Spatiotemporal Dynamics and Topographic Controls of Soil Moisture on Dune Slopes in a Semi-Arid Sandy Region

In arid and semi-arid agroecosystems, soil water availability is a critical regulator of coupled carbon–water (C–W) cycling, vegetation dynamics, and ecosystem resilience under environmental change. This research investigated the temporal evolution and spatial patterns of soil moisture across sand dune slopes within the Mu Us Sandy Land. Data were collected via a combination of continuous high-frequency in situ monitoring spanning 20 months and manual sampling campaigns. We analyzed moisture levels at various depths and slope positions (windward vs. leeward) to understand their distribution and reaction to precipitation. Statistical analysis of all rainfall events that triggered measurable soil moisture responses showed that precipitation was the primary determinant of soil moisture fluctuations. Specifically, shallow soil (10 cm) reacts rapidly to rainfall events > 4.6 mm, whereas intermediate layers (20–50 cm) require > 8.6 mm. Conversely, deep soil moisture (>100 cm) remains stable, responding only to substantial storm events (>50 mm). Topography exerts a strong control over spatial variance; notably, slope toes consistently exhibit higher moisture than upper sections, particularly during wet seasons, indicating strong topographic control on moisture redistribution and possibly reflecting lateral subsurface transfer. Additionally, a nonlinear correlation was observed between mean moisture content and its variability, peaking under intermediate moisture conditions. The results provide a mechanistic basis for understanding agroecosystem responses to climate variability and offer valuable insights for adaptive land management, vegetation restoration, and hydrological modeling in water-limited regions.