1.   Purpose and objectives

     A package for modelling saturated and unsaturated groundwater flow, based on the finite element method. To calculate the heads, fluxes and path lines in heterogeneous anisotropic multi-aquifer systems.

2.   Description

     The flow is described by Darcy's law and the mass continuity equation. The Galerkin finite element method is used to approximate the differential equations as a set of algebraic equations, which are then solved by the conjugate gradient method. Both steady-state and transient systems can be modelled.

     The system can handle the following cases:

     Multi-layered aquifer-aquitard system, neglecting vertical resistance within aquifers and assuming vertical flow in aquitards. Fully 3-D schematisation by using up to 99 model aquifers as layers and 
aquitards for the vertical hydraulic conductivity. The aquifers can be laterally heterogeneous and anisotropic. The aquitards can be laterally heterogeneous. Wells fully penetrate the relevant aquifers. Phreatic conditions. Flow in vertical cross-sections. Systems with linear or non-linear relationships for fluxes from wells, rivers, top-systems (recharge calculated using a relation between hydraulic head and recharge parameters) and model boundaries. Systems with a sharp interface between the flowing 
water in the model and a stagnant fluid with higher density, according to the Badon Ghijben-Herzberg principle. Various types of boundary conditions along model periphery.

     The TRIWACO package consists of several separate programs:

     -  TESNET to generate the finite element grid (triangles including
        river, source and boundary positions);
     -  ARPADI to discretize the input data varying in space
        (e.g. transmissivities) with respect to the finite element grid;
     -  ARCADO for inputting parameters from the GIS ARC/INFO;  
     -  FLAIRS to solve numerically the governing equations. The non-linear 
        equations are solved iteratively. The linearized equations are
        solved by means of the conjugate-gradient method.
     -  TRIPLOT and other plotting programs for the presentation of 
        calculation results and viewing input parameters. 
     -  TRACK for calculating and viewing path lines, travel times and 
        response-characteristics.

3.   Input

     Hydrogeologic parameters, wells, rivers, recharge rate, boundary and initial conditions, and finite element grid structure.

4.   Output

     Heads for all aquifers and fluxes through all aquitards, fluxes to rivers, wells, top-systems and through boundaries. All data is stored in standard datasets which are very easy to use in pre-processing programs or for re-inputting. Several pre-processing programs can be used for viewing 
every aspect of the model. Also printed files are outputted with water- balances and fluxes

5.   Operational requirements and restrictions

     IBM compatible personal computer with 640 K memory, mathematical coprocessor and a hard disc. It is recommended having a 80386-processor or higher. The Graphical Kernel System for printing results includes drivers for most plotters and printers. The package is available on PC (DOS,  WINDOWS and OS/2) as well as on mainframes (UNIX). The user-friendly WINDOWS version is available from the end of 1996.

     Easy installation according to manual. Requires a short time to acquire proficiency in handling. As the FORTRAN 77 source code is not supplied, no FORTRAN compiler is needed by the user.

6.   Form of presentation

     Run version of the computer program package on a number of floppy disks. A comprehensive users manual in English.

7.   Operational experience

     The model has been widely used in The Netherlands since 1988. Many water supply companies and regional authorities have TRIWACO models for (operationally orientated) groundwater investigations. It has also been used in other countries like Hungary, Egypt, Indonesia, Belgium, Germany.

8.   Originator and technical support

     IWACO B.V., Consultants for water and environment, Rotterdam, The Netherlands.

9.   Availability

     From the HOMS National Reference Centre for The Netherlands.

10.  Conditions on use

     A charge will be made for supplying the model. Updates of the new run versions will be forwarded to users automatically during a certain period of time.