Characterisation of groundwater-surface water interaction using field measurements and numerical modelling: A case study from the Ruataniwha Basin, Hawke's Bay, New Zealand

Shallow aquifer systems overlain by rivers constitute normally one hydrogeological entity, because of the interconnection between aquifers and surface water. On the one hand, groundwater abstraction in such aquifer systems may deplete streams. On the other hand, overexploitation of surface water may result in a drop in groundwater level and adverse effects on the environment. It is important, therefore, to understand the relation between rivers and aquifers and to quantify the loss-gain relationship between them. This will help establishing a better water resources management and to reduce or prevent impacts on the environment. In this study, historical rivers flow data in the Ruataniwha Basin in New Zealand has been used to simulate groundwater-surface water, using the finite difference-based MODFLOW model. The model results were checked against six runs of recent concurrent gauging covering the whole basin. The numerical model results show that rivers and aquifers relation varies spatially from one location to another. Quantitatively, rivers gain from the aquifer system much more than what they lose. These results are consistent with the concurrent gauging data.