Bidirectional Coupling of the Plant Canopy Model and Land Surface Model in PALM: A New Module for Improved Soil-Canopy-Atmosphere Interactions

Abstract

ZÁKLADNÍ ÚDAJE: PMC26 Book of Abstracts. Hannover: Leibniz University Hannover, German Weather Service, 2026. s. 30-31. [PMC 26: PALM Model Conference. 23.02.2026-26.02.2026, Hannover]. ABSTRAKT: Urban vegetation plays a critical role in regulating the urban microclimate through transpiration, shading, and energy partitioning. However, in the current PALM model system (version 25.04 and earlier), the Plant Canopy Model (PCM) and Land Surface Model (LSM) operate independently with no exchange of soil moisture information or root water uptake feedback. The PCM uses hardcoded soil moisture values for transpiration calculations, while the LSM handles vegetation transpiration separately through its own parametrisations. This decoupling limits the model’s ability to realistically simulate plant-soil-atmosphere interactions, particularly under water-stressed conditions. This presentation introduces a new coupling module (pcm_lsm_coupling_mod) that establishes bidirectional communication between PCM and LSM in PALM. The module enables: (1) PCM access to actual soil moisture from LSM soil layers for transpiration calculations, (2) root water extraction feedback from PCM transpiration to LSM soil moisture tendencies, (3) computation of Root Area Density (RAD) from Leaf Area Density (LAD) using allometric relationships, and (4) configurable water stress functions including basic threshold stress, the widely-adopted Feddes piecewise linear stress function, and compensatory root water uptake. This implementation follows established approaches: the ECMWF IFS methodology (equation 7.69) for root water extraction distribution, the Feddes et al. (1978) stress reduction function for soil moisture limitations, and Jackson et al. (1996) root distribution profiles for vegetation-type dependent root characteristics. The module integrates into PALM through modifications to module_interface.f90, plant_canopy_model_mod.f90, and land_surface_model_mod.f90, using pointer-based data passing to avoid circular dependencies. Users enable the coupling through a new namelist (pcm_lsm_coupling_parameters) that provides control over stress formulation selection, allometric coefficients, and feedback mechanisms. Comparative simulations using an urban environment test case demonstrate that PCM-LSM coupling produces measurable changes in transpiration rates (+3-6% depending on stress formulation), near-canopy air temperature (-0.27 to -0.33 K cooling within vegetated areas), and soil moisture dynamics. The Feddes stress function produces the strongest coupling effects with physically-based soil moisture response, while basic stress offers a computationally efficient alternative for production simulations. Performance analysis shows that basic stress adds negligible computational overhead, while the full Feddes implementation increases runtime by approximately 35% due to additional per-layer calculations. Future development priorities include diagnostic output for stress factors, crop-specific Feddes parameters, dynamic root growth capabilities, and validation against eddy-covariance flux measurements.