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Allocation compared with renewable freshwater resource

Compared internationally, New Zealand has an abundance of fresh water. It is ranked 4th out of 30 OECD countries on a per capita basis for the size of its renewable freshwater resource. Within New Zealand, allocated water comprises less than 5 per cent of New Zealand’s renewable freshwater resource.

However, not all of the renewable resource is actually available to be used – much of the water needs to be retained in the rivers, lakes and aquifers to maintain the various values of these water bodies (such as ecological, recreational and cultural values). Furthermore, water is not always in the right place at the right time for users. A large proportion of New Zealand’s annual rainfall occurs in winter when demand is relatively low.

This map compares the maximum amount of allocated water from rivers and streams (surface water) as a percentage of the catchment’s modelled minimum river flow expected during dry periods. This shows the ‘worst case scenario’ ie, if full allocation use occurred during the low flow period. Darker coloured areas highlight catchments that have potential allocation pressure during dry periods and may require closer attention.

Darker areas do not necessarily indicate over-allocation, as many regional councils already have plans and management tools in place to reduce the allocation pressure during these periods. Areas with high allocation pressure and no management tools could be prioritised for further investigation and management decisions.

Potential surface water allocation pressure, 2010

Map of New Zealand showing potential surface water allocation pressure, 2010.

Text description

This map of New Zealand colour codes surface water catchments according to the proportion of average annual low flow (MALF) that is currently allocated. There are four categories of potential surface water allocation pressure.

Technical information about the map:

The map has been developed by summing the instantaneous rate of all consumptive takes from surface water within each 5th order planning unit. The seven day mean annual low flow figure is modelled for stream segments by NIWA, and the value used is that of the most downstream segment of the planning unit. The map represents the sum of surface water instantaneous rates as a percentage of the modelled mean annual low flow figure. This map represents 62 percent of consumptive allocation excluding hydro generation.

The low flow model does not consider ephemeral streams, artificial stream diversion/discharge or water movement between groundwater and surface water, meaning more or less water may be available compared with the model. Some groundwater takes will increase potential surface allocation pressure, however, these cannot be identified, on a national scale, so groundwater takes are not included in this analysis.

The map does not consider non-consumptive takes or takes from storage or lake sources as MALF data is not available for these sources. The large consumptive take for hydro generation in Southland is sourced from a lake and is therefore not considered in this map, however, this take removes a large volume from the lower reaches of the Waiau River.

Large lakes and scale issues between large and small catchments also contribute to uncertainties in the MALF model. Despite uncertainties in the modelling, calibration indicates that the model represents an estimate of natural surface water mean annual low flow. This provides a good indication of catchments at risk of allocation pressure and highlights areas to decision-makers that may require further investigation for management options.

Data source: Ministry for the Environment.

Map created by Ministry for the Environment using data from:

Leathwick J R, West D, Gerbeaux P, Kelly D, Robertson H, Brown D, Chadderton W L, Aussail A-G. 2010. Freshwater Ecosystems of New Zealand (FENZ) Geodatabase. Department of Conservation: Wellington.

Rajanayaka C, Donaggio J, McEwan H. 2010. Update of Water Allocation Data and Estimate of Actual Water Use of Consented Takes - 2009-10. Prepared for the Ministry for the Environment by Aqualinc Research Limited. Wellington: Ministry for the Environment.

NIWA provided the modelled mean annual low flow data for Freshwater Ecosystems of New Zealand (FENZ) Geodatabase.

The map shows that eastern regions, such as Otago, Canterbury and Marlborough have a larger proportion of catchments with potential to come under pressure during dry periods. Around 30 percent of the area in Otago has high potential water allocation pressure (over 100 per cent of MALF is allocated), while Canterbury and Marlborough have around 20 percent of their areas with high potential water allocation pressure. Eastern regions tend to have a large proportion of the water allocated for irrigation, which generally occurs during the summer months. In contrast, western regions such as Manawatu-Wanganui and West Coast have almost no catchments (less than 1 percent) with potential to come under pressure during dry periods. These regions tend to have a more even distribution of water allocation throughout the year for industrial and water supply.

Eastern regions also have a large proportion of their water allocated from these potentially pressured catchments. Nearly three-quarters of the allocated water in Otago and over 40 per cent in Marlborough and Canterbury is from potentially highly pressured catchments. These allocations may indicate areas with reduced water security for consent holders, as one management option to reduce the allocation pressure during dry periods may be to restrict abstraction.

This information is from:
Update of Water Allocation data and Estimate of Actual Water Use of Consented takes 2009–10.

 

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Last updated: December 2010