Gravity Monitoring of Ground-Water Storage Change in Central and Southern Arizona

Repeat observations of the absolute acceleration of gravity have been made routinely at 16 gravity monitoring stations in central and southern Arizona since 1998. The observations are gravity control for repeat relative- gravity surveys of networks of stations that monitor aquifer-storage change. The absolute acceleration of gravity is measured twice each year at the stations to an accuracy of about plus or minus 2 microGal, or about a 5 cm thickness of water. Gravity variations during the period of record at the stations have ranged from 4 to 21 microGal. The greatest variations occur at stations located above aquifers where nearby water-level variations document correlative trends in aquifer storage. Gravity variations can result from non-aquifer sources, including vertical displacement of the stations—especially in areas of land subsidence, or fluctuations in near- surface soil moisture. The vertical position of many of the stations, however, is monitored by using the Global Positioning System, and no significant variation has been detected. Gravity variations at stations on crystalline rocks show poor correlation with soil moisture, however, a portion of the gravity variations at stations on alluvial soils correlate with near-surface variations in soil moisture. Removal of the soil-moisture correlation from the record at these alluvial stations decreases the variation in gravity among the stations to 14 microGal and improves the correlation of the residual-gravity variations with water levels.