Wetlands are defined as “those areas that are inundated or saturated by surface or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas” (Tiner, 1996). From a hydrologists’ perspective, the functioning of a wetland may be characterised by the calculation of a water mass balance. The total water budget of a wetland may include components such as precipitation, open water evaporation, and transpiration by vegetation as well as surface water inputs and outputs, which may include environmental and/or managed flows. In areas where shallow water tables are present, such as the South East region, interactions with groundwater may be a significant additional component of a wetland water balance. The aim of the present work was to develop an approach to simulating interactions between Lower Limestone Coast Prescribed Wells Area wetlands and underlying shallow groundwater; specifically, to translate water table variations near a wetland into changes in wetland water level. The industry standard groundwater flow simulation code MODFLOW (Hanson et al., 2014) was used as the basis for this approach. Methods of representing significant components of the water mass balances for both wetland and groundwater domains were assessed. For a wetland, these included precipitation on a wetland catchment area, evaporative losses from inundated areas and evapotranspiration losses from non-inundated areas, as well as additions and losses via both surface water and groundwater flows. For the groundwater domain, significant water mass balance components included lateral flows, including vertical leakage; evapotranspiration from shallow water tables; wetland–groundwater interactions; and changes in aquifer storage volumes. Each of these components was characterised as a time-varying flux. Of particular novelty was the combined approach used to represent recharge and evapotranspiration, which can be represented as a net flux from groundwater rather than by following the traditional approach of compartmentalising the two fluxes. Methods of extracting relevant information from model outputs were also addressed, including producing statistical summaries of wetland surface water persistence and calculating a simple salinisation risk metric in lieu of solute transport simulation.The utility of the approach developed was demonstrated using a synthetic dataset, in lieu of outputs from the regional groundwater flow model for the region that is currently under development, which was unavailable for testing at the time of writing. Results of the synthetic demonstration, including the calculated salinisation risk metric, indicated the potential for managed surface water additions to negate the effects of long-term water table decline on the persistence of wetland surface water levels. Potential means of linking the wetland–groundwater interaction modelling approach to the regional groundwater flow model are also discussed. In future, the modelling approach described here could be used to identify the conditions which may lead to significant changes in wetland hydrological conditions, or to identify the timing of such changes for long-term variations in climate, land use and/or water allocation policy in the region.
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