Abstract: Soil water availability in loess landscapes is strongly modulated by hillslope topography, yet how slope positions shape plant water uptake strategies remain poorly resolved. This study quantified the water sources of Robinia pseudoacacia across six slope positions along a typical loess gully hillslope using stable isotopes and in situ monitoring. The results revealed that soil water content varied nearly twofold along the hillslope, declining from 18.6 % in the riparian zone to 11.1 % at the shoulder line, with persistent dried soil layers at 300 to 500 cm depth on the tops of loess ridges, shoulder lines, and gully slopes. While shallow water uptake of R. pseudoacacia increased across all slope positions during the rainy season, long term water source partitioning diverged markedly. It relied primarily on shallow 0~100 cm soil water on the tops of loess ridges, loess plain ridges, and slope heel lines, whereas those on shoulder lines and gully slopes shifted toward water from 40~200 cm. In the riparian zone, it consistently accessed water from below 200 cm. Partial least squares path modeling identified topography and fine root traits as the primary direct drivers of water source partitioning, together explaining 69 % of the variance. These findings underscore pronounced plasticity in the water use of R. pseudoacacia across slope positions, with uniform deep rooted afforestation should be avoided in loess gully hillslope. A targeted revegetation strategy is therefore essential: drought tolerant shrubs on dry ridges and gully slopes, with deep rooted trees reserved for riparian zones in loess gully hillslope.

Key words: Slope positions, Soil water, Plant water uptake, Robinia pseudoacacia, Loess gully region