1.   Purpose and objectives

     The program SGMP simulates the dynamic behavior of groundwater systems. After calibration and validation using historical data, the model can be used to evaluate the impacts of various water management scenario's on future watertable behavior. The program can also be used to assess 
historical net recharge to groundwater system as a lumped value or to assess drainable surplus and drainage coefficient as function of de-watering depth in case of horizontal subsurface drainage systems.

2.   Description

     The model is based on the two well-known equations: Darcy's law and the equation of conservation of mass. The combination of these two equations results in a partial differential equation for unsteady flow. The solution of a partial differential equation is obtained by finite difference method. This method requires that space be divided into small but finite intervals resulting in an equivalent system of difference- differential equations, the simultaneous solution of which gives the function of h at a finite number of nodes. Since the watertables at the nodes change with time owing to changes in recharge and abstraction, the model also requires a discretization of time. Hence a number of successive 
time intervals have to be chosen. All equations are solved by an implicit numerical integration technique (Gauss-Seidel). This method of integration has the advantage that the magnitude of the time step t does not depend on a stability criterion.

3.   Input

     The program is fully interactive and user-friendly with pop-up menus and help screens to guide the user through the data entry. Basic input data are the type, extent and geometry of the aquifer system under consideration, and the aquifer characteristics like thickness, transmissivity and storativity. The calculations can be made in two modes. In both modes, initial watertable elevations must be prescribed together with time-dependent boundary conditions like heads or fluxes. In normal mode, time-dependent nodal net recharge also need to be prescribed, whereas in the inverse mode time-dependent nodal watertable elevations are required for all nodes in the nodal network.

4.   Output

     The results of the simulations are basically nodal watertable elevations as function of space and time; from these data, nodal groundwater balances are derived. The results of the calculations can be 
directly viewed on the monitor in tabular form and as groundwater hydrographs; these data can also be stored in files to be processed in commercially available spreadsheets and word processing programs. All spatial data can be exported in specific files and subsequently processed with packages like Surfer to produce watertable contour maps, aquifer thickness maps, topographic maps, etc.

5.   Operational requirements and restrictions

     Minimum requirements are a personal computer with 80286 micro- processor, 640 kB RAM, 20 MB hard disc, EGA graphics, MS-DOS operating system; co-processor is recommended. 

     The program is based on the following assumptions and restrictions:
        
     -  The aquifer is treated as a two-dimensional flow system.
     -  The lower aquifer is bounded at the bottom by an impermeable layer.
     -  The upper boundary of the aquifer is either an impermeable layer 
        (confined conditions) or a free watertable (watertable conditions).
     -  The calculation procedures for both confined and unconfined 
        conditions, depending on the relative position of the piezometric
        level with regard to the upper level of the aquifer, have been
        incorporated into the model. The model calculates the change in
        storage with either a specific yield coefficient (watertable
        conditions) or a storage coefficient (confined conditions).
     -  Both specific yield and storage coefficients can vary in a
        horizontal direction.
     -  Transmissivity is calculated as the algebraic product of
        hydraulic conductivity and saturated thickness; both can vary in
        a horizontal direction.
     -  For unconfined aquifers the transmissivity varies with time. The 
        model adjusts the saturated thickness according to the calculated 
        watertable elevation (non-linear conditions). Only the hydraulic 
        conductivity and the bottom of the aquifer must be prescribed.
     -  Both unsteady and steady-state conditions can be simulated by the 
        model.
     -  Darcy's law (linear resistance to laminar flow) and Dupuit's 
        assumptions (vertical flow can be neglected) are applicable for each 
        individual aquifer.
     -  The groundwater model can simulate head-controlled, flow-controlled,
        and/or zero flow boundaries; the first two may vary with time.
     -  Limits between which the water-level in the aquifer is allowed to 
        vary can be prescribed. If the water-level exceeds a certain limit,
        the model introduces an artificial flow rate that will keep the
        calculated water-level within that limit.
     -  The model can be run in the so-called inverse mode. It will then 
        calculate the nodal net recharge values as function of the
        prescribed watertable elevations.
     -  The processes of infiltration and percolation of rain and surface 
        water and of capillary rise and evapotranspiration taking place in
        the unsaturated zone are not simulated by the model. This means that
        the net recharge to the aquifer system must be calculated manually
        and prescribed to the model when the model is run in normal mode.
     -  Feather-edge boundary conditions, where the saturated thickness 
        reduces to zero as the water-level drops below the base of the
        aquifer, are included in the model.

6.   Form of presentation

     The program is available on diskette. A separate publication, ILRI Publication 29, is available on the principles of the model, including guidelines how the required input data can be processed from available field data.

7.   Operational experience

     The program has been used in various groundwater studies in Egypt, Pakistan, and India. More than 1000 copies have been distributed all over the world. The program has also been used successfully both in national and international training courses on groundwater management.

8.   Originator and technical support

     International Institute for Land Reclamation and Improvement ILRI
     P.O. Box 45, 6700 AA  Wageningen, The Netherlands
     Tel: (31 317) 490144, Fax: (31 317) 417187

9.   Availability

     Through the HOMS National Reference Centre for The Netherlands, or directly from ILRI.

10.  Conditions on use

     The program and manual are separately for sale. Discounts are available for educational purposes.