Integrating a subgrid horizontal tracer transport module in HydroBlocks land surface model

HydroBlocks provides a generalized framework for representing subgrid heterogeneity in land surface models. High-resolution environmental data and a hierarchical multivariate clustering scheme are employed to aggregate field-scale grid cells with similar characteristics into coherent Hydrologic Response Units (HRUs), over which the model physics is then simulated individually. This customizable clustering process enables HydroBlocks to approximate fully distributed simulations while maintaining computational efficiency. By preserving the geographic locations of tiles, the model allows for spatially explicit interactions among HRUs. Currently, HRU connectivity has been explored only via subsurface flow, where interactions occur within adjacent height bands. However, connectivity in the lower atmospheric boundary layer is not yet accounted for, despite its critical role in processes like the heat and moisture advection, wildfire spread, and the transport of snow, pollen and dust.

To bridge this gap, we propose three possible approaches for calculating horizontal fluxes between HRUs: the Eulerian Connectivity Matrix (ECM), the Lagrangian Connectivity Matrix (LCM), and the Lagrangian Particle Tracking (LPT). While LPT offers the highest accuracy consistently, it is computationally demanding. In contrast, the performance and computational cost of ECM and LCM are highly dependent on HRU configurations and wind field characteristics. These sensitivities can be brought under control by adjusting the number of HRUs connected, calling for a formal process to determine the optimal parameters for each scheme. To this end, a comprehensive evaluation of ECM and LCM under various HRU configurations and wind conditions is conducted, using LPT as a benchmark. A decision-support model is built accordingly to guide the selection of three approaches and determine appropriate parameter ranges. Incorporating a subgrid horizontal tracer transport scheme into HydroBlocks offers an effective pathway to enhance the representation of spatio-temporal dynamics in land surface modeling.