Soil vapor transport improves soil moisture simulations in drylands

Institute of Atmospheric Physics, Chinese Academy of Sciences

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Half-hourly observed (Obs) and simulated (Sim-vapor) soil moisture at two depths and latent heat flux at site US-SRM in 2007 by CLM5 with the soil vapor transport scheme

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Credit: Lv Bingrong and Zhang Xia

Soil moisture is a key variable in the Earth system, influencing evapotranspiration, surface energy exchange, vegetation activity, and climate feedbacks. Yet in arid and semi-arid regions, land-surface models often struggle to realistically represent soil drying. A common issue is that shallow soils are simulated as wetter than observed, which can affect the representation of drought and land–atmosphere interactions.

 

To address this problem, researchers from the Institute of Atmospheric Physics, Chinese Academy of Sciences, introduced a simplified soil vapor transport scheme into the Community Land Model version 5 (CLM5). The study was recently published in Atmospheric and Oceanic Science Letters.

 

The research targets a long-recognized limitation in conventional land-surface models. Under extremely dry surface conditions, upward liquid water movement becomes very weak or nearly ceases. As a result, near-surface soil layers may dry too slowly in models, leading to unrealistic wet biases.

 

The revised model incorporates an additional pathway: the movement of water vapor through soil pores. Even when liquid transport is strongly suppressed, moisture can still move upward in vapor form. By representing this process, the model produces more realistic drying in near-surface soil layers.

 

“When the surface soil becomes very dry, liquid water pathways can nearly shut down, but moisture does not stop moving altogether. Water vapor can still move through soil pores, and this often overlooked process has important effects on dryland soil moisture simulations,” says Dr. Xia Zhang, corresponding author of the study.

 

The improvement extends beyond soil moisture. Simulations with the revised model show better performance in latent heat flux, including stronger daytime evapotranspiration during dry periods and reduced excessive nighttime condensation compared with the original model. These results suggest that representing soil vapor transport can improve simulations of land–atmosphere exchange under water-limited conditions.

 

The study demonstrates that soil vapor transport is not a negligible process in dry environments, but a key physical mechanism influencing the development of drought conditions in model simulations. Incorporating this pathway may therefore help alleviate persistent wet biases in arid and semi-arid regions.

 

These findings point to a practical pathway for improving drought representation and land–atmosphere coupling in next-generation land-surface and Earth system models. As dryland processes become increasingly important for understanding climate variability and change, a more realistic representation of soil moisture transport may contribute to more reliable simulations of both regional hydrology and climate.

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Journal

Atmospheric and Oceanic Science Letters

DOI

10.1016/j.aosl.2026.100822