Groundwater

It is assumed that the lateral boundaries of subsurface (groundwater) and surface catchments are the same. The groundwater storage (aquifer) is con­ceptualised as a linear reservoir model (cf. “The Linear Reservoir Model” on page 193) with one or two layers. The conceptual structure of the two-layer groundwater component is shown in Figure 7.3. A single layer model only include the shallow (upper) aquifer.

Figure 7.3           Conceptual structure of the groundwater component

As illustrated in Figure 7.3 groundwater interacts with the surface water via groundwater recharge, groundwater discharge and seepage from river branches, reservoirs and connections. Moreover, when the water table of the shallow (upper) aquifer reaches the land-surface, it starts to spill directly into the river. Finally, groundwater from the deep aquifer can be pumped by water users.

Note that groundwater pumping is assumed to take place from the deep aqui­fer unless a one-layer aquifer has been specified (then pumping takes place from the shallow aquifer).

Under the Groundwater tab three types of groundwater properties/parame­ters are specified:

·         Depths

·         Time constants

·         Groundwater recharge time series

Properties can only be specified if a groundwater model has been selected under the general tab.

Depths

Initial water table depth

The initial water table depth (relative to ground surface) determines the mag­nitude of the groundwater discharge and the available water for pumping in the initial period of simulation. Depending on the time constant specified the initial depth may influence the results during a few days or up to several months of the simulation.

Outlet depth

The outlet depth (relative to ground surface) determines the storage capacity of the shallow aquifer and the storage capacity available for baseflow genera­tion in the deep aquifer. For shallow aquifers the water table can vary between the outlet depth and ground surface (depth = 0 metres) at which time overflow of groundwater occurs to the river. For deep aquifers (only rele­vant if a two-layer groundwater model is selected) with no pumping, the water table (used for calculating baseflow) can vary between the outlet depth of the deep aquifer and the outlet depth of the shallow aquifer, at which time over­flow of groundwater from the deep to the shallow aquifer occurs.

Bottom level (only relevant for deep aquifer)

Bottom level is the lowest level (largest depth relative to ground surface) from which water can be pumped from the deep aquifer. This indicates the total storage available for usage from the deep aquifer.

Time constants

A time constant (cf. ‘The Linear Reservoir model’ in the Appendix) is used to calculate the rate at which groundwater discharges into the river (as base­flow). The larger the time constant the lower the rate of discharge. A high time constant reflects a more constant baseflow contribution.

When a two-layer model is selected, separate time constants must be speci­fied for the Shallow aquifer and the Deep aquifer. Additionally, a time constant must be specified for the interface between shallow and deep aquifers. This constant is used to calculate the rate at which water percolates from the shal­low to the deep aquifer.

Groundwater recharge time series

A groundwater recharge time series must be specified if no rainfall-runoff model is selected under the General tab.

Click on the

 button and click Browse... to select an existing time series or click Create a new file… to open the ‘Create a new file’ dialogue.

Groundwater recharge is automatically calculated when a rainfall-runoff model is selected.