What drives the decline in groundwater table? A story of Edendale in New Zealand

The Edendale terrace aquifer in Southland New Zealand has experienced a declining trend of groundwater table over the past two decades. Water abstraction has increased over this time and is associated with farming development, intensification and increased production of local industry. Coincident with an increase of groundwater abstraction is a decrease in annual precipitation. Current granted water allocation is ~55% of the allowable limit of 15% of land surface recharge. Determining the main driver of the declining groundwater table is a first step to improving the sustainability of water use in this area.

In this study, we combined a statistical method and physically based modelling method to analyse the main driving force. In the statistical method, the relationship between precipitation, groundwater abstract, and groundwater table over the past two decades have been analysed and the contributions from decreasing precipitation and increasing groundwater abstract were quantified. In the physically based method, a groundwater model (MODFLOW) was coupled with a hydrologic model (TopNet) to simulate the groundwater flow, and scenarios of groundwater abstract and precipitation were assessed using this coupled hydrologic model and groundwater model.

The modelling result above is used for sustainable water allocation management by the regional government, and this methodology can be used for groundwater management in other regions with declining groundwater table.