Drought poses serious threats to plant growth in water-limited regions and is expected to intensify under climate change. Understanding plant responses to drought is essential for developing effective water management strategies. Black locust (Robinia pseudoacacia L.) covers about 60 % of the Loess Plateau's forests and plays a key role in soil and water conservation. Understanding its drought response is essential for sustaining ecosystem stability amid increasing water limitations, but comprehensive research on these responses remains scarce. This study investigates the water use characteristics of black locust, through a long-term throughfall exclusion experiment simulating moderate (40 %, MD) and extreme (80 %, ED) drought treatments alongside an ambient condition (CK). Under CK conditions, black locust primarily extracted water from the shallow soil layer (0-30 cm, 50.5 %), whereas drought reduced shallow soil water uptake and increased the relative contribution from deeper soil layers (30-200 cm) to 63.0 % and 68.0 % under MD and ED, respectively. However, absolute water uptake from these deeper layers remained lower than under CK conditions. Both stable carbon isotope analysis and sap flow measurements revealed significantly higher intrinsic water use efficiency (WUEi) and lower transpiration under drought. Importantly, WUEi and transpiration exhibited clear nonlinear responses to increasing drought severity: transpiration decreased markedly from CK to MD but plateaued under ED, while WUEi increased under MD and showed no further rise at ED. By integrating transpiration and predawn leaf water potential (Psi pd) data across the three treatments, a fitting analysis revealed a significant exponential relationship, indicating a nonlinear response. A similar exponential pattern between WUEi and Psi pd further underscores the nonlinear physiological adjustments of black locust to drought stress. These nonlinear patterns highlight complex physiological adaptations of black locust to drought stress and provide crucial insights for sustainable water management and afforestation practices in semi-arid ecosystems.