Notes:

The maps and graphs shown here are intended to present the same information used in the National Climate Response Network for Virginia
web pages. There are minor differences to be consistent with the surface-water flow-duration data presented on other Virginia web pages. The principle differences between data presented here and the National Climate Response web page are,

1. Frequency of updates to the ground-water level statistics.

Statistics presented in the National Climate Response Network web pages are updated weekly while statistics presented in these web pages are updated less frequently--usually in December when the 2-year graphs are shifted and in the spring when the ground-water data book for the previous water year is released. Differences in statistics should be minor except when new highs or new lows are measured. The same computation methods are used for both web sites.

2. Division of percentile classes.

The National Climate Response Network uses single percentile classes for data values less than the 10th percentile and greater than the 90th percentile. The data presented here separates data values less than the 10th percentile into two classes, the 10th percentile to the 5th percentile and less than the 5th percentile. Data values greater than the 90th percentile are separated into similar classes, the 90th to the 95th percentile and greater than the 95th percentile. These additional divisions are necessary for consistency with the surface-water flow-duration web pages.

Water-level duration plots depict specific water-level statistic values selected from water-level duration curves. The water-level duration curve is a cumulative frequency curve that shows the percent of values that specified water levels were equaled or less than (percentile), or the percent of values that specified water levels were equaled or exceeded (percent exceedance). Percent exceedance is 100 minus the percentile. Example: the 75th percent exceedance (the value that 75 percent of the water-levels are equal to or greater than) is the 25th percentile (the value that 25 percent of the water-levels are equal to or less than). While the water-level duration curve does not show the chronological sequence of water-levels, it is an important means of representing water-level characteristics of an aquifer throughout the range of water levels on a single curve.

The water-level-duration plots presented here represent the water-level-duration curves of daily water-level data grouped by months. All the daily water-level values for each month are arranged according to magnitude and the percentile for each daily water-level value is computed. The value for a specific percentile is determined by interpolation between the computed values. From the data, the 5th, 10th, 25th, 50th, 75th, 90th and 95th percentiles are determined and plotted. Colors are used to separate above normal (blue), normal (green), and below normal (brown) conditions. The maximum and minimum daily values for each month are selected from the data set.

Information on how sites are selected for inclusion in the National Climate Response web page, definitions, and explanations of computations are presented at the Groundwater Watch Help Page.