Abstract: Groundwater evapotranspiration (ETg), as a critical discharge component of groundwater, has long been a key research focus in groundwater resource assessments. Due to the ease of obtaining groundwater dynamics data, the method of estimating ETg using diurnal groundwater fluctuations (White method) has attracted widespread attention from researchers. However, traditional White method and its variants face limitations in accurately quantifying groundwater recovery rates due to the mechanistic complexity of delayed water table recovery caused by phreatic water release driven by ETg. This study innovatively combines the analytical solution of the Boussinesq equation with dynamic harmonic regression analysis, avoiding direct calculation of groundwater recovery rates while enabling rapid, long-term ETg estimation at the same frequency as groundwater observations. Applying this method to estimate ETg during the Tamarix shrub growing season (August to September, 2023) in the Golmud River Catchment. The results show that: ① During the monitoring period, groundwater levels exhibited typical diurnal dynamics characteristic of groundwater-dependent vegetation, marked by nocturnal recovery and daytime decline. ② Cumulative ETg estimated by this method reached 55.96 mm in August and 88.14 mm in September, showing strong consistency with conventional ETg estimation approaches in the Golmud River Basin, with a mean deviation below 27%. ③ The groundwater specific coefficient (K_c,gw), calculated using water table amplitude, aligns with the growth cycles of Tamarix, reflecting general patterns of groundwater depletion by plant transpiration. However, the introduction of this dimensionless coefficient indicates potential for further refinement. ④ The core challenge of this time-series-based estimation method lies in identifying and extracting water table fluctuations solely attributable to ETg. Building on prior research estimating ETg for Salix shrubs in the semi-arid Hailiutu River Basin, this approach demonstrates applicability across both arid and semi-arid regions of northwestern China.