CHES - Grey River Catchment West Coast

This study is an example of the application of NIWA’s Cumulative Hydrological Effect Simulator (CHES) tool.

This tool can be used to provide support to the management decisions on water allocation in a spatially complex region which experiences diverse water uses both in stream and out of stream. Using the Grey catchment, West Coast, as a case study the West Coast regional council (WCRC) has been able to highlight the capabilities and practicality of CHES in an active water management setting, using it to provide an example of how modelling can be used to explore water allocation scenarios whilst also considering both environmental and anthropogenic values on a system.

CHES was used in this case study in order to characterise the current status of the Grey catchment as well as being able to model future consequences of water use in the catchment. In addition to this, differing water use and usage limit scenarios were be explored improving the foundation on which future water allocations could be planned.  The CHES model was particularly valuable in that it takes into account and models both out of stream (reliability of supply) and instream (physical habitat) values, this was particularly valuable in the comparison and weighting of environmental condition of the waterways when compared to different water usage and water allocation scenarios. 

The CHES model is an add-in to ArcGIS, the background hydrologic data is modelled on ~40 years (5-Jan-1974 to 5-Nov-2015) of TopNet data which can be used to characterise a catchment; in this case the Grey Catchment. This use of data from the past 40 years allows the user to characterise the overall hydrologic impacts of water takes with respect to natural variation of a system (dry vs. wet years) in the context of the past few decades of water use and as such CHES can better be used in planning for the future.   

The steps involved in the initial Set up for the Grey River Catchment were:

  1. Define catchment boundary and river network.
  2. Calibrate underlying hydrologic model.
  3. Generate the existing water take database.
  4. Define and run the reference scenario.
  5. Set-up and run alternative allocation scenarios.

Outputs from the CHES modelling are focused on Mean Annual Low Flow within the catchment. From this data estimations can be made surrounding water availability for allocation as well as habitat gains or losses which in this study have been linked proportionately to water flow and volume. Outputs from this model facilitate visualisation of catchment states and changes to flow throughout the study area.    

 

CHES grey river

Grey River catchment area

 

It is highlighted in the report that CHES model scenario outputs are based on TopNet models (hydrograph inputs) and as such outputs from this model have potential to differ from reality, with inputs differing from measured flow. It is also noted that this is an unavoidable reality given that historical and real time data is not available for every reach of a catchment.

Also the model does not take into account interactions with groundwater; this raises concerns given that in the Grey River Catchment a portion of water allocation is from ground water takes. This is something to be considered when applying results in decision making.

From comparisons of the CHES models to observation in this study it should be noted that the reality of flows and water takes on a day to day basis was a variable that could not be accurately modelled and as such highlighted the limitations of this model. The reality was that modelling was based on the assumption that each abstractor of water took the maximum allocation per day that they were entitled to when the reality was not such. This coupled with the modelled TopNet flows were two variables that provided uncertainty in applications such as this study. 

 

CHES grey river outputs1

 The difference in MALF between the climate change scenario and the current climate as a percentage of change

Associated Models

Cumulative Hydrological Effects Simulator (CHES)

NIWA's new CHES (Cumulative Hydrological Effects Simulator) software tool predicts how water flows in a catchment will change with multiple water uses (e.g.

References

Hoyle J, Diettrich J and Franklin P. 2016. Applying the Cumulative Hydrological Effects Simulator (CHES) for managing water allocation: A demonstration of CHES in the Grey catchment, West Coast. NIWA Client Report No: CHC2016-074.   Prepared for West Coast Regional Council.

Hoyle J, Diettrich J and Franklin P. 2016. Applying the Cumulative Hydrological Effects Simulator (CHES) for managing water allocation: A demonstration of CHES in the Grey catchment, West Coast. Prepared for West Coast Regional Council. Presented at the Water Infrastructure and the Environment conference in December 2016: CHES for the Grey River catchment