Diurnal and nocturnal transpiration behaviors and their responses to groundwater-table fluctuations and meteorological factors of Populus tomentosa in the North China Plain

•Groundwater depth (GT) controlled transpiration dynamics of Populus tomentosa.•Diurnal (Qday) and nocturnal water use (Qnight) had different seasonal trends.•Groundwater recharged Qnight and reduced the water consumption from the stem.•Qnight helped poplars refill the consumption of water stored in the stem (Re).•When GT < 170 cm of the surface, Re/Qnight increased with increasing GT.