potential climate change impacts on water resources in the great plains

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATIONVOL. 35, NO.6 AMERICANWATER RESOURCES ASSOCIATION DECEMBER 1999

POTENTIAL CLIMATE CHANGE IMPACTS ON WATERRESOURCES IN THE GREAT PLAINS'

Dennis Ojima, Luis Garcia, E. Elgaali, Kathleen Miller, Timothy G. F Kittel, and Jill Lackett2

ABSTRACT: This paper reports on the current assessment of cli-mate impacts on water resources, including aquatic ecosystems,agricultural demands, and water management, in the U.S. GreatPlains. Climate change in the region may have profound effects onagricultural users, aquatic ecosystems, and urban and industrialusers alike. In the central Great Plains Region, the potentialimpacts of climate changes include changes in winter snowfall andsnow-melt, growing season rainfall amounts and intensities, mini-mum winter temperature, and summer time average temperature.Specifically, results from general circulation models indicate thatboth annual average temperatures and total annual precipitationwill increase over the region. However, the seasonal patterns arenot uniform. The combined effect of these changes in weather pat-terns and average seasonal climate will affect numerous sectorscritical to the economic, social and ecological welfare of this region.Research is needed to better address the current competitionamong the water needs of agriculture, urban and industrial uses,and natural ecosystems, and then to look at potential changes.These diverse demands on water needs in this region compound thedifficulty in managing water use and projecting the impact of cli-mate changes among the various critical sectors in this region.(KEY TERMS: agricultural hydrology; aquatic ecosystems; climatechange impacts; water demand; water management; water supply.)

INTRODUCTION

The boundaries of the U.S. Great Plains are vari-ously defined, but we will use the one defined on thewest by the Rocky Mountains, and on the north byCanada's southern border. The other boundaries arebased on changes in climate and vegetation type. Theeastern boundary is the edge of the bluestem area,and the southern boundary is the edge of the desert)

scrub. The Great Plains include portions of 10 states(Montana, North Dakota, Wyoming, South Dakota,Nebraska, Colorado, Kansas, New Mexico, Oklahoma,and Texas) and occupy the central third of the landmass of the United States. The area covered is rough-ly 1.3 million km2. Currently 6.2 million inhabitantslive in the Great Plains, with a population density ofless than five persons per km2.

The Great Plains is characterized by its semiaridconditions, but humans have managed to transformthe land to overcome this limitation. Water has been acritical component of that transformation, whichmakes the issue of a continued sufficient supply ofwater of particular concern to the inhabitants. Watersources for these activities include rain, surface waterin rivers, streams, and lakes primarily from snowmeltand groundwater in aquifers. Flows from thesesources have been altered by humans through diver-sion, impoundment, and irrigation for urban and agri-cultural uses. Rainfall is not sufficient, particularly inthe western portion of the Plains, to support the foodproduction systems there now. Surface water sources,both from storage and direct flow, are important inColorado. Kansas and Nebraska, on the other hand,rely heavily on irrigation from aquifers, which makestheir depletion a serious concern.

Agriculture is an important economic activity inthe Plains; it is also the main user of water. Eightypercent of the consumptive use of water in the aridwest is estimated to be from agriculture (Solley,1997). Approximately 20 million of the 200 million

1Paper No. 99100 of the Journal of the American Water Resources Association. Discussions are open until August 1, 2000.2Respectively, Senior Research Scientist, Natural Resource Ecology Laboratory Colorado State University, Fort Collins, Colorado 80523-

1499; Associate Professor and Graduate Student, Department of Chemical and Bioresource Engineering, Colorado State University,FortCollins, Colorado 80523-3281; Scientist III, Environmental and Societal Impacts Group, National Center for Atmospheric Research, P.O. Box

3000, Boulder, Colorado 80307; Scientist/Acting Section Head, Ecosystem Dynamics and the Atmosphere Section, CGD, National Center forAtmospheric Research, P.O. Box 3000, Boulder, Colorado 80307; and Research Associate, Natural Resource Ecology Laboratory, Colorado

State University, Fort Collins, Colorado 80523-1499 (E-MaillOjima: [email protected]).

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1443 JAWRA

Ojima, Garcia, Elgaali, Miller, Kittel, and Lackett

acres of cropland in the Great Plains are irrigated,over half of which is with groundwater (Riebsame,1990). Irrigated land increased from 3.5 million acresin 1950 to 15 million acres in 1990 (Opie, 1996; Solley,1997). The South Platte River Basin alone has over30,000 wells providing 30 percent of the water usedfor agriculture. The depletion of the aquifers is associ-ated with the areas of increased irrigation. For exam-ple, about 16.6 million acre-feet are withdrawn fromthe Ogallala aquifer each year, and generallyrecharge has not made up for the withdrawals sincethe 1940s when irrigation became more prevalent inthe region (Rosenberg et al., 1999).

The other uses of water in this region are urban ordomestic use, wildlife habitat and recreation. Forinstance, the Platte River is critical for food and habi-tat for migratory waterfowl and shorebirds and hasundergone extensive changes in its seasonal flows. Inaddition, the prairie pothole area is an importantbreeding region for migratory birds and has sufferedgreatly from loss of wetland habitats.

Nearly all freshwater ecosystems in the GreatPlains have been modified by direct or indirect humanactivities and land uses, including habitat destructionfrom dams, diversions and channelization, alteredgroundwater flow patterns as a result of pumping,erosion, and the alteration of thermal regimes. Pointand non-point source pollution have introduced awide array of organic chemicals, toxic metals, and fer-tilizers, such as nitrogen and phosphorous, into theecosystem. The alteration of vegetation, the introduc-tion of non-native plant and animal species, and over-harvesting of native species have also negativelyimpacted the aquatic ecosystem. Considerable pollu-tion results from sedimentation, fertilizer, pesticide,herbicide, and waste runoff. This results in increasedsalinity, nutrient loading, turbidity, and siltation ofstreams. Shallow aquifers are also suffering fromthese pollution problems. Agricultural runoff con-tributes two-thirds of the river pollution and one-halfof the lake and reservoir pollution in this country(NRCS, 1996). Drinking water quality is reduced as aresult of pollution, particularly in small towns. Thisdecrease in water quality has affected food produc-tion, human consumption, and wildlife habitat.

Water shortage is already a problem. Most water isdistributed by prior appropriation laws, but costly liti-gation results from numerous water rights battles.These laws sometime inhibit efficient use of the waterby constraining the flexibility of water users' imple-mentation of various management practices due tocourt-enforced regulations. Water supply issues willbecome increasingly important where competitionbetween urban development and agriculture increas-es. Changes in water utilization are leading todepletion of the aquifers, increased tensions over

multi-state compacts, intra-state water transfers, andincreasing competition between agriculture and otheruses. These changes will be further affected by cli-mate changes in this already highly variable climateregion. Thus the concern over water issues, such asallocation and control, is a current and increasinglygrowing concern.

The impact of climate changes will further stressthe water resources of the Great Plains. For example,in the Central Great Plains region (i.e., the Colorado,Kansas, Nebraska, and Wyoming areas), the potentialimpacts of climate changes are anticipated to affectwinter snowfall and snow-melt, growing season rain-fall amounts and intensities, minimum winter tem-peratures, and summer average temperatures, Thecombined effect of these changes in weather patternsand average seasonal climate will affect numeroussectors critical to the economic, social, and ecologicalwelfare of this region. Given these potential impactson the water resources of the region, it is interestingto note that there is not a long-term strategy.

It must be noted that there is tremendous uncer-tainty regarding regional patterns of precipitationchange under global warming. The coarse resolutionof the general circulation models (GCMs) that areused for climate change projections limits their abilityto reproduce the spatial complexity of actual precipi-tation (Giorgi and Mearns, 1991; Giorgi et al., 1998).In addition, there are larger differences across modelsin projected precipitation changes than in projectedtemperature changes (Grotch and MacCracken, 1991;Kattenberg et al., 1996; Miller, 1997),

Runoff changes will depend on changes in precipi-tation, temperatures and other climatic variables, andwarmer temperatures may cause runoff to declineeven where precipitation increases (Schaake, 1990;Nash and Gleick, 1993; Leavesley, 1994). As part ofthe U.S. National Assessment of the consequences ofclimate variability and change, Wo]ock and McCabe(1999) computed annual runoff projections for the 18major water-resource regions of the continental Unit-ed States using the output of two GCM scenarios.They found very little agreement between the modelsthe Canadian Center for Climate Prediction andAnalysis (CCC) model and Hadley Center for ClimatePrediction and Research (HAD) regarding the direc-tion of change in average annual runoff. For themajor river basins in the Central Great Plains region,the Missouri and the Arkansas-White-Red, they foundthat the CCC model predicted reduced annual runofffor the decade 2025-2034, while the HAD model pre-dicted insignificant changes for that decade. Bothmodels predict increases for the decade 2090-2099,but given the uncertainties in those estimates, theauthors conclude that they cannot be considered reli-able.

JAWRA 1444 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

Potential Climate Change Impacts on Water Resources in the Great Plains

Even though projected changes in annual runoffremain highly uncertain, projected changes in theseasonal pattern of runoff are more robust. Snowmeltfrom the Rocky Mountains is an important componentof surface water availability in this region, particular-ly in Colorado, where irrigated agriculture relies moreheavily on surface sources than is the case in Kansasor Nebraska. Several studies of currently snowmeltdominated systems show similar seasonal shifts instreamfiows as a result of warmer temperatures anda shorter snow accumulation period (see e.g., Cooley,1990; Rango and Van Katwijk, 1990; Rango, 1995;Fyfe and Flato, 1999; Wilby et al., in press). Thechanges projected for such systems include lowersummer flows, warmer summer water temperatures,reduced summer water quality and increased winterflows. Given limited opportunities to expand artificialwater storage infrastructure to offset such changes,declines in summer water availability for irrigationand competing human and environmental uses arequite possible, particularly in the western portion ofthis region.

A few studies have examined the societal impactsof, and responses to, previous drought periods ordeclining aquifer levels. These can provide some evi-dence, by analogy, as to the possible consequences ofdrier future conditions. The MINK study, for example,used the 1930s drought in Missouri, Iowa, Nebraska,and Kansas as a climate change scenario and exam-ined the possible consequences of a repeat of thatdrought if it were to occur with the economy expectedin that region in the year 2030. The study concludedthat the overall economic consequences of such adrought would be relatively small due to the smallplace of agriculture and other sensitive sectors in theprojected future economy of the region. The authorsnoted that this result rested on their "reductionist"sector-by-sector approach and argued that considera-tion of synergisms between sectors, or a scenario withwarmer temperatures than were experienced duringthe 1930s, could lead to significantly more severe pro-jected impacts (Crosson and Rosenberg, 1993). As forwater-related impacts, the MINK study estimatedthat yields of irrigated corn and sorghum woulddecline and production costs increase under this sce-nario, due to increased groundwater pumping costs.The study also projected that irrigated acreage inwestern Nebraska and Kansas would decline (Easter-ling et al., 1993).

In Colorado, evidence for possible responses tofuture drought episodes can be drawn from recentdroughts. During the 1976-78 drought, Howe et al.(1980) found that many of the rural entities that sup-ply water for irrigation in eastern Colorado and near-by towns cooperated in various ways to make efficientuse of available water supplies. For example, some

senior right holders agreed not to "call" for theirwater, and some water users pooled their availablesupplies in a single reservoir to reduce evaporationand provide carryover for possible continuation of thedrought. However, in some places the division engi-neers encountered difficulties in enforcing waterrights, and in Division I (Northeastern Colorado), thedrought resulted in "severe drawdown of groundwa-ter" (Howe et al., 1980:46). In addition, in Northeast-ern Colorado both towns and irrigators mitigatedsome of the impacts of the drought by trading waterthrough active rental markets for Colorado-BigThompson shares and ditch-company shares (Maasand Anderson, 1978; Howe et al., 1980).

This paper reports on the current assessment of cli-mate impacts on water resources in the Great Plains,and reflects the current regional assessment contribu-tion to the National Assessment of Climate ChangeImpacts.

Climate and Environment

The continental climate of the Great Plains is char-acterized by two primary gradients which form theboundary for the area and determine the environ-ment; these are precipitation (decreasing east to west)and temperature (decreasing south to north). Thesegradients have interacted to affect human settlementand use of the area (Riebsame, 1990). Total annualprecipitation ranges from 30 inches in the east to lessthan 15 inches in the west (Figure la). Precipitationis least in winter and greatest in summer. Meanannual temperatures display a strong north-southgradient (Figure ib). The region is noted for its highinterannual climate variability, as well. The coeffi-cient of variation for precipitation in the region ishighest in the northcentral and the southwest por-tions of the Great Plains. This variation is the resultof interannual differences in the moisture deliveredfrom the Gulf of Mexico into the interior of the conti-nent during the summer and the strength of the polarjets in the winter. Growing season varies from 110days in the north to 300 days in the south (NRCS,1996).

The lack of moisture results in a steppe or semi-arid grassland ecosystem which has evolved underfrequent fires, persistent grazing pressures, andextreme weather, including droughts and floods.Much of the natural vegetation has been transformedto crop and rangeland, and altered by other humanimpacts such as tree-planting for windbreaks. Plantgrowth is limited by precipitation and nutrient avail-ability. The combination of precipitation, tempera-ture, and wind characteristics of the Great Plainsleads to extreme weather patterns, including extreme



Figure 1. U.s. Great Plains Regional Contour Maps for (a) Mean Annual Precipitation and(b) Mean Annual Temperature from 1961-1990 (VEMAP weather data).


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heat and cold, droughts, floods, blizzards, tornadoes,and hail. This makes plant and animal production inthe area very vulnerable to climate variability.Drought has always been a factor in these grasslands,with the degree and timing controlled by tempera-ture, precipitation, and the ratio of precipitation topotential evapotranspiration.

During the past 100 years, the region has experi-enced a 1.8 to 3.6°F (1-2°C) warming (Easterling etal., 1997; Karl et al., 1996). The northern portion ofthe region has experienced the greatest warming (Fig-ure 2a). Seasonal trends indicate a slightly greaterwarming in the winter months. Precipitation duringthe past 30 years tends to be higher than during theearlier part of the century (Figure 2b), althoughthroughout the past 100 years there has been consid-erable interannual variability.

Despite these climatic limitations, humans havemanaged to transform this area through theirresource use, overcoming various setbacks. The DustBowl of the 1930s shows an extreme example of thesocial disruption caused by mismanagement of thistype of environment. The experiences of the DustBowl led to significant improvements in land usepractices (e.g., reduced tillage, land retirement) thathave so far prevented a disaster of that scale again.While the environment has constrained land uses,technology has overcome some of them, but not with-out problems. Introduced technologies have resultedin the widespread transformation of the original land-scape by humans. Some of the environmental prob-lems from agricultural uses have been siltation ofwaterways from erosion, increased salinization fromirrigation, increased alkalinity of soil, reduced carbonstorage of soil, and increased runoff. A 1980 USDAstudy estimated 16 percent of the cropland exhibitedserious water erosion, and 25 percent showed winderosion (Riebsame, 1990). Ten percent of the NorthernGreat Plains landscapes are affected by salinity, witha 10 percent annual increase (NRCS, 1996). "Tillagepan" from soil compaction is found in half of the culti-vated land in the region (Riebsame, 1990).

Land Use Systems

The primary land use that has transformed theGreat Plains grassland has been agriculture, withurban and industrial use increasingly becoming a fac-tor. Over 90 percent of the land is in farms and ranch-es, and 75 percent is cultivated (Riebsame, 1990).Agriculture is more important to the Great PlainsRegion than to any other in the country. There arefive major production systems in the Great Plains:range livestock, dryland or rain-fed crop fallow,

groundwater irrigation (aquifer-dependent), river val-ley irrigation (snowmelt-dependent), and confinedlivestock feeding. Great Plains agriculture is land-extensive and uses relatively few chemical inputs andlabor per unit of land compared with agriculture inother regions of the U.S.

The crops grown vary according to the climatic gra-dients. Corn and soybean are rainfed and produced inthe east, with dryland wheat and sorghum productionin the west (NRCS, 1996). As productivity decreasesalong this east-west moisture gradient, risk increases.Spring wheat is planted in the north and winterwheat in the south. Cool season (or C3) plants do bet-ter farther north than warm season (or C4) plants(Tieszen et al., 1996). Increasing irrigation to the westallows for more corn production. The rangeland graz-ing is concentrated in the drier areas to the west.Wheat is considered the "indicator species" of theGreat Plains, with its changes in production reflectingvarious forces at play during different periods. TheGreat Plains is the most productive dryland wheatarea in the world, and pivotal to world grain supplies.Great Plains production accounts for 51 percent of thenation's wheat, 40 percent of its sorghum, 36 percentof its barley, 22 percent of its cotton, 14 percent of itsoats, and 13 percent of its corn. It produces 40 percentof the nation's cattle.

CLIMATIC IMPACTS ON WATER RESOURCES

Changes in land use and climate will affect waterquantity and quality. Projections of some of the gener-al circulation models indicate that both annual aver-age temperatures and total annual precipitation willincrease over the region during the coming century.However, the seasonal patterns are not uniform.According to the Hadley Center scenario, a 7.2°F (4°C)increase is projected for the winter period at the endof the next century for the Colorado-Wyoming area.This coupled with possible changes in winter-timeprecipitation would greatly modify the amount andtiming of snow-melt from the Rocky Mountains(Miller, 1997). During the summer months, minimumtemperatures (nighttime) are increased more than themaximum temperatures (daytime). The change inminimum temperatures may affect plant communitiesby increasing the amount of cool season plant species(Alward et al., 1999). These changes in temperaturesmay increase evaporation, and in turn precipitation,so the hydrological cycle may also be affected, result-ing in more intensive convective storm activity.

Water is a critical component of the socio-economicactivities contributing to the land transformations



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taking place in the Great Plains. Thus, the issue ofwater quality and supply is of particular importanceto the inhabitants. About 10 percent of the GreatPlains cropland area, or 20 million acres, is irrigatedcropland. The lack of water availability, due to

increased temperatures and evaporation, can exacer-bate the soil moisture stress of irrigated and non-irrigated regions of the Great Plains. Soil moisturedepletion can greatly reduce yield of range forage andof crops. In addition, many parts of the Great Plains

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1895-1993, and projections from the CCC and Hadley GCMs for 1994-2099.

are showing decreasing water supplies for agricul-ture, partly due to the higher prices urban users arewilling to pay for water.

A consumptive use model incorporating a monthlyestimation method was implemented for this work.The monthly estimation method that was used is the

SCS-Blaney Criddle method (USDA, 1970) expressedas follows:

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where u = monthly consumptive use (in); kc = cropcoefficient reflecting the growth stage of the crop; kt =climatic coefficient related to mean temperature; tmean monthly temperature (CF); and p = monthly per-centage of daylight hours.

The model allows for water supply information tobe accounted for. In this case, it was assumed thatwater supply is at all times adequate. The model wasused to compute the Consumptive Use (CU - rainfall)and the Irrigation Water Requirements (IWR). TheCU model was used to compute CU and IWR for threedifferent crops using climate data generated by sever-al climate scenarios. The results indicate that underthe Canadian Climate Center Model (McFarlane etal., 1992), perennial crops, grass, and alfalfa pasturewould experience a 50-60 percent increase in the con-sumptive demand toward the end of the next century(Figure 3a). The application of the Hadley ClimateCenter scenario (Johns et al., 1997) resulted in slight-ly less of an impact on the consumptive demand, 50percent for grass pasture, and approximately 30 per-cent for alfalfa (Figure 3b). Interestingly, the irrigatedcorn did not show any strong increase in consumptivedemand. This result is most likely due to the slightincrease in growing season precipitation, especially inthe Hadley Climate Center scenario.

POTENTIAL CONSEQUENCES OF CLIMATECHANGE FOR WATER RESOURCES

This section will go beyond discussion of the directeffects of climate change on water resources in theGreat Plains to integrative factors which interact todefine the consequences of climate change in theregion. The three examples chosen for discussion arewater supply, wetlands, and agricultural water man-agement.

Water Supply

Increased demands for water have promoted inno-vative methods of increasing water storage, for exam-ple, through artificial groundwater recharge.Population growth in the western Great Plains isalready creating additional water demands that mayincrease with climatic variability. There will beincreasing demands for water storage to maintain asustainable water supply. Changes in the flow regimeand the social acceptability of structural solutions willrequire a reexamination of infrastructure and sys-tems operations. Water development has moved awayfrom structural development solutions. In the past

years, we have seen extreme events in the GreatPlains (e.g., floods along the Mississippi River) thatmight require the reevaluation of the current waterimpoundment structures.

Although some large federally-funded infrastruc-ture projects may have been designed with excesscapacity, which serves as a reserve, many smaller,locally-funded projects do not have such excess capac-ity. Examples might include reservoir capacity, heightof dikes, and capacity of flood-diversion channels.Consequently, climate change could cause such sys-tems to be unable to perform their functions, leadingto increased stress on local resources.

The scientific basis for determining water needs ofaquatic ecosystems under current and climate changescenarios should be further refined. Aquatic ecosys-tems in the Great Plains face numerous existingstresses caused by competing demands of agriculture(including water quality issues such as nitrogenrunoff) and urban uses. Climate changes will placethe aquatic ecosystems under additional pressure.Temperature and precipitation changes will have awide range of effects on already vulnerable ecosys-tems. These effects range from dry wetlands andstream beds to extreme instream flow variability toincreased demands on water supplies from agricul-ture and urban/residential users. Warmer air temper-atures will affect water temperatures and the abilityof exotic/non-native species of pests, fish, and plantsto migrate into Great Plains aquatic ecosystems anddisrupt these already stressed systems.

Wetlands

Wetlands are already under significant stressthroughout the world due to human activities such asdraining for agriculture and urban expansion. Global-ly, about half of all wetlands have been lost sinceapproximately 1940. In the U.S., excluding Alaska,about 40-60 percent of wetlands have been lost, withpercentages in some states, such as Ohio, as high as90 percent. Wetlands in the Great Plains have under-gone similar losses; in central Nebraska near thePlatte River, most wetlands have been lost. Changesin the timing, areal distribution, intensity, or form ofprecipitation (rain, snow, hail, etc.), coupled withincreased evaporation/transpiration rates, vi 11 affectbiological/agricultural resources in the Great Plains.Already, regulation of streams and rivers have result-ed in loss of the natural hydrograph reducing riparianhabitat. Instream flow requirements required for bio-logical diversity increasingly will compete with con-sumptive uses. Furthermore, problems associatedwith salinity and other pollutants in surface and


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Figure 3. Results of(a) a Consumptive Use and (b) and Irrigation Demand Model for the Historical Period, andProjected to 208 1-2090, by Both the CCC and Hadley GCMs. Results are presented for grass, nlfalfa, and corn.

groundwater will increase with an intensified hydro-logic cycle.

Wetlands provide critical hydrological, biological,and biogeochemical functions which have correspond-ing benefits valued by society. They provide flood con-trol and water storage, assist in pollution filteringand waste processing, provide critical habitat and

breeding ground for birds and other species, andassist in the global cycling of carbon and nitrogen.Any wetland loss or degradation that interferes withthese functions would have corresponding effects onthe benefits valued by society. In the Great Plains,prairie potholes have traditionally provided criticalwater storage and waterfowl habitat. It is estimated


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that prairie wetlands contribute to more than half ofthe annual waterfowl population produced in NorthAmerica. The central section of the Platte River hasprovided both a source of food and protection forwaterfowl and shorebirds during spring and fallmigrations.

Wetlands will be affected by any changes in tem-perature, precipitation, and evaporation/evapotran-spiration, which alter the water supply to the wetlandthrough changes in runoff, streamfiows, and ground-water recharge. Both the seasonal patterns and inten-sity of precipitation events are important. In thenorthern prairies, it is likely that if evapotranspira-tion increases and snowmelt runoff decreases, a dra-matic loss of meadow and shallow marsh lands willresult. Wetlands in the central flyway in Nebraskawould be adversely affected by any drop in riverwater levels during migration periods. However, wet-lands could be enhanced if increased precipitationexceeds evapotranspiration so as to consistently pro-vide increased streamfiow and groundwater supply.

Prairie potholes are considered particularly vulner-able to climate change due to their inability to adaptthrough migration or other processes. This has beenfurther exacerbated by human development in sur-rounding areas. Riverine ecosystems tend to have ahigher potential to adapt through natural or assistedmigration due to the high degree of spatial and tem-poral variability of their natural environment.

Agricultural Water Management

Consumptive water use by agriculture involvesboth flood irrigation from surface waters, particularlyin the western portion of this region, and spray irriga-tion derived from both surface waters and groundwa-ter. Management of both sources is a major task forthis region, a task that will be burdened by anydecrease in available water concomitant with climatechange. Considerable water is drawn from riparianaquifers which can be recharged quickly throughappropriate riverine management. More difficult arethe regional groundwater aquifers which are, to someextent, not rechargeable in the present climateregime. The most widespread and best known amongthese is the Ogallala Aquifer. There is a need tomatch the drawdown from the Ogallala Aquifer to therecharge from precipitation in order to avoid deple-tion of the water resource. Changes in precipitationand evapotranspiration demand may lead to changesin recharge and drawdown respectively. Use of sur-face water for irrigation is another water manage-ment issue. More efficient application methods (waterpulsing, etc.) could decrease water needed. Wateravailability in dryland systems and irrigated systems

can both be affected by residue management andtillage practices.

Quantity of water available for particular usesdepends on politicallsocialleconomic means of alloca-tion and control. Water quality is compromised bysalinity and runoff of fertilizers and wastes. Pressuresfor more high quality water stresses regulatory anddecision mechanisms. These stresses will be exacer-bated if climate change reduces water availability.

In addition to water quantity issues, furtherresearch is needed to help determine the effects of cli-mate change on water quality and ways to best cor-rect and mitigate water quality problems. Salinitymanagement is an important issue in certain areas ofthe Great Plains. Rivers become more saline due torunoff and percolation through highly saline soils.High salinity affects crop production and adverselyimpacts fish and wildlife habitat. Drinking waterquality in the Great Plains is also an important issue.Many small towns struggle to meet current drinkingwater standards. Non-point source pollution can con-tain contaminants from fertilizers, herbicides, pesti-cides, livestock wastes, salts, and sediments thatreduce the quality of both surface water and ground-water drinking water supplies.

Changes in climate may also affect the numbersand types of pests. Pest control operations have thepotential to affect water quality as water drainsthrough crop fields. Currently, poor water quality andquantity have severely impacted fish and wildlifehabitat in the Great Plains. Deterioration in waterquality could have additional adverse impacts on theecosystem.

COPING STRATEGIES

By applying and experimenting with new storagetechniques such as groundwater storage reservoirs,snowpack storage in mountain forests through betterforest management practices, snow capture and stor-age in alpine basins, crop management practices thatenhance soil moisture through crop stubble, windbreaks, mulches, and snow management strategies onthe Plains, the quantity of stored water can increaseto meet water demands in the future. Increased irri-gation demand may lead to a wide-spread adoption ofthe scientific approach to irrigation scheduling,adjustment of yield target to match available water,and/or the change of cropping system/land use in theevent that irrigation costs exceed the worth ofincreased production due to irrigation.

JAWAA 1452 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION


RESEARCH NEEDS

Research needs to build capability to better addressthe current competition among the water needsof agriculture, urban and industrial uses, and naturalecosystems. There are creative and innovativemeans of storing water that should be explored inorder to augment traditional surface water storage.New reservoirs could be less damaging to aquaticecosystems than traditional reservoirs that disruptnatural flows, destroy riparian areas, change thermalregimes, and fragment migration corridors. A detailedunderstanding of water budgets and cycles at regionalscales needs to be achieved. Regional water budgetsneed to be quantified so that temporal and spatial dis-tribution of water availability is better understood.These budgets need to address all components of thewater cycle, including water content in the atmo-sphere, soils, surface and groundwater reservoirs, andvegetative cover. The interaction between these com-ponents needs to be linked to current and future cli-mate regimes and made available with high spatialresolution for land management decisions. If runoff ischanging (due to hydrologic or land use changes), weneed to understand the capabilities of the currentreservoirs to provide flood control and/or urban andagricultural needs. Research should focus on reservoiroperation and the evaluation of any additional struc-tural requirements and/or reservoir managementchanges necessary to adapt to new climatic condi-tions.

Aquatic Ecosystem Needs

Current understanding of the needs of aquatic sys-tems for survival under current demands and climateis incomplete. Aquatic systems research includes bothpreservation of species habitat and biogeochemicalcycling. However, both are a function of the hydrologi-cal regime of wetland systems, how these are alteredby climate change, and how these changes interactwith other human stressors. Water apportionmentdecision making between aquatic ecosystems andhuman needs must be reassessed. We must begin toevaluate the effects that projected climate change willhave on Great Plains aquatic ecosystems. To resolveboth current and future water issues we must developan institutional mechanism that fosters informationflow, tolerance, and compromise, so that the naturalcapital inherent in our aquatic resources is not lost inhasty or one-sided decision making.

Agricultural Water Management

More study of water application technology is need-ed to increase irrigation efficiency. On sprinkler sys-tems, sprinklers formerly at the top of center pivotsystems have been moved down into the canopy. Thishas significantly reduced spray losses. More efforts todevelop precision irrigation in the context of precisionagriculture are needed. Scientific irrigation schedul-ing should be expanded to include the movement ofagro-chemicals in conjunction with water applied andthe movement of salts in the crop root zone. Efforts todevelop and implement policy are needed to avoid sig-nificant depletion of the Ogallala and other ground-water reserves in the region. Further research isneeded on the effects of the possible reduction in soilmoisture and on the effects of extreme precipitationevents that can affect runoff and water quality. Forexample, research is needed on how to best managelivestock wastes during extreme precipitation events.

A fundamental research consideration that per-tains to most of the issues is the importance of main-taining and augmenting strategically selectedlong-term monitoring systems. In many respects,monitoring systems initiated in the 1970s, such as forwater quality, are being cut back. Some of these neednew support, but other monitoring systems, such asfor diagnostic population fluctuations, lake levels,etc., need to be instituted.

CONCLUSIONS

Diverse demands on water needs in this regioncompound the difficulty in managing water use andprojecting the impact of climate changes among thevarious critical sectors in this region. Therefore, theassessment needs to involve members of the wateruse and supply sectors to better understand the com-peting water needs among the agricultural sectors,urban and industrial uses, and natural ecosystems.Understanding of the needs of aquatic systems forsurvival under current water demands and climate isincomplete. Water apportionment decision- makingbetween aquatic ecosystems and human needs mustbe more clearly assessed. We have begun to evaluatethe effects that projected climate change will have onGreat Plains aquatic ecosystems, agriculturaldemands, and water management.



ACKNOWLEDGMENTS

This research effort was sponsored by the National ClimateAssessment of the United States Global Change Research Programand is funded by the Department of Energy in support of the Cen-tral Great Plains Assessment. The technical assistance, StevenMackie, Susy Lutz, and Laurie Richards, was provided by staff atthe Natural Resource Ecology Laboratory. The material developedfor this paper is the outcome of discussions conducted during theCentral Great Plains Workshop held at the Sylvan Dale Ranch inMarch 1999. Climate data and ecosystem analysis is partially sup-ported by the Vegetation Ecosystem Modeling and Analysis Project(VEMAP) Team. The support of Timothy Kittel and Kathleen Millerwas provided by the National Center for Atmospheric Researchwhich is sponsored by the National Science Foundation.

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potential climate change impacts on water resources in the great plains

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