Onstream Farm Dam SRG

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

Open

Figure 1 Water balance of an onstream farm dam

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:

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)

Open

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):

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

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:

The total upstream flow at the farm dam is then calculated as in Equation 5:

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).

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 ActDivFlow_t, 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:

Rainfall

The Rainfall volume to the farm dam is estimated by Equation 9:

The intermediate storage volume at the farm dam is then estimated by Equation 10:

Losses

The seepage volume from the farm dam at time step t is estimated by Equation 11:

If ActSeepageVol_t is greater than the available water V_t, the estimation is then continued by Equation 12, else by Equation 13:

The evaporation volume from the farm dam at time step t is calculated by Equation 14:

If ActEvapVol_t is greater than the available water V_t, the estimation is as in Equation 15, else by Equation 16.

Available water, Demand and Releases

The available water for an onstream farm dam can then be calculated by Equation 17 as:

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:

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, the actual water demand ActDemandVol_t is zero.

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.

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:

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:

Downstream Flow

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

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.