RESEARCH: Analytical equations used in ground-water studies. UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY Water Resources Division Washington 25, D.C. In reply refer to: January 7, 1958 Code 4051 0001 GROUND WATER BRANCH MEMORANDUM NO. 58.23 To: District Chiefs and Staff Officials From: Chief, Ground Water Branch Subject: RESEARCH: Analytical equations used in ground-water studies. The development of computational aids, such as a slide rule with special scales, and the general progress of several research projects (particularly Aquifer Analytical Methods) would be greatly furthered by knowing which theoretical equations are being used most frequently in analyzing ground-water hydraulics problems. Unquestionably the Theis nonequilibrium formula is used more than any sought as to those equations that should be next in order on a list arranged according to decreasing frequency of use. It would appear, for example, that the drain and leaky-aquifer formulas are used quite often, but their proper place on the list can best be judged from the replies to this memo. Each district supervisor and staff official is asked to provide the Washington office with a selection of the equations and empirical relationships which, in his experience in pursuing ground-water investigations, have been most commonly used. Kindly restrict the selection to no more than 6 items arranged in order from the most to the least frequently used. Each item may be presented as an equation name or number, complete with the page and reference work where it can be found. The Ground Water Notes series, particularly Note No. 28, should be one of the handiest reference sources and should be cited wherever possible. In addition to listing the equations you adjudge most useful in describing ground-water movement, your response to the following questions would be especially helpful: 1.) Considering the investigations you are directing, or have directed, what ground-water hydraulics problems can you identify (and describe) which are not amenable to satisfactory analysis using presently available equations? 2.) If your reply to question (1) embodies descriptions of several problems, how would you rank them in order from top priority to lowest priority? It is not intended to burden the district supervisors and staff officials with a detailed review of available quantitative methods. It is expected that the information and opinions sought can be furnished readily by briefly reflecting upon drawing from past field experience. A prompt reply will be appreciated. (s) A. N. Sayre Distribution: A, B, GS, GFO