Relationships between Leaf Area Index and Evapotranspiration and Crop Coefficient of Hilly Apple Orchard in the Loess Plateau

Drought and water shortage are the key factors that restrict the sustainable development of the apple industry in the Chinese Loess Plateau. The accurate prediction of ET can provide a scientific basis for water management of apple orchards. A study on the relationship between LAI, ET and crop coefficient Kc under water deficit is particularly necessary for the accurate prediction of ET in apple orchards. In this work, the crop coefficient Kc under water deficit was defined as the product of the crop coefficient Kc_I under no water stress and the water stress coefficient Ks, namely Kc = Kc_I × Ks. LAI and ET of the hilly apple orchard were measured from April to September in 2019 and 2020. The results showed: (1) The LAI of the apple orchard showed a trend of rapid increase—moderate increase—declined during the growth period, with 0.26–2.16 [m² m⁻²] variation range; (2) The ET of the orchard was greater than the rainfall, the maximum ET was in July or August. The maximum components of ET in the apple orchard was E, with 47.8–49.1% of ET; T accounted for 42.5–43.9% of ET; Ic accounted for only 9.1–9.6% of ET; (3) There was a significant exponential relationship between the LAI and T or ET. The crop coefficient Kc_I under no water stress changed with the development of the apple tree canopy. The variation of water stress Ks was basically consistent with the variation of rainfall; (4) There is a significant exponential relationship between LAI and crop coefficient Kc under water deficit (Kc = 0.1141e^1.0665LAI, R² = 0.7055, <i>p</i> < 0.01). This study demonstrates that LAI could be used to estimate the crop coefficient Kc of apple orchards under water deficit in the Loess Plateau, and the actual evapotranspiration of apple orchards in this region could be predicted.