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Ground-Water-Level Measurements: Why the Choice of Frequency Matters

U.S. Geological Survey Circular 1217

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The frequency of measurement is one of the most important considerations in the design of a water-level monitoring program. The development of a plan for water-level monitoring that will be used for each well in the observation network is dependent on the objectives of the program and the intended use and level of analysis required of the data. The frequency of measurement should be adequate to detect short-term and seasonal ground-water-level fluctuations of interest and to discriminate between the effects of short- and long-term hydrologic stresses (Figure B–1).

Figure B-1. Common environmental factors that influence the choice of frequency of water-level measurements in observation wells

Water-level monitoring may involve “continuous” or periodic measurements. Continuous monitoring involves the installation of automatic water-level sensing and recording instruments that are programmed to make measurements in observation wells at a specified frequency. Continuous monitoring provides the highest level of resolution of water-level fluctuations. Hydrographs constructed from frequent water-level measurements collected with continuous monitoring equipment can be used to accurately identify the effects of various stresses on the aquifer system and to provide the most accurate estimates of maximum and minimum water-level fluctuations in aquifers. For these reasons, it is often advisable that new observation wells initially be equipped with continuous monitoring equipment to identify stresses on the aquifer and the magnitude and frequency of water-level fluctuations. Continuous monitoring may not be required where the hydraulic response of an aquifer to stresses is
slow and the frequency and magnitude of water-level changes in an observation well are not great. However, it is often the best technique to use for monitoring fluctuations in ground-water levels during droughts and other critical periods when hydraulic stresses may change at relatively rapid rates. Near real-time data collection also can be accomplished using a continuous recording device and telecommunication or radio transmitter equipment.

Periodic ground-water-level measurements are those made at scheduled intervals (weeks, months, or years) and are generally used for water-table or potentiometric surface mapping and to reduce the costs of long-term monitoring. Periodic water-level measurements are made by manually using electronic-sensor tapes, chalked metal tapes, or acoustic sounding devices. Potential drawbacks to periodic monitoring are that hydraulic responses to short-term stresses may occur between measurements and may be missed, extreme water-level fluctuations cannot be determined with certainty, and apparent trends in water levels potentially are biased by the choice of measurement frequency.

Synoptic water-level measurements are a special type of periodic measurement in which water levels in wells are measured within a relatively short period and under specific hydrologic conditions. Synoptic water-level measurements provide a “snapshot” of heads in an aquifer. Synoptic measurements commonly are taken when data are needed for mapping the altitude of the water table or potentiometric surface, determining hydraulic gradients, or defining the physical boundaries of an aquifer. Regional synoptic measurements made on an annual or multiyear basis can be used as part of long-term monitoring to complement more frequent measurements made from a smaller number of wells.

An example of the effects of different measurement frequencies is provided by water-level hydrographs for an observation well in Massachusetts. The well is completed in bedrock to a depth of 740 feet, and the characteristics of this well fall in the middle range of the temporal response categories shown in Figure B–1.

A daily water-level hydrograph for the Massachusetts well and hydrographs that would have been obtained for the same well if measurements had been made only monthly or quarterly are shown in Figure B–2. Comparing the effects of different measurement frequencies on the hydrographs illustrates several features. First, monthly water-level measurements for this well generally are adequate to discern overall seasonal patterns in water levels and long-term trends but miss some short-term effects from pumping or recharge. Second, unless quarterly measurements correspond with regular patterns of seasonal variability of water levels, it can be difficult or impossible to discern anything beyond simple long-term water-level trends. Figure B–3, which overlays the daily and quarterly hydrographs from Figure B–2, illustrates how less frequent water-level measurements lead to lower estimates of the range of fluctuations in water levels in an observation well.

Figure B-2. Water-level hydrographs for well PDW 23 in western Massachusetts, based on daily, monthly, and quarterly measurements, plotted to same scale but vertically offset.

Figure B-3. Overlaid daily and quarterly hydrographs for well PDW 23 in western Massachusetts.

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