Managed aquifier recharge: Sustainable water resource management on the Canterbury plains

Demand on water resources for irrigated agriculture on the Canterbury Plains is strong. Against a backdrop of tightening regulatory controls, demand for water has not abated, which is perhaps unsurprising given the significant economic benefits to be generated within the region from irrigated agriculture. Current demand is tangibly impacting on surface water resources in some places and there is concern about unrealised impacts from consented takes that have not yet been exercised.

Cumulative impacts from the various surface water and groundwater abstractions are the key issue associated with water allocation on a regional scale. However, understanding and quantifying cumulative impacts is complex because of the interaction between surface waters and aquifers - particularly within shallow riparian aquifers, which have different hydrodynamic functionality to deeper aquifers.

The degree of coupling between surface waterways and aquifers in a fluvial-glacial sedimentary aquifer system generally decreases with depth. The heterogeneous and vertically anisotropic nature of the sediments, impart progressive confinement at depth on the aquifer system, with the compaction of fine grained materials from the weight of the sediment pile above.

The implication for water allocation of this conceptual hydrogeological understanding of the Canterbury Plains is that the system behaves significantly differently with depth below the surface, with longer residence times and flow paths. If the system is managed in a manner cognisant with environmental flow safeguards (i.e. what we are trying to protect) and the conceptual understanding of the systems’ hydrodynamic functionality, opportunities arise for optimising water allocation.

The complex hydrodynamic functioning of the aquifer-surface water system, coupled with the significant increase in demand for water and the need to manage water resources sustainably, has meant that in the absence of perfect knowledge of the system, water is regulated in a precautionary or conservative manner. This management approach in principal is not at fault, but there are well documented concerns over the rate at which the resource management process (investigation, policy development, and implementation) has adapted to the significant increases in demand.

The question that resides is “what can be done about this resource management conundrum and quickly?”. This paper explores a method for optimising water allocation using the concept of managed aquifer recharge (MAR) on the Central Plains in Canterbury. From the preliminary analysis presented in this paper, MAR would appear to offer significant cost benefits when compared to the current Central Plains Water (CPW) Irrigation Scheme, and represents a more sustainable outcome for enhancing water allocation from the entire system.