Historically, functionality for modelling Farm Dams in Source was available through the FarmDams plugin. This Functionality was moved to core Source in Source version 5.30. While the implementation between the plugin and the core version might differ slightly, the principles and logic are the same and have been applied exactly as found in the plugin - for each dam, Source conducts a water balance, considering inflows, bypass, rainfall, evaporation, seepage, water usage (demand), and overflow (spill) components in each time step.
Scale
This node is treated as a point location even though the farm dam represented may have large dimensions. It can then be considered to be site scale. It is used at every model time step.
Principal developer
Sinclair Knight Merz (SKM) and the Western Australian Department of Water (DoW); eWater Solutions.
Scientific provenance
The water balance in the farm dam has been used from the earliest days of engineering hydrology.
Version
Source version 5.30
Structure and processes
Assumptions:
The solution technique assumes inflows, loss and gain fluxes, and outflows are averaged over a model time-step. This is consistent with the approach used in other parts of Source, such as link routing,
The water is balanced at the dam,
The dam storage is assumed to have a water surface area/level pool, and
The water losses such as infiltration (seepage) and evaporation occur before the water demand at any farm dam
Dependencies
Bulk Change Node Model tool and Farm Dams Observation Point Node were initially developed for use together with a Farm Dam node. However, both can be used for broader purposes.
Theory
For each dam, Source performs a water simulation based on the water balance between inflows, rainfall, evaporation, seepage, water usage and overflow components in each time step. Assumptions and detailed calculations for each element are described as follows.
Water surface area
The water surface area in the farm dam (A) is used to convert a depth added (e.g., rainfall) or subtracted (e.g., evaporation or seepage) at each time step to a volume which can then be added to/subtracted from the current storage volume at that time step. The area is estimated as a function of storage volume and other parameters, with three possible options:
Option 1: Assumes a constant surface area at each time step, as defined by the Max dam surface area input at the farm dam UI (Equation 1)
Where:
At - The surface water area at time step t
C – a constant defined by the input parameter Max. Dam Surface Area
Option 2: Allows a variable surface area per time step calculated as a non-linear function of storage volume at that time step using default coefficients according to a regression relationship as defined by McMurray (2004) 1 (Equation 2):
Where:
At - The surface water area at time step t
Vt-1 - The surface storage volume at the previous time step. If t is 1 (i.e., the first modelled time step), the initial water volume at the farm dam is used as defined by the Initial Storage Percentage
0.0006367522 and 1.071 are default values for coefficients a and b, respectively, as defined in Equation 3 below
Option 3: Allows a variable surface area per time step calculated as a non-linear function of storage volume at that time step using user-defined coefficients according to a regression relationship as defined by McMurray (2004) 1
Where:
At - The surface water area at time step t
a and b are user-inputted regression coefficients, as defined by McMurray (2004)
Upstream flow
The total collected flow at an Onstream Farm Dam node consists of two parts:
the flow from the upstream node(s), and
the interstation inflow generated between the upstream node and this dam. When farm dam functionality is integrated with catchment rainfall-runoff modelling, the concept of interstation inflow to an Onstream Farm Dam node is replaced by sub-catchment lateral flows. In this case the Interstation Flow Scaling Factor remains at its default of 1 and the Reference time series Value remains a default constant of 0.
When applied, interstation inflow is determined as in Equation 4:
Where:
ActISFlowt - the Actual Interstation Flow volume at the time step t. This is the Interstation Flow Reference series scaled down/up by the Scaling Factor
ISFlowFact - the Scaling Factor, an input constant parameter
ISFlowReft - The Interstation Flow Reference Time Series at time step t
The total upstream flow at the farm dam is then calculated as in Equation 5:
Where:
ToTFlowt - total upstream flow at time step t
USFlowt – total flow from upstream node(s) at time step t
ActISFlowt - contributing Interstation Flow
Total upstream flow is split in two directions: (i) flows diverted into the dam, and (ii) flows bypassed directly to the downstream of the farm dam. The Actual Diverted flow is defined by Equation 6 below, and the remainder of the total upstream flow will be accounted in the downstream flow (Equation 23).
Where:
ActDivFlowt - The actual diverted flows into the farm dam at time step t
DivFract - The percentage or proportion of total upstream flow will be involved in the water balance calculation in the edited farm dam
However, the actual inflow into the farm dam is related to the dam bypass capacity. As defined in the original plugin, if the bypass flow capacity for the farm dam is larger than or equal to the estimated actual diverted flow ActDivFlowt, all flow will bypass and no flows will be diverted into the farm dam, and actual bypass flow and actual inflow are estimated as in Equation 7. Else, they are estimated by Equation 8:
Where:
ByPassFlowCap - The farm dam’s bypass flow capacity, an inputted parameter
ActByPassFlowt - The actual bypass flow at time step t, controlled by the bypass flow capacity and actual diverted flows ActDivFlowt
ActInFlowt - The actual inflow to the farm dam at time step t, also controlled by the bypass flow capacity and actual diverted flows
Rainfall
The Rainfall volume to the farm dam is estimated by Equation 9:
Where:
ACTRainVolt - The actual rainfall volume to the farm dam at time step t
Raint - The inputted rainfall timeseries parameter at time step t
At - The surface water area at time step t
The intermediate storage volume at the farm dam is then estimated by Equation 10:
Where:
Vt – The intermediate storage volume at time step t
ActInFlowt - The actual inflow to the farm dam at time step t
ACTRainVolt - The actual rainfall volume to the farm dam at time step t
Losses
The seepage volume from the farm dam at time step t is estimated by Equation 11:
References
1 McMurray, D, 2004. Farm Dam Volume Estimations from Simple Geometric Relationships. Department of Water, Land and Biodiversity Conservation. South Australia. Report No. DWLBC 2004/48.