What is it?
A two-way head dependent boundary condition assigned to a zone allow water to enter or leave the aquifer system depending on the relative difference between the surface water stage and the hydraulic head in the adjacent aquifer model cell.How to use it:
Check the radio box next to ‘Two-way’. The drop-down menu next to ‘Name’ provides different naming options for the zone feature that will be used in displaying results, e.g., when performing water balance analyses.Stage
The stage/top elevation of the water source/sink can be assigned as:- a constant value that does not change with time – select the radio button next to ‘Const:’ and enter the desired value)
- a constant value that is time-varying – after selecting the radio button next to ‘Const:’, check the radio box next to the ‘Transient’ button. (Click on ‘Transient’ button to open the ‘Transient Data’ input menu.)
- the aquifer top elevation minus some prescribed value; by default, the stage is set to the aquifer top elevation, as indicated by the value of zero in the text box next to ‘Top E ‘.
From coupled SW/GW modeling
Check this option to compute stage as an output from integrated aquifer-lake simulation (i.e., aquifer head and lake stage are solved simulatanously for each time-step of simulation). Click the 'From coupled SW/GW modeling' to launch the interface needed to specify additional inputs for modeling (lake balance terms).River Bed
River/lake bed elevation is needed in cases where groundwater head is lower than river/lake bed elevation. When the aquifer head drops below the river bed (which does not happen often, but we have to take this into account in a general software system):Leakage rate = leakance x (river water level elevation - bed elevation)
In other words the aquifer head at the bottom of bed sediment thickness = bed elevation because the aquifer pressure head there = 0. We have multiple ways to define river/lake bed: 1 - Input depth, then bed elevation=Stage-depth (select 'Stage minus' radio button) 2 - Input elevation, then bed elevation=Input elevation (select 'Elevation' radio button) 3 - Using a bathymetry spatial layer
The first option is commonly used for real-world sites; the second option is available for simplified/synthetic models.
Bathymetry input
Select the radio button next to 'IsBathymetry' to use a bathymetry spatial layer. A prompt will apppear indicating how to assign a reference elevation for use with the bathymetry layer, which provides depth information (not absolute elevation). IGW-NET will use the reference elevation (depth=0) to compute the elevation of the river/lake bed. Enter '-999999' to use DEM as the reference elevation.The file format for uploading is ASCII raster file format (.txt file). It should include a few lines of header data followed by lists of cell values. The header data includes the following keywords and values:
ncols – number of columns in the data set
nrows – number of rows in the data set
xllcorner – x-coordinate of the lower-left corner of the lower-left cell.
yllcorner – y-coordinate of the lower-left corner of the lower-left cell.
cellsize – cell size for the data set.
nodata_value – value in the file assigned to cells whose value is unknown. This keyword and value is optional. The nodata_value defaults to -9999.
NOTE:
1)The coordinates should be expresed in the same projection system that is used in the model. (This is automatically recommended/chosen by IGW-NET, but you can set a different system in the Miscellaneous tab of the Domain Attributes interface.)
2) The assumed units for water depth is meters. Use the 'multiplier to m' input to convert from different units to meters (e.g., if depth is in units of feet, use 0.3048 as the mulitplier).
3) Apply an offset to all depth measurements by entering a non-zero values as the 'minus' input. A positive value subtracts from the depth; a negative value adds to the depth.
Leakance
Use the text box next to ‘Leakance:’ to enter a value . The leakage rate per unit area for a leaky polygon or lake = leakance x DH, where DH is the head difference across the bed sediment thickness. The leakance for a lake (or leaky polygon) is defined as a ratio: bed sediment conductivity / bed sediment thickness with a dimension of [1/T or 1/day]. Leakance = leakage rate per unit area per unit head difference across the sediment.Concentration source
The two-way source/sink can be assigned a solute concentration for the purposes of transport modeling (in the case that the zone is acting as a source). Enter a non-zero value in the text box next to ‘Conc:’ (units: ppm) to apply a source concentration to the source/sink.To assign time-varying source concentration, check the radio box next to the ‘Transient’ button, and then LM click the ‘Transient button Check the radio box next to the ‘Transient’ button to open the Transient Data for Lake Concentration sub-menu