In groundwater models of regions with shallow groundwater, piezometric surfaces and fluxes to groundwater dependent ecosystems (GDEs) can be highly sensitive to the parameterisation of recharge and evapotranspiration (ET) processes. For most regional scale models however, recharge has typically been conceptualised based on a percentage of rainfall, while ET is often defined by an arbitrarily defined maximum ET rate, declining to zero with increasing depth to water table. While shallow groundwater processes require an improved conceptualisation, over-parameterisation and increased computational time are challenges for unsaturated zone – groundwater model coupling.
A regional scale groundwater model was developed for part of the South East of South Australia using FloPy, a Python based interface for MODFLOW 2005. The FloPy interface allowed for transparency and reproducibility in the modelling and facilitated rapid and flexible model development that could easily couple with other models or site-specific processes. A lookup table approach for modelling net recharge (gross recharge minus ET from groundwater) was established from previous research and incorporated into the transient FloPy groundwater model. The lookup table was developed from unsaturated zone modelling using WAVES and was based on climate data, soil clay content mapping and the time-varying South Australian Land Cover Layers. This allowed the complex unsaturated zone behaviour to be captured, without significantly increasing the number of model parameters.
The modelling resulted in similar calibration performance for groundwater heads as the regional model, but with a water table that better followed the land surface between the observation bores. Sensitivity of modelled groundwater heads to recharge and evapotranspiration parameters varied, depending on depth to water table and proximity to drains and other boundary conditions. Groundwater heads near GDEs were very sensitive to recharge and ET parameterisation.