Mechanisms by which irrigation regimes influence soil water deep percolation

Substantial deep percolation induced by irrigation practices has emerged as a critical constraint on the efficient utilization of agricultural water resources. However, the mechanisms through which irrigation regimes influences deep percolation remain unclear. During March 2022–March 2024, a combined approach integrating field experiments with HYDRUS-1D modeling was employed to investigate soil moisture distribution characteristics and the mechanisms regulating deep percolation under four distinct irrigation regimes: rainfed, single irrigation, double irrigation, and triple irrigation. The results demonstrate that soil moisture in the upper 100 cm profile exhibits marked sensitivity to irrigation and precipitation inputs. Irrigation is conducive to the formation of stable infiltration channels in the soil profile, accelerating water infiltration, resulting in thickening of high moisture content zones below 100 cm, higher soil moisture content can enhance the stability of soil moisture status to climate change. Root water uptake was more sensitive to irrigation regimes than soil evaporation. Under different irrigation quota conditions, root water uptake of double irrigation was 94.1–118.4 mm higher than that of single irrigation, while the soil evaporation was only 13.1–14.1 mm higher. The irrigation regimes significantly regulate the deep percolation coefficient, which ranges from 0.19 to 0.31: the differential in the deep percolation coefficient between double irrigation and single irrigation transitions from negative to positive (-0.01–0.03) and continues to increase with increasing irrigation quotas per application. The complementary relationship between the increment of root water uptake per unit irrigation increase and the increment of deep percolation per unit irrigation increase reveals that irrigation regimes regulate crop water stress responses to alter root hydraulic redistribution, thereby inducing dynamic variations in deep percolation, and finally forming a dynamic feedback mechanism of water transport process with the deep percolation coefficient as the characterization parameter. This study provides a theoretical basis for improving the efficiency of farmland water use.