Exploring the Hidden Interplay: Moisture and Vegetation Dynamics in the Nabkhas of Omani Coastal Dunes

Soil moisture content is a critical factor in the hydrological cycles of terrestrial ecosystems, especially in sandy environments. The spatial variation in soil water content is influenced by both dynamic and static factors. To understand the ecohydrology of desert environments, a detailed analysis of the vadose zone and topsoil in coastal dunes is essential. This study focuses on the soil hydrology of coastal mini-dunes (Nabkhas) in the Al-Hail North area of Oman, particularly examining moisture redistribution following a 13 mm rainfall event. The area, characterized by a sabkha landform with a shallow water table (approximately 1.4 meters below the surface), is interspersed by an array of Nabkhas. The length and height of three Nabkhas (N1, N2, and N3) were measured. Native plants were present in all Nabkhas: Haloxylon salicornicum in N1 (alive) and N3 (dead), and Salvadora persica in N2. Soil samples were collected from the interdune and Nabkha cores for grain size analysis. Decagon EC-05 sensors were installed at depths of 0 and 20 cm in the vertical profiles of N1, N2, and N3 to monitor diurnal variations in volumetric water content (ϴ_v).

A significant increase in ϴ_v in the top sensor immediately after the rain event was detected, while the bottom sensor showed a minimal increase over time. The top sensor's ϴ_v peaked at 0.1 m³/m³ on the last day of the rain event, then decreased to 0.054 m³/m³ after 8 days due to evaporation. The bottom sensor's ϴ_v reached a maximum of 0.58 m³/m³ on the final recording day. The spatial and temporal variation in ϴ_v is also influenced by vapor condensation from humid air and around native shrubs. High moisture content in the top layers of dunes significantly impacts vegetation patterns.

Another field investigation examined soil moisture variability using excavated profiles at four locations, including three sites under Nabkhas and a vegetation-free control plot. Analysis of volumetric water content demonstrated clear moisture stratification throughout the profiles. Near-surface soil layers showed minimal moisture levels, consistent with the residual water content (θ_r) typical in desert sandy soils. Moving downward through the profile, a significant increase in moisture content was detected, with lower horizons reaching near-saturation conditions (θ_s). This enhanced water retention in deeper layers was associated with both finer soil textures and water table influence. Such moisture-rich deeper soil zones appear to provide continuous capillary water movement to Nabkha vegetation root systems, enabling water redistribution throughout the soil-vegetation-atmosphere interface.

This study contributes to the conservation/restoration of desert vegetation and understanding the resilience of small-scale soil-water-plant ecosystems in arid regions. Further research on soil properties, water availability, and microclimate close to Nabkhas is necessary better to comprehend plant distribution and functioning in these landforms.