Changes in the hydrological regime of the Wairau Plains Aquifer

Abstract

The Wairau Aquifer covers a small proportion of the Wairau catchment in the Marlborough District of New Zealand just prior to the river discharging into the sea. The aquifer is almost exclusively recharged by surface water from the Wairau River and serves as the major resource for drinking water and irrigation in the region. A constantly declining trend in aquifer levels and spring flows have been observed over the past decades, which triggered a range of investigations on the Wairau Plains but also for the entire Wairau catchment (3430km²). The current state of knowledge shows a strong relation between declining trends in aquifer levels and similar trends in Wairau river flows. Experimental evidence and numerical modelling of the interacting river–groundwater exchange flows suggest that the river is perched in the upstream regions and that aquifer recharge is particularly sensitive to an increase of days with extreme low river flows. Since the Wairau River flows and therefore also the fate of the Wairau Plains aquifer are strongly linked to the hydrological processes in the entire catchment, investigations are under way to analyse the hydrological regime and potential trends or changes thereof on the catchment scale. The analysis of a 50-year period of precipitation from NIWA’s Virtual Climate station network already revealed that there is potentially also a declining trend in annual mean catchment precipitation. However, questions remain due to uncertainties in the hydrological data and concurrent land use changes in some parts of the catchment.