Making sense of Ngaruroro River science

TANK Group members today heard the latest science from Regional Council hydrologists about flow statistics for the Ngaruroro River, and it contained some surprises.

TANK is the name of a collaborative process being used to draft a Plan Change for catchments including Tutaekuri river, Ahuriri estuary, Ngaruroro river and Karamū stream.

Today’s science on river flows, plus new modelling about how ground and surface water abstraction affect the Ngaruroro River, will help decision-making about managing water in the TANK catchments.

Developing the Heretaunga Plains ground and surface water model included a data review to calculate flow statistics called Mean Annual Low Flow or MALF.  MALF is an average of the low flows observed in a river each year.  This information helps decide how to manage abstractions that affect river flows while protecting fishery and aquatic ecosystem health.  MALF is often used as a basis for informing the management of flow regimes, allocation limits and restrictions on abstraction. 

The TANK Group also learned in February that groundwater resources over the whole Heretaunga Plains are more closely linked than previously thought, meaning the previous approach to stream-depleting takes is likely to change substantially. 

To calculate MALF, abstractions of surface water and the stream depletion effects of ground water takes are added back into flow measurements so that the MALF reflects a natural or ‘naturalised’ flow. 

In working out the new MALF, the modelling team included new data from a groundwater recharge scheme that operated from the 1980s to early-2000s near Fernhill.  The most recent modelling is much more accurate as it includes the flow record from the recharge scheme and updated information about the effects of surfacewater-depleting groundwater takes.

This works shows that the MALF figure is higher than earlier science indicated and has risen from 4,500 to 4,700 litres per second at Fernhill.   This new flow with information about groundwater inter-connectivity will have significant implications for the flow management decision-making for Ngaruroro River and groundwater in the adjacent Plains.  However, the details are still to come as further modelling is carried out. When complete, the TANK Group will consider the impact of various flow and abstraction regimes.

The current restriction regime for irrigation takes includes a ban on water takes when Ngaruroro River flow reaches 2400 litres per second. New water management provisions for the Heretaunga Plains are likely to look substantially different. 

The TANK Group still has difficult technical details to consider before making its final recommendations, anticipated to be later this year. 

More information on the TANK project is at www.hbrc.govt.nz, search: #tank.

17 March 2017

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