Increasing demands on New Zealand’s freshwater resources has led NIWA to develop a smart addition to its suite of tools assisting in the planning, regulation and evaluation of water use.
NIWA’s CHES (Cumulative Hydrological Effects Simulator) software tool predicts how water-flows in a catchment will change with multiple water uses, such as direct abstractions or storage reservoirs, and what the consequences will be to in-stream ecosystems and reliability of water-take.
The image depicts the mean habitat change for adult brown trout, when comparing the natural flows (no abstractions) with the current state consented abstractions. Abstractions (takes) are depicted as “.” in the figure, and for each river segment (NZReach) the physical habitat change for brown trout was simulated, using simulated mean daily flows. Hence river segments that are coloured in green indicate that brown trout has better habitat due to abstractions, whereas river segments that are coloured red indicate that abstractions are causing a reduction in habitat for brown trout.
So, abstractions (taking water from the river) can lead to an improvement in physical habitat, as the river might have been too high or too fast without any abstractions, and taking water 'shifts' the flow environment closer to the optimum conditions for brown trout. (Dave Allen)
New Zealand’s rivers are coming under increasing pressure to supply water for irrigation, hydro schemes, and domestic and industry uses, says NIWA software developer Dr Jan Diettrich. Understanding and managing these pressures are important for resource managers and end users, including regional councils, irrigation companies and communities.
“Users need to know the availability of water from a river channel and reliability of supply of water out of a reservoir. This needs to be done under the constraints of what water’s actually in the river, what’s allocated to other users in the catchment and limits set by regional councils around minimum flow and maximum take.
“They also need to consider the effects of water-take on in-stream environments, such as fish habitat.”
Jan Diettrich points to a graph representing how Bluegill Bully, a small native fish, would be affected under various water abstraction scenarios. Green river segments indicate better habitat due to abstractions, and river segments that are coloured red indicate that abstractions are causing a reduction in habitat. The CHES software predicts how river and stream flows change after water is abstracted or directed to storage. [Photo: Greg Kelly, NIWA]
CHES maps flows everywhere across a catchment on a daily basis. Starting with natural flows, it is able to adjust and estimate flows everywhere throughout the catchment to take into account existing water-takes and changes to flows such as proposed new abstractions.
It also quantifies the consequences for both the overall availability and reliability of the water resource, and the residual flows that determine in-stream environmental effects.
CHES incorporates modelled river-flow time series for New Zealand’s half-million reaches, and includes user-specified abstraction and storage options. It calculates the effects of water use by combining numerical water routing with operating rules.
Embedded into ArcMap as an add-in, CHES enables analysis of the simulated or measured time series of residual river flows, and water-takes, for user-specified scenarios.
CHES works on user-specified scenarios:
- Natural – without any abstraction, i.e., ‘untouched’ river flows
- Current – flow time series calculated with existing takes, input from a consent database
- Proposed – see the impact of potential abstractions on river flow, existing abstractions, and compounded environmental effects
It also allows for mapping of scenario variations.
[This feature appeared in Water & Atmosphere 19]