Alfalfa (Medicago sativa) pasture sustainability in China's Loess Plateau faces critical challenges from water scarcity and productivity decline in older stands. This two-year study (2020-2021) examined root distribution and morphology, biomass allocation, and soil water storage (SWS) dynamics across eight alfalfa stands aged 3-19 years (M3-M19), with annual cropland as control. Key findings revealed distinct ontogenetic shifts: in younger stands (<10 years, M10-), fine roots (Root length density = 0.18 cm cm(-3)) predominated in shallow layers (0-2 m), constituting 92.5 % of root systems, while coarse roots increased to 15.5 % in older stands (>= 10 years, M10+). Notable vertical redistribution occurred in M10+, with fine root density increasing 62.5 % in deep soil layers (5-10 m) compared to M10-. Soil water deficit reached critical levels (740.1-822.2 mm in 0-10 m profile), with older stands exhibiting 57.5 % greater deep soil layer water depletion. Productivity declined sharply in M10+, showing 46.9 % lower aboveground biomass (peak yield at M6) despite stable belowground biomass. Multivariate analysis identified significant (p < 0.01) correlations between root traits (fine/coarse root ratio) and water depletion patterns, modulated by stand age and soil depth. These results demonstrate that stand rejuvenation before six years optimizes water-use efficiency and yield stability in this fragile ecosystem.