Water Table Depth Calculator

The water table is the surface within an unconfined aquifer where pore water pressure equals atmospheric pressure. Its depth below ground is the ground elevation minus the water level elevation, both referred to a common datum such as NAVD88; that subtraction lets readings from scattered wells be contoured into a water table map showing flow directions. Directly above the water table lies the capillary fringe, which is fully saturated but held under tension; it is only a few centimetres thick in gravel but can exceed a metre in silt and clay, so excavations may encounter wet ground above the level a well shows. Within a well, the static water level is the equilibrium depth with the pump off and the pumping water level is the depth once flow stabilises. Their difference is drawdown, and yield divided by drawdown gives specific capacity, an index of well performance in litres per second per metre. The saturated column available to a well is its total depth minus the static water level, and a common rule keeps maximum drawdown within two thirds of that column so the screen is not dewatered. Interpretation carries caveats. Water levels fluctuate seasonally by a metre or more and respond to pumping, barometric pressure and nearby rivers, so a single reading is a snapshot. A well screened in a confined aquifer measures a potentiometric surface, not a true water table, and that level can stand above ground under artesian conditions. Levels in low-permeability ground recover slowly, so a reading taken soon after drilling or pumping may not represent equilibrium.

Wells long predate any theory of the water table, from the qanats of Persia to hand-dug shafts across the ancient world, but quantitative understanding began in the nineteenth century. Jules Dupuit, a French civil engineer, published analyses of well flow in 1863 that introduced the simplifying assumptions about nearly horizontal flow in unconfined aquifers still known as the Dupuit assumptions, and described the cone of depression around a pumping well. Philipp Forchheimer extended the treatment in the 1880s, and Adolph and Gunther Thiem developed the steady-state equation relating discharge to drawdown in two observation wells, published by Gunther Thiem in 1906, which made it possible to derive aquifer properties from measured water levels. Systematic measurement in the field was largely organised by the United States Geological Survey under Oscar Meinzer, who from 1913 led its ground water division and promoted cooperative observation-well programmes with the states; regular water level records expanded substantially during the 1930s as drought and irrigation withdrawals raised concern about declining supplies. Charles Theis of the Survey published the non-equilibrium equation in 1935, giving drawdown as a function of time and turning a routine water level measurement into a tool for estimating transmissivity and storage. Instrumentation moved from wetted chalk on a steel tape to electric contact tapes, then to pressure transducers with data loggers. Long-term records are now published through the Survey National Water Information System.