Root water uptake (RWU) is a critical ecohydrological process, yet its quantification under field conditions remains challenging. Profile soil moisture dynamics offers a viable approach to estimating RWU via analysing diurnal moisture fluctuations. While validated in humid forests, the applicability of this method in other environments, such as shrubs in semi-arid areas, remains underexplored. Focusing on the Chinese Loess Plateau (CLP), a key region for ecological restoration, we evaluated the utility of soil moisture to quantify RWU for Vitex negundo, a dominant xerophytic shrub. Soil moisture was monitored at two profiles (50 and 130 cm from the shrub) across five root-zone depths during the 2022 growing season. By carefully revising the established method, we developed three methods to quantify daily shrub RWU dynamics and validated them against sap flow measurements. Results revealed that the method incorporating fixed RWU start/end times and daytime soil moisture trends outperformed others and was recommended for follow-up studies. Our findings validate the efficacy of profile soil moisture data for quantifying shrub RWU in semi-arid environments and identify key influencing factors, that is, soil moisture status and sensor placement. RWU estimates from the 50-cm profile exhibited a stronger correlation with sap flow (r = 0.63) compared to the 130-cm profile (r = 0.22). This correlation further intensified under high soil moisture conditions without precipitation (r = 0.80 vs. 0.34). This study advances methodological frameworks for RWU estimation in water-limited ecosystems, offering insights to enhance ecological restoration on the CLP.