The problem of safe yield in insular Ghyben‐Herzberg systems

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Abstract
Where the lower boundary of fresh water in a Ghyben‐Herzberg lens is of area comparable to the area of the aquifer, changes in vertical position yield large amounts of water which may be equivalent to several months or years of annual increment from rainfall. Draft on such a system may not immediately develop indication of water shortage but may trend over a long period toward conditions unfavorable to maintenance of a desired quantity or quality of water. Changes in the amount of ground‐water storage are often not practically measurable, even when a reasonable total inventory suggests that storage is being depleted. To determine in such a system what yield is safe is as much a social problem as a hydrologic one, and involves contemporary and future patterns of normal and emergency use, viewed in the light of long‐term conservation aims. Physical and hydraulic data are essential but ultimate evaluation must take account of the probable future state of the community and existence or non‐existence of alternative future supplies.

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