Influence of Model Conceptualization on One-Dimensional Recharge Modeling —Uley South, Southern Eyre Peninsula, South Australia

Groundwater recharge quantification is one of the most fundamental prerequisites for water management. Groundwater recharge, however, is notoriously difficult to determine. One way to estimate recharge is through unsaturated zone modelling, which requires a sound conceptual understanding of the recharge processes, but the available data usually preclude the development of one, unique conceptual model. This study examines the effect of conceptual model uncertainty on the recharge obtained by one-dimensional modelling. The Uley South Basin, South Australia, was used as a case study, and different conceptual models of the unsaturated zone were considered. Models differed in their representation of the geological complexity as well as in their representation of flow process. Models were run for different vegetation types, soil thicknesses, and water-table depths. The results show that groundwater recharge is greatly influenced by the depth to the water table and by the vegetation type, and that the various models predict a wide range of recharge values, ranging from -600 to 200 mm/year, depending on the combination of processes and parameters considered. The different conceptual models, for equivalent simulations (i.e. equal conditions of vegetation type, depth to the water table and soil thickness), produced very different recharge rates – up to 60 mm/month in cases – and a different recharge distribution during the year. These outcomes can be used to assign uncertainty ranges to the recharge modelled, and help to identify which parameters need to be better constrained to arrive at more reliable recharge numbers.