Groundwater-Surface Water Interactions

In many watersheds, surface waters and groundwater are hydraulically connected. A stream can contribute to groundwater recharge (a "losing" stream) or can gain water from the aquifer (a "gaining" stream) depending on the level of groundwater in the aquifer. Groundwater levels respond to natural recharge from precipitation, but can also be influenced by irrigation in the watershed, where a portion of this water may recharge the aquifer rather than be taken up by the target crop.

To simulate groundwater interactions with surface waters, there are four options.

Specify directly the amount of groundwater inflow to or outflow from a particular river or reach
Have WEAP model these interactions (groundwater "wedge" connected to river)
Have WEAP model these interactions (deep soil layer of Soil Moisture method catchment)
Link WEAP to MODFLOW

To select option 1 or option 2, go to particular groundwater node in the Data view under the Groundwater branch of Supply and Resources, and click on the Method tab. For option 3, create one or more catchment nodes with infiltration links to the groundwater node and enter data for the deep soil layer. If you are linking WEAP to MODFLOW, you should leave all surface water-groundwater interaction variables blank.

If you choose to specify the amount of groundwater inflow from or outflow to a river or reach (option 1), the values can be input under the "Groundwater Inflow" and "Groundwater Outflow" tabs in the Inflows and Outflows section for that river or reach. Inflows to the reach from groundwater are entered as water volumes; outflows from the reach to groundwater are entered as a percentage of streamflow.

WEAP can also model groundwater-surface water interactions using a stylized representation of the system (option 2). Groundwater can be represented as a wedge that is symmetrical about the surface water body, such as a river; recharge and extraction from one side of the wedge will therefore represent half the total rate. The recharge or extraction volumes are dependent on the elevation between the groundwater table (the surface representing full saturation of aquifer pore spaces) relative to the wetted depth of the river (see definition below). The additional parameters required to use this method are:

Hydraulic Conductivity: a measure of the ability of the aquifer to transmit water through its pores, represented in units of length/time.

Specific Yield: the porosity of the aquifer, represented as a fractional volume (a number greater than 0 and less than or equal to 1) of the aquifer.

Horizontal Distance: a representative distance for the groundwater-river geometry, taken as the length from the farthest edge of the aquifer to the river.

Wetted Depth: the depth of the river. This value is used as the reference for comparison to the simulated groundwater elevation.

Storage at River Level: the groundwater storage volume at which the top of groundwater is level with the river.

Maximum Head Difference: Setting the Maximum Head Difference will limit flow from river to groundwater in cases where the groundwater level is far below the river level. Leave blank if no maximum..

In addition to these aquifer-specific parameters, you will need to enter the Reach Length -- the horizontal length of the interface between the reach and linked groundwater -- as data for each reach that is connected to the aquifer.

Note: Even though some river reaches are connected to a groundwater node using option 2, you may link other reaches to the same node using option 1. if you wanted to do this (it is uncommon), for the reaches using option 1, specify the amount of inflow or percent of outflow, instead of the Reach Length.

Entered on: Data View, Branch: Supply and Resources \ Groundwater, Category: Physical, Tabs: Method, Hydraulic Conductivity, Specific Yield, Horizontal Distance, Wetted Depth, Storage at River Level