Observations of the spatial and temporal distributions of gravity and water-mass change in the western United States

Pool, Don; Kennedy, Jeff; Conway, Brian
US Geological Survey, Tucson, AZ, drpool@usgs.gov

Gravity change has been monitored by the U.S. Geological Survey and Arizona Department of Water Resources at hundreds of stations across several networks in the alluvial basins of Arizona using relative and absolute gravity meters since about 1998. A few stations have records of change using crystalline rock stations as a reference for relative gravity surveys since 1993. Networks are installed to monitor both episodic local groundwater recharge and regional trends in groundwater storage, and therefore have different frequencies and durations of surveys. Local recharge monitoring networks are commonly located near ephemeral channels and have been surveyed frequently, with days to weeks between surveys, immediately following recharge events and tend to be short duration projects. Regional networks maintained by the Arizona Department of Water Resources are surveyed at least once every other year. Regional networks in the Tucson, Sierra Vista, Prescott, and Yuma areas maintained by the U.S. Geological Survey have been surveyed annually and portions are surveyed as frequently as quarterly. Most stations are located in locally elevated areas for the purpose of minimizing the effects of variations in soil moisture. Nearby pumping wells and artificial recharge facilities are also avoided. Where possible, stations are located at monitor wells where depths-to-water observations provide additional information on the vertical distribution of storage change. The monitored periods at these networks have included extended drought as well as one or more brief periods of recharge-producing precipitation. Observed gravity changes can be grouped into two categories: large but short-term increases and slower long-term declines. Large short-term gravity changes of 100 æGal or more occur over periods of days to weeks tend and to be concentrated along ephemeral streams. These rapid increases tend to dissipate over periods of weeks to months as the infiltrated water migrates laterally and spreads across a large area in the subsurface. Long-term declines in gravity occur over periods of months to years between recharge events due to groundwater withdrawals, groundwater subflow, and discharge to streamflow. Additional networks of limited extent have recently been established in mountainous terrain in the Black Hills of South Dakota and in a watershed in the eastern Sierra Nevada Mountains. Early trends at these sites indicate that some stations exhibit tens of æGal of increases in gravity following major recharge periods, while other stations exhibit little to no change. The greatest changes tend to occur near ridges; however, further monitoring and correlation of gravity change with local physiography, soils, and geology is needed to better understand the distributions of change in these areas.