Modelling

To help the NSW Office of Water understand how our river and groundwater systems behave, we start with building simple conceptual models. We then move up to a more detailed understanding of the biophysical processes at play. To do this properly requires enormous amounts of monitoring and information. It is not feasible to generate and collect all the necessary information so the Office works to utilise a range of modelling approaches that make use of existing information and data and supplement this with our understanding. The resultant models then help predict what will happen under a variety of scenarios.

There are many different types of models and modelling approaches and these can be applied at scales varying from very small scale specific site or study levels, through to regional scales. The complexity of the modelling approaches can also vary from very simple conceptual models through to very detailed and data rich approaches. The utility of any of these models is constrained by basic limitations in our knowledge and the availability of data to build the model. Models cannot generate knowledge; they only combine what we know into useful forms.

The main surface water model used by the Office for water sharing and management is the Integrated Quantity and Quality Model (IQQM). IQQM has been developed to assess the impacts of different management strategies on all water users. The models have been developed to simulate the major hydrological processes in river valleys, along with relevant management rules, and have been calibrated to match reservoir levels, diversions and flows over the calibration periods. The models are set up in such a way as to reproduce average long term behaviour of the river system for planning purposes and not specifically to reproduce individual daily flow behaviour in any particular year or forecast any future year.

IQQM models have been developed for most inland river basins and some coastal river valleys. The models can be utilised to obtain a range of information on simulated river system behaviour ranging from average summary statistics to specific event or sequence details. These models are used in water sharing plans, for auditing NSW compliance with the Murray-Darling Ministerial Council Cap, for estimating baseline salinity condition of NSW rivers in the Murray-Darling Basin, together with a range of strategic and operational hydrologic matters.

A range of regional groundwater models have also been established to investigate, audit and derive sustainable yield estimates for the state’s groundwater systems. The models have been used in water sharing plans, land and water management plans together with monitoring of groundwater use throughout the state.

The impacts of climate variability and climate change have been a particular focus of the Office and we have utilised a range of climate and hydrological modelling approaches to help translate estimates of rainfall and evaporation changes from climate change scenarios into impacts on the surface water regimes of river basins across NSW.