GROUND WATER--Computer program for ground-water modeling; "Iterative digital model for aquifer evaluation," by P. C. Trescott. UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY RESTON, VIRGINIA 20242 GW Branch July 30, 1973 4300 5016 GROUND WATER BRANCH TECHNICAL MEMORANDUM NO. 74.02 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