AnAqSim Specifications

Aquifer Heterogeneity and Anisotropy

Confined, unconfined, and hybrid confined/unconfined domains.
Confined interface, and unconfined interface aquifers for fresh/salt interfaces.
Anisotropy in the horizontal may very in ratio and direction from one domain to the next.

Multi-Layer Models

Up to 15 levels in multi-layer (3D) models.
Vertical leakage between levels simulated with spatially-variable area sinks.
The layering scheme can vary from one part of model to another (e.g. 4 levels in the area of interest, transitioning to a single layer in the far-field).

Transient Flow

AnAqSim does fully transient modeling using finite-difference time steps and spatially-variable area sinks to model the storage fluxes. Every feature or element available in a steady model is also available in a transient one.
Time periods and time steps are the same as in MODFLOW, with a time step multiplier. Initial heads can be from a prior simulation (steady or transient) or specified as constant across each domain.

Input variables that may vary in a transient simulation include:

  • Discharges of discharge-specified wells and line boundaries
  • Heads at head-specified wells and line boundaries.  These can also turn on and off in different time periods.
  • Normal fluxes at normal flux-specified line boundaries
  • Stages of river line boundaries
  • Reference heads at head-dependent flux (3rd type) line boundaries
  • The flux rate of a flux-specified spatially-variable area source/sink (e.g. recharge or specified leakage)
  • The head of a head-dependent flux-specified spatially-variable area source/sink (e.g. pond stage)

Pumping Wells

Discharge-specified wells that span one domain
Discharge-specified wells that span multiple domains vertically. Heads at the well screen are matched in all domains spanned. AnAqSim determines the discharge from each domain spanned.
Head-specified wells that span one domain. AnAqSim determines the discharge needed to achieve the specified head.

Line Boundaries

Represented by high-order line elements with up to 10 degrees of freedom (strength parameters) per line segment
Discharge-specified line boundaries (e.g. leaching trench)
Head-specified line boundaries
Normal flux-specified boundaries. Specify the normal component of flow across the boundary. Can have non-zero rate or zero rate (impermeable).
Head-dependent normal flux line boundary (3rd type, like MODFLOW GHB)
River line boundaries. Discharge/length determined based in head difference between aquifer and river stage and the width, vertical K and thickness of the river bed resisting layer. Can dry up or revert to fixed discharge if heads fall below stage or base of resisting bed
Fracture/Drain line boundaries. These conduct discharge along the boundary, proportional to the potential gradient along the boundary and a user-specified conductance.
Leaky barrier line boundaries (e.g. slurry wall or sheet pile wall). These conduct flow normal to the boundary, proportional to the potential difference across the boundary and a user-specified conductance.
Inter-domain line boundaries apply where subdomains abut. Can mix layering across these (e.g. several levels on one side, only one level on the other)

Area Sink/Source

Used to model distributed discharges like recharge, leakage between layers, leakage to surface waters, leakage to aquifers below the model’s lowest level, and storage fluxes in transient models.
In single-layer steady models, can efficiently model uniform recharge with a uniform area sink.
Spatially-variable area sinks (SVAS) are used in transient models or models with multiple layers to model spatially-variable storage fluxes and vertical leakage.
SVAS can be specified by domain, by polygon, and with special spacings around wells

Map-View Plots may include

Contours of head, head difference between layers, or extraction (discharge/area due to recharge + leakage + storage fluxes)
Base map (dxf format)
Layout of model elements, with pop-up text to describe boundary conditions
Locations of control points used in the approximation of boundary conditions
Pathlines, single, multiple on a polyline, multiple spread over an area defined by a polygon, mulitple from a well, or multiple from a circle. Pathlines may include arrows at travel time intervals and include vertical elevation (3-D) information Pathlines can be selectively drawn to include only those captured by selected wells or line boundaries, for zone of capture studies
Vectors that show specific discharge, average linear velocity, or aquifer discharge (specific discharge times saturated thickness)
Calibration residuals at the calibration target location, color-coded for positive/negative residuals
User annotations: text, lines, rectangles, circles, and ellipses

Vertical profiles along a line may show:

Domain boundaries
3D pathlines projected onto the line, including time marker dots
Interface elevations
Aquifer discharges normal to or tangent to the line
Extraction rates (recharge, vertical leakage, storage fluxes)
Transient evolution of heads, vertical leakage, and storage flux through a simulation

Analysis of Results

Check the accuracy of boundary conditions in a variety of ways, text and graphs
Run calibrations with computed statistics. Calibration targets can be heads, head differences, head-specified well discharges, head-specified internal line boundary discharges, or river line boundary discharges.
Table or graphs (transient) of river discharges, head-specified line discharges, well heads, and well discharges

Write discharges across all external line boundaries.

Write vertical leakages over polygon areas or domain

Sum the discharge across a polyline
Hydrograph plots at specific locations show simulated transient head or drawdown. Can include observed hydrograph data read from a file to compare with simulated.
Graphs of transient head profiles along a line
Export the graphic or data from any of these graphs

Printing and exporting plots

Plots may be printed to any installed printer
User my crop the plot to a user-digitized polygon area
Control the area printed, the scale and orientation of the printed plot
Export plots in a variety of formats:

  • Surfer grid data for contouring within Surfer software
  • Vector Draw CAD file formats
  • DXF – CAD drawing exchange formats of various vintages (ASCII file)
  • WMF, EMF, SVG – Vector graphics file formats
  • BMP, GIF, JPG, TIF, ICO, PNG, – Bitmap file formats
  • PDF – Adobe portable document format

Export 3D pathlines, 3D head surfaces via CAD file formats. Can be imported into visualization software such as EnviroInsite.


Use the mouse to digitize points, polylines, circles, and ellipses. Often used in combination with a DXF base map
Digitizing includes snap-to capabilities so you can snap to existing elements, features in your basemap, or a regularly-spaced grid of points (e.g. points on 10m centers).

System Requirements

Windows operating systems: XP, 7 to 10 (32 and 64 bit)
At least 250 MB of hard disk space
At least 1 Gb memory, 8+ Gb of memory helps with larger problems

Microsoft .NET version 4.6

Documentation, Help

Detailed User Guide accessible as pdf or Windows help file through AnAqSim’s Help menu
Videos and tutorials that walk you through creating three models of increasing complexity.
Consulting available to help you apply AnAqSim to your sites and projects.