Modelling of sap flux density of oak in a humid region in China

Abstract

Transpiration plays a vital role in determining the watershed water cycle. However, we still have little knowledge of the characteristics of tree transpiration in the Hanjiang River Basin, which is the water source for the middle route of South‐to‐North water diversion project. Here, we measured sap flux density of oak trees (Quercus, the dominant species here) at the 10‐min resolution for 2 years and explored its response to the environmental conditions. The incoming short‐wave radiation and vapour pressure deficit well explained the variation of daytime sap flux density, and a statistical model was then proposed to calculate the daytime sap flux density correspondingly; then a nighttime sap flux density module was proposed based on the daytime sap flux density calculation. Sap flux density showed clear counter‐clockwise hysteresis response to incoming short‐wave radiation, and clockwise hysteresis response to vapour pressure deficit. Our sap flux density model can well reproduce the corresponding hysteresis response to incoming short‐wave radiation and vapour pressure deficit. This study unravelled the environmental controls of sap flux density of the oak trees in the Hanjiang River Basin, proposed an efficient model for the sap flux density simulation, provided important knowledge for understanding the corresponding forests' water use, which is of critical significance in determining the water availability for the middle route of the South‐to‐North water diversion project.

A Novel Crossed Hysteresis Response Pattern of Sap Flux to Solar Radiation

“…Equations 1 and 2 set the basis for the general mathematical expression of hysteresis indicated by a time lag (Zhang et al, 2014). Figure 1b shows that the response of SF to Rsi can be characterized by a closed counterclockwise hysteresis loop, which is reported in previous studies in the literature (e.g., Liu et al, 2024;Wan et al, 2023;Wang et al, 2019). However, some studies have reported SF might advance Rsi as aforementioned (e.g., Wan et al, 2023;Zha et al, 2017).…”

“…The hysteresis response of SF (or other evapotranspiration components) to the key meteorological drivers (i.e., Rsi and VPD) has been widely reported in the literature over the last few decades, attracting extensive attentions to exploring the onset, underlying mechanisms and implications of these hysteresis phenomena (Dhungel et al, 2021;Xu et al, 2022;Zhang et al, 2014Zhang et al, , 2019b. When exploring the response of SF to Rsi and VPD, a clear hysteresis loop emerges when SF is plotted against Rsi and VPD on a diurnal basis (Liu et al, 2024;Wan et al, 2023). Such diurnal hysteresis patterns of SF or evapotranspiration are usually complicated, and it is difficult to describe these relationships with simple mathematical functions, limiting our abilities to accurately quantify forest water use (Bai et al, 2017;Tu et al, 2019;Zhao et al, 2016).…”