Water Resources Applications Software

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Summary of SHARP

       sharp - A quasi-three-dimensional, numerical finite-difference model
               to simulate freshwater and saltwater flow separated by a
               sharp interface in layered coastal aquifer systems

       When the width of the freshwater-saltwater transition zone is small
       relative to the thickness of the aquifer, it can be assumed that
       freshwater and saltwater are separated by a sharp interface.  The
       sharp interface modeling approach, in conjunction with vertical
       integration of the aquifer flow equations, facilitates regional
       scale studies of coastal areas.  This approach does not give
       information concerning the nature of the transition zone but does
       reproduce the regional flow dynamics of the system and the response
       of the interface to applied stresses.  SHARP is a quasi-three-
       dimensional, numerical model that solves finite-difference
       approximations of the equations for coupled freshwater and saltwater
       flow separated by a sharp interface in layered coastal aquifer
       systems.  The model is quasi-three dimensional because each aquifer
       is represented by a layer in which flow is assumed to be horizontal.

       An implicit finite-difference discretization scheme that is central
       in space and backward in time is used to solve the freshwater and
       saltwater flow equations for each model layer.  Spatial
       discretization is achieved using a block-centered finite-difference
       grid that allows for variable grid spacing.  In the central
       difference approximations for the space derivatives, the thicknesses
       at the grid block boundaries are linearly interpolated, and the
       conductivity terms are estimated using the harmonic mean of nodal
       values.  At blocks containing pumped wells, the amount of freshwater
       and saltwater extracted depends on the position of the interface
       relative to the elevation of the screened interval of the well.  The
       rate of freshwater and (or) saltwater extraction from a block,
       relative to the total fluid extraction rate, is determined linearly
       on the basis of the proportion of screen penetrating the freshwater
       and saltwater zones relative to the total open interval of the well.

       The interface elevation in each finite-difference block is
       calculated using the numerically-determined freshwater and saltwater
       head distributions.  The shape of the interface can be obtained by
       connecting the discretized interface elevations.  The position of
       the interface tip (the intersection of the interface with the top of
       the aquifer) and the interface toe (the intersection of the
       interface with the bottom of the aquifer) are located by linearly
       projecting a line defined by the interface elevations in adjacent
       blocks until it intersects the top and bottom of the aquifer.

       Version 1.1 1999/05/10 - Minor changes to bring code into closer
          adherence to Fortran 77 standard and to improve code portability;
          renamed subroutine "READ" as "READIN" to avoid name conflict with
          Fortran READ statement.

       Version 1.0 1990/09/12 - Initial release

       SHARP requires all of the input parameters typically required by a
       finite-difference ground-water flow model (initial conditions,
       boundary conditions, aquifer properties).  However, because it
       solves both freshwater and saltwater flow equations, it has
       additional input requirements.  The fresh and saltwater specific
       gravities and dynamic viscosities must be specified.  Freshwater
       hydraulic conductivities are specified and saltwater hydraulic
       conductivities are calculated in the model.  Fresh and saltwater
       specific storages, effective porosity and confining layer leakance
       values must be specified.  For interface tip and toe tracking, SHARP
       requires elevations of the base of each layer and the thickness of
       the layer.  Offshore bathymetric elevations are required to
       correctly represent offshore boundary conditions.  See documentation
       for details.

       See documentation.

       SHARP is written in Fortran 77 with the following extension:  use of
       names longer than 6 characters.  The code has been used on UNIX-
       based computers and on DOS-based 386 or greater computers having a
       math coprocessor.

       Essaid, H.I., 1990, The computer model SHARP, a quasi-three-
          dimensional finite-difference model to simulate freshwater and
          saltwater flow in layered coastal aquifer systems: U.S.
          Geological Survey Water-Resources Investigations Report 90-4130,
          181 p.

       Essaid, H.I., 1990, A multilayered sharp interface model of coupled
          freshwater and saltwater flow in coastal systems--model
          development and application: Water Resources Research, v. 26, no.
          7, p. 1431-1454.

       Essaid, H.I., 1992, Simulation of freshwater and saltwater flow in
          the coastal aquifer system of the Purisima Formation in the
          Soquel-Aptos Basin, Santa Cruz County, California: U.S.
          Geological Survey Water-Resources Investigations Report 91-4148,
          35 p.

       Operation and Distribution:
          U.S. Geological Survey
          Hydrologic Analysis Software Support Program
          437 National Center
          Reston, VA 20192

       Official versions of U.S. Geological Survey water-resources analysis
       software are available for electronic retrieval via the World Wide
       Web (WWW) at:


       and via anonymous File Transfer Protocol (FTP) from:

         (path: /pub/software).

       The WWW page and anonymous FTP directory from which the SHARP
       software can be retrieved are, respectively:


       If you would like to obtain the price of and (or) order paper copies
       of USGS reports, contact the USGS Branch of Information Services at:

                  USGS Information Services
                  Box 25286
                  Denver Federal Center
                  Denver CO 80225

       To inquire about Open-File Reports or Water-Resources Investigations
                  Tel: 303-202-4200; Fax 303-202-4695

       To inquire about other USGS reports:
                  Tel: 303-202-4700; Fax 303-202-4693

       mocdense(1) - A two-constituent solute transport model for ground
                     water having variable density
       sutra(1) - Saturated and (or) unsaturated, constant or
                  variable-density fluid flow, and solute or energy
                  transport (2-dimensional finite-element code)

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