Assessment of Evapotranspiration–Yield Relationships in Northern China Tea Plantations: A Basis for Crop Water Productivity Improvement

Global climate warming and freshwater scarcity are intensifying water stress in agricultural fields, severely constraining sustainable agricultural development. As a typical C3 perennial cash crop, tea (Camellia sinensis) is naturally suited to low-latitude regions with abundant heat and evenly distributed precipitation, and it is highly sensitive to environmental factors such as temperature and moisture. In northern hilly tea-producing areas, tea plantations often encounter multiple challenges including uneven rainfall distribution and poor soil water retention, resulting in prominent water supply–demand imbalances that critically limit stable and efficient tea production. To explore efficient water-saving irrigation strategies adapted to such ecological conditions, this study was conducted in the Yushan Tea Plantation, Rizhao City, Shandong Province, China. Based on field monitoring data across three growing seasons (spring, summer, and autumn) from 2021 to 2023, five irrigation treatments were evaluated: conventional sprinkler irrigation (CK), drip irrigation (D), micro-sprinkler irrigation (W), drip irrigation with straw mulching (SD), and micro-sprinkler irrigation with straw mulching (SW). Actual crop evapotranspiration (ETc act) was estimated using the soil water balance method, and actual fresh tea leaf yield (FTLY) and crop water productivity (CWP) were measured. Results showed that the SW treatment significantly improved both FTLY and CWP across all three seasons, with summer FTLY in 2022 increasing by 56.58% compared to CK and maximum CWP in spring and autumn reaching 0.916 kg/m3, demonstrating excellent stability and adaptability. Among all irrigation strategies, the SW treatment also exhibited the best regression fitting and yield prediction accuracy. The regression model validated by leave-one-out cross-validation (LOOCV) for the SW treatment demonstrated strong robustness and reliability (R2 = 0.734; RMSE = 208.12 kg/ha; MAE = 183.31 kg/ha). Notably, the samples with the largest prediction errors across all treatments were nearly all associated with the highest or near-highest ETc act values, indicating that model accuracy tends to decrease under extreme evapotranspiration conditions. The results show the synergistic effect of irrigation–mulching integration on enhancing CWP in northern perennial tea systems, providing empirical evidence and theoretical support for developing efficient irrigation strategies in hilly tea-growing regions of Northern China.