WATER
RESOURCES
RESEARCH,
VOL. 35, NO. 12, PAGES 3723-3733, DECEMBER
1999
Sensitivity of stream temperatures in the United States to air temperatures projected under a global warming scenario Omid Mohseni, Troy R. Erickson, and Heinz G. Stefan St. AnthonyFalls Laboratory,Departmentof Civil Engineering,Universityof Minnesota,Minneapolis
Abstract. To project mean weeklystreamtemperaturechangesin responseto global climatewarmingand for studiesof freshwaterecosystems, a four-parameternonlinear function of weekly air temperatureswas used.One parameter, the upper bound stream temperature,was obtainedby extremevalue analysisfrom streamtemperaturedata, and the other three parameterswere obtainedby least squaresregressionanalysis.The least squaresregressionfunctionwas developedseparatelyfor the warmingseasonand the coolingseason(hysteresis)to take heat storagedue to snowmeltor reservoiroperations into account.There were very weak correlationsbetweenmodel parametersand annual or seasonalair temperatures.To project weekly streamtemperaturesunder a 2 x CO2 climate scenario,weekly air temperaturedata from 166 weather stations,incrementedby the output of the CanadianCenter of Climate Modelling (CCC) generalcirculationmodel (GCM), were appliedto nonlinearstreamtemperaturemodelsdevelopedfor 803 stream gagingstations.An error analysisindicatedthat only 39 streamgagingstationswould not exhibit a significantchangeunder the CCC-GCM 2 x CO2 climate scenario.The projectionsat the remaining764 streamgagingstationsshowedthat mean annual stream temperaturesin the contiguousUnited Stateswould increaseby 2ø-5øC,least near the West Coast and most in the Missouri River and Ohio River basins.On average,there would be a 1ø-3øCincreasein the maximumand minimumweekly streamtemperatures under the 2 x CO2 climate scenario,most in the central United States.It was also found that most streamswould experiencethe maximumchangein weekly streamtemperatures in spring(March-June).The minimumchangesin streamtemperaturesare projectedto occurin winter (Decemberand January)and summer(July and August)throughoutthe United
1.
States.
Introduction
Potential global warming causedby an increase in some atmospheric gases,especiallycarbondioxideandmethane,will influencethe thermal regimeof aquaticenvironments.These changeswill impact the organismsliving in the aquaticenvironments.For example,if maximumwater temperaturein a stream reach exceedsthe thermal toleranceof a fish species, that fish speciesis likely to disappearin that stream reach [Eaton and Scheller,1996]. Similarly, if stream temperature drops below a threshold,it may causethe demiseof a fish speciesthroughosmoregulatorydysfunction.There is also evidencethat fishspeciesstopfeedingor feed onlysporadicallyat weekly streamtemperaturesbelow a threshold,for example, on average,8øC for warm water fish species[Schelleret al., 1999]. To project fish habitat changesunder a 2 x CO2 climate scenario,i.e., a climateafter doublingof carbondioxidein the atmosphere,it is necessary to know the thermal constraintsof different fish speciesand to have accessto the current and projected stream temperature time series. A fish survival model, basedon the streamthermal regime and thermal constraintsof warm water fish species,has been developedby Schelleret al. [1999]. The fish survivalmodel has a weekly timescale.
Copyright1999by the American GeophysicalUnion. Paper number 1999WR900193. 0043-1397/99/1999WR900193509.00
For the estimationof streamtemperaturesas a functionof climate variables,modelswith differing complexityand data inputrequirementshavebeendeveloped.There are (1) models usinga completeheat advection-dispersion equation(see review by Stefanand Sinokrot[1993]), (2) modelsincorporating only surfaceheat transferprocessesand the conceptof equilibrium temperature [Edingeret al., 1968;Brown et al., 1971], (3) modelsemployingseasonal(often sinusoidal)functionsof stream temperature with respect to time [Ward, 1963; Kothandaraman,1971;Hostetler,1991;Roweand Taylor,1994], and (4) modelsdevelopedby leastsquaresregression between stream temperatures and air temperatures [Johnson,1971; Song et al., 1973; Crisp and Howson, 1982; Stefan and Preud'homme,1993;Mohseniet al., 1998]. Regressionsbetween water temperatures at individual streamgagingstationsand air temperaturesat nearbyweather stations provide the easiest practical method to estimate stream temperaturesfor the entire United States.They are also attractivefor climate changeeffect studiesbecauseonly one input variable, air temperature,is used, and general circulation models (GCMs) simulatethis variable better than they simulateother climatevariables[Lau et al., 1996].Among regressionmodels,linear modelsare more commonand easier to utilize. However, a linear function of air temperature is usuallynot sufficientto determine streamtemperaturesyearround. It hasbeen shownthat becauseof evaporativecooling, streamtemperaturestartslevelingoff as air temperatureexceeds---20øC.The overallstreamtemperature-airtemperature
3723
3724
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ET AL.' SENSITIVITY
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13=14.1
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OF STREAM
-5
••'-*
•
0
5
10
-•
15
20
o
*
25
30
35
40
WeeklyAir Temperatures (øC)
Figure 1. Simulatedand observedweekly streamtemperatures in the Salt Fork of the Arkansas River near Jet, Okla-
homa, versusweekly air temperaturesrecorded at Wichita, Kansas. NSC, Nash-Sutcliffe Coefficient.
relationshipoften resemblesan S-shapedfunction [Mohseni and Stefan, 1999]. Mohseniet al. [1998] introduceda fourparameterS-shapedfunction(a logisticfunction)to estimate streamtemperatureas a functionof a solepredictorvariable, air temperature.The modelwaslater modifiedby Ericksonet al. [1998a] for the upper bound streamtemperatureestimation.
In this paper, a brief reviewof the new streamtemperature modelis given.The modelis then appliedto multiyearrecords of streamtemperatureand air temperaturedata to simulate weekly stream temperaturesat 803 stream gagingstations throughoutthe contiguousUnited States,underrecentclimate conditions(1961-1979)andundera projected2 x CO2climate scenario.Projectedstreamtemperaturechangesare then presented and discussed.
2.
Nonlinear Stream-Air Temperature Regression
Model
2.1.
TEMPERATURES
STATES
n
Mohseni et al. [1998] studied the relationship between streamtemperaturesand air temperaturesfor 584 U.S. GeologicalSurvey(USGS) streamgagingstationsin the United States.They observedthat a nonlinearrelationshipbetween weeklystreamtemperaturesand air temperaturesexists.Figure 1 is an example that displaysthe relationshipbetween weekly streamtemperaturesmeasuredat the USGS gaging in the Salt Fork of the Arkansas
UNITED
at the inflection point. Air temperatureswere taken from weather stationsthat were 2-244 km awayfrom the stream gagingstations.The goodness of fit of (1) to weeklydatawas not significantlyaffected by the distancebetween stations. Mohseniet al. noticedthat somestreamsexhibitedhysteresis, whichwasattributedto heat storageeffects[WebbandNobdis, 1997].To takehysteresis into account,separatefunctionswere fitted to the data representingthe risinglimb and the falling limb of the streamtemperature-airtemperaturerelationship. The limbswere distinguished from eachother usingthe associatedweek numbers.The minimumof the meanweekly air temperatureswas usedto set the startingweek of the rising limb (equalto the endingweek of the fallinglimb), and the maximumof the meanweeklyair temperatures wasusedto set the endingweek of the risinglimb (equalto the startingweek of the falling limb). Equation(1) waspreviously fittedto 3 years(1978-1980)or lessof recordeddata at 584 stations[Mohseniet al., 1998]. Subsequently, it wasdeterminedthat a 3-yearrecordcouldnot representthe averagethermalregimeof manystreams[Ericksonet al., 1998b].When (1) wasfittedby leastsquaresregressionto the streamtemperaturerecordsand the associatedair temperaturedata, 56 of the recordsgaveestimatesof upper bound stream temperatures a smaller than four or more weeklyrecordedvaluesfor the periodof study(1980-1983). To avoidthis deficiencyof the model,Ericksonet al. [1998a] developeda new method to estimate a as an extremevalue usinga universalstandarddeviate.In this new approach,the upper bound streamtemperaturea was estimatedindependently(not by the regression method). In thispaper,themodelhasbeenusedfor 993streamgaging stationswith the full available stream temperature record lengthsbetween1961and 1990andwith an independentestimate of a. The averagerecordlengthfor the 993 gagingstationswas12years.The fit betweentheregression model(equation (1)) and the available stream temperature data was measuredby the Nash-SutcliffeCoefficient(NSC) [Nashand Sutcliffe,1970]:
Model Description
station 07150500
IN THE
River near
• (rsimt - robst) 2 i=1
NSC
= 1n
NSC _ 0.85). parametersat 803 streamgagingstationsand latitude,eleva- The four model parametersa, /3, ?, and/x determinedfrom tion, watershed size, and meteorologicalvariableswere ex- historicalrecordswere left unchangedfor the streamtemperplored. No correlationwas found betweenthe model param- ature projections.This impliesthat the physicalsetting(river etersand latitude or the contributingwatershedsizeupstream geometry,width, depth, roughness,groundwaterinput, upof the gagingstations.There alsowas no significantrelation- streamconditions,shading,and wind sheltering)of a stream shipbetweenthe four modelparametersand the mean annual remains unchanged.The parameter a is most sensitiveto and seasonalair temperatures(Table 3). The two strongest streamshading[Mohseniand Stefan,1999].Therefore a may relationshipshad correlationcoefficientsof only0.59 and 0.62, changeunder the 2 • CO2 climate scenarioif the land use, respectively. For climatechangeeffect studiesone can there- especiallyin the forestedareas,changes.Other parametersof fore conclude that the model parameters are unlikely to the logisticfunctionare lesslikely to changeunder a 2 x CO2 changeunderwarmerclimateconditions, becausethe correla- climate scenario if the same weather station is used for both tion betweenthe parametersandthe meanannualor seasonal regressionand streamtemperatureprojection. air temperaturesis weak. This is not to be confoundedwith Beforeprojectingthe streamtemperaturechangesunderthe changesin streammorphometryand streamshadingby trees 2 x CO 2 climate scenario,it wasnecessaryto conductan error whichwould stronglyaffect the model parametera. analysis.There are errorsassociatedwith the projectionof air The above results indicate that air temperature is often a temperaturesby the GCMs and errors in the streamtemperreasonablestream temperaturepredictor on a weekly time- ature model simulations. There is a lack of information on the scaleif a nonlinearmodelis used.The additionof meanweekly errors associatedwith the GCMs even for past climate condistreamflow may or may not improve the predictions. No tions,owingto uncertainties in observed data[Lauetal., 1996]. changein the streamtemperature-airtemperaturerelation- Therefore an error analysisto determinethe significance of the shipwasdetectedwhendatawere sortedaccordingto flowrate weekly stream temperaturechangesunder the 2 x CO2 cliand analyzedin separatebatches. mate scenariowas conductedonly for the streamtemperature Table 2. StatisticalPropertiesof the LogisticFunction Parametersfor the 803 Stream Gaging StationsWith NSCs Larger Than 0.90
model simulations.
3.
Stream Temperature Projections
3.1. The 2 x COz Climate Scenario
The outputfrom a generalcirculationmodel(GCM) developed at the CanadianCenter of Climate Modelling (CCC) [McFarlaneet al., 1992] was used to specifythe 2 X CO2 (equilibrium)climatescenario.The second-generation CCCGCM was chosenbecauseit is a coupled atmosphere-ocean modelwith smallgrids(3.75ø x 3.75ø).Climatevariablesare calculatedat grid points. Figure 3 illustratesmean annual changesin air temperaturesthroughoutthe United Statesprojectedby the CCC-GCM. The largestchangesare projectedto occurin inland areas and at higher latitudes.Projectedmean monthly air temperaturechangesin July and January,also shownin Figure 3, are substantiallydifferent. The maximum changesof air temperaturesin July are 5ø-6øCand are projected to occurin the centralMississippiRiver basin;thosein
It was assumed that errors associated with
the streamtemperaturemodel simulationswould not change under the new climate
condition.
The error analysiswas a one-tail paired t test at the 95% confidence level between the mean of the errors,
Table 3. Function
Correlation CoefficientsBetween the Logistic Parameters
and the Mean Annual
and Seasonal
Air Temperaturesfor the 803 Stream Gaging Stations
Annual Winter
0.39 0.18
0.54 0.40
-0.35 -0.26
0.36 0.37
Spring
0.54
0.59
-0.40
0.33
Summer Fall
0.51 0.35
0.62 0.45
-0.45 -0.25
0.31 0.37
MOHSENI
ET AL.: SENSITIVITY
OF STREAM
TEMPERATURES
IN THE
UNITED
STATES
3727
(a)
(b)
"
Figure 3. Changesin air temperaturesprojectedby the Canadian Center of Climate Modelling (CCC) generalcirculationmodel (GCM) for the 2 x CO2 climateconditions:(a) mean annual, (b) July, and (c) January.
1 n
•:'--//-- E Irsim '. - robsil ,
(4)
i=1
and the mean of the changesunder the 2 x CO 2 climate scenario,
1 n
AT = //- • ITsim, - T2xco2,1 '
(5)
i=1
at each gagingstation for the availablerecordsbetween 1961 and 1979. The resultsshowedthat only 39 stream gagingsta-
3728
MOHSENI
ET AL.:
SENSITIVITY
OF STREAM
TEMPERATURES
Table 4. StationsWith Significantand InsignificantChanges in Weekly StreamTemperaturesUnder the 2 x CO2 Climate
Stations with
Mean Absolute
Mean Absolute
RMSE,
of
Errors,
Stations
øC
Changes, øC
NSC
øC
764
1.68
3.14
0.93
1.84
39
2.23
2.27
0.89
1.89
significantchange Stations with no
UNITED
STATES
streamlocationsfor which the projectedtemperaturechanges are statisticallyinsignificantare most likely affected by hypolimneticreservoirreleases(e.g.,in OregonandWashington) or coolingwater (in Minnesota),whichimposelongupperflat tails on the S-shapedstreamtemperature-airtemperaturerelationships.Among the warm water streams,thoselocatedat midlatitudes(in a belt extendingfrom Oklahomato Pennsylvania) or at high altitudes(e.g.,in the RockyMountains)will experiencestatisticallysignificantchanges.In other warm water streams,especiallyin the southeastern United States,evaporativecoolingdoesnot allow any significantwater tempera-
Scenario
Number
IN THE
significantchange
ture increase at the 95% confidence
Changesin weeklystreamtemperaturesare at the 95% confidence level. Values are the meansof the parameters.
tionsdid not projecta significantchangeat the 95% confidence level under the 2 x CO2 climate scenario(Table 4). The projected weekly stream temperature changesat those 39 streamgagingstationswere of smallmagnitudeand associated with relativelylow goodness of fit (large errors). For someecologicalstudies,maximumand minimumstream temperatures are more important than the mean annual streamtemperatures.The resultsof the error analysesof the maximum/minimumweekly streamtemperaturesshowedthat under the 2 x CO2 climate scenario,only 438 streamgaging stationsexhibita significantchangein maximumweeklystream temperaturesand only 610 exhibita significantchangein minimumweeklystreamtemperatures(Table 5). The reasonfor the drop in numbersis the following:If the slope of the Sshapedfunction(equation(1)) is closeto zeroin the vicinityof the observedmaximumor minimumtemperatures,then even immensechangesin air temperaturesunder a climatechange scenariomay not project changesin maximumor minimum weekly stream temperatureslarger than minor errors in the simulations.
Figures4a and 4b illustratestationswith significantor insignificant changesin maximumweekly stream temperaturesat the 95% confidencelevel. Stream gagingstationslocated at higher latitudes,excludingthe West Coast, experiencemore significantchangesin maximumweekly streamtemperatures. In Figure 4, stationshave been divided into warm and cold water streamsby usingthe upper and lower 50 percentilesof mean annual stream temperaturesof all 803 stream gaging stations,respectively.It is evidentfrom Figures4a and 4b that mostcoldwater streamswill experiencesignificantchangesin maximumweekly stream temperatures.The few cold water
level.
Most cold water streamsexperiencesignificantchangesin minimumweeklystreamtemperatures(Figure4c). Thosevery few coldwater streamswith insignificantchangesin minimum weeklystreamtemperatures(Figure4d) either experiencean ice cover(0øC)for severalweeks(e.g.,Maine) or are affected by hypolimneticreservoirreleases.There are 266 warm water streams,the majorityin the southeastern United States(Figure 4c), that show statisticallysignificantincreasesin minimum weekly water temperatures.There are also 111 warm water streamsfor which the projected changein minimum weekly water temperaturesis statisticallyinsignificant.These stations are most likely affectedby groundwateror reservoirs(e.g., Tennessee,Alabama,and Georgia)whichproducelonglower tails in the S-shapedfunction. The significantdifferencebetweenthe number of stations with significantchangesin maximumweekly streamtemperatures(438) andthe numberof thosewith significant changes in minimumweeklystreamtemperatures(610) canbe explained as follows:Minimum weekly streamtemperatureslocatedon the lowerflat tail of the S-shapedfunctionunderpastclimate conditionscan shiftinto the centralpart of the functionunder a warmerclimate,while maximumweeklystreamtemperatures located on the upper flat tail under past climate conditions remainon the tail. Under a coolingclimatescenario,opposite results would be obtained.
3.3. Projected Seasonal Changes in Weekly Stream Temperatures
Weekly streamtemperaturesobtainedunder past and 2 x CO2 climate conditionswere averagedover the 19-yearsimulationperiod.As an example,Figure5 showsresultsfor USGS gagingstation07150500on the Salt Fork of the ArkansasRiver near Jet, Oklahoma.The gagingstationis at 36.7ø latitude.A 1.6øCincreasein mean weekly stream temperaturesis pro-
Table 5. StationsWith Significantand InsignificantChangesin Maximum/Minimum Weekly StreamTemperaturesUnder the 2 x CO2 Climate Scenario Simulation Characteristics
Error AnalysisResults Number of
Mean Absolute
Mean Absolute
Stations
Errors, øC
Changes,øC
RMSE,
NSC
øC
Maximum WeeklyStreamTemperatures Stationswith significantchange 438 1.41 Stationswith no significantchange 365 2.56
2.31 1.96
0.94 0.91
1.81 1.88
Minimum WeeklyStreamTemperatures Stationswith significantchange 610 1.17 Stationswith no significantchange 193 1.90
2.21 1.65
0.93 0.92
1.81 1.94
Changesin maximum/minimum weeklystreamtemperaturesare at the 95% confidencelevel.
MOHSENI
ET AL.: SENSITIVITY
OF STREAM
TEMPERATURES
(a)
(b)
(c)
(d)
o ß
IN THE
UNITED
STATES
3729
Cold water streams Warm water streams
Figure 4. Locationsof 803 U.S. GeologicalSurvey(USGS) stream gagingstationswith (a) significant increasesin maximumwater temperatures,(b) insignificantincreasesin maximumwater temperatures,(c) significantincreasesin minimumwater temperatures,and (d) insignificantincreasesin minimumwater temperatures.All increasesare at the 95% confidencelevel under the 2 x CO2 climate scenario.
jected to occur in December. The maximum changewith a magnitudeof ---6øCis shownto occur in March and April
changein streamtemperature(rangingfrom 2ø to 13øC)in spring,i.e.,betweenlate March andJune(weeks10-24). More (weeks 13-17). Increasesare projectedto be only 2.5øCin pronouncedchangesare more likely to occur at higher latimid-summerand up to 5.8øCin late September(week39). The tudes(Figure 6b). There are severalstreamsin cold regions, magnitudeof the projectedmean weekly streamtemperature whichare projectedto experiencemaximumchangesof weekly changesunder the 2 x CO2 climate scenariois substantially streamtemperaturesin January.A few streamsare expectedto larger than the error between the observed and simulated experiencemaximumchangesin fall, independentof latitude. weekly streamtemperaturesshownin Figure 5. Minimum changesin weeklystreamtemperatures(ranging This exampleshowsthat maximumchangeoccursin early from 0ø to 3øC) under the 2 x CO2 climate scenarioare springand minimum changeoccursin winter or summer.To obtain a more accuratepicture of the seasonwhen maximum and minimum changesin streamtemperatureoccur,the latitudesof all 764 gagingstationswere plottedversusthe time of occurrenceof the maximumand minimumchangesin Figure 6. Most streams are projected to experience the maximum
35
u 3o
J
F I
M
A
•
M
i
J
i
J I
A
___i
S
i
O
N
D
i Latitude 36.7ø
,- 25
• 20 •15
•0 •
projected to occur in December and January, and July and August (Figure 6c). The time of occurrenceof minimum changesin streamtemperatureseemsindependentof latitude. 3.4.
Projected Mean Annual Stream Temperatures
on the Continental
Scale
Thermal characteristics of streamsin a region are affected by the size of the upstreambasin,shading,reservoirreleases, and effluentsfrom industrialunits and groundwaterinflows. To portray the overall changesin weekly streamtemperatures projected for the contiguousUnited States, we decided to mitigatethe differencesamongstreamswithin a region.Using the 764 streamgagingstations,the mean, maximum,and minimum weekly streamtemperaturesof stream gagingstations were averagedwithin cells with a mesh of 2.5ø x 2.5ø. The averagevalues were plotted at the centfolds of the cells. If
5
there was no station located within a cell, no value was re-
0
portedfor that cell.A 2.5ø x 2.5øgrid cellwassmallenoughto show climatic differenceson a continental scale and large enoughto minimizethe number of cellswith no value. Using the Kriging method for interpolation,the contour maps of mean annual,maximumweekly,and minimumweekly stream temperaturesand their changesunder 2 x CO2 climateconditionswereplottedfor thecontiguous United States(Figures7-9).
Weeks
o Observed(1969-79) --- Simulated(1961-79) --a- 2xCO2 Scenario
Figure 5. Mean weekly simulatedand observedstreamtemperaturesat USGS gagingstation07150500on the Salt Fork of the Arkansas River near Jet, Oklahoma.
3730
MOHSENI
ET AL.:
SENSITIVITY
OF STREAM
TEMPERATURES
IN THE
UNITED
STATES
continentalscale. The continentalaverageof the projected streamtemperatureincreaseis 3.1øC,comparedto air temper-
52 (a)
ature increases of 4.5øC.
• 40
'•
•
•
•
3.5. Projected Maximum and Minimum Weekly Stream Temperatures
•
32 28
4
•.15
....
' .............................. 10 16 22 28 Weeks
34
10
34
40
46
' 52
'
•1o
o
28
52 - (c) 48 • 44
40 •36 32
4
16
22 28 Weeks
40
46
52
Figure 6. (a) Time of occurrenceof the projectedmaximum changein streamtemperatures versuslatitude,(b) magnitude of maximumchangeversuslatitude, and (c) projectedminimum changeversuslatitude.Figures6a-6c showresultsunder the CCC 2 x CO2 climate scenario.
Under the 2 x GO2 climate scenario,mean annual stream temperaturesare projectedto reach --•12ø-16øC in the northern United States(Figure7a). Mean annualstreamtemperatures are projectedto exceed22øCin Texas,Louisiana,and Mississippi.In the Rocky Mountains,mean annual stream temperatures areprojectedto rangefrom 9øto 14øCandon the coastsfrom 12ø to 20øC.The continentalaveragestreamtemperatureunderthe 2 x CO2 climatescenariois projectedto be 15.6øC.
Mean annualstreamtemperaturesare projectedto increase the least(lessthan 2øC)in partsof the RockyMountainsand most (--•5øC)in the MissouriRiver and Ohio River basins (Figure7b). The isothermsfor the changesroughlyfollowthe maximumchangesin meanannualair temperaturesprojected by the CCC-GCM (Figure 3). On average,projectedmean annualstreamtemperaturechangesare from 2ø to 5øCon the
A contourmap of the projectedmaximumweekly stream temperaturesunder the 2 x CO2 climatescenariois shownin Figure8a. Streamsin the Mississippi River basinare projected to experiencethe warmestmaximumweeklytemperaturesin the contiguousUnited States, reaching as high as 30øC in Louisiana.
The maximumweeklystreamtemperaturechangesare projectedto be ---1ø-2øCin the RockyMountainsand on the West Coast(Figure 8b), more than 2øCin the westernand eastern MississippiRiver basin, and more than 3øC in the central Mississippi River andthe Ohio Rivervalley(Indiana).In other southernregions,exceptthe lower Mississippi, the simulated maximumweekly stream temperaturechangesare lessthan 2øC, and on the east coast the simulated maximum weekly streamtemperaturechangesare more than 2øC. A contourmap of the projectedminimumweekly stream temperaturesunder the 2 x CO2 climatescenariois shownin Figure 9a. The projected2øC isothermcoversthe northern Midwest and northern New England region; that is, on average,streamsare not projectedto freezeunder2 x CO2 climate conditions.A 0ø-2øCincreasein the minimumweeklystream temperaturesis projectednorth of 42ø latitudeunderthe 2 x CO2 climatescenario(Figure9b) exceptnear the West Coast where it is 2ø-3øC(latitudesare shownin Figure 4). Southof 42ølatitudethe increaseis 1ø-4øC.In the RockyMountainsthe projectedincreasein the minimum weekly streamtemperatures is between 1ø and 3øC, and along the West Coast the projectedincreaseis below3øC.A 6øCincreaseis projectedat a singlestationin southernCalifornia. In summary,a 1ø-3øCincreaseis projectedunder the 2 x CO2 climate scenariofor the maximum and minimum weekly streamtemperatureson the continentalscale,which can be crucialfor fish habitat [Mohseni,1999]. Unlike the projected air temperaturesunder 2 x CO2 climate conditions,which showlarger changesfor maximum and minimum annual air temperaturesthan for mean annual air temperatures[IPCC, 1996],maximumand minimumstreamtemperaturesare projected to changelessthan mean annualstreamtemperatures. That differencein the responseof streamsas opposedto the atmosphereis related to the S-shapedrelationshipbetween streamtemperatureand air temperature.At high and low air temperatures(i.e., closeto the minimumand maximumof the record),the slopeof the streamtemperature-airtemperature relationshipis near zero, whereasat moderate air temperatures(i.e., closeto the meanof the data), streamtemperature variesaboutthe sameas air temperaturedoes.
4.
Summary and Conclusions
A nonlinearregressionmodel developedby Mohseniet al. [1998] and modified by Ericksonet al. [1998a] was used to determineweeklystreamtemperaturesat 993 streamgaging stationsin the contiguousUnited States from recorded air temperatures.The full availablerecord length,between1961 and 1990, was used for model calibrations. The stream tem-
peraturemodel fitted the data with a Nash-SutcliffeCoefficient(NSC) equalto or largerthan 0.80 for 91% of the gaging
(a)
(b)
Figure 7. (a) Projectedmeanannualstreamtemperatures underthe CCC 2 x CO2 climatescenarioand(b) projectedchangesin the mean annual streamtemperaturesunder the CCC 2 x C02 climate scenario.
-•;• ;:•:,•,-.-.,,--':•:: :•?:'-;•:;•:'•'•:.•: ,r:.•;,...:;::Y:•,::•.m;;:•:•/;• '"-z-':,'•½,' ...... ..:.::.'•'•;:•:
..•a:;.:":•? •'• .... •,•.•.....
-4':.:•:•::•:•-•½f[•:-."'::;-:-.:?,/':•: .•[ ::• •;½::": --'::g::?. '•.•.,; "." .•; •.::',:½-. -':::• ....... ,•½•*'
'
":' :-:':...
.-...•½z•a:•k:•:::. :::::::::::::::::::::::: :' -:•:;•:-'•
(a) .:.
::.:. ........
(b)
Figure 8. (a) Projectedmaximumweeklystreamtemperaturesunder the CCC 2 x CO2 climatescenario and(b) projectedchangesin maximumweeklystreamtemperaturesunderthe CCC 2 x C02 climatescenario.
3732
MOHSENI
ET AL.'
SENSITIVITY
OF STREAM
TEMPERATURES
IN THE
UNITED
STATES
(a)
(b)
Figure 9. (a) Projectedminimumweeklystreamtemperaturesunderthe CCC 2 x CO2 climatescenarioand (b) projectedchangesin minimumweekly streamtemperaturesunder the CCC 2 x CO2 climate scenario.
stations.Forty four percent of the stream gagingstationsexhibited seasonalhysteresisin the streamtemperature-airtemperaturerelationship. For the projectionof streamtemperaturesunder a 2 x CO2 climate scenario,803 streamgagingstationswith NSCs larger than 0.85 were used.The weekly air temperaturemodel input data, for the period 1961-1979, were incrementedusing the output of the 2 x CO2 CCC-GCM. The model parameter valuesa,/3, % and • were kept constant,althoughthe upper bound of weekly streamtemperatures,a, may changeunder a 2 x CO2 climate scenarioif land use changes.It is lesslikely that the lower bound of weekly stream temperatures,•, will changesignificantlyunder the 2 x CO2 climate scenario.An error analysisshowedthat 39 stream gagingstationsdid not projecta significantchangein streamtemperaturesat the 95% confidencelevel under the 2 x CO2 climate scenario.For the remaining764 streamgagingstationsthe streamtemperature model projectsthat mean annual stream temperaturesin the contiguousUnited Stateswill increaseby 2ø-5øC,the least in partsof the RockyMountains(2øC) and the mostin the Missouri River and Ohio River basins(---5øC).The increasein maximum and minimum weekly stream temperaturesunder the 2 x CO2 climate scenariois projectedto be only 1ø-3øC. On average,minimum weekly stream temperaturesare projected to remain above freezing. The maximum changesin weekly stream temperaturesare projectedto occur in spring and fall, and the minimum changesare projectedto occurin summerandwinter. More prominentchangesin weeklystream temperaturesare likely to occurat higherlatitudes.
Acknowledgments. The work reported herein was supported by the Agricultural ResearchService,USDA, Grazing Lands Research Laboratory, E1 Reno, Oklahoma, in cooperation with the MidContinent EcologyDivision, U.S. EnvironmentalProtectionAgency, Duluth, Minnesota. Robert Williams and Naomi Detenbeck were the
project officers.The weekly stream temperature data for the 993 USGS gagingstationswere providedby Robert Schcller.The authors appreciatethe suggestions conferredby the reviewcrs,which helped improvethe quality of the manuscript. References Brown, G. W., G. W. Swank,and J. Rothacher,Water temperaturein the Steamboatdrainage,U.S. For. Serv.Res.Pap. PArIv,119, 17 pp., 1971.
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(ReceivedMarch 1, 1999;revisedJune22, 1999; acceptedJune 23, 1999.)
