Aquifer Top Elevation

What is it?

This interface section is used to specify the top elevation of the aquifer (conceptual) layer.

How to use it:

  A. DEM as Top
  B. Confined Aquifers
  C. Constant Elevation
  D. DataNET Layer
  E. User Raster
  F. Scatter Points
  G. Additional Operations

A: DEM as Top Surface

The top boundary of surficial/unconfined aqufers follow the land surface, which can be represented with detailed Digital Elevation Models (DEMs). Accurate, detailed representation of the land surface improves model structure and calcultated related to the aquifer top surface (e.g., aquifer thicknness, gand roundwater elevation computed from land surface and depth-to-water measurements).

There are multiple DEM resolutions available from the MAGNET4WATER Data Center:

Outside the United States, 1000m, 300m and 90m resolution datasets are available from the ASTER Global DEM dataset. A 30m dataset is available from NASA STRM (this is also used if the user selects sub-30m DEM options, e.g., 10m DEM, outside of the United States).

Inside of the United States, 30m and 10m resolution is available from US Geological Survey National Elevation Dataset (NED), and in a few select locations, 3m or 1m is available.

NOTE ON DEM SELECTION: By default, the top elevation of the model follows the DEM of a resolution consistent with the model cell size (e.g., if the cell size is = 90m, the 90m resolution will be used, if the cell size is ˜30m, the 30m resolution will be used etc.). In areas where there is no DEM available at the selected resolution, the global 30M DEM from NASA STRM will be used.

NOTE ON MODELING WITH DEM: There are areas in which the DEM is locally inaccurate (e.g., existence of artificial depressions, "holes" or data gaps), which can lead to significantly distorted overall flow patterns. The model solution is particularly sensitive to this when the water table is shallow/flat. One may need to locally create a "patch" to fix the problematic areas.

Land Surface as a Drain

In instances where the groundwater head exceeds the land surface elevation, groundwater can leave the aquifer as a sink of water (i.e., groundwater is lost as surface seepage). This approach automatically captures the exchange of groundwater to surface water bodies as part of the robust solution process, as the surface water stages (elevations) are embedded in the high-resolution DEM datasets available on the MAGNET Data Center server.

IGW-NET also includes an Import Shapefile tool to add explicitly streams/rivers, lakes and wetland features as prescribed head or flux boundaries within a model (see the IGW-NET Quick Tutorial on this ).(Warning: prescribing boundary conditions of this nature requires careful application by the user, without which significant errors in the regional water balance can occur (even though the flow patterns may be correct).

Currently under development is an interface that will allow users to automatically extract NHD surface water features and add them as conceptual features to the model. (Eventually, we will include lakes, streams and wetlands on a global basis.)

B. Confined Aquifers

For a confined aquifer, users need to input a top elevation. This can be assigned as a constant elevation or it can be imported as a surface from a user raster file.

C. Constant Elevation

Choose the ‘Constant’ option and use the text box to enter the elevation to be used as the top boundary throughout the model domain (useful for controlled numerical experiments).

D. DataNET Layer

This option allows users to import from MAGNET4WATER DataNET a WCS data layer as a raster file of the aquifer top elevation surface. A "linked" DataNET page needs to be opened to be able to send a WCS layer from DataNET into IGW-NET (a prompt will appear if that is the case, allowing the user to open a new DataNET page to be linked).

Once the WCS layer is transferred into IGW-NET, it will be converted into a geotiff raster file uploaded to your user folder.

See the 'DataNET-based Model' Quick Tutorial for an example.

E. User-provided Raster

User raster files can also be used to represent the aquifer’s top surface. To do this, check the box next to the ‘Import’ link, then select the link to open a file browser. Browse to and select the appropriate file for importing. (NOTE: utilize this option if you wish to use your own DEM raster for an unconfined aquifer.

The following raster file formats are supported for import of user DEM files:
1 - Ascii format with the same projection (PRJ) as your model PRJ
2 - Geotiff (.tif) format with PRJ information embedded in the .tif file
3 - IMG format with PRJ information embedded in the .tif file

F. Scatter Points

You can also import scatter points of bottom elevations using zone features (see the 'Elevation tab').

G. Additional Operations

Multiplier to meter: The user raster file importer assumes that the Top Elevation data are in units of meters. If they are not, use the text field next to 'multiplier to meter' to apply an appropriate conversion (e.g., if the original data are in units of feet, enter 0.3048 to convert to meters).

Offset: It's possible to apply a universal offset to the Top Elevation surface when using the DEM or user raster file options. Simply enter a non-zero value in the text field next to 'minus'. A positive value will be subtracted from the top elevation value in each grid cell; a negative value will be added to the top elevation in each cell. (NOTE: units of entered value should be in meters.)