Sustainable Water Allocation Programme (SWAP) – year in review

Expansion of the NZ economy is based on increased agricultural productivity. This is limited by the availability of water for irrigation. In many regions of New Zealand, water from baseflows and groundwater is already fully allocated, so water harvesting from freshes and floods into storage reservoirs is the only option to meet increased water demand.

This means allocation planning and decision-making needs to consider the effects of changes to the full flow regime (not just base flows), since mid-range and flood flows perform numerous physical and ecological functions (e.g. periphyton and substrate flushing, sediment transport, riparian vegetation control, fish-migration cueing, and river-mouth opening). Thus the goals of the Sustainable Water Allocation Programme are to: 

  1. develop knowledge and understanding of effects of changes to the full flow regime on in-stream values 
  2. develop tools and guidelines to help set limits not only on minimum flows and total take but also to set conditions that ensure an adequate residual frequency and duration of high flow events
  3. develop tools that assist planners and decision-makers to quantify the trade-offs between in-stream and out-of-stream values for water-use schemes and management plans.

Some achievements towards these goals over the past year are as follows: 

  • Environmental Flow Strategic Assessment Platform (EFSAP)

EFSAP was applied in three regions (Canterbury, Auckland and Northland) to investigate the effects of various combinations of water resource use limits (on minimum flow and maximum take) on reliability of water take and in-stream physical habitat for key biota.

  • Plant-flow relationships

Analysis of periphyton samples from rivers in the Manawatu –Whanganui region (collected by Horizons Regional Council) showed that periphyton cover was related to flow characteristics, nutrient levels, and other environmental variables. The rivers fell into two groups based on the relationship between periphyton standing crop and time since the last flood. In each river group, different variables were found to explain the variation in periphyton cover.

A subsequent study of channel geomorphology and hydraulics showed that the two groups were distinguished by the relative mobility of their bed-material. At most of the study sites the flow required to remove periphyton corresponded closely with the flows that mobilise sand - indicating that in situ abrasion is a key removal process. These findings inform on the magnitude of flushing flows required downstream of dams and the need to maintain mid-range flows downstream of water-harvesting schemes.  

  • Fish-flow relationships

Statistical analysis of the New Zealand Freshwater Fish Database (NZFFD) showed that frequency of floods, flood magnitude, and time period between floods are the most important flow variables that influence fishes, although the exact relationships vary between fishes. Analysis of eel data showed that the strength of the relationship between longfin eels and flow magnitude increases with distance inland but this relationship is also mediated by catchment land-use.This study identified the land-use types where eel populations will be more sensitive to flow alteration.

A related study on habitat use of eels found that management of river flows is required to ensure flow regimes that maintain availability of suitable local-scale hydraulic conditions, and that biodiversity conservation efforts need to be targeted at protecting a gradient of rivers across New Zealand's river landscape. 

A new multidisciplinary, numerical method has been developed and tested under sub-contract to Cawthron Institute. This simulates physical habitat occupation by trout using a process-based drift-dispersion and trout net-rate-of-energy-intake modelling package. This considers how much space trout need to forage off drifting invertebrates to sustain their energy requirements for body maintenance, growth and reproduction. The models predict reach-scale trout carrying capacity as a function of flow. The new models launch off traditional hydrodynamic models that predict how water depths and velocities change with flow. The hydrodynamic and invertebrate drift dispersion models predict the physical habitat and food template upon which fish select the most profitable feeding habitat. The modelling package is being used in New Zealand and the USA and offers the prospect of more accurate predictions of the response of sports fish to flow regime change.

  • River ecosystem modelling.

Advances have been made in developing and improving empirical and national-scale models relating flow-regimes to biota. These models were used to inform the National Objective Framework (NOF) process. Examples include prediction of: the probabilistic distributions of periphyton over time; the macroinvertebrate community index (MCI) and percentage pollution sensitive taxa (EPT)and taxon richness; and the probability of occurrence of various invertebrate species.

A physically-based numerical model for Benthic Invertebrate Habitat Simulation (BITHABSIM) in rivers has been tested under sub-contract to Cawthron Institute. The model made reasonable predictions of the variation in benthic invertebrate abundance associated with flow variation. It adds ecological realism to traditional hydraulic-habitat modelling by accounting for disturbance by floods and drying in addition to available physical habitat. The study supports the use of BITHABSIM for improved assessment of effects of flow regime change on benthic invertebrates – the food base for most fishes and river birds. 

Huka Falls. Credit: Dave Allen