Global Climatic Changes And Regional Hydrology: Impacts .
The Influence of Climate Change and Climatic Variability on the HydrologieRegime and Water Resources (Proceedings of the Vancouver Symposium,August 1987). IAHSPubl. no. 168, 1987.Global climatic changes and regionalhydrology: impacts and responsesPeter H. GleickEnergy and Resources GroupUniversity of CaliforniaBerkeley , California, USAABSTRACTAs the atmospheric concentration of carbondioxide and other trace gases increases, changes in globaland regional climatic conditions will lead to a wide rangeof hydrologie impacts, including changes in the timing andmagnitude of runoff and soil moisture. These hydrologiechanges, in turn, will result in diverse economic, social,and political consequences.The nature of the regional hydrologie effects dependson changes in the climatic conditions and the waterresource characteristics of the region. The researchconducted to date has identified a wide range of potentialproblems—as well as some possible advantages—that mightresult from plausible changes in climate estimated bystate-of-the art general circulation models.These hydrologie changes fall into a series of distinctcategories, including: changes in the timing of wateravailability; changes in the magnitude of water availability; changes in the hydrologie variability; and effectson water quality. Similarly, diverse societal responses tothe hydrologie changes are available, including adaptation,mitigation,and prevention. Each of these responsesdepends on the quality of the information available onfuture impacts and on the perceived importance of theeffects. This paper discusses the extent and character ofhydrologie changes that could result from global climaticchanges, together with the options available for hydrologists and water planners.IntroductionGrowing attention is being paid to climatic changes that may resultfrom increasing atmospheric concentration of carbon dioxide and othertrace gases. While the direct effects of changes in climaticconditions can be severe—as can be seen by the effects of existingclimatic variability—we must also pay attention to the wide rangeof indirect effects, such as changes in agricultural productivity,changes in sea-level, and changes in water resources. This lattercategory is one of the most important and yet least well-understoodconsequence of future changes in climate. Hydrologie impacts mayinclude major alterations in the timing and magnitude of surfacerunoff and soil-moisture availability, and changes in the quality offreshwater resources. Associated with these effects will be a wide389
390P.H. Gleickrange of economic, environmental, and societal impacts» This paperdiscusses the likely extent and character of important hydrologiechanges that could result from global climatic changes, together withthe options available to hydrologists and planners for dealing withthe most severe impacts»The limited research conducted to date has identified a wide rangeof potential problems—as well as possible advantages—that mightresult from plausible changes in climate» These hydrologie changesfall into distinct categories, including: changes in the timing andmagnitude of water availability; changes in the frequency andseverity of severe events, and effects on water quality» Similarly,diverse societal responses to the hydrologie changes are possible,including adaptation, mitigation, and prevention» Each of theseresponses depends on the quality of the information available aboutfuture impacts and on the perceived importance of the effects»Future climatic changesDespite the fact that hydrologists need accurate information onclimatic means and variability in order to develop appropriate waterresource designs and rules of operation, details of future climaticconditions cannot yet be predicted with any high degree of confidence»The principal reasons for this inability to clearly identify futureclimatic changes are the complexities of the ocean-atmosphere-landinteractions, the difficulties of developing satisfactory computermodels to reproduce these interactions, and uncertainties about ouractions that affect climatic conditions»The problem is that, at present, while there are many ways inwhich climate may be affected by human actions, we are unable to seeclearly either the direction of future climatic changes or nature oftheir societal impacts» Because we are unable to "do the experiment"directly, we must attempt to model climate and climatic changes—animprecise alternative because of the complexity of the global climatesystem» Much of the effort of trying to understand the atmosphericsystem has focused on the development of large-scale computer modelsof the many intricate and intertwined phenomena that make up theclimate» The most complex of these models - general circulationmodels (GCMs) - are detailed,time-dependent, three-dimensionalnumericalsimulations that include atmosphericmotions, heatexchanges and important land-ocean-ice interactions (see, Manabe1969a, 1969b; Schlesinger and Gates 1980; Manabe and Stouffer 1980;Wetherald and Manabe 1981; Ramanathan 1981; Manabe et al» 1981;Hansen et al» 1983, 1984; Washington and Meehl 1983, 1984)»GCMs permit us to begin to evaluate some of the implications forglobal climatic patterns of increasing concentrations of radiativelyactive atmospheric gases»While many uncertainties remain, aconsensus is now beginning to form about the direction and magnitudeof certain major impacts, such as increases in global-averagetemperatures and changes in the intensity and distribution of theglobal hydrologie cycle»Unfortunately, state-of-the-art general circulation models arelarge and expensive to operate» Furthermore, while GCMs are invaluable for identifying some climatic sensitivities and changes in global
Effects of climatic change on regional hydrology 391climatic characteristics, they have two limitations that reduce theirvalue to researchers interested in more detailed assessments of waterresources: (1) they are unable to provide much detail on regional orlocal impacts, and (2) they are unable to provide much detail onsmall-scale surface hydrology. Until our ability to model climateimproves, we must use other methods to either enhance the informationavailable from GCMs or provide insights now unavailable from them.Plausible future hydrologie changesThe attention focused on large-scale GCMs in recent years results inlarge part from their relative sophistication compared to othermodels. Yet this attention has also highlighted the need for newmethods of hydrologie assessment. Recently there have been someserious efforts to evaluate the regional hydrologie implications ofclimatic changes (Schwarz 1977; Stockton and Boggess 1979, Nemec andSchaake 1982; Revelle and Waggoner 1983; Flaschka 1984; U.S. Environmental Protection Agency 1984, Cohen 1986, Gleick 1985, 1986a,b,1987c). These works have provided the first evidence that relativelysmall changes in regional precipitation andévapotranspirationpatterns might result in significant changes in regional wateravailability.Methods for evaluating the hydrologie impacts of climatic changesinclude using historical data to evaluate the effects of pastfluctuations in precipitation and temperature on runoff and soilmoisture; determining the sensitivity of runoff and soil moisture tohypothetical changes in the magnitude and timing of precipitation andtemperature; and incorporating regionally disaggregated changes intemperature and precipitation predicted by GCMs into more accurateregional hydrologie models. While none of these methods - individually - can provide much reliable information on future changes, eachcan provide insights into specific hydrologie vulnerabilities toclimatic change.Future hydrologie changes: what can we expect?Changes in climate may cause changes in a variety of hydrologievariables, including the timing, location, duration, and extent ofprecipitation, runoff, soil moisture, and extreme events. Theseimpacts can be categorized in a variety of different ways. One usefulmethod, shown in Table 1, is to separate the impacts by the spatialand temporal scales involved, with additional separation for thedifferent statistical moments of interest and the distinction betweenpolitical and geophysical boundaries. In the following sections, themost plausible and worrisome changes in water availability aredescribed. These changes are not the only hydrologie effects thatwill occur,and not all of these will occur at any one place or atany one time. Nevertheless, we should pay particular attention tothese impacts because they are more likely to occur, they are harderto mitigate, and they may be more disruptive than other climaticeffects.
392 P.H. GleickTable 1 Hydrologie effects of climatic changeHydrologie Variable of InterestUseful PrecipitationSurface RunoffAvailable Soil MoistureGroundwaterTemperatureMonsoonality (Onset, Ending, Intensity, Location)Storm EventsTemporal Scale of InterestLong-Term (greater than annual)AnnualSeasonal (two to six months)MonthlyDailySpatial Scale of Interest (Political)GlobalContinentalCountry/RegionLocal10 10 10 10Jkm 2km 2km 2- 10 s km 2Spatial Scale of Interest (Hydrologie)GlobalContinentalRegionalWatershed10 10 10 10 2km2km 2- 10 km2- 10 5 km2Statistical Scale of InterestMeanVariancePersistenceSkevHigher MomentsHydrologie Impact of InterestQuantityQualityPeak Events (High and Low)Source. Gleick (1987a)a) PrecipitationDespite the fact that all GCMs predict an intensification of theoverall hydrologie cycle, particularly increases in global averageannual precipitation rates, this information is only marginallyuseful* As the global average temperature increases, we expect anincrease in the rate of évapotranspiration and precipitations GCMsnow suggest that the annual-average increase in global precipitationmay be on the order of seven to fourteen percento Far more interesting and potentially disruptive are the changes in regional precipitation patterns, which are much harder to models At present, there islittle consensus about specific regional changesoTwo specific vulnerablities need attention: (1) changes in averageprecipitation rates in regions with rainfed agriculture; and (2)changes in the frequency of extreme precipitation events in areasvulnerable to flooding and storms s In the first case, an increase inprecipitation in agricultural regions dependent on rainfall could
Effects of climatic change on regional hydrology 393have a beneficial effect, while a decrease would have the oppositeeffect. Similarly, floods and storms are already responsible forenormous human sufferings Such events could be exacerbated by anincrease in the variability of regional precipitation»There are a number of hydrologie effects that may be drivenprimarily by temperature changes, not precipitation changes» Thispermits the identification of certain impacts that are somewhatindependent of precipitation rates» Among these impacts are changesin soil-moisture availability and changes in the timing of surfacerunoff» Although both of these variables depend heavily on city,precipitation rates, vegetation characteristics, topography, and soildepth and type, some generalizations can be made»b) Soil-moisture availabilitySoil-moisture behavior in general circulation models is very simple,and efforts to improve the representation of moisture in the soilcolumn are now underway (Dickinson 1986; Rind 1987)» For the lastseveral years, there has been a growing interest in soil-moisturechanges because of the possibility that somesignificant—andpotentially adverse—effects on soil-moisture availability may resultfrom increasing concentrations of carbon dioxide» In particular, somegeneral circulation model results suggest that soil moisture in midcontinental regions in mid-latitudes may decrease during summermonths, which is often the critical period for crop productivity(Manabe et al» 1981; Mitchell 1983; Manabe and Wetherald 1986; Rind1987)» Although there are disputes over the magnitude (and sometimesthe direction) of these soil-moisture changes, the present activeresearch in this area may help to resolve the uncertainties»Recently, some detailed hydrologie models have supported thepossibility of decreased summer soil-moisture availability in someregions (Gleick 1986a, 1987c)» In particular, despite increases inannual and seasonal precipitation, increases in temperatures can leadboth directly and indirectly to decreases in soil-moisture availability during summermonths» In regions with winter snowfall andspring snowmelt (in the United States, such regions include largeparts of California, the Rocky Mountains, the Pacific Northwest, andthe Northeastern and Northcentral U»S»), increases in temperaturesmay lead to decreases in the ratio of snow to rain in winter months,increases in the speed of snowmelt in spring months, and an earlieronset of drying in early summer (Gleick 1987c)» Figure 1 shows thedecreases in summer soil moisture in a major California watershedthat result from eight scenarios generated by three state-of-the-artGCMs. The GCMs each predict quite different precipitation, yet theregional model results using these scenarios all show decreasedsummer soil-moisture availability» Similar results were identifiedfor other regions by Mather and Feddema (1986)» This robust result isone example of the type of hydrologie impact that should be morecarefully studied on a regional basis»
394P.H.GleickCHANGE IN SUMMER SOIL MOISTURE UJ \—SUo0-20-i% -14-20-31OinUJ-40-60-310 -80T ONLYT,P(r)T,P(a)Figure 1 Changein summer (June, July and August) soil moisturepredicted by a water-balance model of a major Californiawatershed using precipitation and temperature data fromthree general circulation models: the National Center forAtmosphericResearch (NCAR),the GeophysicalFluidDynamics Laboratory (GFDL), and the Goddard Institute forSpace Studies (GISS). Note that all three models showdecreases in soil moisture. The eight scenarios are:Temperature only, Temperature and relative precipitation,and Temperature and absolute precipitation. See Gleick(1987a) for details of the model, the scenarios, and theuncertainties.c) RunoffSurface runoff shows a sensitivity to increases in temperaturesimilar to the one described above for soil moisture. In certainregions, seasonal runoff appears to be vulnerable to changes in thetiming of surface flows, even when the overall annual runoff does notchange significantly.Figure 2 plots historical average-monthly runoff in a majorCalifornia watershed together with the runoff predicted by a waterbalance model using temperature and precipitation changes predictedby a state-of-the-art GCMo In this case, while the average-annualrunoff volumes do not change significantly between the two cases, themonthly pattern has changed»This can be seen by the large increasein winter runoff and the decrease in summer runoff. The physicalmechanisms at work here are similar to the ones described above
Effects of climatic change on regional hydrology 395MODEL VS. ACTUAL SURFACE RUNOFF5000GCM TEMPERATURE AND PRECIPITATION CHANGESACTUALMODEL4000COu u u3 3000o0 2000o1000 i11— I1111rrJAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DECFigure 2 Average-monthly runoff: actual and model-predicted usingtemperature and precipitation changes developed by theGeophysical Fluid Dynamics Laboratory GCM» The annualrunoff volumes for both of these runs are the same; theseasonal pattern has changed» See the text for details»driving the soil-moisture changes—less total winter snowpack, morewinter runoff, faster snowmelt in the spring, and smaller springrunoff » Figures 3 to 5 show the details of average-monthly changes inrunoff using the temperature and precipitation changes from threeGCMs to drive a regional water-balance model of the Sacramento Basinin Northern California - perhaps the most important watershed inCalifornia (Gleick 1987b)« In all of these cases, summer runoffdecreased and winter runoff increased, while average-annual runoffwas only slightly changed (Gleick 1987c). These runoff changes canincrease the frequency of flood events by shifting more runoff topeak runoff months, even if overall average runoff doesn't change.Similarly, regions dependent on minimum summer flows may be adverselyaffected»The vulnerability of water resources to climatic conditionsThe availability of freshwater for agricultural, industrial, residential, and commercial use is sensitive to existing climatic variabili-
396P.H.GleickC l i m a t e - I n d u c e d Change In RunoffNCAR TEMPERATURE AND ABSOLUTE PRECIPITATION— 100Figure 3J1111111r1ir1 —JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DECPercentchangein monthly runoff between the NCARtemperature and absolute precipitaion run and the longterm average runoff. Note the increase in winter runoffand the decrease in summer runoff.ty. The sensitivity varies with supply and demand, water quality, andthe specific needs of the users» As the climate begins to change, themost severe pressures rn available water resources are likely to comein regions where the existing water resources are already constrainedduring certain times. This section discusses existing vulnerabilitiesthat might be either exacerbated or mitigated by climatic change.Regions with natural deficits: Arid and semi-arid lands are, bydefinition, regions with natural water deficits: the potentialévapotranspiration exceeds natural water inputs during part or all ofthe year* At the same time, these lands are often thought to hold thegreatest potential for future agricultural development assuming thatwater can be made available for irrigation, and that the soil qualityis high enough (Rosenberg 1981; Gleick 1987a)» Improvements in thehydrologie conditions of these regions would require increases in theaverage water availability. Since évapotranspiration is likely toincrease following a doubling of atmospheric carbon dioxide, such anincrease in average availability must come through precipitation orwater transfers into the basin. At the same time, if the variabilityof water resources availability were to increase, the vulnerabilityof these regions to climate could remain high. For a climatic changeto be most advantageous to arid and semi-arid regions, there wouldhave to be an increase in mean water availability and a decrease inthe variability. It is important to note here, however, that while
Effects of climatic change on regional hydrology397Climate—Induced Change In RunoffGFDL TEMPERATURE AND ABSOLUTE PRECIPITATIONo-100Figure 4J111111i11111—JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DECPercentchangein monthly runoff between the GFDLtemperature and absolute precipitation run and the longterm average runoff. Note the increase in winter runoffand the decrease in summer runoff.such changes might be beneficial to agricultural productivity orother human uses, they can lead to dramatic shifts in the naturalcharacter of the existing ecosystems (Gleick. 1987a).Regions with high societal demands: In many regions of the world,the demand for water approaches the available supply during certainperiods. In these regions, efforts are often already underway tomodify either the available supply or demand. Changes in climate thatexacerbate these demands or reduce the overall supplies will havenegative consequences for the region, while overall increases inwater availability could ease some problems. As with the firstexample, the most advantageous climatic change would be increasedmean availability and decreased variability. An increase in variability would increase the frequency of severe events and may not resultin net benefits to the region.Flood-prone regions: Areas prone to flooding, such as low-lyingfloodplains, would benefit from a decrease in the variability ofprecipitation and runoff and suffer f rom an increase in both the meanand the variability
The Influence of Climate Change and Climatic Variability on the Hydrologie Regime and Water Resources (Proceedings of the Vancouver Symposium, August 1987). IAHSPubl. no. 168, 1987. Global climatic changes and regional hydrology: impacts and responses Peter H. Gleick Energy and Resources Group University of California Berkeley , California, USA
Describe certain climatic phenomena that occurs on a global level. (S9ES-IIIf31) After going through this module, you are expected to: 1. describe climatic phenomena that occur on a global level such as global warming, climate change, El Niño and La Niña; 2. determine
INDIAN MONSOON Based on different parameters like Temperature, Precipitation, Humidity, etc., the planet is divided into different climatic regions. E.g., Equatorial climate, Siberian, etc., India is placed in the monsoon type climatic region as it is the climatic pattern in the south and south-east Asia. Monsoon type of climate is well described as the seasonal reversal of winds in a year.
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