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

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, R2 = 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.