Groundwater Safe Yield and Sustainability

By Darrel Dunn, Ph.D., PG, Hydrogeologist

(Professional Synopsis 🔳)

Safe Yield is a concept that has been used in water resource management for more than 80 years. Todd (147) defined safe yield in a 1959 textbook as the amount of water which can be withdrawn from a groundwater basin annually without producing an undesired result. Groundwater basin was loosely defined as a physiographic unit containing one large aquifer or several connected and interrelated aquifers. Undesired results were generally considered to be (1) exceedance of the long-term mean annual water supply to the basin, (2) excessive cost of extracting the water, (3) reducing the quality of the water to an unacceptable level, and (4) interference with prior water rights. In subsequent years undesired results have been expanded, and the term safe yield has been partially supplanted by the term sustainability. Sustainability includes a longer list of undesired results, which may include (1) depletion of streams and springs, (2) drying wetlands, and (3) adverse effects on water-dependent ecosystems.

Estimating safe yield has been associated with overly simplistic solutions to the problem rather than addressing sustainability with sound hydrologic analysis and appropriate technologies. One of the simplistic solutions is the assumption that the natural recharge rate represents a safe yield rate. In recent decades such simplistic solutions have sometimes been replaced by computer models of the hydrologic systems (MODFLOW, for example). These models are capable of more realistic representation and quantitative estimation of the effects of water management decisions. The models can be adjusted through time as the water resource is developed and more data becomes available. Such adjustment allows for beneficial water management flexibility. Integrated water resource planning is feasible. Water resource management is no longer constrained to simplistic methods.