Records of stage and water discharge may be complete or partial. Complete records of stage or discharge are those obtained using a continuous or specified time-interval stage-recording device through which either instantaneous or mean daily discharges may be computed for any time, or any period of time, during the period of record. Occasionally, other parameters such as tainter gate openings and stream velocity will also be needed to compute discharges. Stations for which daily mean discharges or gage heights are published are referred to as "daily stations."
By contrast, partial records are obtained through discrete measurements without using a continuous stage-recording device and pertain only to a few flow characteristics, or perhaps only one. The nature of the partial record is indicated by table titles such as "Crest-stage partial records," or "Low-flow partial records." Records of miscellaneous peak discharge at selected sites or of measurements from specific studies, such as low-flow seepage studies, may be considered as partial records and these are presented under the appropriate heading. Locations of all complete-record and crest-stage partial-record stations for which data are given in this report are displayed by activating the appropriate theme on the user interface.
Data Collection and Computation
The data obtained at a complete-record gaging station on a stream or canal consist of a continuous record of stage, individual measurements of discharge throughout a range of stages, and notations regarding factors that may affect the relations between stage and discharge. These data, together with supplemental information, as weather records, are used to compute daily discharges.
Continuous records of stage are obtained with devices that record stage values at selected time intervals or with analog recorders that trace continuous graphs of stage. Measurements of discharge are made with current meters using methods adapted by the Geological Survey as a result of experience accumulated since 1880. These methods are described in standard textbooks, in Water-Supply Paper 2175, and in U.S. Geological Survey Techniques of Water-Resources Investigations (TWRI), Book 3, Chapters A1 through A19 and Book 8, Chapters A2 and B2. The methods referenced above are consistent with the American Society for Testing and Materials (ASTM) standards and generally follow the standards of the International Organization for Standards (ISO).
In computing discharge records, results of individual measurements are plotted against the corresponding stages, and stage-discharge relation curves are then constructed. From these curves, rating tables indicating the approximate discharge for any stage within the range of the measurements are prepared. If it is necessary to define extremes of discharge outside the range of the current-meter measurements, the curves are extended using: (1) logarithmic plotting; (2) velocity-area studies; (3) results of indirect measurements of peak discharge, such as slope-area or contracted-opening measurements, and computations of flow-over-dams or weirs; or (4) step-backwater techniques.
Daily mean discharges are computed by applying the daily mean stages (gage heights) to the stage-discharge curves or tables. If the stage-discharge relation is subject to change because of frequent or continual change in the physical features that form the control, the daily mean discharge is determined by the shifting-control method, in which correction factors based on the individual discharge measurements and notes of the personnel making the measurements are applied to the gage heights before the discharges are determined from the curves or tables. This shifting-control method is also used if the stage-discharge relation is changed temporarily because of aquatic growth or debris on the control. For some stations, formation of ice in the winter may so obscure the stage-discharge relations that daily mean discharges must be estimated from other information such as temperature and precipitation records, notes of observations, and records for other stations in the same or nearby basins for comparable periods.
At some stream-gaging stations the backwater from reservoirs, tributary streams, or other sources affects the stage-discharge relations. This necessitates the use of the slope method in which the slope or fall in a reach of the stream is a factor in computing discharge. The slope or fall is obtained by means of an auxiliary gage set at some distance from the base gage. At some stations the stage-discharge relations are affected by changing stage; at these stations the rate of change in stage is used as a factor in computing discharge.
For some gaging stations there are periods when no gage-height record is obtained, or the recorded gage height is so faulty that it cannot be used to compute daily discharge. This happens when the recorder stops or otherwise fails to operate properly, intakes are plugged; the float is frozen in the well, or for various other reasons. For such periods, the daily discharges are estimated from the recorded range in stage, previous and following record, discharge measurements, weather records, and comparison with other station records from the same or nearby basins. Information explaining how estimated daily-discharge values are identified in station records is included in the next two sections, Data Presentation (REMARKS paragraph) and Identifying Estimated Daily Discharge.
Computation of records of lake or reservoir contents requires a
stage-contents relation, which can be obtained from surveys,
curves, or tables defining this relationship. The application of
stage to the stage-contents curves or tables gives the contents
from which daily, monthly, or yearly changes then are determined.
If the stage-contents relation changes because of deposition of
sediment in a lake or reservoir, periodic resurveys may be
necessary to redefine the relation.
Streamflow data in the report are presented in a new format that is considerably different from the format in data reports prior to the 1992 water year. The major changes are that statistical characteristics of discharge now appear in tabular summaries following the water-year data table and less information is provided in the text or station manuscript above the table. These changes represent the results of a pilot program to reformat the annual water-data report to meet current user needs and data preferences.
The records published for each continuous-record surface-water discharge station (gaging station) now consist of four parts, the manuscript or station description; the data table of daily mean values of discharge for the current water year with summary data; a tabular statistical summary of monthly mean flow data for a designated period, by water year; and a summary statistics table that includes statistical data of annual, daily, and instantaneous flows as well as data pertaining to annual runoff, 7-day low-flow minimums, and flow duration.
The manuscript provides, under various headings, descriptive information, such as station location; period of record; historical extremes outside the period of record; record accuracy; and other remarks pertinent to station operation and regulation. The following information, as appropriate, is provided with each continuous record of discharge or lake content. Comments to follow clarify information presented under the various headings of the station manuscript.
LOCATION.--Information on locations is obtained from the most accurate maps available. The location of the gage with respect to the cultural and physical features in the vicinity and with respect to the reference place mentioned in the station name is given. River mileages, given for only a few stations, were determined by methods given in "River Mileage Measurement," Bulletin 14, Revision of October 1968, prepared by the Water Resources Council or were provided by the U.S. Army Corps of Engineers.
DRAINAGE AREA.--Drainage areas are measured using the most accurate maps available. Because the type of maps available at the time of determination of drainage area varies from one drainage basin to another, the accuracy of drainage areas likewise varies. Drainage areas are updated as better maps and funds become available.
PERIOD OF RECORD.--This indicates the period for which there are published records for the station or for an equivalent station. An equivalent station is one that was in operation at a time that the present station was not, and whose location was such that records from it can reasonably be considered equivalent with records from the present station.
REVISED RECORDS.--Published records, because of new information, occasionally are found to be incorrect, and revisions are printed in later reports. Listed under this heading are all the reports in which revisions have been published for the station and the water years to which the revisions apply. If a revision does not include daily, monthly, or annual figures of discharge, that fact is noted after the year dates as follows: "(M)" means that only the instantaneous maximum discharge was revised; "(m)" that only the instantaneous minimum was revised; and "(P)" that only peak discharges were revised. If the drainage area has been revised, the report in which the most recently revised figure was first published is given.
GAGE.--The type of gage in current use, the datum of the current gage referred to mean sea level (see glossary), and a condensed history of the types, locations, and datums of previous gages are given under this heading.
REMARKS.--All periods of estimated daily-discharge record will either be identified by date in this paragraph of the station description for water-discharge stations or flagged in the daily-discharge table. (See next section, Identifying Estimated Daily Discharge.) If a remarks statement is used to identify estimated record, the paragraph will begin with this information presented as the first entry. The paragraph is also used to present information relative to the accuracy of the records, to special methods of computation, to conditions that affect natural flow at the station and, possibly, to other pertinent items.
COOPERATION.--Records provided by a cooperating organization or obtained for the U.S. Geological Survey by a cooperating organization are identified here.
EXTREMES OUTSIDE THE PERIOD OF RECORD.--Included here is information concerning major floods or unusually low flows that occurred outside the stated period of record. The information may or may not have been obtained by the U.S. Geological Survey.
PEAK DISCHARGES FOR CURRENT YEAR.--For stations meeting certain criteria, all peak discharges and stages occurring during the water year and greater than a selected base discharge are presented under this heading. The peaks greater than the base discharge, excluding the highest one, are referred to as secondary peaks. Peak discharges are not published for canals, ditches, drains, or streams for which the peaks are subject to substantial control by man. The time of occurrence for peaks is expressed in 24-hour local standard time. For example, 12:30 a.m. is 0030, and 1:30 p.m. is 1330.
REVISIONS.--If a critical error in published records is discovered, a revision is included in the first report published following discovery of the error.
Although rare, occasionally the records of a discontinued gaging station may need revision. Because, for these stations there would be no current or, possibly, future station manuscript published to document the revision in a "Revised Records" entry, users of data for these stations who obtain the record from published data reports may wish to contact the District office to determine if the published records were revised after the station was discontinued. Data obtained from computer files for discontinued stations will be current since these files are updated with appropriate revisions at the time revisions are made.
Manuscript information for lake or reservoir stations differs slightly from that for stream and stage stations. A paragraph describing the dam, beginning storage date, if known, and pertinent contents and elevation information is included in the description. Normally there is no "REMARKS" section. "EXTREMES" sections are presented only for those reservoirs where daily or more frequent pool elevations are available.
Headings for AVERAGE DISCHARGE, EXTREMES FOR PERIOD OF RECORD, AND EXTREMES FOR CURRENT YEAR have been deleted and the information contained in these paragraphs, except for the listing of secondary instantaneous peak discharges, which are now presented in the PEAK DISCHARGES FOR CURRENT YEAR paragraph, is now presented in the tabular summaries following the discharge table or in the REMARKS paragraph, as appropriate. No changes have been made to the data presentations of lake contents.
Data table of daily mean values
The daily table of discharge records for stream-gaging stations gives mean discharge for each day of the water year. In the monthly summary for the table, the line headed "TOTAL" gives the sum of the daily figures for each month; the line headed "MEAN" gives the average flow in cubic feet per second for the month; and the lines headed "MAX" and "MIN" give the maximum and minimum daily mean discharges, respectively, for each month. Discharge for the month also is usually expressed in cubic feet per second per square mile (line headed "CFSM"); or in inches (line headed "IN."); or in acre-feet (line headed "AC-FT"). Figures for cubic feet per second per square mile and runoff in inches or in acre-feet may be omitted if there is extensive regulation or diversion or if the drainage area includes large noncontributing areas. At some stations monthly and (or) yearly-observed discharges are adjusted for reservoir storage or diversion, or diversion data or reservoir contents are given. These figures are identified by a symbol and corresponding footnote.
Statistics of monthly mean data
A tabular summary of the mean (line headed "MEAN"), maximum (line headed "MAX"), and minimum (line headed "MIN") of monthly mean flows for each month for a designated period is provided below the mean values table. The water years of the maximum and minimum monthly flows are provided immediately below those figures. The designated period will be expressed as 'FOR WATER YEARS ______-_______, BY WATER YEAR (WY)," and will list the first and last water years of the range of years selected from the PERIOD OF RECORD paragraph in the station manuscript. It will consist of all of the station record within the specified water years, inclusive, including complete months of record for partial water years, if any, and may coincide with the period of record for the station. The water years for which the statistics are computed will be consecutive, unless a break in the station record is indicated in the manuscript.
A table titled "SUMMARY STATISTICS" follows the statistics of monthly mean data tabulation. This table consists of four columns, with the first column containing the line headings of the statistics being reported. The table provides a statistical summary of yearly, daily and instantaneous flows, not only for the current water year but also for the previous calendar year and for a designated period, as appropriate. The designated period selected, "WATER YEARS ______-______," will consist of all of the station record within the specified water years, inclusive, including complete months of record for partial water years, if any, and may coincide with the period of record for the station. The water years for which the statistics are computed will be consecutive, unless a break in the station record is indicated in the manuscript. All of the calculations for the statistical characteristics designated ANNUAL (See line headings below.), except for the "ANNUAL 7-DAY MINIMUM" statistic, are calculated for the designated period using complete water years. The other statistical characteristics may be calculated using partial water years.
The date or water year, as appropriate, of each statistic reporting extreme values of discharge is provided adjacent to the statistic. Repeated occurrences may be noted in the REMARKS paragraph of the manuscript or in footnotes. Because the designated period may not be the same as the station period of record published in the manuscript, occasionally the dates of occurrence listed for the daily and instantaneous extremes in the designated-period column may not be within the selected water years listed in the heading. When this occurs, it will be noted in the REMARKS paragraph or in footnotes. Selected streamflow duration curve statistics and runoff data are also given. Runoff data may be omitted if there is extensive regulation or diversion of flow in the drainage basin.
The following summary statistics data, as appropriate, are provided with each continuous record of discharge. Comments to follow clarify information presented under the various line headings of the summary statistics table:
ANNUAL TOTAL.--The sum of the daily mean values of discharge for the year. At some stations, the annual total discharge is adjusted for reservoir storage or diversion. The adjusted figures are identified by a symbol and corresponding footnotes.
ANNUAL MEAN.--The arithmetic mean of the individual daily mean discharges for the year noted or for the designated period. At some stations, the yearly mean discharge is adjusted for reservoir storage or diversion. The adjusted figures are identified by a symbol and corresponding footnotes.
HIGHEST ANNUAL MEAN.--The maximum annual mean discharge occurring for the designated period.
LOWEST ANNUAL MEAN.--The minimum annual mean discharge occurring for the designated period.
HIGHEST DAILY MEAN.--The maximum daily mean discharge for the year or for the designated period.
LOWEST DAILY MEAN.--The minimum daily mean discharge for the year or for the designated period.
ANNUAL 7-DAY MINIMUM.--The lowest mean discharge for 7 consecutive days for a calendar year or a water year. Note that most low-flow frequency analyses of annual 7-day minimum flows use a climatic year (April 1-March 31). The date shown in the summary statistics table is the initial date of the 7-day period. This value should not be confused with the 7-day 10-year low-flow statistic.)
INSTANTANEOUS PEAK FLOW.--The maximum instantaneous discharge occurring for the water year or for the designated period. Note that the secondary instantaneous peak discharges above a selected base discharge are stored in District computer files for stations meeting certain criteria. Those discharge values may be obtained by writing to the District Office. (See address on back of title page of this report.) SURFACE-WATER DATA, GEORGIA 1999
INSTANTANEOUS PEAK STAGE.--The maximum instantaneous stage occurring for the water year or for the designated period. If the dates of occurrence for the instantaneous peak flow and instantaneous peak stage differ, the REMARKS paragraph in the manuscript or a footnote may be used to provide further information.
INSTANTANEOUS LOW FLOW.--The minimum instantaneous discharge occurring for the water year or for the designated period.
ANNUAL RUNOFF.--Indicates the total quantity of water in runoff for a drainage area for the year. Data reports may use any of the following units of measurement in presenting annual runoff data:
Acre-foot (AC-FT) is the quantity of water required to cover 1 acre to a depth of 1 foot and is equivalent to 43,560 cubic feet or about 326,000 gallons or 1,233 cubic meters.
Cubic feet per second per square mile (CFSM) is the average number of cubic feet of water flowing per second from each square mile of area drained, assuming the runoff is distributed uniformly in time and area.
Inches (INCHES) indicate the depth to which the drainage area would be covered if all of the runoff for a given time period were uniformly distributed on it.
10 PERCENT EXCEEDS.--The discharge that has been exceeded 10 percent of the time for the designated period.
50 PERCENT EXCEEDS.--The discharge that has been exceeded 50 percent of the time for the designated period.
90 PERCENT EXCEEDS.--The discharge that has been exceeded 90 percent of the time for the designated period.
There are several exceptions to the above-described format. First, if a station was operated under both non-regulated and significantly regulated flow regimes, two sets of monthly mean and summary statistics are furnished. One set of monthly mean and summary statistics represents the period prior to regulation, and the second set represents the period since flow has been regulated. The summary statistics prior to regulation do not include current calendar or water year statistics since they are included in the SINCE REGULATION summary statistics. Also, in the station manuscript there is an AVERAGE DISCHARGE line heading, which is the arithmetic mean of the complete water-year mean discharges for the entire period of record, and includes both the regulated and non-regulated periods of record. Some AVERAGE DISCHARGE computations may include mean discharges adjusted for reservoir storage or diversion. Another exception occurs when discharge records are fragmentary for various reasons. Then, the monthly mean and summary statistics have been eliminated or modified, based on available information, and EXTREMES FOR PERIOD OF RECORD and EXTREMES FOR CURRENT YEAR line headings have been included in the station manuscript. Extremes may include maximum and minimum stages and maximum and minimum discharges. The highest stage may have been obtained from a graphic, digital, or electronic recorder, a crest-stage gage, or by direct observation. Similarly, the minimum is the instantaneous minimum discharge, unless otherwise qualified, and was determined and reported in the same manner as the maximum.
The daily table of gage-height stations gives mean gage-height for each day. In the monthly summary, the line headed "MEAN" gives the average gage height during the month. The lines headed "MAX" and "MIN" provides the maximum and minimum daily gage heights, respectively, for the month.
Data for reservoirs are presented following the continuous-station data for the basin in which they are located. Month-end elevations, contents, and monthly and yearly change in contents are presented in tabular form following the reservoir station description.
Data collected at partial-record stations follow the information
for continuous-record sites. If collected, data for
partial-record discharge stations are presented in two tables.
The first is a table of annual maximum stage and discharge at
crest-stage stations, and the second is a table of discharge
measurements at low-flow partial-record stations. The data
contained in the partial-record station tables are often
supplemented by information gathered at miscellaneous sites that
are neither continuous record nor partial-record stations. This
information is presented in tables similar to those for the
partial-record stations and the table headings explain the data
that are shown.
Identifying Estimated Daily Discharge
Identifying Estimated Daily Discharge
Estimated daily-discharge values published in the water-discharge tables of annual State data reports are identified either by flagging individual daily values with the letter symbol "e" and printing a table footnote, "e Estimated," or by listing the dates of the estimated record in the REMARKS paragraph of the station description.
Accuracy of the Records
The accuracy of streamflow records depends primarily on: (1) The stability of the stage-discharge relation or, if the control is unstable, the frequency of discharge measurements; and (2) the accuracy of measurements of stage, measurement of discharge, and interpretation of records.
The accuracy attributed to the records is indicated under "REMARKS". "Excellent" means that about 95 percent of the daily discharges are within 5 percent of the true; "good," within 10 percent; and "fair," within 15 percent. Records that do not meet the criteria mentioned are rated "poor." Different accuracies may be attributed to different parts of a given record.
Daily mean discharges in this report are given to the nearest hundredth of a cubic foot per second for values less than 1 ft3/s; to the nearest tenth between 1.0 and 10 ft3/s; to the nearest whole numbers between 10 and 1,000 ft3/s; and to 3 significant figures for values more than 1,000 ft3/s. The number of significant figures used is based solely on the magnitude of the discharge value. The same rounding rules apply to discharges listed for partial-record stations and miscellaneous sites.
Discharge at many stations, as indicated by the monthly mean, may not reflect natural runoff due to the effects of diversion, consumption, regulation by storage, and increase or decrease in evaporation due to artificial causes or to other factors. For such stations, figures of cubic feet per second per square mile and of runoff, in inches, are not published unless satisfactory adjustments can be made for diversions, for changes in contents of reservoirs, or for other changes incident to use and control. Evaporation from a reservoir is not included in the adjustments for changes in reservoir contents, unless it is so stated. Even at those stations where adjustments are made, large errors in computed runoff may occur if adjustments or losses are large in comparison with the observed discharge.
Other Records Available
Information used in the preparation of the records in this publication, such as discharge-measurement notes, gage-height records, temperature measurements, and rating tables are on file in the Georgia District office. Also, most of the daily mean discharges are in computer-readable form, and have been analyzed statistically. Information on the availability of the unpublished information or on the results of statistical analyses of the published records may be obtained from the District office.
Records of Surface-Water Quality