Wang et al. (2025) Beyond the Surface: Understanding Salt Crusts’ Impact on Water Loss in Arid Regions

Identification

• Journal: Land
• Year: 2025
• Date: 2025-10-10
• Authors: You‐Nian Wang, Zhiwei Li, Shuaiyu Wang, Chengzhi Li
• DOI: 10.3390/land14102028

Research Groups

• College of Ecology and Environment, Xinjiang University, Urumqi 830046, China
• Key Laboratory of Oasis Ecology Ministry of Education, Xinjiang University, Urumqi 830046, China
• Xinjiang Jinghe Observation and Research Station of Temperate Desert Ecosystem, Ministry of Education, Bole 833300, China

Short Summary

This study experimentally investigated how salt crust morphology and coverage, regulated by sand mulching thickness, influence soil moisture evaporation in arid regions. It found that while high-coverage crusty salt crusts inhibit evaporation, medium-coverage patchy crusts and internal "salt tree" structures can promote it, challenging the conventional understanding that salt always suppresses evaporation.

Objective

• To elucidate the mechanisms by which salt crusts influence soil moisture evaporation.
• To generate distinct salt crust morphologies (crusty, patchy) by applying six sand mulching thicknesses (1 cm, 2 cm, 3 cm, 4 cm, 5 cm, 6 cm).
• To quantitatively monitor the dynamic evolution of salt crust coverage (SCC) and its coupling with soil moisture evaporation.
• To systematically elucidate how NaCl salt crust morphology and SCC regulate evaporation.
• Hypothesis: A morphological transition from a crusty to a patchy salt crust, induced by a critical sand mulching thickness, would fundamentally alter evaporation dynamics, shifting them from suppression to promotion.

Study Configuration

• Spatial Scale: Indoor soil column evaporation experiments using columns 20 cm in height and 10 cm in diameter, with varying sand mulching thicknesses (1 cm to 6 cm).
• Temporal Scale: The experiment lasted 13 days.

Methodology and Data

• Models used: None explicitly used for analysis; findings contribute to refining regional hydrological models.
• Data sources:
  • Soil moisture evaporation rates measured hourly using electronic balances (accuracy 0.01 g).
  • Salt crust coverage (SCC) and morphology recorded hourly by a high-definition camera and daily by a vertical microscope, processed using Adobe Photoshop.
  • Salt crust thickness measured with calipers.
  • Indoor temperature (average 23 °C) and relative humidity (approximately 30%) recorded by a Vaisala sensor and Campbell Scientific data logger.
  • Experimental materials: Fine sand (100–200 µm) and coarse sand (500–800 µm) for mulching.
  • Brine solution: 4 mol/L NaCl aqueous solution.

Main Results

• Sand mulching significantly reduced soil moisture evaporation; total evaporation decreased by 85.6% for deionized water and 72.4% for brine as mulching thickness increased from 1 cm to 6 cm.
• At a 4 cm sand mulching thickness, the brine soil column exhibited higher total evaporation (44.42 g) than the deionized water column (34.71 g), indicating a promotion of evaporation (maximum promotion rate of 54.15% on Day 4).
• The effect of salinity on evaporation varied with salt crust morphology and coverage:
  • High-coverage crusty crusts (1–2 cm sand mulching): Formed with SCC ranging from 92.62% (1 cm) to 55.93% (2 cm), significantly inhibited evaporation, with a maximum inhibition rate of 60.91% at 1 cm mulching.
  • Medium-coverage patchy crusts (3–4 cm sand mulching): Promoted evaporation, with a maximum promotion rate of 54.15% at 4 cm mulching and 33.61% at 3 cm mulching. However, excessively low (e.g., 0.42%) or high (e.g., 50.14%) SCC could inhibit evaporation.
  • Non-crusting weak influence (5–6 cm sand mulching): No significant surface salt crust formed. Internal "salt tree" structures developed, shortening the water vapor diffusion path and promoting evaporation in the late stage (days 11–13), with a promotion rate of 36.73% at 6 cm mulching.

Contributions

• Challenges the conventional understanding that "salt inhibits evaporation" by demonstrating conditions (e.g., patchy crusts, "salt trees") where salt can promote soil moisture evaporation.
• Provides novel mechanistic parameters for accurately quantifying evaporation fluxes in saline soils.
• Enhances the understanding required for refining regional water cycle models, particularly the module related to atmosphere–soil water vapor exchange.
• First study to observe distinct salt crust morphologies (crusty and patchy) forming on soil with the same coarse particle size, regulated by sand mulching thickness.

Funding

• National Natural Science Foundation of China (grant number 42267043)
• “Key Research and development projects of Xinjiang Uygur Autonomous Region” (grant number 2022B03025-5)

Citation

@article{Wang2025Beyond,
  author = {Wang, You‐Nian and Li, Zhiwei and Wang, Shuaiyu and Li, Chengzhi},
  title = {Beyond the Surface: Understanding Salt Crusts’ Impact on Water Loss in Arid Regions},
  journal = {Land},
  year = {2025},
  doi = {10.3390/land14102028},
  url = {https://doi.org/10.3390/land14102028}
}