Estimate canopy transpiration in larch plantations via the interactions among reference evapotranspiration, leaf area index, and soil moisture

Accurately modelling and predicting forest transpiration under changing environment and canopy structure are essential for understanding the interactions among the atmosphere, soil, and vegetation and precisely integrating forest water management. To enhance water management in larch plantations, the canopy transpiration (T), reference evapotranspiration (ETₒ), canopy leaf area index (LAI), and relative extractable water (REW) of the 0–60 cm soil layer were synchronously monitored during the growing season (May–September) in 2016 and 2018. The response functions of the basal crop coefficient (Kcb) in an ETₒ-based transpiration model to individual driving factors (LAI and REW) were determined using upper boundary lines and then coupled to form the T model using the crop coefficient method. The results showed that (1) the mean (and range) of the daily canopy T was 0.83 (0.003–1.60) mm·d⁻¹ in dry year of 2016, and 0.70 (0.02–1.55) mm·d⁻¹ in wet year of 2018. (2) The Kcb increased with rising LAI, following a positive linear relation. With rising REW, Kcb first increased rapidly and then stabilized gradually following a saturated exponential function. (3) The daily T model was fitted as T=(0.056 + 0.083(1-exp(-3.915REW))) (0.961LAI) ETₒ and had good performance in both the calibration period (R² = 0.78; NSE = 0.77) and validation period (R² = 0.85; NSE = 0.75). (4) The relative importance of ETₒ, LAI, and REW on T were analysed based on the developed T model. The results showed that the average contribution of ETₒ, LAI, and REW to T differed in various years. The dominant factor contributing to the changes in T was ETₒ followed by REW and LAI in dry year of 2016 and LAI followed by ETₒ and REW in wet year of 2018 compared with the reference T under the long-term means of the daily ETₒ, LAI, and REW. The newly developed simple T model in this study represents a key step for the comprehensive integrated management of forests and water and accurately predicts the response of the daily canopy T to varying LAI values under a given REW and ETₒ, which is helpful for designing precise adjustments of stand structure.