This section describes the calibration of the two components of surface water modelling in the Namoi subregion. These components are the Australian Water Resources Assessment (AWRA) landscape model (AWRA-L) and the AWRA river model (AWRA-R).

AWRA-L is regionally calibrated at 11 unregulated catchments using two calibration schemes: one biased towards high streamflow, and another biased towards low streamflow. The high-streamflow and low-streamflow calibrations perform reasonably well for predicting daily runoff for a wide range of streamflow conditions. For the nine hydrological response variables predicted by the model (Section, the high-streamflow calibration outperforms the low-streamflow calibration for predicting annual flow (AF) only. The low-streamflow calibration provides better predictions for the other eight hydrological response variables. Both calibrations tend to over estimate the daily flow rate at the 1st percentile (P01). Conversely, both calibrations under estimate zero-flow days (ZFD), low-flow days (LFD), low-flow spells (LFS) and length of low-flow spell (LLFS). Neither calibration predicts ZFD well, although the low-streamflow calibration slightly outperforms the high-streamflow calibration on a catchment-by-catchment basis.

AWRA-R is calibrated for 23 streamflow gauging sites of the Namoi River and tributaries. Using runoff from both AWRA-L calibrations, two concurrent AWRA-R calibrations are conducted: a high-streamflow calibration and a low-streamflow calibration. Both variants perform well overall, with the high-streamflow calibration outperforming the low-streamflow calibration. However, the AWRA-R low-streamflow calibration is markedly better than the high-streamflow calibration, with smaller model biases and similar interquartile ranges compared to the AWRA-R high-streamflow calibration for the low-streamflow metrics, except for ZFD and LFD, which are poorly simulated in both calibrations.

This section also assesses the AWRA-R model components representing river management (including water resources assessment and allocations, dam storage volumes and dam releases) that were calibrated using simulated or modelled outputs (Section Results show that these components of the model capture relevant aspects of river management for a wide range of climate conditions.

Detailed model calibration was undertaken as part of the surface water modelling in the BA for the Namoi subregion (consistent with the overall BA approach outlined in companion submethodology M06 for surface water modelling (Viney, 2016)). This reflects that the focus of the BA modelling is on the difference between two possible futures (baseline and CRDP), rather than on making an absolute prediction, as well as on the presentation of results within an uncertainty framework. The probabilistic focus means that the model parameters are varied over a wide range of plausible values (i.e. several orders of magnitude) in order to capture the uncertainty inherent in the system. The purpose of model calibration is therefore restricted to ensuring that the model is able to adequately represent the surface water system with optimal parameter values. However, these optimal parameter values are not used further in the modelling, so a detailed and time-consuming optimisation procedure (as commonly undertaken for deterministic modelling) is not followed in the BA. Instead, this calibration methodology means that results are reported for thousands of model runs that cover the range of plausible input parameter values (see Viney, 2016 for further details). This approach, which is not yet widely reported in relevant technical or scientific literature, is different from typical calibration methods used in surface water models which only report results for one optimal model run.

Last updated:
6 December 2018
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