Two-way coupling between sub-grid river networks and the land surface in Earth system models

Abstract:

Over the past decade, there has been appreciable progress towards modeling the water, energy, and carbon cycles at field-scales (10–100 m) over continental to global extents. One such approach, named HydroBlocks, accomplishes this task while maintaining computational efficiency via sub-grid tiles, or Hydrologic Response Units (HRUs), learned via a hierarchical clustering approach from available global high-resolution environmental data. However, until now, there has yet to be a macroscale river routing approach that is able to fully leverage HydroBlocks’ approach to sub-grid heterogeneity, thus limiting the added value of field-scale land surface modeling in Earth system models (e.g., riparian zone dynamics, irrigation from surface water, and interactive floodplains). This presentation introduces a novel river routing scheme that is intertwined with HydroBlocks' modeled field-scale land surface heterogeneity. This is achieved through very high-resolution sub-grid stream network topologies, reach-based river routing, the simulation of floodplain dynamics, and the two-way coupling between the river reaches and their corresponding sub-grid tiles.

The primary features of the routing scheme in the HydroBlocks land surface model include: 1) each macroscale grid cell’s own fine-scale river network is derived from very high resolution (<100 m) DEMs; 2) the inlet/outlet reaches of each macroscale grid cell are linked to assemble the continental river networks; 3) the river dynamics are solved at a reach-level via the Kinematic wave assumption of the Saint-Venant equations; 4) a two-way coupling is established between each tile and the river network. To implement and test the novel approach, we used a 1.0-degree bounding box surrounding the Atmospheric Radiation and Measurement (ARM) Southern Great Plains (SGP) site in Northern Oklahoma (United States). The results show: 1) the implementation of the two-way coupling between the land surface and the river network leads to appreciable differences in the simulated spatial heterogeneity of the surface energy balance; 2) a limited number of tiles (~300 per 0.25-degree cell) are required to approximate the fully distributed simulation adequately; 3) the surface energy balance partitioning is sensitive to the river routing model parameters. The resulting routing scheme provides an effective and efficient path forward to enable a two-way coupling between the modeled very high-resolution stream networks and existing tiling schemes within Earth system models.