Unintended consequences to groundwater from improved irrigation efficiency: Lessons from the Hinds-Rangitata Plain, New Zealand

Abstract

In many countries, including New Zealand, limits on freshwater availability have led to a push toward improved irrigation efficiency via changes in irrigation application technologies, so as to increase the ‘crop per drop’. In this paper, we provide evidence of unintended consequences to groundwater arising from improved irrigation efficiency in parts of the Canterbury Plains, New Zealand. We compare groundwater levels, groundwater nitrate concentrations and stable isotope values from before and after the change in irrigation efficiency. We show that changes in irrigation methods from low efficiency border-dyke irrigation to high efficiency sprinkler irrigation has resulted in reduced recharge to groundwater and reductions in groundwater levels. Groundwater level reductions of up to 17 m have occurred in some areas. The reduced groundwater levels have impacted spring flows and required groundwater users to drill new wells to access the now deeper groundwater. The reduction in irrigation recharge has also likely contributed to increased nitrate concentrations in groundwater. In response to the unintended consequences documented in this study, managed aquifer recharge is being implemented as a means of augmenting groundwater levels, quality, and spring-fed waterways. As such, increased irrigation efficiency may not be reducing water use in this region. A regional water balance would be required to determine if water savings are occurring overall.