Estuarine parameters

Estuarine parameters are quantities that define an estuary's physical characteristics (its geometry and circulation) and the inputs to an estuary (nutrient and suspended solids loads, fresh water flow, sunlight etc.). A large number of model simulations have been run for a broad range of estuarine parameter values. The SERM estuarine parameters are:

The total nitrogen load, total phosphorus load, load status, and total suspended solids load can be calculated directly from the above estuarine parameters.

Introduction

In order to simulate an estuary using the kinds of spatially-explicit, process-based models employed here (see Model Description), it is necessary to specify a fairly extensive data set. The model geometry and bathymetry (length, width and depth of each model cell) must be specified. In the simple hydrodynamic models used here, winds are ignored, and circulation is assumed to be determined by river flow and tidal amplitude. River flows and nutrient and sediment loads as a function of time must be specified at the upstream boundary, and concentrations of nutrients, salinity, and other particulate and dissolved tracers at the downstream boundary. Point source loads into the estuary must also be specified.

The challenge in developing a simple generic model has been to find ways to specify this data set using a relatively small number of parameters. After much deliberation, we have settled on a set of 8 parameters which together define the geometry, bathymetry, and physical and chemical forcing of an estuary. A brief description follows and further details can be found by clicking on each parameter.

The model's geometry and bathymetry is defined by two parameters: the Estuarine type, which specifies both the nature of the circulation: lagoon, tidal, salt-wedge, and the horizontal size and shape of the estuary, and the Depth which specifies the average depth of the estuary.

The river forcing is characterized by four parameters. The Climate zone specifies the seasonal variation in flows and loads (and also temperature). The Fresh water replacement time specifies the mean annual river flow, in terms of the turnover time of the estuary volume. The Catchment clearance specifies the diffuse loads (nutrient and sediment concentrations in river inflow) as a function of catchment clearance. (This constitutes a very simple catchment model. We expect this ultimately to be replaced by catchment loads from the much more sophisticated catchment models used in the Catchment and River components of the Audit.) In a number of Australian estuaries, concentrations of humic acids or coloured dissolved organic matter (CDOM) in river water are very high, and control light attenuation and productivity throughout much of the estuary. We have therefore added CDOM light attenuation as a fourth river parameter.

The specification of the model exchange with the ocean depends on the estuarine type. For lagoon estuaries, it is specified as a oceanic flushing time: this allows for the effects of a constricted entrance, common to most lagoon systems. For tidal and salt wedge estuaries, it is specified as tidal range. Marine concentrations of most tracers are low compared with estuary concentrations, and these have been assigned values which differ according to climate zone.

Finally, the model allows the specification of additional point source loads, with a composition characteristic of secondary-treated STP effluent.

Of these parameters, Estuarine Type, Depth, Climate Zone, Oceanic Exchange and CDOM might generally be regarded as inherent or "natural" properties of the estuary, although Depth and Oceanic Exchange could be modified by engineering or sedimentation due to catchment clearing. Fresh water replacement time, Point Source Loads and Catchment Clearance are likely to be key pressure indicators for catchment modification in most estuaries.

As discussed under Model approach, we elected here to model a large representative set of generic estuaries, rather than try to implement (automatically) specific models of each estuary in the OzEstuaries database. This decision was driven largely by uncertainty over what data might ultimately be available for each real estuary, and when that data would become available. As the Estuarine Database is populated, an obvious extension of the modelling presented here is to provide a capacity to link the database to the model, so that model runs for each estuary are specified by the estuarine parameters in the database.

While this could be done using the current set of Estuarine Parameters, there would be good reasons to revise the Estuarine Parameter set if such a link was attempted. The set chosen here has been driven strongly by the need to keep the total number of parameters low, to make the number of simulations required computationally feasible. However, this constraint would disappear in a project to model estuaries from the database, and would be replaced by the requirement that the data be available. It would be possible in such a project to modify and extend the Estuarine Parameters so as to permit more realistic representations of each estuary.

Specifying Estuarine Parameters in the SERM Interface.

At the SERM interface specification page, the user is asked to specify a set of simulated estuaries by selecting a range for each estuarine parameter. Guidelines for selecting these ranges for individual parameters are provided at the parameter links listed above. For example applications of these guidelines, see the case study estuaries.

For estuarine parameters with categorical values (estuarine type and climate zone) the SERM interface specification page offers a choice of the available categories. For estuarine parameters with numerical values (depth, inflow colour, fresh water replacement time, ocean exchange or tidal range, point source loads, and catchment clearance), the SERM interface specification page offers ranges which are based around the values used in simulations. For example, the database contains results for simulations run with catchment clearance percentages of 2, 10, 50 and 90 % cleared. At the SERM interface specification page, the user is offered corresponding ranges of 1-5, 5-20, 20-80 and 80-100 % cleared. We offer ranges rather than particular values to help the user interested in a particular real estuary select simulations which are "closest" in behaviour to that estuary, taking into account the nonlinear dependence of the model on parameters. For example, in the model a 25 % cleared catchment behaves more like a 50 % cleared catchment than a 10 % cleared catchment. The user with a 25 % cleared catchment who chooses a range of 20 - 100 % cleared sees runs with a 50 % cleared catchment. Of the available simulations, these most closely correspond to 25% cleared.

SERM was developed as part of the Estuarine Health Theme
of the National Land and Water Resources Audit.