CLIMATE CHANGE IMPACTS ON NEW YORK CITY'S WATER SUPPLY SYSTEM

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATIONVOL. 36, NO. 2 AMERICAN WATER RESOURCES ASSOCIATION APRIL 2000

CLIMATE CHANGE IMPACTS ON NEWYORK CITY'SWATER SUPPLY SYSTEM1

Reginald Blake, Reza Khanbilvardi, and Cynthia Rosenzweig2

ABSTRACT: It has been well established that the greenhouse gasloading of the atmosphere has been mcreasmg since the mid 19thcentury. Consequently, shifts in the earth's radiative balance areexpected with accompanying alterations to the earth's climate.With these anticipated, and perhaps already observable, changes inboth global and regional climate, managers of regional waterresources seek insight to the possible impacts climate change mayhave on their present and future water supplies. The types anddegrees of impacts that climate change may have on New YorkCity's water supply system were assessed in a study of a watershedat Allaben, New York. Hypothetical scenarios of future climate andclimate change projections from three General Circulation Models(GCMs) were used in conjunction with the WatBal hydrologicalmodel and the Palmer Drought Severity Index (PDSI) to ascertainhow runoff and soil moisture from this watershed may change in awarmer climate. For the worst case predictions, the results indicatethat within the century of the 2000s, the watershed's air tempera-ture may increase up to about 11°F, while its precipitation andrunoff may decrease by about 13 and 30 percent, respectively. Ifthis watershed is typical of the others within the New York Citywater supply system, the system's managers should considerimplementing mitigation and adaptation strategies in preparationfor the worst of these possible future conditions.(KEY TERMS: climate change; PDSI; GCM; water supply; soilmoisture; WatBal; New York.)

INTRODUCTION

Atmospheric concentration of greenhouse gases hasbeen rapidly increasing since the beginning of theIndustrial Revolution (IPCC, 1990). Anthropogenicactivities — the burning of fossil fuels and deforesta-tion — have increased the atmospheric concentrationof CO2 by about 25 percent in about a century (Rosen-zweig and Hillel, 1993). As more and more heat-trapping, greenhouse gases load the atmosphere, the

likelihood of substantial global climatic warmingincreases as the climate system becomes perturbed.The probable climate perturbation is usuallyexpressed as changes in the mean values of specificclimate parameters, such as temperature or precipita-tion. With the mean global temperature expected toincrease between 2.7-6.3°F by the middle of thisupcoming century (IPCC, 1990), researchers antici-pate changes in global and regional rainfall patterns,increases in the frequency and intensity of hurri-canes, alterations in the timing and magnitude ofrunoff, greater fluctuations in soil moisture storage,changes in lake levels, sea level rise and adverseimpacts on water quality, among a host of other cli-matic, hydrological, medical and socio-economicchanges. These expected changes could be a source ofserious concern for regional water management.

New York is the biggest, most diverse city in theUnited States. It is the center of global commerce,tourism, finance, and technology On a daily basis, anaverage of slightly less than 1.5 billion gallons ofdrinking water are needed to supply the nine millionpeople in and around the New York City area. TheCity's drinking water is obtained from the Delaware,the Catskill, and the Croton water supply systems.These combined systems are located about 125 milesto the north and the northwest of the City, and theyspan a watershed area of approximately 1,900 mi2(Figure 1). The systems are linked in a very innova-tive and intricate network of storage reservoirs, tun-nels and aqueducts that distributes drinking waterpredominantly via gravity control to the City asshown in Figure 1. Of the three systems, the Crotonsystem is the oldest. It was first used in 1842, and it

'Paper No. 99142 of the Journal of the American Water Resources Association. Discussions are open until December 1, 2000.2Respectiveiy, Post Doctoral Fellow, NASA/Goddard Institute for Space Studies, 2880 Broadway, New York, New York 10025; Professor and

Director, Center for Water Resources and Environmental Research, City University of New York, City College, Convent Ave. at 138th St.,Room T-107, New York, New York 10031; and Senior Research Scientist, NASA/Goddard Institute for Space Studies, 2880 Broadway, NewYork, New YOrk 10025 (E-Mai]JBlake: [email protected]).

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 279 JAWRA

Blake, Khanbilvardi, and Rosenzweig

Figure 1. New York City's Water Supply System.

JAWRA 280 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

Climate Change Impacts on New York City's Water Supply System

has 12 reservoirs and three controlled lakes. The sys-tem has a maximum storage capacity of 70.7 billiongallons (BG), and its drainage basin area is 318.0 mi2.The Catskill system's two storage reservoirs, theAshokan and the Schoharie (first used in 1917 and1927, respectively), give it a maximum storage capaci-ty of 147.5 BG, and its drainage basin area is 571 mi2.The newest of the three systems is the Delaware sys-tem which was completed in 1965. It has a maximumstorage capacity of 325.9 BG, and it drains an area of1,010 mi2.

The anticipated changes in global climate areexpected to have an impact on the regional water sup-piy systems of New York City. The water demand isexpected to increase dramatically, and the water sup-piy systems would in turn become stressed. A warmerclimate would increase both water demand and evap-oration and, therefore, may reduce watershed runoffand thus exacerbate water supply shortages. Thispaper investigates the possible impacts climatechange may have on the Allaben sub-watershed of theAshokan reservoir watershed (Figure 2). This majorwatershed and its subwatershed at Allaben are locat-ed within the Catskill water supply system.

Site Description

The Allaben watershed has a drainage area of 65mi2. It is nestled in the irregular, rugged CatskillMountains, and it flows through Ulster County, NewYork. The watershed is natural and uncontrolled. It islocated 90 miles north of New York City and 50 milessouth of Albany, the State Capitol. Its elevation isapproximately 1,200 ft above mean sea level, and thenative soil texture is a sandy loam. The mean maxi-mum temperature is 60.6°F, and the mean minimumtemperature is 4 1.2°F. Average precipitation is about3-4 inches per month.

The Allaben watershed supplies approximately 20percent of the water in Esopus Creek, which in turnsupplies water to the Catskill system's Ashokan reser-voir. Esopus Creek at Allaben is one of the threemajor interior waterways of Ulster County. The creekis used for recreation, wildlife habitat, and irrigation.It is also one of the most celebrated trout fisheries inthe northeastern United States. Twenty-five years ofhydrometeorological data (relative humidity, sunshinehours, temperature, runoff, and precipitation: 1970-1994) from the Allaben watershed were used in thestudy.

Models

One hydrological model (WatBal), a soil moistureindex (PDSI) and three General Circulation Models(GCMs) were used to assess the possible climatechange impacts on the Allaben watershed.

WatBal, an acronym for the WATer BALance hydro-logical model (Kaczmarek, 1993; Yates, 1994; Yatesand Strzepek, 1994), is an integrated water balancemodel designed primarily to assess climate changeimpacts on watershed runoff. This one-dimensional"bucket-type" model computes the mass balance with-in the soil moisture zone by using the water balancevariables of precipitation, runoff and actual evapo-transpiration to account for changes in soil moisture.It extracts soil moisture via the Priestly-Taylor radia-tion approach for potential evapotranspiration. Themodel can simulate direct runoff, surface runoff, sub-surface runoff, effective precipitation, and evapotran-spiration (as a function of the Priestly-Taylorpotential evapotranspiration).

The Palmer Drought Severity Index (PDSI), devel-oped by (Palmer, 1965), remains one of the mostaccepted indices used to characterize and provide asingle measure of meteorological drought severity.Using this standardized index to express soil mois-ture conditions allows for convenient soil moisturecomparisons and assessments (Riebsame et al., 1991;Briffa et al., 1994; Scian and Donnari, 1997). ThePDSI uses monthly time series of hydrologic data todetermine the amount of moisture required for "nor-mal" monthly weather. It then computes a droughtseverity index based on soil moisture anomalies andcharacteristic local coefficients of the water balancevariables.

Climate change impact studies are usually per-formed with a suite of GCM projections. Though themodels in the suite may differ in their climate projec-tions due to different parameterizations of climateprocesses, they are very useful in providing ranges(low end and high end) and trends of the climaticvariables. For this study transient, linearlyinterpolated predictions from three GCMs [CanadianCentre for Climate Modeling and Analysis (CC), Unit-ed Kingdom Hadley Centre (HC), and the GoddardInstitute for Space Studies (GI)] provided the climatechange scenarios (the first two GCMs are being usedin the U. S. National Assessment of Climate Variabili-ty and Change). The scenarios were obtained fromGCM simulations with sulphate aerosols (CCGS,HCGS, and GIGS) and without sulphate aerosols(CCGG, HCGG, and GIGG). Sulphate aerosols have acooling effect on climate.


METHODS

2


Figure 2. The Ashokan Reservoir Watershed.

Procedure

Trend analyses were performed on Allaben's 25-year hydro meteorological record of temperature, pre-cipitation, and runoff to detect trends in theseclimatic variables. The WatBal model was calibrated,validated, and used in sensitivity experiments to

determine how sensitive the watershed's runoff is tochanges in its precipitation and temperature.

The PDSI soil moisture accounting model wasapplied to the watershed for a set of hypotheticalclimate change scenarios to provide insight to thepossible changes the soil moisture may undergo.These changes in Allaben's soil moisture were then


2 0 2 4 6 8 10 Kilometers 4


compared to the corresponding changes in New YorkCity's (Central Park, Manhattan) soil moisture.

The extreme events of droughts and floods for thewatershed were then assessed by applying statisti-cal/probability techniques to the Palmer droughtindices obtained from the hypothetical climate changescenarios. The return periods for drought and floodmagnitudes that currently occur once every 100 years(the 1 percent drought and the 1 percent flood) andfive times every 100 years (the 5 percent drought andthe 5 percent flood) were calculated. These returnperiods indicate the sensitivity and probability ofrecurrence of these extreme events under the hypo-thetical climate change scenarios.

The return periods for the same extreme eventswere again calculated. This time, however, they werecalculated for climate change scenarios obtained fromthe GCMs.

Finally, the predicted hydroclimatology (tempera-ture, precipitation, PDSI, and runoff) of the water-shed throughout the upcoming century were obtainedand calculated from the GCMs (temperature, precipi-tation), the GCMs and the Palmer soil moistureaccounting model (PDSI), and the GCMs and WatBal(runoff).

RESULTS AND DISCUSSION

The trend analyses of the 25-year hydrometeorolog-ical record of temperature, precipitation, and runofffor the Allaben watershed reveal that no significantclimatological shifts have occurred within the water-shed over that time period. The analyses of Figure 3.show slightly negative trends in all three variables;however, the precipitation trend line has a more pro-nounced negative slope than the others.

the base temperature and the base temperature plus2°C and 4°C, respectively. For precipitation, P + 0, P ±10, P ± 20 represent the base precipitation, ± 10 per-cent and ±20 percent changes in the base precipita-tion, respectively. The numbers in the table indicatethe percentage change in runoff caused by the hypo-thetical changes in temperature and precipitation.They reveal that a 10 percent increase or decrease inprecipitation leads to at least a 7 percent increase ordecrease in runoff, while a 20 percent increase ordecrease in precipitation leads to at least a 17 percentincrease or decrease in runoff. The table also indi-cates that in general runoff is reduced by 1.5-2.0 per-cent for each 1°C (1.8°F) increase in temperature.

50 . .—, y° -00285, + 4.1251

A__. A

WatBal

The WatBal sensitivity results are tabulated inTable 1. In the table, T + 0, T + 2 and T + 4 represent

Figure 3. Allaben's Hydrometeorological Data (1970-1994).

TABLE 1. WatBal Sensitivity Experiment Results, Percent Changes in Runoff.

Assumed TemperatureChanges, C

Assumed Precipitation Increases and Decreases (percent)P - 20% P - 10% P +0% P + 10% P + 20%

T + 0 -22.0 -10.2 0.0 14.5 24.2

T +2 -24.5 -13.7 -3.5 11.0 20.7

T + 4 -26.7 -15.0 -5.8 7.7 17.5


ALLABENS MEAN ANNUAL TEMPERATURE 11970-19941

'U _________________

540 —i I I I I I I I I 4-,-44- I I

1970 1975 1980 1985 1890

YEAR

ALLABENS MEAN ANNUAL PRECIPITATION 11970-1994)

0-—z

H fIII11970 1975 1980 1985

YEAR

1990

ALLABENS MEAN ANNUAL RUNOFF (1970 -19941

1970 1975 1980 985 1990

YEAR


PDSI

Sensitivity analyses were also conducted for thewatershed's PDSI. The results are shown in Table 2,and the PDSI classes are explained in Table 3. Theyindicate that the combination of increased tempera-ture and decreased precipitation leads to soil mois-ture depletion while the converse — decreasedtemperature and increased precipitation — tendstoward soil moisture replenishment. The sensitivity ofthe watershed's PDSI to changes in temperature andprecipitation is highlighted in Figure 4. As expected,the figure shows that if temperature increases andprecipitation remains constant, the watershed wouldexperience drought conditions with the worst casescenario being a mild drought for a 3°C (5.4°F)increase in temperature. The figure also shows thatfor a fixed temperature, soil moisture conditionsdepend on fluctuations in precipitation. With a fixedtemperature and a 10 percent decrease in precipita-tion, the watershed experiences mild drought, whilefor that same temperature and a 20 percent increasein precipitation, the watershed becomes moderatelywet. Comparing Allaben to New York City (NYC), Fig-ure 5 shows that for the same sensitivity criteria,NYC on average would become slightly drier thanAllaben.

TABLE 2. PDSI Sensitivity Tests for the Allaben Watershed.

Scenario Mean PDSI

Base T, P 0.024

T + 1C , Base P -0.543

T + 2C ,Base P -1.198

T + 3C , Base P -1.820

Base T, P - 10% -1.215

T + 1C , P - 10% -1.826

T+2'C,P-lO% -2.486

T+3C,P-10% -3.176

Base T, P + 10% 1.157

T+1'C,P÷lO% 0.614

T+2C,P+10% 0.097

T + 3C , P + 10% -0.517

Base T, P + 20% 2.250

T+1C,P+20% 1.723

T+2C,P+20% 1.232

T+3C,P+20% 0.704

TABLE 3. PDSI Classes for Wet and Dry Conditions.

PDSI Classes

> 4.00 Extremely Wet3.00 to 3.99 Very Wet2.00 to 2.99 Moderately Wet

1.00 to 1.99 Slightly Wet0.50 to 0.99 Incipient Wet Spell0.49 to -0.49 Near Normal

-0.50 to -0.99 Incipient Drought-1.00 to -1.99 Mild Drought-2.00 to -2.99 Moderate Drought-3.00 to -3.99 Severe Drought

<-4.00 Extreme Drought

PDSI TEMPERATURE SENSITIVnY FOR ALLABEN

00 — ——

.04 -

.08

—____________________________BASE T*1(C) T2(C) T3(C)

TEMPERATURE CHANGE

PDSI PRECIPITATION SENSITMTY FOR ALLABEN

25 — ———--— —.--——- —20 - —''

BASE T. P - 0% 1. P° 10% T P • 20%

PRECIPITATION CHANGE

Figure 4. PDSI Temperature and PrecipitationSensitivity for Allaben.

Extreme Events - Hypothetical Scenarios

The probability of recurrence for the extreme condi-tions of the 1 percent and the 5 percent drought andflood for hypothetical climatologies based on PDSI isrecorded in Table 4. In one scenario, the drought thatcurrently occurs once every hundred years could occurevery 11 to 12 years if the temperature increased bythree degrees and the precipitation remains the same.



TABLE 4. Sensitivity of Drought and Flood Return Periods for Allaben (1970-1994).

Sensitivities

Events1 PercentDrought

5 PercentDrought

1 PercentFlood

5 PercentFlood

Base 1.0 5.0 1.0 5.0

T+1°C, P 1.6 9.6 0.4 0.7

T+2°C, P 4.6 18.9 0.3 0.6

T÷3°C, P 11.6 31.0 0.2 0.4

T, P - 10% 2.6 16.9 0.0 0.3

T÷1°C, P-10% 7.2 28.3 0.0 0.2

T+2°C, P-10% 19.3 45.0 0.0 0.2

T+3°C, P-10% 35.0 61.1 0.0 0.

T, P +10% 0.0 1.6 16.3 27.81

T + rc, P+10% 0.5 3.9 7.6 17.7

T + 2°C, P+10% 1.4 6.9 1.9 6.7

T + 3°C, P+10% 2.9 12.9 1.2 2.4

T, P +20% 0.0 0.9 33.0 39.8T + 1°C, P÷20% 0.0 1.6 28.0 34.5T + 2°C, P+20% 0.3 3.2 19.9 29.1

T + 3°C, P+20% 1.3 5.9 11.5 21.1

If the temperature remains the same and the precipi-tation increased by 20 percent, the flood that current-ly occurs five times per hundred years could increaseits frequency by almost a factor of eight.

GCM Predictions

The projected changes in Allaben's temperature,precipitation and PDSI according to the three GCMsare shown in Figure 6. Since the projections arederived from linear interpolation between gridboxes,a degree of uncertainty is associated with them. Someuncertainty is always inherent whenever GCM projec-tions are applied to regional scales. With this in mind,Figure 6a shows that the Canadian Centre GCM pre-dicts a temperature increase of 5.4°F (3°C) throughoutthe decades of the 2040s and the 2050s for Allaben.By the 2090s, the model expects the temperatureincrease to almost double the mid-2000s increase. TheHadley Centre model projects about a 4.2°F (2.3°C)warming for the 2050s and about a 6.4°F (3.6°C) forthe 2090s. The Goddard Institute model expects a2.7°F (1.5°C) warming by the 2050s and about 6°F(3.3 °C) by the 2090s. On average, the inclusion of sul-phate aerosols reduces the temperature increases byabout 1°F (0.6°C) in all the models.

In general, GCM predictions of regional precipita-tion vary more from model to model than temperaturepredictions do. Figure 6b shows that by the 2050s, theCanadian Centre models expects a 4-7% decrease in

precipitation, the Hadley Centre model projects a 13percent increase in local precipitation, while the God-dard Institute model expects a -2 to +1 percent precip-itation change. By the 2090s, the Canadian Centremodel expects a -3 to +5 percent precipitation change,the Hadley Centre model projects a 23-30 percentincrease in precipitation, while the Goddard Institutemodel predicts a 2-5 percent increase in precipitation.

The PDSI projections of Figure 6c show that theCanadian Centre model, with its higher temperatureincreases and greater precipitation decreases, expectsa greater degree of soil moisture deficiency than theother two models. By the 2050s, that model predicts amoderate drought, and by the end of that century, thedrought severity worsens to a classification ofextreme. For this model, the watershed's current1 percent drought would become 30 times more fre-quent by the 2050s and about 55 times more frequentby the 2090s, while the current 5 percent droughtwould become 11 and 15 times more frequent by thesame two respective time periods (Figure 7).

With a lower temperature projection and, by far,the greatest precipitation projection of the three mod-els, the Hadley Centre model predicts slightly wettersoil moisture conditions for the watershed. Indeed,from Figure 8, the model expects the watershed's cur-rent 1 percent flood to occur five times more frequent-ly by the 2050s and 25 times more frequently by the2090s.

The seemingly conservative Goddard Institutemodel predicts an incipient drought by the 2050s and



Figure 6. GCM's Projected Change in Allaben's Temperature, Precipitation, and PDSI.

a mild drought by the 2090s. This model expects thewatershed's current 1 percent drought to triple in fre-quency by the 2050s and to increase ten-fold by the2090s.

In general, the PDSI analyses indicate that the soilmoisture may have a stronger dependency on temper-ature than it does on precipitation. For example, fromFigure 6, even a 30 percent increase in precipitation


Projected Change in Allaben's Temperature

111 LiNhJh2000s 2010$ 2020s 2030s 2040s 2050s 2060s 2070s 2080s 2090s

Decade

A 9CCGG DCCGS DHCGG DHCGS DGIGG DGIGS

Projected Change in Allaben's Precipitation

35.. . _-.--___-_ —.---——----.—----.--

:412000s 2010s 2020s 2030s 2040s 2050s 2060s 2070s 2080s 2090s

Decade

B DCCGG DCCGS OHCGG DHCGS DGIGG OGIGS

Projected Change in Allaben's PDSI

4

I:(I,

-6

Decade

c I0CCGG DCCGS DHCGG DHCGS DGIGG DGIGS

2000s 201 Os 2020s 2030s 2040s 2050s 2060s 2070s 2080s 2090s


JIlL JIlL mdt6ItkT]lIk lili ilL iIL:llJFL ,

Figure 8. GCMs' Projected Change in Aliaben's 1 Percent and 5 Percent Flood Probabilities.


PROJECTED CHANGE IN ALLABEN'S 1% FLOOD

30

25

2000 ________________15'-10

IL

0

2000s 2010s 2020s 2030s 2040s 2050s 2060s 2070s 2080s 2090s

DECADE

DCCGG DCCGS DHCGG DHCGS QGIGG DGIGS

35

PROJECTED CHANGE IN ALLABEN'S 5% FLOOD

30

25

2000-JU-

I)10 ill ifi

5 ill [IL Ill

0

III Ii i•lI

Jl II.!

2000s 2010s 2020s 2030s 2040s 2050s 2060s 2070s 2080s 2090s

DECADE

DCCGG DCCGS DHCGG DHCGS EIGIGG 0 GIGS


can produce only a slightly wet watershed because ofa 6°F (10.8°C) temperature increase.

When the GCM climate change scenarios of tem-perature and precipitation are used to drive the Wat-Ba! model, estimates of percent changes in runoff canbe obtained as shown in Figure 9. By the 2050s, theCanadian Centre model predicts a 7-10 percentdecrease in runoff, and this worsens to a 22-28 per-cent decrease by the 2090s. The Hadley Centre model,on the other hand, expects a 7-8 percent and a 12-19percent runoff increase for the same two periods,respectively. For its projection, the moderate GoddardInstitute model expects runoff decreases of less thanfive percent by the 2050s and decreases between 8and 10 percent by the 2090s. Overall, the figure indi-cates that the projected changes in runoff follow close-ly the patterns for each model's predicted changes inprecipitation and PDSI.

Summer 1999

The last summer of the 20th century was unusual-ly hot and dry for New York City. The watershed areaof Figure 1 received below-normal amounts of precipi-tation which exacerbated the already long-term mois-ture deficiencies. The PDSI for the watershed areaindicated a moderate drought with portions of it evenexperiencing extreme drought. Several droughtwatches and warnings were issued by the NationalWeather Service, and individuals were asked to con-serve water. Tap water restrictions were also imposedon other residents within the watershed area.

Rivers and creeks ebbed below their normal sum-mer minimums, wells dried up or yielded little water,both shallow and deep-rooted vegetation exhibitedstress, grass became brown, the soil in many areaswas parched, and the reservoir levels decreasedsteadily (Figure 10). The combination of both recordbreaking high temperatures (Table 5) and minimalrainfall (Table 6) brought the City and its watershed

DCCGG DCCGS DHCGG EIHCGS DGIGG DGIGS

Figure 9. GCMs' Projected Change in Allaben's Runoff.


0

0

aC.)

aUaa.

20

15

10

5

0

-5

-10

-15

-20

-25

-30

2000s 201 Os 2020s 2030s 2040s 2050s 2060s 2070s 2080$ 2090s

Decade

TABLE 5. New York City's Hottest Julys(mean temperature in °F).

Year Temperature

1952 80.3

1955 80.9

1993 80.2

1999 81.5

TABLE 6. New York City's Driest Julys(mean rainfall in inches).

Year Rainfall Amount

1910 0.49

1924 0.89

1954 0.96

1955 0.51

1987 0.891999 0.44

area to the brink of a drought emergency. The City'swater managers, The New York City Department ofEnvironmental Protection, should perhaps consider1999s summer conditions as a prelude of things tocome.

SUMMARY AND CONCLUSION

It is important that water supply managers becomeaware of the impacts that climate change may haveon water resources. With state-of-the-art GCMs andstandard hydrological models, hydro-climatologistscan provide water managers with plausible climatechange scenarios and their impacts on waterresources. These can be used as guides to prepare forfuture water supply planning.

As for the Allaben watershed of the New York Citywater supply system, this study concludes that evenwith their inconsistencies, especially for precipitationprojections, overall, the GCMs indicate that thewatershed may be drier and more stressed in awarmer climate. For worst case scenarios: Allaben'stemperature is expected to increase by about 5.4°F



Ua(UC.)(U

E

'I)

0

(U(U(U

'I-0C(UU(U0.

—G— Normal

—6—Actual

Oct-98 Nov-98 Dec-98 Jan-99 Feb-99 Mar-99 Apr-99 May-99 Jun-99 Jul-99 Aug-99

Months

Figure 10. New York City Reservoir System Capacity.


(3°C) by the 2050s and by about 10°F by the 2090s inthe worst (most dire) GCM prediction. Its precipita-tion is projected to decrease by about 13 percent dur-ing the 2070s, and its runoff is predicted to decreaseby about 30 percent during the 2090s. Additionally,the frequency and severity of droughts at Allaben isexpected to increase more so than the frequency andmagnitude of floods.

It is probable that the unusual sauna-type weatherexperienced during the summer of 1999, especially inJuly, may be more the norm than the exception in thecoming century. Little or no rainfall, above normaltemperatures, smaller reservoir quantities, lowstreamfiows and empty wells may set the coming cen-tury apart from all others preceding it. As droughtprobabilities for the New York City area increase, theCity's worst drought on record — a drought thatoccurred during the early 1960s and peaked in the1965 when: precipitation for the entire year was 28inches, reservoir levels fell below 40 percent of theircapacity and the PDSI was as low as -5.7 — may notonly be a thing of the past but also a trend for thefuture.

As resilient as the New York City water supply sys-tem is touted to be, in light of the recent and perhapsmore severe and more frequent future droughtepisodes, prudence dictates that water managersbecome more serious about addressing climate changeissues. Mitigation and adaptation strategies must bedeveloped and be ready for immediate deployment.An excellent set of adaptation methodologies has beensubmitted by Hansler and Major (1999). Their recom-mendations include stronger infrastructural, institu-tional and operational ties between the New YorkCity water supply system and neighboring systemslike that of the Delaware River and an expansion ofNew York City's Chelsea Pumping Station.

Let us learn a lesson from the ant who prepares forthe time of need during the time of ease.

ACKNOWLEDGMENTS

The authors gratefully thank Mr. Richard Goldberg ofNASA/GISS and Dr. D. Yates of NCAR for their assistance and sup-port throughout this project.

LITERATURE CITED

Briffa, K. R., P. D. Jones, and M. Hulme, 1994. Summer MoistureVariability Across Europe, 1892-1991: An Analysis Based on thePalmer Drought Severity Index. Int. J. Climatol. 14:475 - 506.

Hansler, G. and D. Major, 1999. Climate Change and the WaterSupply Systems of New York City and the Delaware Basin:Planning and Action Considerations for Water Managers. In:Proceedings of Speciality Conference on Potential Consequences

of Climate Variability and Change to Water Resources of theUnited States, D. Briane Adams (Editor). American WaterResources Association, Middleburg, Virginia, TPS-99-1, pp. 327-330.

Intergovernmental Panel on Climate Change, 1990. World Meteoro-logical Organization United Nations Environmental Pro-gramme. University Press, Cambridge, United Kingdom.

Kaczmarek, Z., 1993. Water Balance Model for Climate ImpactAnalysis. ACTA Geophysica Polonica 41(4):1-16.

Palmer, W. C., 1965. Meteorological Drought. U.S.Weather Bureau,Research Paper No. 45, Washington, D.C., 58 pp.

Riebsame, W. E., S. A. Changnon, and T. R. Carl, 1991. Droughtand Natural Resources Management in the U.S.: Impacts andImplications of the 1987-89 Drought. Westview Press, BoulderColorado.

Rosenzweig, C. and D. Hillel, 1993. Agriculture in a GreenhouseWorld. National Geographic Research and Exploration.

Scian, B. and M. Donnari, 1997. Retrospective Analysis of thePalmer Drought Severity Index in the Semi-Arid PampasRegion, Argentina. Tnt. J. Climatol. 17:313-322.

Yates, D., 1994. WatBal — An Integrated Water Balance Model forClimate Impact Assessment of River Basin Runoff. IIASA Work-ing Paper, WP-94-64, Laxenburg, Austria.

Yates, D. and K. M. Strzepek, 1994. The Impact of Potential Evapo-transpiration Methodology on the Determination of RiverRunoff. IIASA Working Paper, WP-94-96, Laxenburg, Austria.


CLIMATE CHANGE IMPACTS ON NEW YORK CITY'S WATER SUPPLY SYSTEM

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