Challenges Calibrating Hydrology for Groundwater-Fed Wetlands: a Headwater Wetland Case Study.

This study aims to adapt the Soil and Watershed Assessment Tool (SWAT), a ubiquitously used watershed model, for ground-water dominated surface waterbodies by accounting for recharge from the aquifers. Using measured flow to a headwater slope wetland in Alabama's coastal plain region as a case study, we present challenges and relatively simple approaches in using the SWAT model to predict flows from the draining watershed and relatively simple approaches to model groundwater upwelling. SWAT-simulated flow at the study watershed was limited by precipitation, and consequently, simulated flows were several times smaller in magnitude than observed flows. Thus, our first approach involved a separate stormflow and baseflow calibration which included the use of a regression relationship between observed and simulated baseflow (E NASH = 0.67). Our next approach involved adapting SWAT to simulate upwelling groundwater discharge instead of deep aquifer losses by constraining the range of deep losses, ß deep parameter, to negative values (E NASH = 0.75). Finally, we also investigated the use of artificial neural networks (ANN) in conjunction with SWAT to further improve calibration performance. This approach used SWAT-calibrated flow, evapotranspiration, and precipitation as inputs to ANN (E NASH = 0.88). The methods investigated in this study can be used to navigate similar flow calibration challenges in other groundwater dominant watersheds which can be very useful tool for managers and modelers alike.

Modeling Nitrogen and Carbon dynamics in wetland soils and water using a mechanistic wetland model.

In this article, extension and application to variably-saturated wetland conditions of a process-based wetland model, namely WetQual is demonstrated. The new model described in this article is an improved version of an earlier model, which was only capable of capturing nutrient dynamics in continuously ponded wetlands. The upgraded model is capable of simulating nutrient cycling and biogeochemical reactions in both ponded and unsaturated zones of the wetland. To accomplish this goal, a comprehensive module for tracking water content in wetland soil was implemented in the model, and biogeochemical relationships were added to explain cycling of nitrogen (N) and carbon (C) in variably saturated zones of wetlands. The developed model was applied to a small, restored wetland receiving agricultural runoff, located on Kent Island, Maryland. On average, during the two year study period, the ponded compartment of the study wetland covered 65% of the total 1.2 ha area. Through mass balance analysis, it was revealed that the mass of nitrogen lost to denitrification at the variably saturated compartment of the study wetland was about 3 times higher than that of the ponded compartment (32.7 ± 29.3 kg vs. 9.5 ± 5.5 kg) whereas ammonia volatilization at the variably saturated compartment was a fraction of that of ponded compartment (1.2 ± 1.9 kg vs. 11.3 ± 11.8 kg). Sensitivity analysis showed that cycling of carbon related constituents in variably saturated compartment had high sensitivity to temperature and available soil moisture.

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

A common phenomenon observed in natural and constructed wetlands is short-circuiting of flow and formation of stagnant zones that are only indirectly connected with the incoming water. Biogeochemistry of passive areas is potentially much different than that of active zones. In the research reported in this paper, the spatial resolution of a previously developed wetland nutrient cycling model was improved in order to capture the spatial variability of concentrations and reaction rates regarding nitrogen and carbon cycles throughout active and passive zones of wetlands. The upgraded model allows for several compartments in the horizontal domain, with all neighboring compartments connected through advective and dispersive/diffusive mass transport. The model was applied to data collected from a restored wetland in California that was characterized by the formation of a large stagnant zone at the southern end of the wetland due to close vicinity of the inlet and outlet structures in the northern end. Mass balance analysis revealed that over the course of the research period, about 23.4±3.9% of the incoming total nitrogen load was removed or retained by the wetland. It was observed that mass of all exchanges (physical and biogeochemical) regarding nitrogen cycling decreased along the activity gradient from active to passive zones. Model results also revealed that anaerobic processes become more significant along the activity gradient towards passive areas.