Estimating groundwater evapotranspiration by a subtropical pine plantation using diurnal water table fluctuations: implications from night-time water use

Exotic pine plantations have replaced large areas of the native forests for timber production in the subtropical coastal Australia. To evaluate potential impacts of changes in vegetation on local groundwater discharge, we estimated groundwater evapotranspiration (ET_g) by the pine plantation using diurnal water table fluctuations for the dry season of 2012 from August 1st to December 31st. The modified White method was used to estimate the ET_g, considering the night-time water use by pine trees (T_n). Depth-dependent specific yields were also determined both experimentally and numerically for estimation of ET_g. Night-time water use by pine trees was comprehensively investigated using a combination of groundwater level, sap flow, tree growth, specific yield, soil matric potential and climatic variables measurements. Results reveal a constant average transpiration flux of 0.02 mm h⁻¹ at the plot scale from 23:00 to 05:00 during the study period, which verified the presence of night-time water use. The total ET_g for the period investigated was 259.0 mm with an accumulated T_n of 64.5 mm, resulting in an error of 25% on accumulated evapotranspiration from the groundwater if night-time water use was neglected. The results indicate that the development of commercial pine plantations may result in groundwater losses in these areas. It is also recommended that any future application of diurnal water table fluctuation based methods investigate the validity of the zero night-time water use assumption prior to use.