in 2001–2010, and 0.84 in 2011–2020, indicating a continuously increasing trend. In the past four decades, the yield of winter wheat had increased by 42.4%, and K_c had increased by 11.6%. The increase in ET was the main reason for the increase in K_c. The ET during the past four decades increased with increasing crop production, and with a relatively stable ET₀, the K_c increased. Therefore, the K_c varied with changes in crop grain production, which was related to biomass production and canopy size. Under the current growing conditions, leaf area index and biomass were important factors that affected K_c. When the leaf area index reached a certain level, K_c was mainly affected by the atmospheric evaporation potential determined by the saturated water vapor pressure difference and atmospheric temperature. The K_c during the recent three years was 0.79 for 2017–2018, 0.86 for 2018–2019, and 0.79 for 2019–2020. The average ET was 442.3 mm during the three years, and the average K_c at different growing stages of winter wheat were 0.70 from sowing to winter dormancy, 0.42 during winter dormancy, 0.76 from recovery to jointing, 1.18 from jointing to heading, 1.39 during heading to grain-fill, and 0.96 during maturity. Thus, the water requirements for winter wheat after winter dormancy increased sharply and reached the highest values during the heading to earlier grain-filling stages. The results from this study indicate that K_c varies with changes in the crop growing conditions and should not be taken as a constant value. K_c developed during three recent seasons in this study could be used to determine the crop water requirements for irrigation scheduling under the current growing conditions.