Stream/Aquifer Interaction Package

The Stream/Aquifer Interaction package is used to simulate the interaction between surficial streams and the groundwater. It is similar to the River package in that water can move from the stream to the aquifer or from the aquifer to the stream depending on the relative differences in the stream stage and the water table elevations. However, unlike the River package, flow is routed through the stream using simple channel hydraulics and Manning's equation is used to compute the stage in the stream.

NOTE: GMS does not support parameters for the stream package.

Stream Network

When the Stream package is used, a complete, ordered stream network must be defined as shown in the following figure:

Ordered Stream Network Used by the Stream Aquifer Interaction Package.

A stream network is composed of reaches and segments. A reach is the portion of a stream that lies inside a single cell. A single cell may contain multiple reaches. A segment is a group of reaches that forms one section of the stream. The reaches within a segment are always numbered from upstream to downstream. The segments should also be numbered from upstream to downstream.

The incoming flowrate must be defined for the topmost segments (segments 1, 4, and 6 in the above figure). Flow is then routed and combined to get the incoming flowrate for the other segments. An exception to this is a diversion such as the canal at segment #2 in the above figure. In this case the flowrate into the diversion should be specified. The flow to the diversion would be subtracted from the flow coming out of segment #1 and whatever is left over would be routed to segment #3.

In the input file for the Stream package, each reach is assigned the following values:

ijk indices of cell
segment ID
reach ID
stage (typically starting stage)
conductance
Elevation of the bottom of the streambed
Elevation of the top of the streambed

Each segment is assigned the following values:

incoming flow (for top segments and diversions)
width
slope
roughness coefficient
IDs of tributaries (upstream segments)
ID of upstream segment (for diversions only)

When Manning's equation is used to route the flow through the network, the stream channels are assumed to have a rectangular cross section.

Defining Streams

Unlike other stream packages in GMS, a stream network cannot be created by selecting cells in the grid and manually assigning values to the cells. Due to the complex nature of the input and the requirement that the data be ordered in a specific fashion, a stream network can only be created using a series of Stream Arcs in the Map module. The cell-by-cell values are automatically created and properly ordered when the Map -> MODFLOW command is selected.

The Stream Package Dialog

Once the Map -> MODFLOW command is selected and the stream data have been assigned to the grid cells, the data can be viewed and some of the input values can be edited using the Stream Package dialog. The top section of the dialog is used to specify the output options. The Compute stages toggle is used to specify whether the Stream package should compute the stages (specified stages are initial values only) or whether the specified stages should be used directly. The first spreadsheet lists the reach values and the second section lists the segment values.