Advancing Land-Surface Modelling in ecLand Through a Unified Hydro-Thermal Framework

Land surface models (LSMs) play a central role in simulating land-atmosphere interactions by representing coupled soil, vegetation, and energy-water-carbon processes. In most current LSMs, soil hydraulic and thermal properties are treated independently and are coupled only indirectly through soil moisture content, without an explicit linkage between the underlying parameters that define the shape of the curves characterising hydro-thermal properties. Although several independent studies have demonstrated strong correlations between soil hydraulic properties (SHPs) and soil thermal properties (STPs), these relationships have not yet been incorporated into land surface models to assess their impacts on land-surface states and fluxes.

For the present study, we developed a unified hydro-thermal framework for ecLand that explicitly integrates soil hydraulic and thermal properties, thereby improving the representation of coupled soil moisture and heat transport and associated land–atmosphere interactions. First, the van Genuchten (1980) soil water retention curve (SWRC) was replaced by formulations that explicitly represent adsorbed and capillary water components (e.g. Lu, 2016; Peters-Durner-Iden, 2024), leading to a more physically consistent description of soil hydraulic properties, particularly under dry soil conditions. Second, the thermal conductivity formulation of Peters-Lidard et al. (1998), currently used in ecLand, was replaced by an approach that directly links thermal conductivity to SWRC parameters (Lu & McCartney, 2024), ensuring a consistent coupling between soil hydraulic and thermal properties.

We first quantified the impacts of these developments on soil states (soil moisture and soil temperature at multiple depths) and land-surface fluxes (latent and sensible heat) using a series of controlled sensitivity experiments. These experiments were designed to isolate the response of the coupled hydro-thermal system to variations in soil texture, soil depth and discretization, and climatic regimes with an emphasis on more arid conditions. Through this sensitivity analysis, we examined how the unified hydro-thermal framework influences moisture–temperature feedbacks, vertical heat transport, and surface energy partitioning across contrasting hydro-climatic environments. The performance of the unified framework was then evaluated at selected in situ sites by comparing simulations from the original and updated ecLand configurations against observations of soil moisture, soil temperature, and latent and sensible heat fluxes. We find substantial differences between the two formulations in the simulated surface energy fluxes, particularly for soils with high sand content, where discrepancies in latent and sensible heat reach approximately 40 W m^-2 for loamy sand. Future work will focus on implementing this framework in global ecLand simulations to assess impacts on near-surface land states and surface fluxes, and subsequently in fully coupled land-atmosphere simulations within IFS to evaluate potential improvements in near-surface atmospheric variables (e.g. 2 m air temperature and relative humidity) and the reliability of ECMWF’s sub-seasonal to seasonal forecasts.