Composite Groundwater-Level Hydrograph Help Document

A composite hydrograph provides a broad overview of groundwater levels in a Principal Aquifer. The purpose of the composite hydrographs of the Principal Aquifers of the United States is to show the 'average' response of groundwater levels in the Principal Aquifer over time. This should indicate if there is ongoing depletion, or longer term climatic changes in each Principal Aquifer. Composite groundwater-level hydrographs have been calculated for those Principal Aquifers of the United States that had enough wells passing certain criteria (number of wells, length of record). The length of each composite hydrograph is 30 years.

Two different composite indices are presented for the Principal Aquifers of the United States. The 'Composite Water-Level Hydrograph' starts with the median annual groundwater level for the period of interest for each index well. The average and median values of all the index wells for the particular year are then calculated to determine a composite water level. The composite water levels representing the average or median water level of all the index wells are then presented on a hydrograph. The 'Normalized Composite Water-Level Hydrograph' uses the same baseline data, but is based on a calculation of the difference of the median annual groundwater level from the annual average median groundwater level for each well as a percentage of the historical range of median values. The normalized composite groundwater-level hydrograph shows the variation of groundwater levels in the Principal Aquifer around the average value. Computation details for each composite hydrograph are provided below.

If a Principal Aquifer is not listed, it is not available because it did not have enough wells with a sufficient period of record to calculate a composite hydrograph or the distribution of wells was deemed to be less than optimal.

There are three different types of pages available for the composite groundwater-level hydrographs of the Principal Aquifers of the United States.

The home page (https://groundwaterwatch.usgs.gov/compositehome.asp) is the gateway to all the information available. This page provides a clickable list of all the aquifers available. Clicking on the name of the aquifer will bring the user to a summary page of the composite hydrographs for a specific aquifer.

The summary page for each aquifer is reached once an aquifer is selected from the home page. The summary page provides a map showing the location of the index wells, a thumbnail of the 30-year annual composite groundwater-level hydrograph, and a thumbnail of the 30-year annual normalized composite groundwater-level hydrograph for the aquifer selected. Clicking on the “View Source Data and Full-size Hydrograph” bar below each hydrograph brings the user to another page that contains more detailed information for each hydrograph.

Individual hydrograph pages accessed by clicking on the “View Source Data and Full-size Hydrograph” bar provide a larger hydrograph and a data table of the values used to construct the hydrograph. There are two different individual hydrograph pages, one for the 30-year annual composite groundwater-level hydrograph and one for the 30-year annual normalized composite groundwater-level hydrograph.

The home page describes the purpose of the Composite Groundwater-Level Hydrographs and provides a list of the Principal Aquifers for which the composite hydrographs are available. To get to the hydrographs of a specific aquifer, click on the aquifer’s name. This page also has additional resources at the bottom of the page with links to: the Composite Hydrographs help page, a Map of the Principal Aquifers of the United States, the National Aquifer Code Reference List, and the Principal Aquifer Active Well Count.

The summary page has a scalable map showing the location of the wells used to calculate the composite hydrographs. The wells selected to calculate the composite hydrograph must have 30 years of measurements with at least one measurement in every year. In addition, there must be at least nine wells in a Principal Aquifer for it to be available on the home page list of aquifers. Below the map is an explanation of the colors and shapes of the well markers on the map. This explanation is the same as is used throughout the groundwater watch site (https://groundwaterwatch.usgs.gov/) for all the networks.

Below the map are two small hydrographs showing the 30-year annual composite groundwater-level hydrograph and the 30-year annual normalized composite groundwater-level hydrograph. The value for each data point on the hydrograph can be observed by hovering the cursor over the curve. Under each curve is information indicating the number of wells used in the calculation. If there is additional information that is useful in interpreting the hydrograph for a specific aquifer, it is provided in a text box above the hydrographs. Below each hydrograph is a “Print Hydrograph” button that will allow the hydrograph to be printed as a separate image.

At the bottom of the page there is a button labeled “Return to National Aquifers Page” that will return the user to the home page.

The 30-year annual composite groundwater-level hydrograph page has a full size version of the hydrograph at the top of the page. The teal curve is the hydrograph based on the mean value of the annual median water level measurement for the index wells. The dark red curve is the median value of the annual median water level measurement for the index wells. As with the smaller graphs on the summary page, the value for each data point on the hydrograph can be observed by hovering the cursor over the curve.

Below the graph are the data used to produce the composite hydrograph. For each index well, the median water level for each year is shown. For index wells with continuous data, the annual median is calculated from all daily values and periodic measurements in the year. For index wells with only periodic data, the annual median is calculated from all periodic measurements available for the year. For example, if there is only one measurement in a year, that measurement is the annual median water level. If there are 3 periodic measurements in a year, then the annual median water level is the median of the 3 periodic measurements. Thus, the annual median water level is always the median of available daily values and periodic measurements.

The median water levels from all index wells are used to calculate both a mean composite water level and a median composite water level. In other words, for a specific year, the mean (average) of the medians and the median of the medians for all index wells is used to generate composite water level hydrographs. The approach for composite hydrographs was chosen to keep the statistics simple and not to overly manipulate the basic data.

There are additional data that can be obtained through the data table. Of the most importance is that all the data used to calculate the composite hydrograph are readily available. The data used to calculate the median value for any well and year can be obtained by selecting (clicking on) the median value of interest in the table. This then opens a new web page that provides all the daily values used to calculate the annual median for the well. The annual median value is highlighted in yellow.

In addition, selection of any index well site ID will direct the user to the Groundwater Watch page for that well. Hovering with the cursor over the icon of a hydrograph under the “Well Info” column shows the most recent Groundwater Watch hydrograph for the entire period of record for that particular index well in a pop-up window. If the BLS (which stands for Below Land Surface) is highlighted in yellow under the “Well Info” column, it indicates that the water levels were converted from Above Sea Level (ASL) measurements and the conversion equation is shown by hovering over the yellow highlighted BLS. At the bottom of the table the calculated mean and median values are provided.

At the bottom of the page there is a button labeled “Return to National Aquifers Page” that will return the user to the home page and a button that provides a kml file that enables the location of the index wells used to be plotted using Google Earth and other applications.

The 30-year annual normalized composite groundwater-level hydrograph page has a full size version of the hydrograph at the top of the page. The teal curve is the hydrograph based on the mean value of the normalized annual median water level measurement for the index wells. The dark red curve is the median value of the normalized annual median water level measurement for the index wells. As with the smaller graphs on the summary page, the value for each data point on the hydrograph can be observed by hovering the cursor over the curve.

Below the graph are the normalized water levels used to draw the composite hydrograph. The normalized value of each annual median groundwater-level of a well is calculated as:

xnorm = -1 * (x – xmean)/(xmax-xmin) * 100

Where:

x = the median annual water level for a well

xmean = the arithmetic mean of the annual medians for the 30 year period

xmax = the maximum annual median water level for a well for the 30 year period

xmin = the minimum annual median water level for a well for the 30 year period

Using the median water level for each year (x), a maximum and minimum median water level is determined for each well for the 30 year period. The range of water levels for each well is then calculated as the maximum minus the minimum for each well. The mean water level for each well is determined from the 30 annual median values. The normalized value is calculated for each year using the above formula. Mean and median normalized composite groundwater-level hydrographs are then calculated for the aquifer from the normalized values of all the wells for each year.

The normalized composite groundwater-level hydrograph represents the variation of the water levels in the Principal Aquifer as a percentage of the range in water levels in relation to the mean value. Because all the median water levels are based on Below Land Surface values, the large numbers indicate a lower water level. Thus, the calculated normalized value includes a negative one multiplier to indicate that negative normalized values (percentages) refer to water levels below the 30 year median. A 20 percent index indicates that water levels in the aquifer are 20 percent of the known range of median water levels higher than average. A -30 percent index indicates that water levels in the aquifer are 30 percent of the known range of median water levels lower than average. A zero index indicates that water levels are average for the 30 year period of record.

When an individual normalized value for a particular year and well is hovered over, the calculation of the normalized value is shown in a pop-up box. All the values used in the calculation are shown. The annual median values used to calculate x, xmean, xmax, and xmin are available from the “view source data” link at the bottom of the page.

As with the 30-year annual composite groundwater-level hydrograph, selection of any index well site ID in the data table will direct the user to the Groundwater Watch page for that well. Hovering with the cursor over the icon of a hydrograph under the “Well Info” column shows the most recent Groundwater Watch hydrograph for the entire period of record for that particular index well in a pop-up window. At the bottom of the table the calculated mean and median values are provided.

Also, at the bottom of the page there is a button labeled “Return to National Aquifers Page” that will return the user to the home page and a button that provides a kml file that enables the location of the index wells to be plotted using Google Earth and other applications.