The use of stable isotopes to determine optimal application of irrigation-water to a maize crop

Aim Understanding the water use of a maize-based cropping systems can help people improve water management strategies. We conducted a two-year field experiment at the Shiyang River Basin to study the effect of flood irrigation on crops' water uptake depth and estimated the reasonable irrigation amount for a maize field. Methods We collected water samples from the plant xylem, soil, irrigation-water, and precipitation during the crop growing season in 2018 and 2019 for hydrogen and oxygen stable isotope analysis. The Iso-Source model was used to calculate crops' water uptake depth. We determined the irrigation depth and estimated the reasonable irrigation amount based on the soil water balance and isotope mass conservation. Results Maize used water from the surface 0.2 m soil layer when irrigation-water was applied. On average, only 40 +/- 1% (179 mm) of the irrigation-water remained in this soil layer and an average of 24 +/- 8% (124 mm) of the irrigation-water and precipitation moved below 1.0 m soil depth per year. Conclusions Low-volume and high-frequency irrigation strategies can reduce soil water evaporation and drainage. The amount of irrigation-water to apply is a function of the crop's growth stage. At the jointing stage, the target is to apply water to the 0.05 m soil depth. For the heading stage, the target soil depth is 0.05 to 0.2 m, and for the filling stage, the target soil depth is 0.05 to 0.6 m. The irrigation amount corresponding to the three growth stages is 83 mm, 93 mm and 78 mm.