CHES: Comparing the catchment-wide effects of water-use scenarios on water-take reliability and in-stream environmental values

The increasing demands on New Zealand’s freshwater resources, and the complexities of planning, regulating and evaluating the impacts of water use, underpin a new GIS modelling tool.

The increasing demands on New Zealand’s freshwater resources, and the complexities of planning, regulating and evaluating the impacts of water use, underpin a new GIS modelling tool.

New Zealand’s rivers are under increasing pressure to supply water for irrigation, power, and domestic and industrial supply. They are also highly valued for recreational pursuits, and for their ecological diversity and scenic beauty.

Planning or regulating future water use at the catchment scale and evaluating the effects of individual new projects has become more complex as:

  • new developments need to be considered in the context of existing schemes, what would have occurred naturally, and what might occur under future climate change
  • effects need to be evaluated and balanced across a wide range of in-stream and out-of-stream values
  • there is increasing involvement of community groups in the planning and decision-making processes, so effects-evaluation/decision-making tools need to be easy to use, provide user-focused outputs, and deliver rapid answers to “what if?” questions.

These ideals underpin the design of the CHES (Cumulative Hydrological Effects Simulator) tool.  It first predicts how water flows across an entire catchment will change under a combination of water uses, then forecasts what these flow changes would mean in terms of reliability of water supply and effects on in-stream environmental values. CHES is an ArcMap add-in, which facilitates data input and output displays, and while designed for water quantity management it also has the capability to display static water quality information.

CHES is underpinned by a hydrological rain-runoff model (Topnet) that hindcasts daily-mean flow time-series records at each node across the drainage network, typically for a 30 year reference period. Surface-water abstractions, diversions, storage reservoirs and associated operating rules are then added into CHES, and the flow changes are routed downstream. Operating rules include options for setting water allocation limits and the spatial strategy for applying these (e.g., at a single reference station or on a reach-by-reach basis). Ground/surface-water exchanges are currently only captured in relatively simple ways, but the facility exists to couple CHES to an integrated surface/ground-water model.

Scenarios are built from combinations of the underpinning hydrological model output and water-use schemes. Typical baseline scenarios are the natural flow regime without any water use, or the existing regime incorporating existing water use and regulations. Typical future scenarios may include a proposed regional water-use plan, changed abstraction limits, a new water-use scheme, and climate change.

CHES output includes a suite of hydrological statistics calculated over the simulation period for each scenario. These statistics are then transformed into quantifications of effects on attributes that define in-stream and out-of-stream values. Out-of-stream values include reliability of water supply and indications of over- or under-allocation. Instream values include ecosystem health (represented, e.g., by periphyton cover and physical habitat), recreational fishing, geomorphic function, and cultural values. Value states, including exceedence of bottom-line conditions, are distinguished in line with New Zealand’s National  Objectives Framework, and differences between scenarios can be inspected either in map view (in the ArcGIS sphere) or in summary tables.