Representing the Interactions of Soil Moisture, Groundwater, and Biogeochemistry in Earth-System Models

Soil moisture is a crucial control on surface energy fluxes, vegetation properties, and soil carbon cycling. Its interactions with ecosystem processes are highly nonlinear across a large range. Nevertheless, Earth System Models (ESMs) generally only represent an average soil-moisture state in grid cells at scales of 50-200 km, and as a result are not able to adequately represent the effects of subgrid heterogeneity in soil moisture, particularly in regions with large wetland areas.

Several approaches have been attempted to address this limitation. Simple groundwater models are being included as a supplement to the 1-dimensional hydrology in ESMs, for instance by representing a uniform unconfined auqifer with a water-table depth state. This water-table depth can be used to diagnose the saturated fraction in a gridcell using TopModel, and this fraction can be used for some of the biogeochemical processes represented, such as methane fluxes. High-resolution, 3-dimensional groundwater models have also been coupled into climate models, but these require extensive input data and are prohibitive to run at a global scale. Ongoing work is attempting to include a reduced-order version of these models to enhance computational efficiency.

Another avenue is to represent subgrid heterogeneity explicitly. With collaborators, I included a representation of hillslope-scale topographic gradients, TiHy (Tiled-hillslope Hydrology), into the Geophysical Fluid Dynamics Laboratory (GFDL) land model (LM3). LM3-TiHy models one or more representative hillslope geometries for each gridcell by discretizing them into land model tiles hydrologically coupled along an upland-to-lowland gradient. Each tile has its own surface fluxes, vegetation, and vertically-resolved state variables for soil physics and biogeochemistry. In marginally wet regions around the globe, LM3-TiHy simulates shallow groundwater in lowlands, leading to higher evapotranspiration, lower surface temperature, and higher leaf area compared to uplands in the same gridcells. Moreover, more than four-fold larger soil carbon concentrations are simulated globally in lowlands as compared with uplands. With further improvements, the model could provide a new approach to investigating the vulnerability of Boreal peatland carbon in ESMs.