The Load calculator is a tool to determine the amount of pollutants that are absorbed by the water in its path through catchments and groundwater areas towards the river network. The pollutants are computed considering different pollutant sources related to farming, agriculture, or industrial activities, including animal or human waste, sewage sources, or use of fertiliser, etc. The input consists of the pollutant sources and amounts, and the output consists on the actual flux of pollutants that makes it into the river network.
The tool can be applied as a stand-alone tool for calculating average mass fluxes of pollutants for individual sub-catchments (e.g. kg/catchment/year) or on a raster grid basis (e.g. kg/grid/year). Alternatively, it can be used to provide the pollution loading for a MIKE HYDRO Water Quality model. Pollution loads may include both point and non-point sources.
All loads are initially calculated as constant mass fluxes for each sub-catchment, e.g. kg/year, however when applying the Load Calculator together with e.g. the MIKE HYDRO Water Quality model there are several ways to translate the constant mass fluxes into mass flux time series depending on e.g. runoff time series or any other known temporal variations.
Distance specific decay or retention of pollutants can be included taking into account the distance between the location of the pollution sources and the presumed outlet in the river network.
The Load Calculator dialogue consists of four main tabs that will hold the required information for the engine to construct the loads:
· Sources - for specifying pollution sources,
· Catchments - for specifying the transport,
· Decay - for specifying the retention of pollutants, and
· Output - for specifying how the output is to be stored.
The sources tab is used to define the various sources that are to be modelled in the setup. The individual sources are inserted by clicking the ‘Add’ button. The user will then have to enter the name of the source e.g. ‘manure’.
This will generate a new sub tab within the main source tab. The dialogue may hold multiple sources, each added or removed using the buttons at the lower right corner of the main tab. Each source must be defined using the associated dialogue.
Shape file
Click the browse ‘...’ button to browse for and select a shape file that includes information about the source, e.g. data on fertiliser application, population numbers, etc. The shape file may either be a point or a polygon file. The text field cannot be edited but will be populated with the path to the shape file. The shape file is used to define the source and the following two attributes must be defined using the drop-down menues:
· Name Field: Select a field in the attribute table of the layer that includes a unique ID or Name of the administrative or statistical unit.
· Value field: Select a field in the attribute table of the layer that includes data of the fertiliser application, head count, release amount, etc.
This field allows an implicit aggregation of point sources. It defines sub sets of the total loads, in such a way that the contribution of each category can be disaggregated from the total, in order to have an overview of the relative contribution of each category in relationship with the total load.
Select the components that are to be transported in the system. The combobox is populated through querying the MIKE ECO Lab template in the setup of the constituents available (see section 13 Water Quality (p. 169)).
Note that only ‘State variables’ where ‘Transport = ADVECTION_DISPERSION’ will be shown and handled in the Load Calculator.
The user may select to modulate the load with the runoff. This is done by enabling ‘Distribute according to runoff’. The runoff time series is applied as a temporal multiplication factor to distribute the mass fluxes of the pollution source in time for the current load source. Time distribution of components can be defined individually using an alpha series stored in a dfs0 file.
The table is populated by the user inserting a line. The districts are selected from a combobox based on the IDs in the shape file. The columns are added based on the components selected above. Modifications factors not supplied are equal to unity (n=1).
The catchment tab holds information specific to the catchments. It consists of two main group boxes. The top one holding information concerning runoff hydrographs the lower focusing on the concentrations of the baseflow in the catchments.
Click ‘Load all catchments’ to load the existing catchments in the setup. For updating existing catchments click ‘Update all catchments’, this will add missing catchments to your tables.
This box is holding information concerning runoff hydrographs. For non-point sources, pollutant transport to the river (as a flux) is typically positively correlated with the Rainfall runoff. Non-point sources include all sources specified as Livestock or Fertiliser source types. To account for this runoff dependent flux of non-point pollutants, it is assumed that pollutant concentrations in the runoff are constant.
The Runoff Start and End times are applied to specify a period for which annual non-point loads (e.g. kg/year) are translated into an average pollutant concentration (e.g. mg/l). The total annual load (i.e. total mass) is divided by the total accumulated runoff (i.e. total volume) for the specified period to calculate the mean runoff concentration (e.g. in mg/l) of each pollutant originating from non-point sources. To provide a variable load flux input time series, this concentration is then multiplied by the runoff time series specified for each Catchment in the setup.
Typically, the period specified will be of one year duration and represent the calibration period for a MIKE HYDRO Water Quality model.
If the assumption of a constant concentration of a pollutant in the runoff originating from non-point sources is not satisfactory, it is possible to apply an alpha time series to distribute the non-point sources in time.
Catchment baseflow concentrations
The table holds the baseflow concentration values of the constituents for each catchment. The columns are added based on the selected components (see section Sources (p. 212)).
This option is only relevant if a groundwater runoff component has been explicitly included in the setup. This must be included as separate time series files, either as a user specified time series or as a NAM model simulation result. When a groundwater component is included, only the fraction of the total load corresponding to:
(Total Runoff - Groundwater runoff)/Total Runoff
will be added to the catchment node in the river network. The rest of the loads are ignored. Instead a user specified baseflow concentration must be specified for each pollutant component representing the expected pollutant concentration in the groundwater discharging into the river section. Baseflow concentrations can be identified from water quality measurements as concentrations found during low flow situations in parts of the river where domestic and point sources are absent. Thus, the baseflow concentration is often a calibration parameter.
This approach has been introduced based on the assumption that most non-point loads are derived from the overland or drainage flow components of the hydrological cycle. This is often seen in rivers dominated by non-point sources resulting in a high concentration of pollutants during high flow and low concentrations during low flow. The fluctuation of concentrations in this case is typically determined by the relation between groundwater and surface/drainage water discharge to the river. Baseflow concentrations are typically considerably lower (e.g. <10%) compared to surface/drainage flow.
In some cases, though, transport through groundwater may be significant. In those situations it may be recommended not to include groundwater separately as described above. Instead use the Distance decay function to describe the overall retention of pollutants in the total runoff not distinguishing between different types of runoff components.
The decay tab is only active if ‘Apply Distance Decay’ has been enabled. This is then applied globally to all catchments.
A river network shape file is used to calculate the distance to the nearest river network point. Please note that the shape file may use a finer resolution than the one used for the modelling of water movement in the rivers.
A fixed temperature is used throughout the model.
The Output tab holds the information on the output type along with the mechanism for launching the load calculation.
A switch ‘Basin’ controlling whether the output should be saved as Basin data or not.
Statistics in the form of a shape file with attributes describing the statistics will be produced if the switch ‘Maps and Statistics’ is checked.
Calculation process is started with ‘Generate loads’ button and a progress bar is updated based on the number of catchments handled.
