The Impact of Extreme Precipitation on Soil Moisture Transport in Apple Orchards of Varying Ages on the Loess Plateau

The long-term cultivation of apple trees with deep root systems can significantly deplete moisture from the deep soil layers, while extreme rainfall events can rapidly replenish this moisture. Therefore, it is of great academic significance to investigate the influence of extreme precipitation on soil water dynamics in apple orchards of varying ages. This study was conducted on agricultural land and apple orchards of 12 years, 15 years, 19 years and 22 years (12 y, 15 y, 19 y and 22 y) to examine the impact of extreme precipitation on soil moisture transport. Soil moisture content and hydrogen and oxygen isotope (2H, 18O and 3H) data were collected before (October 2020 and May 2021) and after the extreme precipitation event (May 2022). This comprehensive analysis focuses on two aspects: soil moisture distribution and soil water recharge. The following main conclusions were drawn: (1) Extreme precipitation significantly enhanced deep soil water recharge in apple orchards: the depths of soil water supply for apple orchards of 12 y, 15 y, 19 y and 22 y were recorded as 282 mm, 180 mm, 448 mm and 269 mm, respectively. Correspondingly, the recharge depths were measured at approximately 12, 10, 10 and 7 m, respectively. It was observed that the recharge depth decreased with increasing age of the orchard. (2) Extreme precipitation did not have a significant impact on the values of δ2H and δ18O of deep soil moisture due to a limited infiltration depth through the piston flow mechanism (the maximum infiltration depth being around 3 m). (3) In agricultural land as well as apple orchards of 12 y, 15 y and 22 y in 2020, the tritium peak occurred at soil depths of 7.2, 6.9, 6.7 and 5.7 mm, respectively; in 2022, the corresponding values increased to 7.9, 8.7, 6.7 and 5.9 mm, respectively. This indicates that planting apple trees hindered the transport of soil moisture. The peak concentration of tritium in both agricultural land and different-aged apple orchards decreased after experiencing extreme precipitation. The findings will provide a scientific basis for water resource management and efforts toward ecological restoration on the Loess Plateau.