Abstract
Abstract. Salinity modelling in river systems is complicated by a number of processes, including in-stream salt transport and various mechanisms of saline accession that vary dynamically as a function of water level and flow, often at different temporal scales. Traditionally, salinity models in rivers have either been process- or data-driven. The primary problem with process-based models is that in many instances, not all of the underlying processes are fully understood or able to be represented mathematically. There are also often insufficient historical data to support model development. The major limitation of data-driven models, such as artificial neural networks (ANNs) in comparison, is that they provide limited system understanding and are generally not able to be used to inform management decisions targeting specific processes, as different processes are generally modelled implicitly. In order to overcome these limitations, a generic framework for developing hybrid process and data-driven models of salinity in river systems is introduced and applied in this paper. As part of the approach, the most suitable sub-models are developed for each sub-process affecting salinity at the location of interest based on consideration of model purpose, the degree of process understanding and data availability, which are then combined to form the hybrid model. The approach is applied to a 46 km reach of the Murray River in South Australia, which is affected by high levels of salinity. In this reach, the major processes affecting salinity include in-stream salt transport, accession of saline groundwater along the length of the reach and the flushing of three waterbodies in the floodplain during overbank flows of various magnitudes. Based on trade-offs between the degree of process understanding and data availability, a process-driven model is developed for in-stream salt transport, an ANN model is used to model saline groundwater accession and three linear regression models are used to account for the flushing of the different floodplain storages. The resulting hybrid model performs very well on approximately 3 years of daily validation data, with a Nash–Sutcliffe efficiency (NSE) of 0.89 and a root mean squared error (RMSE) of 12.62 mg L−1 (over a range from approximately 50 to 250 mg L−1). Each component of the hybrid model results in noticeable improvements in model performance corresponding to the range of flows for which they are developed. The predictive performance of the hybrid model is significantly better than that of a benchmark process-driven model (NSE = −0.14, RMSE = 41.10 mg L−1, Gbench index = 0.90) and slightly better than that of a benchmark data-driven (ANN) model (NSE = 0.83, RMSE = 15.93 mg L−1, Gbench index = 0.36). Apart from improved predictive performance, the hybrid model also has advantages over the ANN benchmark model in terms of increased capacity for improving system understanding and greater ability to support management decisions.
Highlights
Models are being used increasingly for the management of hydrological systems such as streamflow (e.g. Hsu et al, 2002; Shamseldin et al, 2002; Dessie et al, 2014; Yaseen et al, 2016; Gibbs et al, 2018), reservoir inflow
When the artificial neural networks (ANNs) model designed to primarily capture saline groundwater accessions is added, the Nash–Sutcliffe efficiency (NSE) increases to 0.38 and the root mean squared error (RMSE) reduces to 30. 46 mg L−1, which is due to improved performance during periods of low flow, as expected (Fig. 9b, d)
This paper introduces a framework for the development of hybrid models for the prediction of salinity in rivers
Summary
Models are being used increasingly for the management of hydrological systems such as streamflow (e.g. Hsu et al, 2002; Shamseldin et al, 2002; Dessie et al, 2014; Yaseen et al, 2016; Gibbs et al, 2018), reservoir inflow Poorlyunderstood processes may not be able to be described explicitly in mathematical form, so the degree of hypothetic influence is necessarily lower This is due to the fact that information contained within measured data of the underlying system behaviour has to be relied upon to a greater degree for model development (e.g. for determination of the functional form of the model, as well as model calibration). In order to illustrate and test the utility of the conceptual high-level framework introduced, it is applied to a case study of the 46 km reach between Lock 5 and Lock 4 on the Murray River in South Australia (Fig. 4). This particular location is chosen due to the following:. A greater understanding of, and the ability to, predict salt accession under different conditions along this reach will improve the ability to manage and operate this infrastructure once constructed
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