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 - for each dam, Source conducts a water balance, considering inflows, 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 1 (Equation 2):
Where:
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
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.