Different supplemental irrigation methods result in discrepant water use efficiency and yield by changing steps of water use process in dryland wheat

Supplemental irrigation (SI) at critical growth stages is a practical way to mitigate water shortage of winter wheat in dryland. However, the yield performance after SI is unstable in different precipitation years. To obtain a better understanding about the applicability of SI in dryland, the utilization efficiency of precipitation and irrigation water (IUE) was divided into 5 sequential ratios: water storage ratio, water consumption ratio, water transpired ratio, transpiration efficiency and harvest index. In this study, those ratios were investigated under four SI methods: no irrigation (W₀), SI once at jointing (W_j), SI once at booting (W_b) and SI twice at jointing and booting (W_j+b) throughout four years. Our results showed that water storage ratio was increased under all SI treatments, but water transpired ratio was only significantly increased under W_j and W_j+b, due to their greater development of plant population compared to W₀. Thus, wheat yield was greatly improved by 7–12 % under W_j and W_j+b than that under W₀ in dry and normal years. However, IUE was significantly decreased under W_j+b. Compared with W₀, W_j+b had higher evaporation during the fallow period and lower water consumption ratio. Furthermore, 100–300 cm subsoil water utilization was decreased under W_j+b from jointing to harvest time due to the low root length density (RD_L) in subsoil. Under W_j, evaporation, subsoil water utilization and RD_L were not negatively affected and water use efficiency was increased compared to W₀. Thus, SI once at jointing stage is a more suitable practice in dryland wheat farmland when considering the dual goal of effectively using water while increasing yields under worse precipitation year.