Michelle Sneed (Northern California)Hydrologist
United States Geological Survey
Land Subsidence: The Lowdown on the Drawdown
Land subsidence caused by groundwater withdrawal in California, particularly in the San Joaquin Valley, has recently received increased attention from water-science professionals and the media because two recent droughts, 2007-09 and 2012-present, have triggered high rates of groundwater withdrawal and historically high rates of land subsidence (as much as about 1 foot per year). The compaction of susceptible aquifer systems caused by excessive groundwater pumping is the single largest cause of subsidence in California, and the 5,200 square miles affected by subsidence in the San Joaquin Valley during the better part of the 20th century has been identified as the single largest human alteration of the Earth’s surface topography. In some areas that historically depend on surface-water resources, groundwater pumping has increased during periods of drought to compensate for reduced surface-water availability, resulting in large and rapid groundwater-level declines. In some areas where surface water is a minor component of the water supply or where land use has changed to more water-intensive uses, groundwater levels have declined during both drought and non-drought periods. While more focus has been placed on the highly visible infrastructure damage from subsidence, which generally can be repaired, compaction of the aquifer system, sight unseen, permanently decreases its capacity to store water such that subsidence occurring today is a legacy for all tomorrows. This presentation will include discussions of subsidence processes, measurements, analyses, and consequences by exploring selected case studies throughout California, including the San Joaquin Valley, the Coachella Valley, and/or the Mojave Desert.
Michelle Sneed is a hydrologist with the U.S. Geological Survey and has been with the California Water Science Center since 1994. She received her B.S. and M.S. degrees in geology from California State University, Sacramento, where she periodically teaches geology classes. She has published many studies on land subsidence related to fluid-pressure changes in areas throughout California and other areas in the Western United States. Ms. Sneed integrates various methods of land-surface elevation (and elevation change) measurement, including spirit leveling, Global Positioning System, extensometry, and Interferometric Synthetic Aperture Radar (InSAR) techniques, to leverage the diverse spatial and temporal scales of the datasets. Analyses and simulations have focused on the preconsolidation stress, vertical hydraulic conductivities, and the elastic and inelastic compressibilities (storage) of aquifer-system components based on hydrogeological structure, land-surface elevation changes, and groundwater-level changes. Recent studies in the San Joaquin and Coachella Valleys explore the impact of subsidence on water-conveyance infrastructure, and have been featured in the news media. She is a member of AGU, GRA, and NGWA, and recently was invited to join the UNESCO Working Group on Land Subsidence, the recognized leader in promoting global land subsidence studies.