A frequency‐method of evaluating ground‐water levels

Water‐levels in wells, which are utilized by the hydrologist as a measure of ground‐water storage, customarily are measured in terms of distance below a convenient measuring point and expressed with reference to a fixed datum. Datum‐planes or surfaces of several types have been used—each serving some particular purpose advantageously. These include: “Planes” of regional extent, such as mean sea‐level; irregular surfaces defining a particular hydrologic condition, such as low‐water level (if known) or the water‐level on some particular date; and local reference‐planes at each observation‐well, such as a horizontal plane through the measuring point or at the average height of the land‐surface. Recently the Geological Survey has considered the standard use of a land‐surface datum, precisely defined for each observation‐well. This procedure has the practical advantage that the water‐levels can be recorded in final form as soon as the initial measurement is made, without leveling to establish a regional datum or waiting to accumulate sufficient data for defining a particular hydrologic condition.