Canterbury groundwater - allocation policy and the underlying science
In February 2004, Environment Canterbury announced an interim policy on limits for groundwater use in the region. The policy rests on two main principles. First, it establishes groundwater zones, each covering an area of similar hydrogeology and recharge sources. Second, it recognises that recharge to Canterbury’s groundwater has two distinct sources: a relatively steady component (from rivers), and a seasonally variable component (from rainfall, irrigation, and intermittent streams).
Groundwater abstraction results in a progressive lowering of groundwater levels. Environmental effects include local reductions in groundwater levels, reductions in spring flows, and increased risk of saltwater intrusion. From a resource management perspective it is useful to distinguish between localised and whole-aquifer (or zone) effects. Localised effects can be managed by requiring adequate separation between wells or by enforcing short-term reductions in pumping rates. Effects over a whole zone result from pumping from many wells over an extended time. Management requires some understanding of aquifer dynamics.
The new policy draws on the results of several years of research into aquifer dynamics. Since 1999, scientists from the Institute of Geological and Nuclear Sciences, Environment Canterbury, University of Canterbury, and University of Otago have been measuring recharge to groundwater from rainfall using a network of four monitoring stations in central Canterbury on non-irrigated unimproved pasture. In 1999 and 2000, the measured groundwater recharge from rainfall ranged from 26% to 37% of the rainfall. Modelled estimates of rainfall recharge from May 1999 to March 2001 ranged from 23% to 44% of rainfall.
The same models have been applied to aquifers over the whole region, and it was estimated that the average recharge to groundwater from rainfall is about 30% of rainfall. The initial allocations set in the new policy are based on half this value, i.e., the amount allocated will be 15% of the annual rainfall. This precautionary limit of 50% of the seasonally variable component of recharge is based on Environment Canterbury’s assessment of the "Canterbury Strategic Water Study", a Lincoln Environmental study that considered the cumulative effects of groundwater abstractions on spring-fed streams.
An initial estimate of groundwater recharge based on rainfall in each zone indicated that the maximum groundwater use set by this policy is less than the existing allocation in eight of the Canterbury groundwater allocation zones (the red zones on the map). The steady component of recharge from rivers will not initially be allocated to users until further study of aquifer dynamics demonstrates that the environmental effects of further development are acceptable. Thus, the effect of the policy has been to place a constraint on further allocation of groundwater in the red zones.
Public response to the new policy has been generally supportive; for example, an editorial in the Christchurch Press (‘Water, precious water’, 24 April 2004) approved of the ‘sound precautionary approach’.
Ongoing work by Environment Canterbury and the Institute of Geological and Nuclear Sciences aims to refine the groundwater recharge estimates by including recharge from irrigation. This is likely to result in a revision of both rainfall recharge estimates and the maximum groundwater allocation in each zone.
For further details of the policy and underlying science, refer to: www.ecan.govt.nz/water/groundwater/groundwater-allocation.html
P.A. White, Institute of Geological and Nuclear Sciences ([email protected]) D.M. Scott, Environment Canterbury ([email protected])