This is the first of three papers concerning the extraction of aquifer parameters through the application of time series analysis to water level data from boreholes. The analysis is performed on data from six wells located in the Nubian desert near the shores of Lake Nasser, Egypt. These wells are ideally suited to this study since they are remote from most sources of hydrological noise and hence respond primarily to forcing functions that can be measured or modeled. For four of the wells, which were part of a monitoring system installed prior to impoundment of Lake Nasser, monthly (at least) water level measurements began in 1964. In 1985, two additional wells were drilled, and high precision piezometers that sampled data every 0.2 hour were installed in five of the wells. Thus data are available which span the period band 23 years to 0.4 hour. This paper provides background information and stratigraphy relevant to assessing the significance of the aquifer parameters derived using spectral methods. A regional-scale three-dimensional hydrogeological model for the study area is developed, based on the 23-year record of water level changes that occurred at the various well sites in response to the changing level of Lake Nasser.

The model that best fits the data, and is consistent with recent stratigraphic studies of the Nubian formation in the area, consists of three horizontal units of total thickness 400 m. The lowermost unit, overlying the granitic basement, is an aquifer of fluvial sandstones and has a large-scale (several kilometer) horizontal permeability of 0.32--0.43 μm2. The middle unit is an aquaclude that extends unbroken under the lake and ''leaks'' at periods longer than several years. Thus the lower aquifer is confined up to periods of several years. The uppermost unit is the water table aquifer, which is composed of 25--30% porosity sandstones with interspersed claystone lenses and has a large-scale permeability of 1.0--1.5 μm2. Of the wells studied, two are screened in the basal sandstone and four are screened in the water table aquifer. Analysis of tidally and atmospherically forced fluctuations in five of these wells is presented in the companion papers. ¿American Geophysical Union 1991