Water Quality must be enabled by the user if it should be included in the simulation and in the MIKE HYDRO tree view. To activate Water Quality it is required to enable the ‘Water quality’ checkboxes under Simulation specifications\Modules.
The Water quality definitions page includes the following tab-pages:
· Forcings
· Output
Solution method
Water quality calculation is based on predefined MIKE ECO Lab templates which contains a model defined by a number of coupled differential equations, which is solved through numerical integration and interactions between each equation.
Solution method is a selection of the Integration Method for solving the coupled ordinary differential equations defined in the MIKE ECO Lab file. Three different built-in integration routines (solution methods) are available (please consult the MIKE ECO Lab Reference Manual for details on the methods for solving the coupled linear differential equations in the MIKE ECO Lab framework):
· Euler: Euler or Linear Solution
A very simple numerical solution method for solving ordinary differential equations.
· RK4: Fourth order Runge-Kutta.
A classical numerical solution method for solving ordinary differential equations. It has normally higher accuracy than the Euler method but requires longer simulation times. The fourth order Runge Kutta method requires four evaluations of equations per time step.
· RKQC: Fifth order Runge-Kutta with Quality Control
A numerical solution method for solving ordinary differential equations. The accuracy is evaluated and the time step is adjusted if results are not accurate enough. The method requires 6 evaluations at each time step to take a so-called Cash-Karp Runge Kutta step and the error is estimated as the difference between a Runge Kutta fourth order solution and the Runge Kutta fifth order solution.
The accuracy (and the computing time) varies for the three integration routines.
The most accurate result will be calculated when using RKQC. However, in some cases the same results can be obtained - using less computational time - with the less advanced options; RK4 or EULER.
In general, it is recommended to use the RKQC routine. RK4 ad EULER methods are generally only applied during the set-up and initial calibration phase of a project. If the RK4 or the EULER routines are used, it is strongly recommended to run an additional simulation with the RKQC routine and compare the two results (RKQC versus RK4/ EULER) before making any conclusions based on the model.
In the case of a very dynamic model system with steep concentration gradients in one or more of the components, integration may not be possible when using the RKQC routine, and an error message will appear. Reducing the time step will help in most cases, but sometimes the gradients are so steep, that they cannot be solved accurately. The Quality Control of RKQC ensures that all components are calculated within an accuracy of 1 µg/l. Using the second best routine (RK4), where no Quality Control is included, the steep gradients can be solved in a relatively accurate way and RK4 is therefore recommended when integration is impossible with the RKQC routine.
Update frequency
The update frequency is a parameter that allows to define how often the water quality processes will be calculated during the simulation. The update frequency is defined as a multiplum of the simulation time step used for the HD- and AD-simulation and therefore determines the frequency for simulating water quality processes on top of the standard Advection and Dispersion processes which are calculated at every time step. The Update Frequency must be an integer above zero. Definition of the frequency should be based on careful considerations. The user should recognise that the dynamics of the advection dispersion is comparable to the dynamics of the process descriptions. Thus it is strongly recommended to use an update frequency of 1.
The selection of the Time Step of the MIKE ECO Lab model, and hereby the Update Frequency, has to be based on considerations of the time scales of the processes involved. Please notice that this selection can be rather decisive for the precision of the numerical solution as well as for the CPU time of the simulation. A large Update Frequency will decrease the precision as well as the CPU time. It is therefore advisable to perform a sensitivity analysis on the Update Frequency before making the final selection.
Disable calculation of processes. AD results only.
MIKE ECO Lab templates are often used to define a range of state variables (or AD components) and it therefore often beneficial to make a template that suits the need of the project. The integration of MIKE ECO Lab in MIKE HYDRO also offers the possibility for executing a simulation which does not include any water quality process solutions from the MIKE ECO Lab framework. By enabling the ‘Disable calculation of processes. AD results only’ tickmark, the simulation will include only the transport and spreading of model components (state variables) and not include any biological or chemical processes at all.
The simulation results is in that case identical to a situation where the components are defined and simulated through the Advection Dispersion module.
Enabling this checkmark may be feasible in an early stage of model creation where stability of the Hydrodynamic and Advection-Dispersion simulations are performed. Water quality processes can then be activated as a second phase of model definition.
Model selection
The water quality model to be applied is defined in a MIKE ECO Lab template file and the model selection is about selecting the MIKE ECO Lab template that must be used for the project. It is possible to select between a range of MIKE ECO Lab templates defined by DHI from the drop-down selection (press ‘arrow down’ in the Select model field. Alternatively, use the file selection button
to browse for the requested MIKE ECO Lab file.
Summary
Summary section list the overall content of the selected MIKE ECO Lab template. Description is the water quality model description defined in the MIKE ECO Lab template.
Initial state variables enable the specification of initial values for each state variable defined in the MIKE ECO Lab template. Initial values are applied at start of simulation. Default value is 0 for all the variables unless other values are defined.
Initial state variables can be defined as Global values and Local values. Global values a per default applied for the state variables in all points in the model, and Local values can then be defined where local exceptions to the global values has to be defined.
Global
Global values of initial conditions for state variables are defined here. The table presents all state variables derived from the MIKE ECO Lab template and initial values can be defined for all state variables.
Some of the fields in the table are not editable; Name, Description, Transport, Type and Unit of the variable.
Changes can be made to the following columns:
Fixed value. Initial global value of the actual state variable.
Factor. Scaling factor for the parameter’s Fixed value. The default value is 1.
Local
Local exceptions of the globally defined initial values for state variables are specified in this page.
Note that if you want to specify local variations at the same locations for different state variables, you must define this in individual lines for each State Variable.
Use the Add button ‘+’ to create the required number of lines in the table (one line for each location where an individual state variable needs to have a local value defined). The delete button ‘-’ deletes highlighted rows from the table.
Name. Click inside the Name field to open a drop down menu, from which the required state variable can then be selected.
Branch name. The branch along which the variable assumes a local value can be selected from a drop down menu listing all branches in the model setup.
Distributed. Checkbox defining if the value of the variable is specified in a single point or distributed in a certain reach of a river.
From chainage. River chainage from which the specific variable assumes a local value. if distributed is enabled. River chainage of local value if point value is the option.
To chainage. River chainage to which the specific variable assumes a local value in case distributed has been enabled
Fixed value. Initial local value of the state variable.
Factor. Scaling factor for the parameter’s Fixed value. The default value is 1.
Constants can be any parameters (physical constant, coefficient, rate, etc.) defined in the expressions of in the MIKE ECO Lab model. Constants are potentially spatial varying bot constant in time.
The Constants are essentially divided into two groups:
· Built-in Constants, and
· User defined Constants
The built-in Constants are automatically provided by the MIKE HYDRO engine during execution, whereas the user-specified Constants must be specified in the present dialogue.
Depending on the Spatial Variation of the Constant, it can be specified as a Global value or as Local values.
Global
Global values for Constants are defined here. The table presents all Constants derived from the MIKE ECO Lab template. Global values can be defined for all Constants defined as ‘User defined’ in the MIKE ECO Lab template.
Some of the fields in the table are not editable; Name, Description, Transport, Type and Unit of the variable.
Changes can be made to the following columns:
Fixed value. Initial global value of the actual Constant.
Factor. Scaling factor for the parameter’s Fixed value. The default value is 1.
Interpolate. Interpolate checkbox relate to the overall definition on how to interpret the potential definition of local values for the specific variable. If Interpolate is enabled, multiple local values for the same variable at different locations in the same branch will be interpolated between the specified locations. If not enabled, values will be interpreted as local, point exceptions to the global value. That is, If this checkbox is enabled, interpolation will be activated, otherwise local values are applied only at the specific location.
Local
Local exceptions of the globally defined values for Constants are specified in this page.
Note that if you want to specify local variations at the same locations for different Constants, you must define this in individual lines for each Constant.
Use the Add button ‘+’ to create the required number of lines in the table (one line for each location where individual Constants needs to have a local value defined). The delete button ‘-’ deletes highlighted rows from the table.
Name. Click inside the Name field to open a drop down menu, from which the required Constant can then be selected.
Branch name. The branch along which the variable assumes a local value can be selected from a drop down menu listing all branches in the model setup.
Distributed. Checkbox defining if the value of the variable is specified in a single point or distributed in a certain reach of a river.
From chainage. River chainage from which the specific variable assumes a local value. if distributed is enabled. River chainage of local value if point value is the option.
To chainage. River chainage to which the specific variable assumes a local value in case distributed has been enabled
Fixed value. Initial local value of the Constant.
Factor. Scaling factor for the parameter’s Fixed value. The default value is 1.
Forcings can be defined as any input parameter (physical property, rate, etc.) in the MIKE ECO Lab model. Examples of Forcings are: Temperature, salinity, solar radiation and water depth.
Forcings are essentially divided into two groups:
· Built-in Forcings, and
· User defined Forcings
Built-in Forcings are automatically provided by the model system during model execution, whereas the user defined Forcings must be specified in the present dialogue. Depending on the Spatial Variation of the Forcing, as defined in the MIKE ECO Lab model, it can be specified as a “Constant” or a “Varying in time”.
Global
The global values of the model forcings must be entered in the table in this page. The table contains some fields that cannot be changed; these are Name, Description and Unit of the forcing. You can edit the following fields:
TS type. You can specify in this field if the value of the forcing is Constant or Time Varying.
Fixed value. Enter the constant value of a forcing.
File. If the forcing assumes a Time Varying value, you can browse from this field to the appropriate .dfs0 file or, alternatively, create and automatically upload a new .dfs0 file for the forcing.
TS item info. This field is for Time Varying forcings only. It displays which item is selected when the uploaded .dfs0 file has more than one item.
Auxiliary variables
Auxiliary variables (or help processes) are intermediate calculation variables that may be defined in the MIKE ECO Lab template. If defined, they can be stored as output in an additional output result file.
Name, Description and Unit fields are derived from the template file and cannot be edited.
Selected. If this checkbox is selected, the specific auxiliary variable is included in the output results.
Process
Processes are rates describing the change of a state variable over time. Processes are defined in the MIKE ECO Lab template and may be marked an output variable. Process variables marked as output can be saved to the additional result file from the simulation.
Name, Description and Unit fields are derived from the template file and cannot be edited.
Selected. If this checkbox is selected, the specific process is included in the output results.
Derived output
Derived outputs are additional output calculations defined in the MIKE ECO Lab template. An example could be the sum of various state variables (e.g. Total N = Organic N and Inorganic N) which is useful to save in the additional output file to reduce the need for manual post-processing of the main model results.
Name, Description and Unit fields are derived from the template file and cannot be edited.
Selected. If this checkbox is selected, the specific derived outputs are included in the output results.
