Effects of extreme climate on hydrological dynamics in dryland apple orchards: a modeling study

In the past decades, extreme precipitation and drought have increased in both intensity and frequency in global drylands, threatening the sustainability of agricultural systems. To address this challenge, this study refined the process-oriented STEMMUS (simultaneous transfer of energy, mass, and momentum in unsaturated soil) model by introducing a dynamic leaf area index (LAI) development sub-module, and defined scenarios incorporating variations in precipitation amount, precipitation intensity, and temperature. These scenarios elucidate the response patterns of shallow and deep soil moisture and apple orchard evapotranspiration to climatic fluctuations. Key findings reveal that, at the interannual scale, increased growing-season precipitation significantly enhanced soil water storage in both shallow (0–200 cm) and deep (200–450 cm) layers. High-intensity precipitation partially increased soil water storage, particularly under reduced precipitation scenarios, though its contribution to deep soil remained limited. Compared to ambient temperature conditions, 2°C warming resulted in maximum reductions of 30.6 mm and 29.7 mm in shallow and deep soil water storage, respectively. Growing-season cumulative canopy transpiration (T), soil evaporation (E), and T/ET all increased significantly with greater precipitation. Conversely, high-intensity precipitation and 2°C warming reduced cumulative transpiration by 9.7–16.4 % and 7.2–18.3 %, respectively, while T/ET decreased by 4.0–9.4 % and 8.9–15.5 %. Notably, 2°C warming markedly amplified cumulative soil evaporation by 11.5–15.7 %, whereas high-intensity precipitation had no significant effect on soil evaporation. These findings provide a theoretical foundation for developing sustainable water management and climate adaptation strategies in dryland agroecosystems.