Wetland functionality is provided in Source to allow modelling of quasi-two-dimensional situations where flow is governed by hydraulic considerations. Wetlands that may be modelled include both simple wetlands consisting of a single water body, and more complex systems such as Hattah Lakes, which consist of several interconnected wetlands.
Key modelling features provided are:
- Hydrological behaviour of perennial or ephemeral water bodies in a wetland. This includes the interaction between storage volume, groundwater seepage, rainfall, evaporation, inflow and outflow. This is modelled using the Storage node.
- Connections within the wetland, and between the wetland and other parts of a river system. This includes inlets to and outlets from water bodies that can either be controlled (regulated) or uncontrolled (unregulated). In cases where flow is governed by head difference or may be in either direction, connections between water bodies are modelled via the Wetland Link. The Hydraulic Connector is used to model situations where the flow into or out of a wetland link is too small to have a material effect on the water level in the river. The Storage node is used when a water body, such as a lake or weir, is part of a wetland system.
- Conservative constituents. When this modelling option is enabled for a scenario, the movement of conservative constituents such as salt can be tracked through a wetland.
- Demand. Wetlands may have ecological, recreational, cultural or consumptive requirements. All demands, including those for wetlands, are modelled via a combination of a Water User node with the appropriate demand model, and Supply Point node(s) that associate water user demand with a physical source of supply.
|Note: Groundwater fluxes are not considered in the wetlands hydraulic solution as these have negligible effects on the outcome.|
On a spatial scale, wetland functionality may apply to a single water body, or a complex system of interconnected wetlands (each made up of one or more water bodies). Every water body and connection may be modelled. Flow of water, its ownership (if relevant) and constituents through a wetland is calculated and reported at every model time-step.
This version of Wetland modelling has been developed by eWater CRC for Source.
The approach to wetland modelling used in Source is the Cells Model. The Cells Model was used in the formulation of the first Mekong model (Zanobetti et al 1970). This approach is employed by IQQM, which has been used in Australia for many years.
None. A wetland exists in a Source scenario once a Storage node is created.
The following assumptions and constraints apply:
- A uniform water level (ie. level pool) is assumed across each storage compartment
- The variation of water level and flow is assumed to be slow enough that the hydraulic conveyance can be estimated based on a quasi-steady-state assumption.
- The response of a wetland is slow enough that all groundwater, management and ecological responses can be made based on information of the last time-step.
- Daily or sub-daily time-step is small enough for the quasi-steady-state hydraulic response.
Wetland clusters and the "Cells model"
The group of nodes and links that represent a wetland in Source are referred to as a "wetland cluster". These can be Storage nodes, Wetland links and Hydraulic Connector nodes.
Figure 1 illustrates some types of wetland that may be represented in Source by nodes in a wetland cluster:
- A wetland without river flow (green box). Examples include upland swamps, or spring fed wetlands on a floodplain.
- A wetland with a single water body (purple box). Examples include billabongs and oxbow lakes.
- A complex wetland with multiple water bodies (grey box). There are multiple paths of river flow through this type of wetland. Examples include the Macquarie Marshes, Hattah Lakes and Menindee Lakes.
Boundary conditions in the Source model shown in Figure 1 occur whenever the clusters (boxes) intersect with a non-Wetland link (dashed line).
In Source each wetland cluster in a scenario is treated as a modelling unit, which is processed using a Cells Model. In this type of model, a wetland consists of a number of storage cells with the movement of water between them described by a set of functions (that are specified by the modeller) and boundary conditions. The main elements of the cells model and the Source components that are used to represent them are shown in Table 1.
Table 1. Cells model vs Source components
Wetlands Hydraulic Connector node, Storage node inlet or outlet
Water surface elevation (WSE or "Reduced level")
The Cells Model approach assumes that the movement of water within a wetland is related to the head difference between the water bodies that make up the wetland. To enable this in Source, the water surface elevation (WSE) is determined for nodes in a wetland cluster.
Wetlands Hydraulic Connector nodes and boundary conditions
Boundary conditions are points at which water can flow in to or out of a wetland. In Source, these points are at either a Wetlands Hydraulic Connector node or a Storage node’s inlet or outlet link (Figure 1). The Wetlands Hydraulic Connector node is used to represent situations where a wetland interacts with a river reach and the inflows and outflows are too small to have an effect on the water surface elevation in the river.
Figure 2 shows a representation of this type of wetland in Source, where the wetland’s boundary is modelled at the Wetlands Hydraulic Connector node.
Figure 2. Wetland boundary at an hydraulic connector
If the outlet of a Storage node in a wetland cluster is connected to a conventional link then that storage is a boundary of the wetland cluster. Examples in the Murray include:
- Lock 3, which could be modelled as a Storage node at a boundary of a wetland system that includes Lake Bonney, and;
- Euston Weir, modelled as a Storage node at a boundary of the Euston Lakes wetland system.
Figure 3 shows a representation of this type of wetland in Source, where the wetland boundary is modelled at the Storage node’s outlet.
Figure 3. Wetland boundary at an on-river storage/weir
The rules governing outflow from a boundary depend on the type of node:
Wetlands Hydraulic Connector nodes:
- Mass balance: Downstream flow is the difference between upstream inflow and total flow in connected wetland links.
- Flow in each connected wetland link is a function of water surface elevation (WSE) at the hydraulic connector node and the node at the other end of the link. See the Wetland Links (Connectors) section for more information.
- The modeller specifies a relationship between upstream inflow and WSE at the hydraulic connector node.
- If the storage node is a weir, and this is within its target operating range, releases are set so that the storage remains at the same volume where possible.
- When there is a spillway, there is a minimum volume "spilled". This amount varies with water surface elevation.
- The release from a storage is determined by the volume ordered, and is limited by outlet capacity (which varies with water surface elevation). It also takes into account spill.
Wetland links (connectors)
In a Cells Model, connectors allow water to move between cells. In Source, connectors are represented by the wetland link. Flow rate in a connector can be described as a function of water surface elevation (WSE) at each end of the link:
fQc is a function that returns flow rate along connector c.
Qc is the flow rate along connector c. A positive value indicates a flow from WSE1 to WSE2.
WSE1 and WSE2 are the water surface elevations at each end of connector c.
Flow in a wetland link can be in either direction (unless the modeller specifies otherwise). The default flow direction is set when the modeller selects one node before the other when creating the wetland link. It is indicated by the direction of the arrow on the link in the Schematic Editor. Flow that moves in the default direction is represented by a positive number, while flow in the other direction is represented by a negative number.
Currently Source has only one type of wetland link function implemented to represent a channel. The specific form of wetland link flow function for a channel implemented in Source is:
Qc is the wetland link’s flow rate (m3/s).
C() by the square root of the channel length (m2.5/s).
ΔWSE is the drop (m) in surface water elevations along the channel.
In cases where ΔWSE<0.01m, the flow function is adjusted to prevent computational issues due to an infinite slope of the curve at the point where ΔWSE approaches zero. This adjusted function is:
The value for hydraulic conveyance will vary according to where it is calculated. To model this, a piecewise relationship between modified conveyance c and water surface elevation ( referred to as reduced level) is configured using the wetland link’s feature editor. The WSEc at the point at which the modified conveyance is calculated is estimated as:
WSE1 is the water surface elevation at the originating node
WSE2 is the water surface elevation at the destination node
weighting is a spatial weighting factor configured by the modeller in the wetland link’s feature editor. It is specified in relationship to the from end of the link.
Source interpolates between points in the defined modified conveyance versus water surface elevation relationship to find the value of C that corresponds to WSEc. The originating and destination nodes are defined by the modeller by which direction they drag the wetland link into their system and is shown as an arrow on the link.
Source models regulated flows in wetland links by allowing the modeller to specify a target flow rate. This limits the rate of flow on the wetland link to the target value. , ie. if a positive target flow rate is configured, Source will not permit flow in the opposite direction on the wetland link (ie. the value will be set to zero if the flow calculation returns a negative value, and vice versa). The following calculations adjust the channel flow rate to implement the regulated channel.
Where target flow rate Qtarget ≥ 0
Where target flow rate Qtarget < 0
Storage nodes (Cells)
In Source, storage nodes are used to represent the "cells" or wetland compartments in which water can be stored. A storage node has a relationship between water surface elevation and surface area that can be used to represent the spatial distribution of water in a wetland. Another function relates its water surface elevation WSEws to the volume of water stored:
Aws is the surface area of storage node ws.
Sws is the volume stored in storage node ws.
fAws is a function that relates surface area to water surface elevation of the storage node ws.
fSws is a function that relates storage volume to water surface elevation of the storage node ws.
WSEws is the water surface elevation of storage node ws.
The functions above are based on relationships configured by the modeller in the Storage node Dimensions table (in the Storage node's Feature Editor). The Level parameter found in this table is translated into WSE by addition of the node's Elevation parameter (set in the same dialog).
Mass balance equations are used to model the behaviour of a storage node as a cell within a wetland cluster’s cells model. As an implicit Eulerian numerical scheme is used, for each storage node the mass balance over an arbitrary time period is:
S1ws is the volume of water stored in storage node ws at the start of the period
S2ws is the volume of water stored in storage node ws at the end of the period
Inws is the volume that flowed into storage node ws over the period
Outws is the volume that flowed out of storage node ws over the period.
The storage node’s WSE is an input parameter to its cells model calculations (the process by which this is ascertained is described later starting at Model Initialisation). Elements of each storage node’s mass balance equation are calculated as follows:
End of time-step storage volume S2ws is determined using the function described above that relates this attribute to WSE.
Volume of inflow to the storage node:
Iws is the volume of upstream inflow entering storage node ws from outside the wetland cluster (ie. via its inlet link).
Groundwaterws(Levelws) is the groundwater flow volume over the time-step in storage node ws. The relationship between groundwater and level is defined in the Seepage table, configured by the modeller on the storage node. WSE is converted to Level by subtracting the base Elevation.
NetEvaporationws is the net evaporation flow volume for the time-step in storage node ws. This is a combination of the Rainfall and Evaporation functions configured by the modeller on the storage node. These would usually have surface area Aws as a parameter.
WetlandFlowws is the total flow volume over the time-step on wetland links connected to storage node ws. See the Wetland Links (Connectors) section.
Volume of downstream outflow from the storage node (leaving the wetland cluster): This calculation depends on whether the storage node represents a weir, and if so, where the weir’s WSE is in relation to its operating range:
- Weir: If WSEws ≤ minimum target WSE, downstream outflow is zero.
- Weir: If WSEws ≥ maximum target WSE, downstream outflow is set to the maximum of the orders and (entering the node via the inlet link).
Where the storage node is not a weir, or the weir’s WSEws is within its target range:
dt is the length of the time-step.
fomax() is the outlet path capacity relationship that relates the storage node’s WSE to the maximum flow rate that can be made through the outlet path. Note that this capacity includes a spill rate that is also determined by WSE.
fSpillo() is the spillway relationship for outlet path o that relates the storage node’s WSE to the flow rate leaving the storage node over the spillway.
nows is the number of outlet paths from storage node ws.
o is a storage node outlet path.
Ordero is the flow rate ordered to go down outlet path o.
Wetlands and ordering
Orders can be placed on behalf of a wetland. This may be modelled in Source by connecting a water user node with an in-stream demand model to a supply point node upstream of the wetland cluster for which the water is to be ordered, and using "target flow" functions on the wetland links to move the water into the wetland once it arrives.
The order processing for both the above configurations is described in the Scientific Reference Guide entries for the water user node, supply point node and ordering system.
Storage nodes on the boundary of a wetland cluster may also be used to supply downstream orders. The interaction of the storage node and ordering system are described under Scientific Reference Guide entries for these items.
Conservative constituents in wetlands
Source treats all water bodies modelled using the storage node as fully mixed. This is discussed in more detail in the Scientific Reference Guide entry on the Storage node. Conservative constituent modelling in wetland links is not yet implemented.
At the start of the scenario run, the schematic network analyser works from upstream to downstream to identify the nodes and links that make up each wetland cluster (representing a wetland). The following rules are used:
- Node types that may belong to a wetland cluster include the hydraulic connector and the storage node. A node is a part of a wetland cluster only if it has a wetland link connected to it.
- Each node can only belong to one wetland cluster.
- If a node has not been assigned to a cluster yet, and is connected via a wetland link to another node that has been assigned, it is assigned to the same cluster as that node.
Source keeps track of the number of nodes that are in each wetland cluster, referred to as NodesInCluster.
When the schematic network analyser encounters a node in a wetland cluster during the flow phase, it processes the cluster together as a unit before running the model for individual nodes and links that make up the cluster. The component that performs the cluster’s Cells Model processing will be referred to here as the "cluster processor".
Cluster processing results in an estimate of the water surface elevation in each storage node and the direction and rate of flow in each wetland link. The model that is later run for each individual storage node determines the node’s final water surface elevation for the time-step. The storage node model is described in the Scientific Reference Guide entry for the Storage node.
As the order of execution of model components cannot be specified, the cluster processor may need to process storage and hydraulic connector nodes for which the current time-step’s inflow rate has not yet been calculated. In these cases, the previous time-step’s inflow values are used.
Flow phase processing of each wetland cluster will only occur once in every time-step, no matter how many connections there are between wetlands or rivers modelled.
The cluster processor uses an iterative method to find and return a set of water surface elevations that satisfy the mass balance equation for every one of its storage nodes. The initial iteration uses the following initial values for WSE:
- Hydraulic connector nodes: The Reduced Level value for the current rate of inflow in the piecewise relationship between Average Inflow and Reduced Level specified by the modeller.
- Storage nodes: The WSE at the end of the previous time-step.
At each subsequent iteration a new set of trial values is created and fed into storage node mass balance equations. This involves the following steps:
- The trial WSEs are used to determine flow rate and direction in the cluster’s wetland links as described under Wetland Links (Connectors).
- The mass balance equation is evaluated for every storage node in the cluster. This equation is described under Storage Nodes (Cells).
- The precision of the solution is determined and compared to the cluster’s convergence limit.
- Precision is calculated as the root mean square of storage node mass balance volumes:
ws is a storage node in the wetland cluster
MassBalancews is the result of the mass balance equation for storage node ws.
nws is the number of storage nodes in the wetland cluster
The wetland cluster’s convergence limit is calculated based on the scenario’s wetland node convergence limit that was specified by the modeller.
Precision is compared to the cluster’s convergence limit to determine whether a solution was found:
- If the Precision < ClusterConvergenceLimit, or the maximum number of iterations has been reached, the process of solving the cluster is finished.
- If the Precision ≥ ClusterConvergenceLimit and more iterations are allowed, parameters for each storage node in the cluster are adjusted ready for the next iteration:
- The mass balance error is subtracted from the end of time-step storage volume estimate
- The corresponding WSE for the adjusted storage volume is determined using the Storage Dimensions relationship (between storage volume and level).
The solution technique implemented in Source allows other types of node to be used within wetland clusters that like the hydraulic connector node have a WSE that is not impacted by wetland link flow. At present this functionality is not used as the only types of node that may be connected by wetland links are storage nodes and hydraulic connector nodes.
All input data is configured via user interfaces. The parameters are summarised in the following tables:
Table 2. Wetland Configuration parameters
The maximum number of sets of node WSEs (trial solutions) to examine for a wetland cluster in finding an acceptable one, ie. where Precision ≤ Convergence Limit. Equation 13 of the Flow phase section describes this further. On rare occasions this number will be exceeded by up to 2.
Integer > 0
The upper threshold on error per node for an acceptable wetland cluster solution. At each solution iteration of a wetland cluster’s Cells Model, its Precision is compared to the Convergence Limit to determine whether it is acceptable (and hence the final one).
Real number > 0
These parameters are accessed via the scenario Edit menu, and apply to all wetland clusters in the scenario.
If log diagnostics are required, there are also parameters that determine the location of the log file, the time-steps to be logged, and whether to only log those in which no acceptable Cells Model solution could be found. These parameters are described in more detail in the Source User Guide.
Table 3. Location Control parameters
Note that this information is also available in the Schematic Editor on the node’s tool tip.
Table 4. Hydraulic Connector node parameters (average inflow vs WSE)
Average inflow rate
Average inflow rate associated with the WSE
Real numbers ≥ 0
Table 5. Wetland link parameters
Name of the node connected to the link from which outflow is given a positive value and inflow a negative value.
First node connected
Name of the other node connected to the link from which outflow is given a negative value and inflow a positive value.
Second node connected
Indicates whether flow can move in both directions
Bidirectional or Unidirectional
Represents the point at which conveyance is calculated. Determines the influence of each end of the link’s WSE on conveyance. A value of 0.5 gives each end equal weighting.
Real numbers between 0 and 1
Modified Conveyance vs Water Surface Elevation (or Reduced Level) Table
Modified conveyance for a water surface elevation
Real numbers ≥ 0
Water surface elevation (WSE)
WSE for a modified conveyance
Flow Regulation Parameters
Indicates whether the link is regulated.
Yes or No
Table 6. Storage node parameters
Storage Dimensions Table: This describes a relationship between the storage’s Level, Volume and Surface Area
Maximum depth of water in the storage
Row 1: 0
Row 2: 100
Real numbers ≥ 0
Volume of water stored in the storage
Row 1: 0
Real numbers ≥ 0
Surface area of water in the storage.
Row 1: 0
Row 2: 1000
Real numbers ≥ 0
Initial storage level
Initial depth of water in the storage.
Volume: Default m
Real numbers ≥ 0
Minimum operating level
Minimum depth of water to operate the storage at. Below this level no releases are made.
Volume: Default m
Real numbers ≥ 0
Maximum operating level
Maximum depth of water to be kept in storage. Releases are made to attempt to ensure this depth is not exceeded.
Volume: Default m
Real numbers ≥ 0
Rainfall and Evaporation: Used to calculate NetEvaporationws
Value returned by the selected time series, function or scenario. Used to represent storage inflow/gain due to rain falling on the surface of the water stored.
distance/time: Default mm/day
Value returned by the selected time series, function or scenario. Used to represent storage outflow/loss due to evaporation from the surface of the water stored.
distance/time: Default mm/day
Seepage Table: Used to calculate Groundwaterws
Depth of water in storage associated with the rate of seepage.
Row 1: 0
Real numbers ≥ 0
Rate of seepage associated with a depth of water in storage.
Row 1: 0
Outlet Level versus Discharge Tables: Used to calculate fSpillo(WSEws) and fomax(WSEws)
Depth of water in storage associated with a discharge rate.
Row 1: 0
Real numbers ≥ 0
Use of these parameters is described in Storage Nodes (Cells). For a full list of Storage Node parameters, see the Source User Guide.
Output data is viewed using the Recording Manager. The recorded attribute values (results) are reported for each time-step of the model run. Results relevant to wetland Cells Model processing are summarised in the following tables:
Table 7. Wetland cluster (Cluster solver) attributes
Number of solutions (sets of storage node WSE) that were tried for the time-step before one was found with acceptable (ie. Precision ≤ Convergence Limit) or the maximum iteration limit was reached. See Equation 13.
Integer, between 0 and the Maximum Iterations setting for Wetlands on the Edit menu.
Table 8. Storage node attributes
Cell mass balance error
Value returned from the storage node’s Cells Model mass balance equation, described under Storage Nodes (Cells). This result comes from the final trial solution for the wetland cluster in each time-step.
Table 9. Hydraulic connector node attributes
Average Reduced Water Level
Elevation at the surface of the water at the point in the river represented by the hydraulic connector node. This is based on the upstream inflow rate.
Rate of flow (or flux) to/from the wetland(s) connected to the river at the hydraulic connector node.
Table 10. Wetland link attributes
Average flow rate
Rate and direction of flow in the wetland link. See Qc in the Wetland Links (Connectors).
Total flow volume
Total volume and direction of flow in the link over the time-step. Equals Average flow rate • time-step length.
Average reduced water level
Elevation of the water surface at the point at which the link’s conveyance is calculated; WSEc described in Wetland Links (Connectors)
Real number. Should be between From node reduced level and To node reduced level.
From node reduced level
Elevation of the water surface, calculated by the cluster solver, at the end of the link configured to be upstream by default.
To node reduced level
Elevation of the water surface, calculated by the cluster solver, at the end of the link configured to be downstream by default.
Maximum permitted flow rate on the link. Also indicates the permitted flow direction. See the sub-section in Wetland Links (Connectors).
For a full list of output data for wetland nodes and links, refer to the Source User Guide.
Zanobetti, D., Lorgeré, H., Preissmann, A., & Cunge, J.A., 1970, Mekong Delta mathematical model program construction, J. Waterways and Harbors Division, American Society of Civil Engineers, vol.96, no.2, pp.181-199.