...
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.
Data requirements
For details of the input data requirements, parameter settings and descriptions, and output data, please see the Farm Dam Node chapter in the Source User Guide.
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:
...
a and b are user-inputted regression coefficients, as defined by McMurray (2004)1
Upstream flow
The total collected flow at an Onstream Farm Dam node consists of two parts:
...
Vt- The intermediate storage volume at time step t from Equation 12 or 13
Available water, Demand and Releases
The available water for an onstream farm dam can then be calculated by Equation 17 as:
...
DamCap - User input farm dam capacity
If the ActDemandVolt is greater than the AvailableWatert calculated in Equation 17 above, Demand and Available Water are calculated according to Equation 21. Else Equation 22 is used.
...
Where:
SupplyDemandVolt – The water to supply the actual demand volume from the farm dam at time step t
Vt- The storage volume at time step t according to Equation 15 or Equation 16
AvailableWatert – Water available for release at time step t from Equation 21 or Equation 22
If available water from Equation 21 or Equation 22 is zero, there is no Release at this time step, as shown in Equation 23. Else, Release is calculated as in Equation 24 and Equation 25.
...
If the required ReleaseVolt is greater than AvailableWatert as calculated in Equation 22 above, the model continues the estimation according to Equation 24, else Equation 25 is used:
...
Where:
ReleaseVolt – The Release Volume requested at time step t
ActReleaseVolt – The actual water volume released from the farm dam for Release Volume request at time step t
Vt- The storage volume (i.e., the available water) at time step t from Equation 21 or Equation 22
Spill
Spill occurs if the storage volume Vt as calculated by Equation 25 is greater than Dam Capacity and Storage volume and Spill volume are calculated according to Equation 26. Else according to Equation 27:
...
Where:
SpillVolt – The actual spill volume from the farm dam at time step t
Vt - Final storage volume at time step t
Downstream Flow
Finally, flow directly downstream from the farm dam is calculated by Equation 28:
...
Where:
DSFlowt – The actual water volume downstream of the farm dam at time step t
The Source simulation of onstream farm dams is based on a generalised water balance, and the final flow output downstream of the farm dam takes into account all components such as actual Interstation flow, actual diverted flows, actual inflow volume, actual bypassed flows, actual rainfall volume, actual evaporation volume, actual seepage, storage volume and downstream flow.
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.