Climate Change and Water Resource Availability: An Impact Assessment for Bombay and Madras, India

Climate Change and Water Resource Availability: An Impact Assessment for

v Bombay and Madras, India

by Robin hi. Leichenko Deparlmenr of Geograph!. 302 Walker Building Pennsylvania Stale Uni~ersily UNIVERSITY PARK R4 16603 U.S.A.

ABSTRACT

Global climare change associaled wit11 rising almospheric concenrrations ofgreenhouse gases may alrer regional lemperal~rre and precipilalion pallerns. Such changes could rhrealen /he availabili/y of water resources for rapid/>. gron,ing Thrrd World cities. many of which are already experiencing seveie waler supp1.v dejciencres. This paper invesligales [he porenlial impacts of climale change on waler resource availabilily for lit.o lndiarr cilies. Bornbay and .Madras. The paper begins by discussing furrrre [rends for pop~rlarion grou.th and ~valer denland in each ci1.1; ,Ye.yr, rrsing clima~e change scenarios based on lhree general circ1rlalio11 rnodels IGC.Ws), [he pajer assesses how clirnale change rnay afecl tvarer availabililv in the riro urba~r regions. The assessri?~nt is condtrcled rhrotrgh /he use of a monlhlj, drvness index measuring potenlial maporranspirmion and precrpilalion. For each region, the dryness index under "normal" clbnalic condilioris is compared with iride.res created using GCM scenarios. The resltlts of this assessment indicate ihar, ~oiless large increases in regional precipitation accompan.~. climale warming. higher rules 01' evapotranspiralion n,ill mean reduced water availabilily for bolh cities. The paper concl~rdes by discirssb~g so~ne implications for ivater managernen/ in Third World cities.

Global warming caused by rising atmospheric con- centrations of greenhouse gases threatens to change temperature and precipitation patterns in regions throughout the world. These changes could affect the availability of water resources by altering the timing and magnitude of runoff. In developing countries. large and rapidly growing cities. such as Karachi. SSo Paulo. and Mexico City, may be especially vulnerable to water resource changes associated with global warming. Many of these megacities face increasingly severe water resource problems including shortages in supplies, deterioration of water quality. flooding, and inadequacies in facilities for treatment and dis- posal of sewage [ 1,2]. Climate change could exacerbate the water problems of large Third World cities by increasing water demand and reducing availability of water supplies.

This study explores the potential impacts of climate

change on water resource availability for two mega- cities in India, Bombay and Madras. Bombay and Madras were selected for investigation because the water supply arrangements of both cities make them especially vulnerable to potential changes in regional temperatures and precipitation. Both cities depend primarily on seasonal precipitation for their water supplies (Fig. I ) , and both cities suffer from severe water shonages during periods of drought.

STUDY APPROACH

Numerous studies have examined the regional hy- drologic effects of climate change in developed coun- tries 13-51. Recently, climate change researchen have turned their attention to the impacts of global warm- ing on water resources in the developing world (6-81. But one area of research that has received little attention deals with impacts of climate change on the

Figure 1. Annual rainfall over India.

water resources of Third World cities [I]. This inves- tigation of Bombay and Madras takes an initial step into that research area.

The study uses an impact assessment methodology similar to that employed in past research. This meth- odology involves three steps: ( I ) construction of future climate scenarios using output from genecll circula- tion models. sensitkity analysis. or historical ana-

logues; (2) assessment of the effects of the altered climate scenarios on regional hydrology through the use of empirical or deterministic hydrologic models or through application of water-balance methods; and (3) consideration of potential impacts of the hydrol- ogic changes on regional water resource availabilit):

Climate change scenarios for this study were con- structed using output from three general circulation

models: the Goddard Institute for Space Studies model (GISS) [9.10]. the Geophysical Fluid Dynamics Lab- oratory model (GFDL) 19.1 I], and the United King- dom Meteorological Office model (UKMO) [I2]. The evaluation of the hydrologic impacts ofclimate change is conducted through a water-balance assessment in- volving the creation of a dryness index [13]. The dryness index, which Oldekop [I41 developed and Budyko 1151 modified, evaluates runoff potential for a region as follows [13]:

Dryness Index = Evapotranspiration/Precipitation.

Tests of the dryness index in regions throughout the world indicate that mean annual runoff has a reliable correlation with the index (13,161. This study compares the monthly dryness index under normal climatic conditions with the indexes for each climate change scenario. The comparison provides an assess- ment of the potential effects of climate change on water resource availability

URBAN WATER SUPPLY AND DEMAND IN BOMBAY AND MADRAS

The rapid growth of Bombay and Madras has continually strained each city's ability to provide sufficient water supply to meet demand. Table I illustrates current population levels and population growth trends for the cities. The projected future population levels are based on the assumption that present annual growth rates, which are 2 per cent in both cities. will continue (17,181.

Bombay and Madras draw their water supplies primarily from surface reservoirs that monsoonal and postmonsoonal rainfall must recharge annually. Both cities lack sufficient water supplies to meet current demands. Bombay draws its supply of approximately 3,000 million liters per day (Ml/day) from six regional surface reservoin [19]. Current water demand in Bombay is estimated to be approximately 3,500 MI/ day [19]. Madras derives 80 per cent of its total water supply of 290 Ml/day from surface reservoirs and the remainder from groundwater [I 7,201. Madras's water supply is estimated to be approximately 40 per cent below demand [17]. In response to continual short- ages, the two cities ration residential water supply Different areas of each city receive water service for a few hours per day on a rotating basis [17,18].

Although each city plans to increase its water supply

Table 1. Population grnrth trends for Sambay and Madras. Figurer are in millions

Year 1990 :WO 2025 2050

Bombay ' 13 16.5 27 44 Madras 6 7.5 12 '0

Sources 11 7.181

Vol. 18. No. 3 (1993)

through additional regional diversions [17,19], Pop- ulation trends indicate that water demand in both cities will continue to outstrip supplies. The above population projections suggest that within the time- frame in which atmospheric concentrations of CO, are expected to double (approximately 60 years) (211, waterdemand in the two cities could more than triple. Even without climate change, Bombay, Madras, and many other Third World megacities, are likely to experience severe and possibly life-threatening water resource shortages during the next century.

Besides suffering from perpetual shortages, the water supplies of both cities are highly vulnerable to mon- soonal variability. This variability is manifest through extreme drought or flood events, which occur a p proximately every three to five years in most regions of the Indian subcontinent [22]. Over the 114-year period from 1871 to 1984, Bombay has experienced 12 severe droughts and 10 severe floods [23]. Madras has experienced 17 severe droughts and 18 severe floods during the same period. Climatic variability poses difficulties for water management and planning throughout the subcontinent [24]. In the Bombay and Madras regins. even one year of drought leads to water supply shortages (17,181. During fjood pe- riods, the two cities are unable to take advantage of excess rainfall because they lack adequate water stor- age facilities.

CLIMATE CHANGE SCENARIOS

The climate change scenarios for the Bombay and Madras regions are based on output from the GISS, GFDL. and UKMO general circulation models (GCMs). The method used to~construct the GCM scenarios is similar to that employed in many climate impact nudies [3,7,2 I]. For the temperature scenarios, mean monthly changes in temperature were deter- mined by subtracting the temperatures in IXCO, control run from the temperatures in the doubled CO, run for the GCM grid cells that overIay each urban region. The difference was then added to the average monthly temperatures of each urban region. Monthly changes in precipitation were derived in a similar fashion, except that a ratio of the 2XCOJ IXCO, run was used. This ratio was multiplied by the monthly precipitation normals for each urban region.

There are a number of uncenainties associated with using the GCMs for regional predictions of climatic change. First, there is a difference in scale between the GCM grid cells, which are approximately 500 km X 500 km. and the smaller hydrologic regions in which the climate change impacts are examined 171. This study used the predictions for changes in monthly temperature and precipitation for the GCM grid cells

that contain each urban region to predict changes in the climate normals of the urban regions. Second. the GCMs do a poor job of replicating the current climate in many regions of the world [21,?5]. For the lndian subcontinent, the modeled temperatures in the IXCO, control runs for GFDL and GISS are generally too cool, while the UKMO temperatures are too warm [25]. The modeled precipitation con- ditions for the control runs of all three of the GCMs are too high for central and south India (251. Re- garding precipitation seasonality, all of the models do a poorjob in replicating the wet/dry seasonal contrasts of the Indian subcontinent [21,25]. A third problem with the GCMs is that they inadequately reprerent some key processes such as ocean circulation and cloudiness (71. Despite the various uncertainties in using the GCMs for. regional climate prcdictions. GCMs provide the best indication of atmospheric response to increased concentrations of C0:. and they are widely used in climate impact studies [7.11,26].

Before examining the climate change scenarios for each city, the possible effects of climate change on the monsoon timing and variability should be men- tioned. Monsoonal and oostmonsoonal rainfall dom- inate temperature and precipitation patterns in the Bombay and Madras regions. Thus, changes in the timing or variability of the monsoon would be crucial for water supplies. Little research has been conducted on the effects of increased C02 concentrations on the riming of the monsoon. A recent study. however. found no evidence for a significant change in the mean onset date of the Indian monsoon due to rising C O levels [27]. Because the water supplies of the two cities are already vulnerable to monsoon variability: changes in variability could pose a serious threat. Credible predictions about the effects of rising CO: levels on climatic variability are not yet available [7,28.29]. Clearl~ additional research ?nto the effects

annual precipitation for Bombay is 427 cm, an in- crease of 136 per cent from the normal of 181 cm. GlSS is the warmest and driest scenario for Bombay. LnderGISS, average temperatures increase by 3.7 C" in Bombay, and annual precipitation decreases by 20 per cent. UKMO is slightly cooler than GISS for Bombay, predicting a 3.5 C' annual temperature increase. UKMO is also wetter than GISS, predicting a 40 per cent increase in annual precipitation in Bombay

.\fadrar Climare Scenurios

The Madras region is warmer and dryer than Bombay. Average temperatures range from 24'C in February to 33'C in May, and annual precipitation is approximately I15 cm [30]. Most of this rainfall occurs during the postmonsoon period in the months oiOctober through December. Although low-pressure areas in the Bay of Bengal cause Madras's postmon- soon rainfall, the rains of October-December are popularly referred to as the "northeast monsoon" [jl]. Figures 4 and 5 depict the normal climate and the climate change scenarios for the Madras region. .As in the case of Bombs?. GFDL is the coolest and u.ettest scenario. Under GFDLI the annual average increase in temperature for Madras is 1.3 C'. GFDL innual precipitation for Madras is 321 cm. 153 per cent more than the 127 cm normal. Under the GISS scenario average temperatures increase by 3.6 C' in Madras, and annual precipitation is virtually un- changed. UKMO is slightly warmer than GISS for Madras. predicting a 4.2 C annual temperature in- crease. UKMO predicts a 10 per cent increase in annual precipitation in Madras.

of climate change on monsoon timing and variability is necessary. For the purposes of this study, it is CLIMATE CHANGE IMPACTS ON 3ssumed that climate change will not alter the timing WATER AVAILABILITY or variability of the monsoon.

.

Bon7bo.v C l~n la~e Scenarios

Average temperatures in Bombay range from 13°C in February to 24'C in May, just before the onset of the monsoon 130). The Bombay region. among the wettest in India. has an annual precipitation of ap- proximately 200 cm [30.3 I]. Nearly all of Bombay's precipiration occurs during the southwest monsoon season in the months of June through September 1301. Figures 2 and 3 depict the normal climates and the GCM dimare change scenarios for the Bombay region. GFDL is the coolest and wettest scenario. Under the GFDL scenario. the annual avenge in- crease in temperature for Bombay is 2.2. (P. GFDL

To assess the water resources impacts of climate change. a monthly dryness index was created for each urban region. first using the normal temperature and precipitation data, and second, using the climate change scenarios. The dryness index measures runoff potential by dividing total monthly evapotranspira- rion by total monthly runoff. Runoff may only occur if the dryness index is less than one (i.e., if total precipiration is greater than total evaporation) (131. Evapotnnspiration (ET) was calculated using the Thornthwaite method [XI. The Thomthwaite method. which is based on air temperature and length of day [ 161, has been applied in assessments of water balances for the Indian Subcontinenr and for regions through- out the world (33,341.

22 JAN FEB MAR APR MAY JUN JUL' i i t i ~ SEP OCT NOV.DEC

I

MONTHS

- NORMAL -+- GlSS ++ GFLD + UKMO I

Figure ?. Bombay region temperalure scenarios.

Impacts for Bornbay

Under normal climatic conditions, runoff occurs in the Bombay region during the months of June- September (Table 2). The discussion will focus on changes in the dryness indexduring those four months (Fig. 6). The GFDL scenario predicts that the dryness index will decrease in the Bombay region during each of the four monsoon months, indicating that runoff will increase. This prediction is not surprising since GFDL is the wettest and coolest climate change scenario. UKMO predicts little change in Bombay's dryness index during the months of June-August, but predicts that the index will increase from 0.66 to 1.25 during September. Such a shift in the index to a value greater than one suggests that runoff may decrease significantly during this month. The GISS results are of even more concern because two normally wet months. August and September. show dryness index values greater than one. Under climatic conditions similar to those predicted by GISS. the Bombay region may experience a severe reduction in water avail- ab~lity.

Table 2. .\.lonrhly dryness index for the Bombay region under n o r d md dtered climatic conditions

Month Normal GISS GFDL UKMO

].AN 27.67 34.22 69.84 65.64 FEB 28.87 30.61 60.53 IW.70 4IAR 41.78 59.92 85.52 41.05 .APR 55.53 141.76 71.64 59.15 MAY 12.77 16.09 27.08 21.52 JUN 0.42 0.56 0.23 0.38 IUL 0.26 0.72 0.16 0.30 .AUG 0.43 1.97 0.23 0.50 SEP 0.66 1.86 0.15 1.25 OCT 2.43 3.96 1.29 2.61 NOV 10.46 18.00 7.72 11.82 DEC 35.37 42.W 81.33 57.58

In the Madras region, runoff normally occurs,during the months of October through December (Table 3). The discussion will focus on those three months: changes during the monsoon months of August and September are included in fig. 7 for illustration but will not be discussed because rainfall during these monsoon months contributes little to regional water

Voi. 18. No. 3 (1993) I51

....... - .......................

E 0

.

JAN FEE MAR APR MAY JUN JUL AUG SEP OCT NOV DEC MONTHS

1 - NORMAL + GlSS + GFLD + UKMO I Figure 3. Bombay region precipiutioa scenarios.

supplies [31]. As in the case of Bombay, the GFDL scenario predictsincreased runoff (a lower dlyness index) in the Madras region. The UKMO scenario shows little change in the dryness index during Oc- tober and November, but a large increase during December from a normal value of 0.61 to a new value of 2.27. This increase in the dryness during December indicates that runoff could be significantly

reduced during this month. The GISS model predicts an increase in the October index to a value just over one. For November and December, GISS predicts slight dec- in runoff. As in the case of Bombay, the shifts in the dryness index under the UKMO and GISS scenarios to values of over one during normally wet months signal a possible reduction in total water availability in the Madras region.

Table 3. Mnnthlv drvness index for the M a d m repion under , , - normal and altered dimatic mnditions CONCLUSION Monrh Normal GlSS GFDL UKMO

JAN FEB MAR APR MAY JUN JUL 4UG SEP OCT NOV DEC

22.41 2.87 3.71 78.35 Population growth trends suggest that Bombax 10.77 14.19 14.51 107.26 Madras, and many other Third World megacities will 17.87 27.76 43.33 IS.01 34.76 27.64

48.89 face increasingly severe water shortages into the fu- 20.61

14.43 34.39 1 1 . 2 7 29,44 ture. Global warning may exacerbate those shonages 7.1 1 14.65 6.97 15.09 by altering the timing and availability of water sup- 2.95 5.93 1.77 1.94 3.90 1.75

6.73 plies. but the precise water resource impacts of climate 2.34

1.87 3.71 1.1 i 3,07 change remain uncertain. The results for this study 0.51 1.07 0.09 0.77 bring out this uncertainty While the GISS and UKMO 0.30 0.44 0.22 0.61

0.42 scenarios indicate that global warming would reduce 0.85 0.69 2.17 water availability in the two urban regions, the GFDL

152 Warer Inrernarrofial

JAN FEB MAR APR MAY JUN JLlL AUG SEP OCT NOV DEC MONTHS

1 - NORMAL + GlSS + GFLD + UKMO I Figure 4. Madras region lemperafure scenarios.

scenarios predict that climate change might actually increase water availability. Not only are there uncer- tainties with regard to the GCM scenarios, but some researchers also raise questions about whether or not a CO, related climate change will even occur 1351.

In light of these various uncertainties, specific water policy and management recommendations for re- sponding to climate change are not yet possible [36,37]. But what the results of this climate change study do emphasize is the need for expansion of the range of urban water management and planning adjustment alternatives both as a response to water problems of today and in anticipation of future climatic uncer- tainty [38]. For urban water managers in the devel- oping world, expansion of the range of adjustment alternatives might entail consideration of alternative technologies such as subsurface storage or reuse of waste water. Such technologies are less climate de- pendent than conventional surface reservoirs, and may therefore prove less vulnerable to current climate variability 1241. Expansion of adjustment options

might also involve assessment of alternative water management policies. Recent studies suggest that innovative policy approaches such as domestic and industrial water demand control measures and water pricing can reduce water use in Third World cities 1391. Questions remain, however, about the broader applicability of such measures [40], indicating that this area warrants further investigation.

Like the majority of research on climate change and water resources, this study has focused on the potential impacts of climate change on water resource availability. It must be emphasized, however, that Third World megacities suffer from a wide range of interrelated water resource problems, many of which could worsen with climatic change. To help Third World cities better cope with today's water problems and, at the same time, prepare for possible climatic change. future study should be directed toward ex- panding the range of water management alternatives for addressing the interrelated water resource prob- lems of these cities.

Vol. 18, No. 3 (1993) 153

40

20 .... ..................

0 JAN FEB MAR APR MAY JUN JUL. AUG SEP OCT NOV DEC

MONTHS

- NORMAL + GlSS ++- GFLD -s- UKMO 1 I 1

Figure 5. Madras region PreejpibIion scenarios.

ACKNOWLEDGMENTS

T h e author thanks K e n Strzepek, Diana Liverman. J im Wescoat, Gilbert White, Rob Rose, Dave Mc- Ginnis, Karen Borza, a n d two anonymous reviewers for advice on the research and for helpful comments on earlier drafts.

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