Conceptual Groundwater:Conceptual Groundwater

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 input file. Also, the location of the streams that receive the base flow is defined with the LINKS card, followed by the name of the LINKS index map.

Cards in the project file

card file

CONCEPTUAL_GW *.cep

LINKS *.lik

Conceptual Model Input File

The *.cep file contains the parameters and varaibles used in the conceptual groundwater. The first card in the file is the GSSHA_CONCEPTUAL_GW card. Followed by the cards that define the parameters.

SLOW_MAX is the 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 slow_content/slow_max - initial condition, 0-1.

FAST_CONTENT fast_content/fast_max - initial condition, 0-1.

UNDERFLOW_PERCENT is the percent of groundwater flow that does not contribute to stream.

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

GSSHA_CONCEPTUAL_GW

SLOW_MAX 100.0

FAST_MAX 500.0

SLOW_RATE 0.003181

FAST_RATE 0.0312

SLOW_CONTENT 0.36600

FAST_CONTENT 0.2113

UNDERFLOW_PERCENT 0.00

Lateral base-flow contribution to channel segment

*.lnk file is an index map that shows the location of the streams that receive base flow. The amount of groundwater contributing area to a stream link is assumed to be the same as the contributing surface water area of the stream link.

References

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