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

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.