October 5, 2017
Long-Term Groundwater Depletion
Development of global groundwater resources for agricultural, industrial, and municipal purposes greatly expanded in the last century, and economic gains from groundwater use have been dramatic. In many places, however, groundwater reserves have been depleted to the extent that water levels have declined tens to hundreds of meters, well yields have decreased, pumping costs have increased, and detrimental environmental impacts have become evident. These impacts increase the cost and reduce the sustainability of groundwater development. Estimates of depletion in individual aquifer systems based primarily on direct volumetric approaches indicate that cumulative global groundwater depletion since 1900 totals about 4,500 km3 through 2008. This large volume represents a substantial net transfer of water mass from continents to oceans, thereby contributing to sea-level rise (equivalent to a sea-level rise of 12.6 mm—approximately 6% of the observed total rise). The rate of annual depletion has increased markedly since about 1950, with maximum rates occurring during 2000-2008, when they averaged about 145 km3/yr (equivalent to 0.40 mm/yr of sea-level rise, or 13% of the reported rate of 3.1 mm/yr during this time). For the U.S. as a whole during 1950-2005, about 15% of total pumpage was derived from a reduction of storage of groundwater—a depletion fraction of 0.15. But depletion fractions vary widely within the U.S. and even within any given large aquifer system. During 2000-2008 the Central Valley of California had the largest depletion intensity (a measure that accounts for areal extent of aquifer) at an average rate of 0.075 m/yr. Trends in the annual number of days of zero streamflow and 7-day-minimum streamflow values were evaluated in several areas of known groundwater depletion. Results clearly show that in some cases streamflow depletion is related to (and probably caused by) groundwater depletion. However, an unexpected finding was that about as many stream gages within each aquifer’s boundaries showed upward trends in annual minimum flows as showed downward trends, despite the overall trend of declining groundwater levels. Groundwater depletion must be confronted on local and regional scales, where water managers will necessarily have to take actions to reduce demand (primarily in irrigated agriculture) and/or increase supply through managed aquifer recharge, desalination, and/or developing alternative sources.
Leonard Konikow received a B.A. in geology from Hofstra University, and an M.S. and Ph.D. from Penn State University. He started working for the U.S. Geological Survey as a hydrogeologist in 1971 and is now an Emeritus Scientist. His research interests include the development and application of simulation models for groundwater flow and contamination problems, groundwater-surface water interactions, coastal submarine groundwater discharge processes, and groundwater depletion. He is a member of the National Academy of Engineering, Fellow of the American Geophysical Union, and has received the M. King Hubbert Science Award from the National Ground Water Association and the O.E. Meinzer Award from the Geological Society of America. He also served on several committees of the National Research Council and in leadership positions in several professional societies.