Abstract

All groundwater pumping comes from capture; the greater the intensity of pumping, the greater the capture. Capture comes from decreases in natural discharge and increases in recharge. Natural discharge supports riparian, wetland, and other groundwater-dependent ecosystems, as well as the baseflow of streams and rivers. Capture depends on usage, and it is not related to size or hydrogeological characteristics of the aquifer, or to the natural recharge. The traditional concept of safe yield, which equates safe yield with natural recharge, is flawed and has been widely discredited. It has now been replaced with sustainable yield.

Sustainable yield depends on the amount of capture, and whether this amount can be accepted as a reasonable compromise between a policy of little or no use, on one extreme, and the sequestration of all natural discharge, on the other extreme. A reasonably conservative estimate of sustainable yield would take all or suitable fractions of deep percolation. On a global basis, deep percolation is about 2% of precipitation. Sustainable yield may also be expressed as a percentage of recharge. Limited experience suggests that average percentages may be around 40%, with the least conservative around 70%, and reasonably conservative around 10%.

Sustainability may be fostered by enlightened management which seeks to capture rejected recharge, encourage clean artificial recharge, and limit negative artificial recharge. A holistic approach to groundwater sustainability considers the hydrological, ecological, socioeconomic, technological, cultural, institutional and legal aspects of groundwater utilization, seeking to establish a reasonable compromise between conflicting interests. Communities are beginning to consider baseflow conservation as the standard against which to measure groundwater sustainability. In the end, sustainability reflects resource conservation policy; the more conservative a policy, the more sustainable it is likely to be.

Authors' Biographical sketch

Victor Miguel Ponce has forty years of experience in hydrologic engineering, including education, research, consulting, and service. His areas of expertise are surface-water hydrology, surface-groundwater interactions, computational hydrology, and environmental hydrology. Specific areas of current interest are the sustainability of groundwater resources, computer modeling of hydraulic and hydrologic systems, and online hydrologic engineering. He is the author of a popular textbook (Engineering Hydrology, Principles and Practices, Prentice Hall, 1989), and more than 480 publications, including fifty refereed journal papers.

Victor M. Ponce
Department of Civil and Environmental Engineering
San Diego State University
San Diego, CA 92182-1324

E-mail: ponce@ponce.sdsu.edu