Depth-dependent soil water-salt relationships in the Hetao Irrigation District, arid Northwest China: Implications for sustainable management

Soil salinization threatens agricultural productivity in arid gravity irrigation systems worldwide, yet the depth-stratified relationships between soil water and salt content remain poorly characterized. We monitored soil moisture and electrical conductivity (EC) at six depths (0–100 cm) across 15 sites in the Hetao Irrigation District, China, during the 2025 growing season. Our results indicate a depth-dependent divergence in water–salt associations. The EC-derived salinity index varied from 0.08 to 42.91 g/kg, exhibiting extreme spatial heterogeneity (coefficient of variation (CV) = 185.2%) across the salinity gradient. However, moisture-salinity relationships showed marked depth dependency. Surface soils (0–20 cm) exhibited non-statistically significant correlations ( r  = 0.07), whereas deep layers (60–100 cm) showed a statistically significant but weak positive correlation ( r  = 0.25, p  < 0.01, R ²  = 0.06), indicating limited explanatory power. This pattern is consistent with a subtle tendency of downward salt redistribution. The EC-derived salinity index showed a 42% increase in deep layers (from 1.59 to 2.26 g/kg) over the season, compared to a 136% fluctuation in surface layers. These results highlight limitations of surface-focused monitoring approaches and indicate patterns consistent with subsurface salt redistribution. Compared with previous studies, this work provides a depth-resolved, multi-site observational dataset combined with a stratified analytical approach, offering additional evidence for interpreting soil salinity dynamics under heterogeneous field conditions.