Proceedings of the 2003 Georgia Water Resources Conference, held April 23-24, 2003, at the University of Georgia.The Apalachicola-Chattahoochee-Flint
(ACF) Basin has experienced three major drought
events in the past 50 years. A cumulative deficit
evaluation of the drought events shows that the greatest
cumulative rainfall deficit occurred during the mid
1950’s event, but the greatest flow deficit occurred
during the 1999–2002 drought event. An evaluation of
the unimpaired flow set, being used to develop an
Allocation Formula for the ACF Basin, indicates that
this data set is not consistent with these findings,
suggesting further evaluation may be needed to address
the adequacy of the dataset as the basis for future policy
decisions. Caution should be used in using and in
interpreting model results from this time period

Dowd, John

Holmbeck-Pelham, Skelly A.

Leitman, Steve

Scholarly Materials And Research @ Georgia Tech

AN EVALUATION OF OBSERVED AND UNIMPAIRED FLOW ANDPRECIPITATION DURING DROUGHT EVENTS IN THE ACF BASINSteve Leitman 1, John Dowd2 and Skelly Holmbeck-Pelham3___________________________________________________________________________________________________________________AUTHORS: 1Consultant, 5812 Old Federal Road, Quincy, Florida, 32351; 2Geology Department, The University of Georgia, Athens GA 30602;and 3Policy Director, Upper Chattahoochee Riverkeeper, 3 Puritan Mill, 916 Joseph Lowery Boulevard, Atlanta, GA 30318.REFERENCE:  Proceedings of the 2003 Georgia Water Resources Conference, held April 23-24, 2003, at the University of Georgia.  KathrynJ. Hatcher, editor, Institute of Ecology, The University of Georgia, Athens, Georgia.__________________________________________________________________________________________________________________    Abstract.  The Apalachicola-Chattahoochee-Flint(ACF) Basin has experienced three major droughtevents in the past 50 years.  A cumulative deficitevaluation of the drought events shows that the greatestcumulative rainfall deficit occurred during the mid1950’s event, but the greatest flow deficit occurredduring the 1999–2002 drought event.  An evaluation ofthe unimpaired flow set, being used to develop anAllocation Formula for the ACF Basin, indicates thatthis data set is not consistent with these findings,suggesting further evaluation may be needed to addressthe adequacy of the dataset as the basis for future policydecisions. Caution should be used in using and ininterpreting model results from this time period. .INTRODUCTION    The ACF Basin begins in the Piedmont of northernGeorgia, and extends through middle and southernGeorgia to the Florida panhandle.  The 19,700 squaremile basin hosts a wide variety of plant and animalspecies.  More than 4 million human inhabitants call theACF Basin home, relying on the streams for watersupply, fisheries, recreation, waste assimilation, powergeneration, and irrigation.  The generally high qualityof life in the ACF Basin has spurred economic growth.This growth has occurred mainly in the Atlanta area.    It is this population growth and the associatedincreased demands on water resources that sparked theTri-state Water Wars in 1990.  Rather than resolvingthe issues through litigation, the states are workingtogether to formulate a water sharing agreement.  In1997, US Congress enacted the ACF Compact, whichallows Alabama, Georgia, and Florida to determine anallocation formula for the basin.  The deadline fornegotiations, which has been extended more than adozen times, is currently set for July 2003.  Thisformula will have far-reaching and long-termconsequences, governing stream and reservoirmanagement and thus affecting stream habitat, thehealth of fisheries, recreational uses, water availabilityand economic growth for the next 50 years.    Thispaper evaluates whether the current flows in the basinhave been affected by human consumption and theconsistency of the U.S. Army Corps of Engineersunimpaired flow set with recently observed flow andprecipitation trends during a major drought event. Theunimpaired data set plays an important role in modelingassociated with developing the ACF AllocationFormula.    The basis of the evaluation is an examination of flowand precipitation data.  Flow data is evaluated at threelocations: the USGS gage at Chattahoochee, Florida onthe Apalachicola River, the Columbus, Georgia gage onthe Chattahoochee River and the Newton, Georgia gageon the Flint River.  The Apalachicola at Chattahoocheegage was used because it monitors below the pointwhere the Flint and Chattahoochee Rivers converge andwhere the cumulative effects of human activities can beevaluated.  The Chattahoochee at Columbus gage andthe Flint at Newton gage are the furthest downstreamgages on the rivers at which reliable, long-termobserved data are available.  Although gages furtherdownriver exist on both the Chattahoochee and FlintRiver, these gages are affected by backwater effectsfrom reservoirs and are not considered reliable.    Observed data, precipitation data and an unimpaireddata set were evaluated to gage the effects of humanactivities on instream flow.  The unimpaired data set isa synthesized data set prepared by the U.S. Army Corpsof Engineers, Mobile District and the States ofAlabama, Florida and Georgia for modeling purposes inthe ACF Comprehensive Water Resources Study andongoing negotiations between the states.  Theunimpaired data set was constructed by removing fromobserved data the influences of consumptivewithdrawals and returns and reservoir management andevapo-precipitation effects at reservoirs.  The observedflow data used in this paper were taken from the USGSweb site (http://waterdata.usgs.gov).  The unimpairedflow set used for this analysis is the most recent setavailable from the Corps of Engineers as of January2003.  At the time this paper was written this data sethad not been approved by the three states.  Theprecipitation data were obtained from the web site forthe National Climatic Data Center(http://lwf.ncdc.noaa.gov/oa/climate/onlinerod/drought/xmgrg3.html).  The precipitation data were divided bythe Climate Division of the National Climate DataCenter and were converted into a weighted average forthe basin above the Chattahoochee gage using an Excelspreadsheet prepared by the Northwest Florida WaterManagement District and modified to include data from1994 to 2001.    The analysis for this paper focuses on the low flowcomponent of the data set for the ACF Basin.  Lowflows will be the aquatic bottom line for the ACFsystem.  If low flows are insufficient, poorly timed, ortoo frequent, aquatic species, recreational users,discharges, and all water users could suffer negativeeffects.  Furthermore, the ratio of reservoir storage toflow in this basin is low; the reservoir system only hasthe capacity to affect low to median flows in the lowerparts of the basin, and cannot store adequate water toaugment low flows for a long duration of time or at alarge magnitude for a sustained period of time.RESULTS    The cumulative deficit of observed flows,unimpaired flows and precipitation during three majordrought events are shown in Figures 1, 2 and 3.  Thecumulative deficits were computed by comparingobserved data that occurred during the drought eventsto observed average monthly values for the period of1939 to 2001. All of the dataset comparisons begin onJanuary 1.  The drought events occurred during themid-1950s, the mid-1980s and 1999 to 2002, so one ofthe events occurred before consumptive demands in theACF basin began to grow significantly, one in the midstof the growth of consumptive demands and one whenconsumptive demands had reached their current level.    In developing these comparisons, it was ourexpectation that the unimpaired flow results wouldcorrespond to fluctuations in the rainfall data over theentire period of record.  If any differences in therelative comparisons were to occur, we expected thatthey would be only in the observed flow data set.Affects of consumptive and evaporative losses shouldbe apparent in the gage data.  Because the unimpaireddata set was only extended to December 31, 2001, inFigure 2 the 1999 to 2002 drought event is onlyanalyzed through that date.Figure 1.  Observed flow cumulative deficit.Figure 2.  Cumulative precipitation deficit.Figure 3. Unimpaired flow cumulative deficit.-500000-400000-300000-200000-10000001000001 7 13 19 25 31 37 43MONTHS INTO DROUGHTFLOW DEFICIT (cfs-days)Jan 1, 1954 - July 31, 1957Jan 1, 1985 - July 31, 1988Jan 1, 1999 - July 31, 2002-30.00-25.00-20.00-15.00-10.00-5.000.005.001 7 13 19 25 31 37 43MONTHS INTO DROUGHTPRECIPITATION DEFICIT (inches)Jan 1, 1954 - July 31, 1957Jan 1, 1985 - July 31, 1988Jan 1, 1999 - July 31, 2002-500000-400000-300000-200000-10000001000001 7 13 19 25 31 37 43MONTHS INTO DROUGHTFLOW DEFICIT (cfs-days)Jan 1, 1954 - July 31, 1957Jan 1, 1985 - July 31, 1988Jan 1, 1999 - July 31, 2002DISCUSSIONIn reviewing Figures 1, 2, and 3 it can be seen that inthe observed flow set, the 1999 to 2002 drought eventhad the largest cumulative deficit in flows, even thoughthe precipitation data indicated the 1954 to 1957drought was the most severe drought event.  Theunimpaired flow set also shows the greatest deficit forthe 1999 to 2002 drought event.  The magnitude of thedifferences in the cumulative deficits in the observeddata set is often in the 50,000 to 100,000 cfs-days rangeand the magnitude of the differences in precipitation inthe five to ten inches range.  The differences in theunimpaired set are of less magnitude than in theobserved dataset.  The unimpaired flow set shouldcorrelate to the precipitation data, not the observed flowdata. The reasons for this discrepancy could be theresult of 1) an undercounting of consumptive demandsin the basin, 2) an undercounting of evaporative lossesfrom the basin or 3) other errors or assumptions in thedataset used to compute the unimpaired flow set.Consumptive Demands    The consumptive demands used in the STELLA andHEC5 models can be broken up into municipal andindustrial demands and agricultural demands.These models serve as the means to technicallyevaluate alternative allocation formula proposals andtherefore in the decision-making process of whether aspecific proposal is acceptable.  Municipal andindustrial demands used to construct the unimpairedflow set were reported data provided by the States andare the best data available.  The data were metered bythe various water users and represent a reasonableestimate of consumptive water use in the basin.Agricultural demands, however, were estimated andderived from average annual withdrawals.  In dry andwet years, the agricultural demand estimates were giventhe same value in the unimpaired data set although inreality the demand does vary; this could be a significantsource of error.  This potential error has beencompensated for in modeling efforts, but was notaddressed in the unimpaired data set.  The data used toestimate agricultural demands were computed bymultiplying estimated acreage of irrigation, source ofirrigation water (surface or groundwater), andapplication rates.  If the source of water wasgroundwater, further assumptions had to be made withregard to how agricultural withdrawals in the karstDougherty Plain region effect surface flows.  All ofthese estimates represent potential sources of error.101102103104105Flow (cfs)12/21/39 12/18/49 12/16/59 12/13/69 12/11/79 12/08/89 12/06/99DateFigure 4.  Unimpaired flow at Columbus gage onChattahoochee RiverEvaporative losses    In computing the evapo-precipitation effects for theunimpaired flow set, the Corps accounted only forevaporative losses at the four major storage reservoirsin the basin (U.S. Army Corps’, 1997), which covernearly 150,000 acres at full pool.  However, there areapproximately an additional 100,000 acres ofimpoundments in the ACF basin where evaporationoccurs but was not accounted for in developing theunimpaired data set (Georgia Department ofTransportation).  About 60% of this acreage is in theFlint basin and 40% the Chattahoochee.  In the summermonths during a dry year this translates into amaximum of about 300 to 400 cfs-days of lost flowunaccounted for in the unimpaired flow set.The Unimpaired Flow set    The “unimpaired flows” represent the Corps’estimate of the flow that would have occurred from1939-2001 if no human activity had taken place.Figure 4 shows the unimpaired flow estimated for theColumbus gage on the Chattahoochee River for the1939-2001 period, with the daily flows plotted on a logscale.  One check on the hydrologic consistency ofthe record is to plot the flow duration curve, a plotof the (log) cumulative average daily flows on anormal probability axis. If the variation in theunimpaired flows is hydrologically correct, the flowduration curve for data in the first half of the recordshould be similar to a flow duration curve in the secondhalf of the record.  The data were split at the end of1974 because this represents the approximate mid-pointof the series; human influence on the watershed wasalso substantially different after 1974.  The pre-1974data fits a log-normal distribution (straight line), while102103104105Averag eDailyFlo w-cfs1/5/39-12/31/741/1/75-12/31/011/1/75-12/31/981/5/39-12/31/9850.1 1 5 10 30 70 9095 99 99.9Percent Flow Equaled or ExceededFigure 5.  Unimpaired Flow duration curve forColumbus gage on Chattahoochee River103104105106Flow (cfs)12/21/39 12/18/49 12/16/59 12/13/69 12/11/79 12/08/89 12/06/99DateFigure 6.  Unimpaired Flow at Chattahoochee gageon Apalachicola Riverthe 1975-2001 data do not (Figure 5).  It has a numberof anomalously low data points.  When the years 1999-2001 are removed, the data are similar to the pre-1974curve, demonstrating that the inconsistent data is in thelast three years of record, the recent drought.  Toconfirm this finding, the data from 1939-1998 wereplotted and found to fit log-normal distributionindistinguishable from the pre-1974 curve. Similaranalyses of flow trends at the Apalachicola River atChattahoochee and Flint River at Newton gages werealso conducted.  The disparity in the data persists at theChattahoochee gage site (Figure 6), but does not persistat the Newton site (Figure 7).  The same trends seen inFigure 4, 5 and 6 with one-day flows persist when thedata is converted running seven-day flows.  Thissuggests that the problem associated with theunimpaired set is with the Chattahoochee basin, not theFlint Basin even though the agricultural andevaporation data earlier suggested problems with Flintbasin.  These results demonstrate that the one-dayunimpaired flows derived for the 1998-2001 period of102103104105Flow (cfs)12/21/39 12/18/49 12/16/59 12/13/69 12/11/79 12/08/89 12/06/99DateFigure 7.  Unimpaired Flow at Newton gage on FlintRiverdrought are not reliable, especially when it isrecognized that this time period did not have as great ofa precipitation deficit as occurred in the 1954-1957drought.CONCLUSIONS   This analysis shows flows at the Apalachicola Riverat Chattahoochee gage during drought events have beenimpacted by consumptive demands and that the Corpsof Engineers daily unimpaired flow at the Columbusgage for the years 1999-2001 do not totally reflect theseimpacts. This suggests that in making policy decisionsin the Allocation Formula negotiations that cautionshould be used in drawing conclusions from flows inthe 1999 to 2001 time period.  The limits of the datasetsused to develop the unimpaired flow set must berecognized in modeling exercises and in interpretingmodel outputs. Further analysis at additional gages isnecessary to address the adequacy of the dataset as thebasis for future policy decisions.ACKNOWLEDGEMENTS  We extend special thanks to the Mott Foundation andthe Turner Foundation for their support.REFERENCESGeorgia Department of Transportation, Lakes and Pond  data, undated, available through Georgia GIS Data  Clearinghouse.

An evaluation of observed and unimpaired flow and precipitation during drought events in the ACF basin

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An evaluation of observed and unimpaired flow and precipitation during drought events in the ACF basin

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