A debate continues around the appropriate selection of models (empirical, conceptual and physically-based models) for use in hydrological studies. Physically-based models are often considered by physical scientists to be superior to the others. The implication is that the use of equations that are theoretically correct at a certain scale can be applied at any scale with equally good results, implying a degree of accuracy that may not actually exist (Grayson et al., 1996) particularly when field data is not available at the scale(s) used by the model. In reality processes that are important at one scale may not necessarily be important at other, larger or smaller, scales (Sivapalan et al., 2003). Whilst each type of model has its advantages and disadvantages, it is important to see the different model approaches as complementary, and not competing, with each approach providing different insights into a system. The selection of model type and modelling approach ultimately depends on the research objectives and constraints.
5.4.1 Modelling objectives and selected approach
The key modelling objectives within the context of this thesis are: 1) to consider how water allocation models might be improved through an understanding of aquifer-river interactions; 2) to quantify the impacts of groundwater extraction on river flows within the connected aquifer-river systems of the Namoi River catchment; 3) to better inform water policy on groundwater extraction; and 4) to be able to utilise the model in future integrated assessment of water allocations options at the catchment scale (Letcher et al., 2004). The scope of the research has been further refined to limit the study to an analysis of the alluvial aquifers (from which most groundwater is extracted) that interact with the gauged and unregulated river systems. The gauged and unregulated river systems were selected for two reasons: 1) the river gauges provide a source of data by which to measure river flow characteristics, including any changes to flow characteristics; and 2) the unregulated river systems and their associated ecosystems may be more vulnerable to altered hydrology. The regulated river systems are thought to be of lesser concern in the first instance because they have already been heavily altered by dam flow releases throughout much of the irrigation season.
River and groundwater resources are separately managed and allocated in Australia. River water is often allocated using rainfall-runoff models and/or monitoring of upstream flows, and groundwater resources are allocated based on the ‘sustainable yield’, which is an estimate of the long term average annual recharge to the exploited aquifer system. A shortcoming of managing aquifer systems based on their sustainable yield is that the interactions between groundwater and river systems are not considered, and hence the impacts of groundwater allocations on river flows usually remain unassessed. Combining a rainfall-runoff model with a simple groundwater model is a sensible progression in water allocation model development in order for aquifer-river interaction processes to be considered.
The groundwater resources in the Namoi River catchment are managed by groundwater management zones (refer to Section 3.3.2). The water sharing plans specify the permissible annual groundwater extraction volumes for each groundwater zone based on an estimation of the sustainable groundwater yield. Stream gauging stations can generally be found at the junction of each groundwater zone, with water policies for both surface and groundwater directed at the subcatchment level. The model or models developed for this study then ideally need to be detailed enough to model the fluxes
between groundwater and surface water systems at the subcatchment scale, and yet be general enough to inform water allocation and other management initiatives. The type of model envisaged in order to achieve the research objectives would be a parsimonious-conceptual style of model, making use of lumped parameter estimates, which can adequately apportion dynamic exchanges of water volumes between surface and groundwater systems within the subcatchment on a daily time step. A simple model is preferred for this research for a number of reasons including: the requirement for subcatchment-scale water budget accounting in order to assess water sharing plans; the requirement to model streamflows, including baseflows, on a daily time step; the requirement for a model that could be later used in integrated assessments; the limited data pool and time with which to parameterise a complex model; and the uncertainties associated with validating models that are over-parameterised.
5.4.2 Top-down Modelling Approaches
A top-down or downward approach to model building was first introduced to the discipline of hydrology by Klemeš (1983). The top-down modelling approach was described in the context of predicting overall catchment response based on the interpretation of observed responses at the scale of interest. A key characteristic of the top-down approach to modelling is that the model structure, including what processes to explicitly include and in which way, is inferred from the data (Sivapalan et al., 2003). This style of model building focuses on attempting to learn from the data about how a system works by trying to relate the inputs of a system to the outputs, for example through the use of transfer functions, without being overly concerned as to the physical processes occurring (Beven, 2000a). Developing an understanding of the rainfall-runoff relationships and the influence of groundwater extraction and climate variability on river flows are fundamental to this study if we are to better understand how groundwater extraction might alter river hydrology over time. Such a downward approach to modelling may give insights into the key driving factors as evidenced by the data itself and the information contained within the data alone.