Three-Dimensional Volume Averaged Soil-Moisture Transport Model: A Scalable Scheme for Representing Subgrid Topographic Control in Land-Atmosphere Interactions

Abstract

In this research, the 3-D Volume Averaged Soil-moisture Transport (VAST) formulation is derived from the Richards equation to incorporate the lateral flow and subgrid heterogeneity due to topography. A conjunctive surface-subsurface flow model at a large scale, a 1-D Diffusion Wave (DW) surface flow model interacting with the 3-D VAST model, is developed for the comprehensive terrestrial water and energy predictions in LSMs. For the implementation of the model, the mixed numerical scheme using a time splitting method is employed for each flow component. This research also focuses on the development and construction of appropriate Surface Boundary Conditions (SBCs) for mesoscale Regional Climate Model (RCM) applications. This conjunctive flow model is substituted for the existing 1-D scheme in the Common Land Model (CLM), one of state-of-the-art LSMs. The new coupled model (CLM+VAST) performance is investigated using the new SBCs and the North American Regional Reanalysis (NARR) forcing data in the off-line mode for a study domain around the Ohio Valley region. The simulation results show that the lateral and subgrid fluxes play a significant role in total soil-moisture dynamics, and the interaction between surface and subsurface flows and the flow routing scheme improve the runoff predictability significantly in the new model. The new coupled model using realistic SBCs can provide a full suite of modeling capability to characterize surface water and energy fluxes.