Conceptual Groundwater:Conceptual Groundwater

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A conceptual groundwater scheme based on the SAC-SMA model (Burnash, 1995) can be used to simulate groundwater discharge to the stream without the need to solve the full 2D lateral free surface groundwater flow equations. This method will provide an estimate of base flow, but not information about the groundwater heads in the watershed, the location of saturated source areas, etc. As described by Burnash (1995)the groundwater is conceived as two layers, an upper zone with fast flow and a lower zone with slow flow. Flow to the upper layer is from the GSSHA infiltration routine. Once the upper zone storage is filled, water can percolate to the lower zone and move laterally as interflow. Once the storage in the lower layer is filled, water in the lower zone moves to two reservoirs. Outflow from the two reservoirs represents short and long term baseflow. The outflows from these reservoirs is added to the stream each update.

A special card based input file is used to specify the inputs for the conceptual groundwater model. To use the conceptual groundwater model, the CONCEPTUAL_GW card is placed in the project file *.prj, along with the name of the input file. Also, the location of cells in the grid with streams that receive base flow is defined with the LINKS card, followed by the name of the LINKS index map.

Cards in the project file

LINKS *.lik

Conceptual groundwater model input file

The *.cep file contains the parameters and values used in the conceptual groundwater model. The first card in the file is the GSSHA_CONCEPTUAL_GW card, followed by the cards that define the parameters and their values.

SLOW_MAX - maximum storage in slow aquifer (mm).
FAST_MAX - maximum storage in fast aquifer (mm).
SLOW_RATE - rate constant between 0-1, determined by calibration.
FAST_RATE - rate constant between 0-1, determined by calibration.
SLOW_CONTENT - initial content of the slow reservoir, 0-1.
FAST_CONTENT - initial content of the fast reservoir, 0-1.
UNDERFLOW_PERCENT - percent of groundwater flow not contributing to the stream.

So that a conceptual groundwater input file, *.cep, would look something like this.

SLOW_MAX 100.0
FAST_MAX 500.0
SLOW_RATE 0.003181
FAST_RATE 0.0312

Lateral base-flow contribution to channel segment

The *.lik file is an index map that shows the location of cells in the grid where streams receive base flow. Base flow is apportioned to these cells via groundwater contributing area. The groundwater contributing area to a cell is assumed to be the same as the contributing surface water area of the cell.


Burnash R.J.C., 1995, The NWS river forecast system - catchment modeling, Computer models of watershed hydrology, Edited by V. P. Singh, pp. 311-366.

GSSHA User's Manual

21 Conceptual Groundwater