The response of the water level in a well to Earth tides and atmospheric loading under unconfined conditions can be explained if the water level is controlled by the aquifer response averaged over the saturated depth of the well. Because vertical averaging tends to diminish the influence of the water table, the response is qualitatively similar to the response of a well under partially confined conditions. When the influence of well bore storage can be ignored, the response to Earth tides is strongly governed by a dimensionless aquifer frequency Q′u. The response to atmospheric loading is strongly governed by two dimensionless vertical fluid flow parameters: a dimensionless unsaturated zone frequency, R, and a dimensionless aquifer frequency Qu. The differences between Q′u and Qu are generally small for aquifers which are highly sensitive to Earth tides. When Q′u and Qu are large, the response of the well to Earth tides and atmospheric loading approaches the static response of the aquifer under confined conditions. At small values of Q′u and Qu, well response to Earth tides and atmospheric loading is strongly influenced by water table drainage. When R is large relative to Qu, the response to atmospheric loading is strongly influenced by attenuation and phase shift of the pneumatic pressure signal in the unsaturated zone. The presence of partial penetration retards phase advance in well response to Earth tides and atmospheric loading. When the theoretical response of a phreatic well to Earth tides and atmospheric loading is fit to the well response inferred from cross‐spectral estimation, it is possible to obtain estimates of the pneumatic diffusivity of the unsaturated zone and the vertical hydraulic conductivity of the aquifer.

As part of the Parkfield, California, earthquake prediction experiment, water level is monitored in a well 460 m from the main trace of the San Andreas fault on Middle Mountain, in the preparation zone of the anticipated Parkfield earthquake. The well configuration allows water level to be monitored in two fluid reservoirs at depths of 85 and 250 m below land surface. During 1987, water level changes were recorded during 12 of the 18 episodes of accelerated fault creep detected by a creep meter spanning the fault trace 750 m northwest of the well. The creep‐related water level changes in the shallow reservoir have durations of less than 1 day, whereas in the deeper reservoir the changes persist for as long as 2 months. These data suggest that the transient nature of the water level changes in the shallow interval is due to vertical flow to the water table and is not evidence that creep events propagate past the well. Phase leads of earth tidal constituents in the water level data from the shallow interval relative to the same constituents in the local volume strain tide support the interpretation of significant flow to the water table at periods of 1 day or less. The form of the water level changes in the deep interval is affected by horizontal flow to the well bore. This effect can be removed from the water level records using a theoretical response curve constrained by the phases of earth tidal constituents in the deep interval relative to the local volume strain tide. For the events where the signal in the shallow interval has been large enough to measure, the sizes of the simultaneous water level changes in the two reservoirs are consistent with the same amounts of volume strain occurring at both depths.

thirds of the Central Valley of California It is 250 mi long, averages 40 mi in width, and encompasses 10,000 Land subsidence due to groundwater overdraft in the San Joaquin mi~ excluding the rolling foothills that skirt the valley Valley began in the mid-1920's and continued at increasing rates until surface water was imported through major canals and aqueducts in the on three sides. The pertinent geographic features of 1950'sandlate1960's. In areas wbere surface water replaced withdrawal the area discussed in this report are those in the of ground water, water levels in the confined system rose sharply and southern four-fifths of the valley (fig. D. subsidencesiowed or eseentially eased. Land subsidence due to groundwater withdrawal The three major subsiding areas in the San Joaquin Valley in this report are the Los Banos-Kettleman City area, largely in western began in the San Joaquin Valley in the mid-1920's and Fresno County; the Tulare-Wasco area, mostly in Tulare County; and locally exceeded 28 ft by 1970 IPoland and others, the Arvin-Maricopa area, in Kern County. 1975); in December 1977, subsidence reached a maxi-The latest areawide leveling was in 1972 in the Los Banos-Kettlemunl of 29.6 ft in western Fresno County. More than man City area and in1969-70 in the Tularo-Wasco and Arvin-Maricopa 5,200 mi = of irrigable land, one-half the entire valley, areas. The 1972 leveling in the Los Banos-Kettleman City area showed that subsidence rates had decreased sharply with the importation of has been affected by subsidence.

No abstract

The purpose of the report was to present to the Department of the Navy results of a detailed groundwater investigation of the main groundwater basins at a Marine Corps Training Center that was under construction. This training center has evolved into the Marine Corps Air Ground Combat Center (MCAGCC), Twentynine Palms, California. Administrative reports may not be cited or quoted or released to the public. However, this report contains valuable information on the geology and hydrology of the area that has important implications on the availability, source, and movement of ground water. For these reasons, and because of the importance of ground water to the MCAGCC and surrounding area, this report has been retyped for public release at this time. In some respects, this report does not conform to current USGS editorial standards and it uses technical terms that are no longer used by the USGS. Also, as is common in reports produced for other Federal agencies, direct recommendations are given by the author. To avoid confusion, however, the report has been reproduced largely as originally written. Because reproducibles are not available, photocopies were made of the illustrations (some of which are of poor quality because of the deterioration of paper and ink); the photocopies were retouched and placed at the back of the report. It is our belief that these deficiencies are outweighed by the advantages to the public of making the information in this report available to potential users.