Sustainable water resource management using surface-groundwater modelling: Motueka-Riwaka Plains, New Zealand

New Zealand’s freshwater resources in coastal regions are under increasing pressure from demands for drinking, irrigation and industrial uses, and climate change effects, particularly sea level rise. The management of water resources needs to balance values associated with (but not limited to) economy, society, ecology, culture, recreation and water quality. To assist the management of water resources in the Motueka-Riwaka Plains, New Zealand, a numerical integrated surface-groundwater model was developed. Calibration of the transient integrated surface-groundwater model was achieved with a root mean square error (RMSE) of 2.6% for groundwater levels. Model verification against groundwater levels not used in the calibration also resulted in an RMSE of 2.6% for groundwater levels. Model outputs, coupled with groundwater trend analyses, were used to identify the impact of groundwater abstractions on rivers, streams and groundwater-fed springs. Trend analyses showed that groundwater levels are lowering in the central area of the plains. Model scenarios predict that additional groundwater abstraction of over 30,000 m³/day from a well field located near the central area of the plains (adjacent to the Motueka River) is unlikely to increase the risk of saltwater intrusion and will result in surface water flow depletion within acceptable limits.