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Pacific Islands Water Science Center
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Synoptic Groundwater-Level Measurements for the Pearl Harbor Area, Oahu
What Are Synoptic Groundwater-Level Measurements?
Synoptic groundwater-level measurements are a special type of 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 (and an indication of freshwater storage) in an aquifer. Synoptic measurements commonly are taken when data are needed for mapping the elevation of the water table or potentiometric surface, determining hydraulic gradients, or defining the physical boundaries of an aquifer. Regional synoptic measurements are made periodically for selected aquifers to complement more frequent measurements made from a smaller number of wells.
Periodic water-level measurements are made by manually using electronic-sensor tapes or chalked metal tapes and may contain errors associated with (1) the measuring tapes used, (2) the measuring-point elevations at the top of the well, and (3) the plumbness of the well. Each of these sources of error is discussed briefly here.
Available Data for the Pearl Harbor Area
Available synoptic groundwater-level measurements for the Pearl Harbor area are summarized in the following table and maps. Water-level measuring tapes were calibrated during October 2002 and during July and August 2011. Water levels for the synoptic surveys of August 17, 2011 and April 26, 2012 reflect measuring-point elevations resurveyed to a common datum established by the National Geodetic Survey using differential Global Positioning System (GPS) methods during 2011.
In general, water levels are lowest near the coastal, southern part of the Pearl Harbor area and are highest in the inland, northern part of the area. Water-level records from wells 3-2101-03 and 3-2256-10 provide temporal context for the synoptic water-level measurements.
Water-level measurements were made possible with the assistance of the Honolulu Board of Water Supply and the State of Hawaii Commission on Water Resource Management. USGS Volunteer Joe Kennedy assisted in the field with the 2011 and 2012 groundwater-level measurements.
Comparison between October 31, 2002 and May 15, 2003
On October 31, 2002, water levels measured ranged from 13.6 to 20.4 feet above mean sea level in 24 wells. On May 15, 2003, water levels ranged from 13.1 to 19.7 feet above mean sea level in 23 wells. Water-level measurements on October 31, 2002 and May 15, 2003 shared a total of 21 wells in common. For three wells in the western part of the Pearl Harbor area, water levels differed by 0.1 foot or less. For the remaining 18 common wells, water levels measured on May 15, 2003 were about 0.4 to 1.6 feet lower than water levels measured on October 31, 2002.
Comparison between August 17, 2011 and April 26, 2012
On August 17, 2011, water levels measured ranged from 15.2 to 22.3 feet above mean sea level in 28 wells. On April 26, 2012, water levels ranged from 15.0 to 22.2 feet above mean sea level in 27 wells. Water-level measurements on August 17, 2011 and April 26, 2012 shared a total of 27 wells in common. At each of these common wells, water levels measured on April 26, 2012 were lower than the water levels measured on August 17, 2011. For four of the wells in the southeastern part of the Pearl Harbor area and one well in the northwestern part of the Pearl Harbor area, water levels decreased by 0.1 foot or less. For the remaining 22 common well sites, water levels measured on April 26, 2012 were 0.14 to 0.41 feet lower than water levels measured on August 17, 2011.
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