GROUND WATER--Computer program for ground-water modeling; "Iterative digital model for aquifer evaluation," by P. C. Trescott.

                          GEOLOGICAL SURVEY
                       RESTON, VIRGINIA  20242

                                                      GW Branch

                                                  July 30, 1973
                                                      4300 5016


Subject:  GROUND WATER--Computer program for ground-water
                        modeling; "Iterative digital model for
                        aquifer evaluation," by P. C. Trescott.

A recently open filed report entitled "Iterative digital model for
aquifer evaluation," by P. C. Trescott, provides documentation for
a computer program that in part supplements and in part supersedes
the 1970 documentation by G. F. Pinder carrying the same title.
We request that you bring this memorandum most particularly to the
attention of ground-water specialists involved in modeling of
ground-water systems.

Trescott's preliminary documentation report is not a self-
contained report and requires familiarity with Pinder's original
digital model and its documentation.  Requests for copies of
Trescott's documentation and/or program deck should be limited to
those individuals with experience in modeling ground-water
systems.  Copies of his computer model were distributed to
attendees at the WRD Seminar on Digital Modeling of Ground Water
Flow held in Denver in January 1973.  A more detailed
documentation that will be self contained is planned in time for
the next ground-water modeling school scheduled in February 1974,
and subsequent release as an open-file report.

Trescott's program permits consideration of an aquifer that is
heterogeneous, anisotropic, and irregularly bounded.  The aquifer
may occur under water table or confined conditions at any point,
or part confined and part water table depending on the relation of
the head to the elevation of the top of the aquifer at that point.
This option permits consideration of the case where the head
changes enough to convert the aquifer at some places from confined
to water table conditions or vice versa.  The storage coefficient
assigned to a node depends on whether the aquifer is confined or
not at that time.  Also, the program limits leakage into the
aquifer when the head declines below the top of the aquifer.

Water may be derived from storage in the aquifer, uniform areal
recharge, recharge wells, constant head boundaries and leakage
from confining beds (or stream beds) in which the effects of
storage are considered.  Discharge may be from wells, constant
head boundaries, and evapotranspiration.  The model assumes
evapotranspiration decreased linearly from a specified maximum
value at the land surface downward to a given depth where
evapotranspiration ceases.  The model accomodates multiple pumping
periods.  Several improvements in the data input and output have
been made over Pinder's earlier program.

                                   (s) Gerald Meyer
                                   Chief, Ground Water Branch
                                   Water Resources Division

WRD Distribution:  A, B, S, FO, PO