Measurement of sap flux density using a single probe through a lumped capacitance method

Accurate measurement of sap flux density (SFD) is crucially important in plant transpiration studies. A novel single-probe method with pulse heating was established to measure SFD, based on both the lumped capacitance method and a proposed logarithmic relationship between the heat exchange coefficient and SFD. Peach, apple, and jujube trees were selected to evaluate the model reliability and accuracy. The results of experiments showed that the proposed method exhibited good predictive accuracy, with relative errors (REs) of less than 12%. For all tree species, SFD measured by the proposed method showed strong agreement with the real SFD determined by the weighing method, with root mean square errors (RMSEs) ranging from 0.353 to 0.712 L dm−2 h−1. Field tests showed that the daytime outputs from the proposed method closely matched those measured by the transient thermal dissipation method (linear model), with R2 > 0.95. The slopes of the linear fit for SFD predictions were 0.818, 0.981, and 1.041 for peach, apple, and jujube trees, respectively, with corresponding intercepts of 0.518, 0.338, and 0.647 L dm−2 h−1. In addition, the field SFD measurements were significantly and linearly correlated with reference evapotranspiration (ETo). The Pearson correlation coefficients between predicted SFDs and ETo were 0.84, 0.58, and 0.85 for peach, apple, and jujube trees, respectively, supporting the rationality of predictions by the proposed method. Similar to existing models, the proposed model still requires species-specific calibration and shows reduced accuracy under near-zero SFD conditions. This study provides an accurate and energy-efficient alternative for sap flow measurement, while reducing wounding effects compared with multi-probe methods.