SERM forcing

Introduction

The physical and chemical forcing of the SERM model is specified through the estuarine parameters. This document details how the forcings (which include boundary conditions) are determined. The forcings applied to the model can be categorised as mass fluxes (nutrient loads, TSS loads), heat fluxes (by imposing an annually varying temperature), and sunlight. The mass fluxes can be the result of inputs (such as river inputs, point source loads) or boundary conditions (the ocean concentration of nutrients and solids). A final section lists the initial conditions used for the simulations.

Sunlight

Plant growth is forced by an average daily irradiance at the surface of the water. The daily average is calculated using a planetary geometry model (Brock, 1981), and a cloud model assuming a constant 3 oktas of cloud (Evans and Parslow, 1985). The yearly shape and magnitude of irradiance changes with latitude, and are graphed for each climate zone.

Heat fluxes

The estuarine water temperature is based on measured air temperature from the particular climate zone. While this is very simplistic, a heat budget (balancing radiative gains with evaporative and other losses) would have been difficult to implement. The seasonal temperature profiles are graphed for each climate zone.

Mass fluxes

The volume flow rate [m³ d^-1] of fresh water to the estuary is given by:

flow rate = SA * depth / fresh water replacement time

where SA is the surface area of the estuary,

The nutrient and suspended solids pools considered in the ecological model are:

• NH₃ [mg N m^-3] - ammonia

• NO₃ [mg N m^-3] - nitrate (and nitrite)

• DIP [mg P m^-3] - dissolved inorganic phosphorus

• Lab_Det_Benth [mg N m^-3] - Labile detritus at a molecular C:N:P ratio of 550:30:1

• Lab_Det_Plank [mg N m^-3] - Labile detritus at a molecular C:N:P ratio of 106:16:1

• PIP_unfloc [mg P m^-3] - phosphate absorbed to unflocullated TSS

• PIP_floc [mg P m^-3] - phosphate absorbed to flocullated TSS

• TSS_unfloc [kg m^-3] - unflocullated suspended solids

• TSS_floc [kg m^-3] - flocullated suspended solids

As stated above, mass fluxes into the model result either directly from inputs (such as river loads), or as a result of specifying ocean boundary conditions. We consider the direct fluxes first.

The direct input of nutrient and suspended solids is modelled as two independent components: a point source load (the sum of point source loads such as sewerage treatment plants or aquaculture) and a diffuse load. The point source load is assumed to be constant throughout the year, contains only NO₃, NH₃, and DIP, and is specified as a mass of N per m² per day. The diffuse load is a function of catchment clearance, fresh water replacement time, and depth, and co-varies in time with the local rainfall (as specified by the climate zone). The diffuse load contains NO₃, NH₃, DIP, Lab_Det_Benth, Lab_Det_Plank, PIP_unfloc, and TSS_unfloc.

Calculation of point source loads

The mass ratios of NO₃, NH₃ and DIP in point source loads are based on the composition typical of secondary-treated sewerage treatment plant loads:

Point source loads are assumed to be constant throughout the year and are specified per m² of estuary area (in mg N m^-2 d^-1). To determine the total load in the estuary, multiply the load by the estuary surface area.

Calculation of diffuse loads

Diffuse loads are specified as a concentration in the fresh water. As a result, the actual diffuse load (in mass per unit time) is a function of the catchment clearance percentage (which determines the concentrations in the inflow), the fresh water replacement time, estuary surface area and depth (which together specify the volume of fresh water input), and the climate zone (which determines the seasonality of the fresh water inflow).

Note: The SERM model assumes that all catchments throughout Australia, independent of native vegetation, soil type, relief, etc. will result in the same concentration of loads in the fresh water, as a function of catchment clearance only.

Calculating the diffuse N.

The diffuse loads are calculated from the uncleared and cleared portions of the catchment, and then summed. The total nitrogen (TN) concentrations in water draining from cleared and uncleared catchment portions are:

The percentage of NH₃, NO₃, Lab_Det_Benth, Lab_Det_Plank and DON that makes up the TN in water draining from cleared and uncleared catchment portions are:

For intermediate clearance rates, the concentration of nutrients from the cleared and uncleared sectors are calculated individually, and summed to give the total diffuse loads. Note that because runoff from cleared land contains 10 times the concentration of nutrients and solids, a 10 % cleared catchment will have a load composition (although not magnitude) half-way between a pristine and totally cleared catchment.

Calculating the diffuse P loads.

The ratio of total nitrogen to total phosphorus (TP) is also different in runoff from cleared and uncleared catchments:

In contrast, the fraction of DIP and DOP making up TP is assumed to be independent of clearance:

DIP = 0.3 * TP
DOP = 0.17 * TP

To assign fractions of TP to Lab_Det_Benth, DIP, DOP, and PIP_unfloc, calculate the TN:TP ratio for the whole catchment:

catchment TP = [(2000*(catchment clearance/100) /10 + (200*(1- catchment clearance)/100)/30]*depth / fresh water replacement time

Assign 30% of TP to DIP, and 17% to DOP. Now remove the amount that would be held in Lab_Det_Benth:

Lab_Det_Benth_P = ((30.97/14.01)/30)*[(2000*(catchment clearance/100)*0.02 + (200*(1- catchment clearance)/100)*0.25]*depth / fresh water replacement time

Finally, the rest of the P is assigned to PIP_unfloc. i.e.

PIP = TP - DIP - DOP - Lab_Det_Benth_P

Calculating the diffuse TSS loads.

For intermediate clearance rates, the concentration of TSS from the cleared and uncleared portions are calculated individually, and summed to give the total diffuse TSS loads, in the same way that the calculations was done for total diffuse nitrogen loads.

Boundary Conditions

The only boundary in the estuarine model is the ocean. The boundary conditions are identical for all climate zones, with the exception of dissolved phosphorus and nitrate (see Table). In particular, notice that the uniform rainfall climate zone (UNR) contains time varying boundary concentrations of nitrate and phosphate. This represents the seasonal cycle of mixed layer nutrients in temperate coastal waters off southern Australia, with maximum concentrations in winter. The amplitude of this cycle varies with latitude, and an intermediate amplitude has been used here.

These boundary concentrations (in combination with the estuarine parameters tidal range or oceanic flushing time, depth, fresh water replacement time and climate zone that determine volume exchange rates) determine the net mass fluxes between the estuary and ocean.

Initial Conditions

The initial values of state variables in the water column, sediment and epibenthos were assigned the same value in all simulations. Simulations were run for 10 years, and in most cases, this is long enough for initial values to have negligible effect on model outputs in the last year.

It should be noted that using an initial condition of a viable benthic community may not be appropriate in an already highly degraded system, without viable seagrass and/or macroalgae benthic communities.

The initial values are listed below for completeness.