Nutrient Dynamics in Flooded Wetlands. I: Model Development

Abstract

Wetlands are rich ecosystems recognized for ameliorating floods, improving water quality, and providing other ecosystem benefits. This part of a two-paper series presents a relatively detailed process-based model for nitrogen and phosphorus retention, cycling, and removal in flooded wetlands. The model captures salient features of nutrient dynamics and accounts for complex interactions among various physical, biogeochemical, and physiological processes. The model simulates oxygen dynamics and the impact of oxidizing and reducing conditions on nitrogen transformation and removal, and approximates phosphorus precipitation and releases into soluble forms under aerobic and anaerobic conditions, respectively. Nitrogen loss pathways of volatilization and denitrification are explicitly accounted for on a physical basis. Processes in surface water and the bottom-active soil layer are described by a system of coupled ordinary differential equations. A finite-difference numerical scheme is implemented to solve the coupled system of ordinary differential equations for various multiphase constituents' concentrations in the water column and wetland soil. The numerical solution algorithm is verified against analytical solutions obtained for simplified transport and fate scenarios. Quantitative global sensitivity analysis revealed consistent model performance with respect to critical parameters and dominant nutrient processes. A hypothetical phosphorus loading scenario shows that the model is capable of capturing the phenomenon of phosphorus precipitation and release under oxic and anoxic conditions, respectively

Capturing Spatial Variability of Biogeochemical Mass Exchanges and Reaction Rates in Wetland Water and Soil through Model Compartmentalization

“…The vertical soil layer is partitioned into aerobic and anaerobic zones where the boundary between the two zones fluctuates up or down based on competing oxygen supply and removal rates. Readers are referred to Hantush et al (2013), Kalin et al (2013), and Sharifi et al (2013) for more detailed information regarding model structure, governing equations, and case study applications of WetQual.…”

“…However, diffusion and dispersion are modeled in a similar manner, thus their effects are considered collectively in a mass transport term as dispersion/ diffusion. Similar to Hantush et al (2013), for the dispersive/ diffusive mass transfer rate across the i; j boundary, β i:j can be obtained by conserving mass flow in the schematic compartmental system depicted in Fig. 2.…”

“…In the research reported in this paper, the spatial resolution of a previously developed wetland nutrient cycling model, namely the WetQual model (Sharifi et al 2013;Hantush et al 2013), was improved in order to capture the spatial variability of concentrations and reaction rates regarding nitrogen, phosphorus, and carbon cycles throughout active and passive zones of wetlands. A previous version of the WetQual model was comprised of three compartments in the vertical domain (z), including (1) standing water, (2) aerobic soil, and (3) anaerobic soil, and lumped in the horizontal domain (xy-plane), thus qualifying WetQual as a quasi-onedimensional (1D) model.…”

Carbon dynamics and export from flooded wetlands: A modeling approach

“…Once l 1 is computed, the thickness of the lower anoxic layer would be l 2 = H − l 1 where H is the thickness for active sediment layer [L]. Refer to Hantush et al (2012) for more details on oxygen dynamics in WetQual model. Definitions for rest of the parameters are either presented earlier or could be found in Table 1.…”

“…The named scheme was previously employed and explained by Hantush et al (2012). The selected numerical integration time step is t = 0.01 day, however to save memory storage, results are aggregated to daily averages.…”