Page Comparison

...

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:

...

V_t- 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:

...

V_{t- The intermediate storage volume at time step t from Equation 15 or 16}

If AvailableWater_t at time step t is less than or equal to zero, there is no water available to meet demands and releases at this time step and demand and release volumes are set to 0, as shown in Equation 18:

...

If AvailableWater_t at time step t is greater than zero, the model will calculate the water supplied for demands and releases as outlined in Equation 19 to Equation 25:

The average annual water demand for a farm dam is calculated by Equation 19:

...

Where:

AvgYearDemand - The average annual demand volume from the daily reference demand data

DemandTS_{i- The user input demand timeseries at time step i,}

and the total number of time steps in the demand time series is n

The actual water demand from a farm dam at time step t is estimated by equation 20 if AvgYearDemand from Equation 19 is not zero. Else, theactual water demand ActDemandVol_t is zero.

...

Where:

ActDemandVol_t – The actual water demand volume requested from the farm dam at time step t

DemandTS_t  - The user input demand timeseries at time step t

DemandFactor - User input Demand Factor to convert the actual water demand from the referenced water demand timeseries

DamCap  - User input farm dam capacity

If the ActDemandVol_t is greater than the AvailableWater_t calculated in Equation 17 above, Demand and Available Water are calculated according to Equation 21. Else Equation 22 is used.

...

Where:

SupplyDemandVol_t – The water to supply the actual demand volume from the farm dam at time step t

V_{t- The storage volume at time step t according to Equation 15 or Equation 16}

AvailableWater_t – 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 ReleaseVol_t is greater than AvailableWater_t as calculated in Equation 22 above, the model continues the estimation according to Equation 24, else Equation 25 is used:

...

Where:

ReleaseVol_t – The Release Volume requested at time step t

ActReleaseVol_t – The actual water volume released from the farm dam for Release Volume request at time step t

V_{t- 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 V_t 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:

SpillVol_t  – The actual spill volume from the farm dam at time step t

V_t  - Final storage volume at time step t

Downstream Flow

Finally, flow directly downstream from the farm dam is calculated by Equation 28:

...

Where:

DSFlow_t – 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.