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/or 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 timestep.

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 contribution to the channel must be defined with the LINKS card, followed by the name of the LINKS input file.

Cards and files used in the project file

card file

CONCEPTUAL_GW *.con

LINKS *.lik

CONCEPTUAL_GW card is required to run the conceptual groundwater in GSSHA. This card is associated by the file *.con. This file contains the parameters and varaibles used in the conceptual groundwater.

Conceptual groundwater parameters and example parametric values in *.con file.

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

SLOW_MAX is the maximum storage in slow aquifer in mm.

FAST_MAX maximum storage in fast aquifer in 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, must be 0-1.

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

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

Lateral base-flow contribution to channel segment

*.lnk file is the location of the channel running grids. In developing conceptual groundwater, it is necessary to define the portion of the base flow that enters a channel segment. Groundwater contributing area to stream segments is defined by the contributing area of the stream segment.

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