Sap Flow Measurement as an Instrument for Evaluating Transpiration in Water Balance Studies of a River Basin

Abstract

The article presents the results of instrumental measurements of stem sap flow with the use of sap-flowmeters in the trunks of white fir (Abies nephrolepis), the analysis of the main factors of the process of moisture transport in tree trunks, and the results of modeling hourly series of xylem flow rates. The analysis of factors was made with the use of the method of principal components, and the reproduction of individual series of xylem consumption was based on a multiple linear regression model. The evaluated volume of the transported moisture flux in fir trees over the vegetation period in 2020 varied from 1720 to 5620 L, depending on tree diameter, and the mean daily velocity of stem sap flow in the xylem varied from 0.3 to 1.0 cm/h. Regression analysis was used to find the optimal structure of the empirical model, which includes two predictors—air temperature and humidity. The empirical coefficients of regression equations were determined for each tree based on calibration samples. The results of testing on long enough test samples showed that the model series of xylem discharge on a complete sample (from May to October) for three test trees out of four are close to the measured values: the coefficient of correlation is 0.79–0.88, Nash–Sutcliffe coefficient is 0.62–0.85. The simulation efficiency improves significantly when samples for individual months are used: the correlation coefficient is 0.87‒0.94, the Nash–Sutcliffe coefficient is 0.73‒0.97.