Modeling Soil Moisture Dynamics in Wetlands

Abstract

Soil moisture distribution and fluxes are among the fundamental components of the hydrological cycle. One-dimensional Richard’s Equation (hereafter RE) is often used to represent the physics of water moisture distribution and flow in soils by the forces of capillarity and gravity at the local-field and watershed scales. However, obtaining an accurate and efficient numerical solution of RE has remained challenging due to its high nonlinearity. In this study, a depth-averaged solution to RE was developed to advance the current knowledge of soil moisture modeling in the root zone and vadose zone. We first propose a two-layer approximation of RE, which describes vertically-averaged soil moisture content and flow dynamics in the root zone and the unsaturated soil below. The two-layer solution of RE converted the partial differential equation (PDE) of RE into two-coupled ordinary differential equations (ODEs) describing dynamic vertically-averaged soil moisture variations in the two soil zones subject to a deep or shallow water table in addition to variable soil moisture flux and pressure conditions at the surface. The two-layer model was evaluated using different soil textures, layer configurations, and field observation data. The results showed the robustness of the numerical model in tracking vertically-averaged moisture contents in the roots layer and the lower vadose soil. Next, we extended the two-layer solution of RE to a multiple layer-averaged solution of RE (LARE). LARE was evaluated by applying it to a field site. Results showed that the model provided accurate estimations of moisture contents for multiple soil layers, and it was computationally efficient in accounting for complex, dynamic prescribed boundary conditions without any convergence issues. Last, we integrated the two-layer solution of RE to a process-based biogeochemical model WetQual to simulate soil moisture dynamics in the variably saturated compartments of wetlands. We also improved the plant growth module in WetQual. Plant water uptake was specified for plants in the wetland environment. The primary productivity module was modified to consider environmental factors including temperature stress, water stress, and plant dormancy. The updated WetQual was evaluated by applying it to a restored wetland located on Kent Island, Maryland, USA, by two numerical experiments using different bottom boundary conditions for moisture movement in the variably saturated compartment. The results showed that different bottom boundary conditions applied in the variably saturated compartment had significant influences on NH4, TSS, and TOC transport and changed wetland N, C, and P budgets. The simulated plant biomass and nutrient uptake had very good agreements with the field measurements.