Estimation of Soil Evaporation in Apple Orchards Based on Hydrogen and Oxygen Isotopes

Accurate estimation of soil evaporation and plant transpiration is crucial for the scientific and effective management of water resources in apple orchards. Using apple orchards of varying ages as research subjects, the Craig–Gordon model was employed to quantitatively estimate the soil evaporation loss rate, and water balance analysis was utilized to differentiate between soil evaporation and plant transpiration. This study analyzed the effects of extreme precipitation on soil evaporation in apple orchards of varying ages. The results indicated the following: All stable isotopic compositions of soil water fell within the lower right region of the Local Meteoric Water Line, and extreme precipitation reduced the intercept of the Evaporation Trend Line in apple orchards of varying ages. Soil evaporation loss rates in 2021 and 2022 ranged from 13% to 24% and from 14% to 18%, respectively. In 2022, the soil evaporation of apple orchards of 12 y, 15 y, 19 y and 22 y was measured at 123, 77, 129 and 143 mm, respectively, indicating negative correlation (R = −0.75) between soil evaporation and the leaf area index. The corresponding plant transpiration levels were 617 mm, 552 mm, 522 mm and 330 mm, indicating a linear negative correlation between plant transpiration and age (R = −0.92). These results are helpful to guide the scientific development of irrigation systems for apple orchards and provide a reference for enhancing apple yield and promoting the sustainable development of the apple industry.