Modelling groundwater abstraction scenarios using a groundwater-river interaction model of the Upper Motueka River catchment

The effects of various groundwater abstraction scenarios on stream flow and groundwater levels in the Upper Moteuka River catchment have been investigated using a groundwater-river interaction model. The model operated on a daily time step and had two components: a FEFLOW groundwater model to simulate groundwater losses and gains from the river, and a custombuilt river model to route river flow and to calculate river water levels using Manning's equation. For the 'base case', the model input included climate and abstraction data for the period 1 July 2001 to 30 June 2003, and the model was calibrated by adjusting hydraulic conductivity and streambed conductance to achieve a good match to observed groundwater levels, river flows and river water levels measured in this same time period. Thereafter the calibrated model was then used to predict groundwater levels, river flows and river water levels for six different scenarios: 1) actual groundwater abstraction at the end of 2008; 2) groundwater abstraction assuming the maximum take specified on each existing water permit; 3) groundwater abstraction assuming additional permitted takes sufficient to allow complete irrigation over all potentially irrigable land; 4) actual groundwater abstraction at the end of 2008 in roughly half of the model area combined with complete irrigation of all potentially irrigable land in the remainder of the model area; and 5) actual groundwater abstraction at the end of 2008 assuming river bed elevations were 0.3 m higher or lower than current elevations. For all scenarios, modelled mean and median river flows during the irrigation season were at most 5% less than for the base case, and differences outside the irrigation season were even smaller. Only the third of the tested scenarios resulted in modelled river flows that may breach minimum flow requirements at some locations. For all scenarios, modelled groundwater levels were no more than 1 m below the base case, with the largest drops occurring at four locations along the margins of the catchment and near pumping centres. The results suggest that a groundwater pumping scheme could incorporate existing and new abstraction while still ensuring that estimated low flow conditions are not breached during the irrigation season, and the scenario modelling results can be used by Tasman District Council for groundwater allocation management.