The objective of this study is to analyze potentialimplications of climate change and human demandfor water on the groundwater balance of the SantaCruz Valley Aquifer in the USA−Mexico borderregion. The Southwest USA and adjoining NorthwestMexico, which already experience water scarcity, areex pected to become drier over the 21st century (Karlet al. 2009). The variability of precipitation has ledDiffenbaugh et al. (2008) to identify this region as a
‘climate change hotspot’. Groundwater aquifers, as aresult of the projected declines and greater variabil-ity of surface water, are expected to experience in -creased pumping to meet human demands for water.In this paper, we pay special at tention to the SantaCruz Valley Aquifer, one of 4 aquifers currentlybeing assessed as part of the USA−Mexico Trans-boundary Aquifer Assessment Program (TAAP). TheSanta Cruz Aquifer is subject to climate impacts onfuture water supply and variability, including flood-ing and drought. Evolving institutional arrangements
Effects of climate change and population growth onthe transboundary Santa Cruz aquifer
Christopher A. Scott1,2,*, Sharon Megdal3, Lucas Antonio Oroz4, James Callegary5, Prescott Vandervoet1
1Udall Center for Studies in Public Policy, 2School of Geography & Development, and 3Water Resources Research Center, University of Arizona, Tucson, Arizona 85721, USA
4Comisión Nacional del Agua, Hermosillo, Sonora 83280, Mexico5Arizona Water Science Center, US Geological Survey, Tucson, Arizona 85719, USA
ABSTRACT: The USA and Mexico have initiated comprehensive assessment of 4 of the 18aquifers underlying their 3000 km border. Binational management of groundwater is not currentlyproposed. University and agency researchers plus USA and Mexican federal, state, and localagency staff have collaboratively identified key challenges facing the Santa Cruz River ValleyAquifer located between the states of Arizona and Sonora. The aquifer is subject to recharge vari-ability, which is compounded by climate change, and is experiencing growing urban demand forgroundwater. In this paper, we briefly review past, current, and projected pressures on Santa Cruzgroundwater. We undertake first-order approximation of the relative magnitude of climatechange and human demand drivers on the Santa Cruz water balance. Global circulation modeloutput for emissions scenarios A1B, B1, and A2 present mixed trends, with annual precipitationprojected to vary by ±20% over the 21st century. Results of our analysis indicate that urban wateruse will experience greater percentage change than climate-induced recharge (which remainsthe largest single component of the water balance). In the Mexican portion of the Santa Cruz, upto half of future total water demand will need to be met from non-aquifer sources. In the absenceof water importation and with agricultural water use and rights increasingly appropriated forurban demand, wastewater is increasingly seen as a resource to meet urban demand. We considerdecision making on both sides of the border and conclude by identifying short- and longer-termopportunities for further binational collaboration on transboundary aquifer assessment.
Resale or republication not permitted without written consent of the publisher
Clim Res 51: 159–170, 2012
to address these challenges are asymmetrical onopposite sides of the border, as detailed below. Fig. 1shows the location of the Santa Cruz aquifer, sharedbetween the states of Arizona and Sonora.
Historical antecedents demonstrate that waterresources have always been crucial to human settle-ment and economic development in the Santa Cruzvalley. During the Holocene, native peoples relatedto the present day Tohono O’odham inhabited river-ine environments in the study area. Irrigation waspracticed, but few remnants exist of large-scale irri-gation works such as those employed by the Hoho -kam along the lower Santa Cruz, Gila, and SaltRivers to the north. Europeans first made forays intothe region in the mid-16th century, and establishednumerous settlements along the river. With Mexi-can independence from Spain in 1810, the regioncame under Mexican jurisdiction. Before the USAacquired the area north of the current borderthrough the 1854 Gadsden Purchase (Tratado de la
Mesilla), the Santa Cruz River occupied a more cen-tral location in the Mexican state of Sonora. Thissocio-political shift brought with it new technolo-gies, some of which allowed for increased exploita-tion of groundwater. The middle and upper SantaCruz River basin experienced increasing use ofgroundwater, mostly due to agricultural and urbanexpansion in Tucson and Ambos Nogales (i.e. thetown of Nogales on both sides of the border).Ground water increasingly be came the principalsource of water to supply Tucson, Nogales, Arizona,and Nogales, Sonora (Logan 2002).
During the early 20th century, Mexico and theUSA negotiated a series of agreements related tothe border. Transboundary water resources werethe subject of the 1944 treaty titled ‘Utilization ofWaters of the Colorado and Tijuana Rivers and ofthe Rio Grande,’ which renamed the InternationalBoundary Commission, created in 1889, as theInternational Boundary and Water Commission
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Fig. 1. Location of Arizona (USA)−Sonora (Mexico) transboundary aquifers
Scott et al.: Assessment of Santa Cruz transboundary aquifier
(IBWC; in Spanish, the Comisión Internacional deLími tes y Aguas, CILA), which is composed of bothUSA and Mexican Sections. The treaty emphasizedshared surface waters and left groundwater essen-tially unaddressed. Subsequently, both sections ofthe IBWC passed a resolution (Minute 242) in 1973in an effort to resolve transboundary groundwaterissues. However, although Minute 242 establishesbi national data sharing for groundwater data, itdoes not carry the same weight as the 1944 treaty(Mumme 2000), which results in binational ground-water issues being addressed on a case-by-casebasis. While agreements such as Minute 227 and276 specifically addressed sanitation and wastewatertreatment issues for the Nogales Wash drainage andAmbos Nogales area, no Minute has specificallyaddressed the main stem of the Santa Cruz River.Studies that address individual shared aquiferswere developed in response to local needs. In particular, the Mesilla and Hueco Bolson aquifersunder lying El Paso, Texas, and adjoining southeast-ern New Mexico with Ciudad Juárez, Chihuahua inMexico have re ceived considerable attention, par-ticularly relating to water quality and quantity chal-lenges in this region of rapid growth and scarcewater. Both Mesilla and Hueco Bolson aquifers areincluded in the TAAP.
Following this introduction and brief review of his-torical water use in the Santa Cruz River Valley, wepresent data and methods for our assessment of cur-rent and future demand for water resulting from pro-jected population growth. We apply climate modelsimulations for precipitation under multiple emis-sions scenarios to the water balance and examine theimplications for future water demands. Followingthis, we compare water management regimes onboth sides of the border and review instances of ef -fective cooperation as well as challenges. This sec-tion also presents salient information on the TAAP.The con cluding section addresses future binationalaquifer assessment priorities in light of the findingsof this paper.
2. DATA AND METHODS
The Santa Cruz River originates in the San RafaelValley of southeastern Arizona, flows south intoMexico, and then turns to the north, entering Arizonaabout 10 km east of the urban area of AmbosNogales. The Nogales and Tucson portions of theSanta Cruz River basin have a combined drainagearea of approximately 11 780 km2 (Cevera Gomez1997, Erwin 2007, ADWR 2008), as delimited by the
Santa Cruz (1940 km2) and Tucson(9840 km2) Active Management Areas(AMAs). AMAs are the principal watermanagement units used in Arizona,de fined by the 1980 Ground waterManagement Act. The Santa CruzRiver and the San Pedro River to itseast (which originates in Mexico) areboth tributaries of the Gila River,which drains much of southern Arizona and parts of western NewMexico, and in turn is a tributary ofthe Colorado River. The ColoradoRiver Basin covers 647 000 km2 andflows through 7 USA and 2 Mexicanstates before flowing into the Sea ofCortez.
Fig. 2 shows the Santa Cruzaquifer delineated by Quaternaryand Tertiary alluvial and sedimen-tary rock units on both sides of theborder (Castro-Escárrega et al. 2000,Richard et al. 2000, Drewes et al.2002). On the USA side of the border,the 3 main sedimentary/ alluvial unitsare the Nogales Formation, the Older
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Fig. 2. Likely extent of Santa Cruz aquifer
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Alluvium, and the Younger Alluvium (Erwin 2007,Nelson 2007). These units are also found in theMexican portion of the aquifer as well as in the USAin the San Rafael Valley, which is located in theuppermost portion of the basin. Most wells arelocated in the Younger Alluvium, as hydraulic con-ductivities are typically greater than 30 m d−1
(Towne & Stephenson 2003, Tapia-Padilla 2005).However, storage in Younger Alluvium aquiferstends to be extremely limited, to the degree that theCity of Nogales, Arizona regularly varies pumpingrates from well to well in order to maintain adequatewater levels. Hydraulic conductivities of the OlderAlluvium tend to decrease from north to south andrange from 1 to 10 m d−1 and that of the NogalesFormation is typically less than 0.3 m d−1 except infractured zones where it may be higher. The thick-ness of the Nogales Formation in some areasexceeds 1000 m. Thickness of the Older Alluvium
ranges from a few meters to greater than 300 mwhile that of the Younger Alluvium is typically lessthan 50 m with widths highly variable and rangingfrom less than 100 m to about 5000 m.
Key elements of the water budget, in millions ofcubic meters (MCM) and thousands of acre-feet(kAF), in the USA portion of the Santa Cruz basin areshown in Fig. 3. Evapotranspiration (ET) from naturalvegetation is the largest component, with ground -water extraction for human uses split 51 and 49%,respectively, between agriculture and municipal/industrial uses. Water availability linked to climaticfactors is highly variable (Figs. 4 & 5). This variability,coupled with the locally limited extent and high per-meability of the aquifer, result in significant intra-and inter-annual fluctuation in water levels (Shamiret al. 2007), raising the need for careful management,not just of the aggregate water budget, but moreimportantly, of the spatial and temporal distribution
of pumping and recharge.The water budget in the Sonoran
portion of the Santa Cruz does notaccount for riparian ET, a deficiencythat the planned assessment processaims to address. The human usecomponents are presented in Fig. 6.Note particularly the preponderanceof urban/ industrial use compared toagriculture, indicating the relativelylimited ability to meet future demandfor urban growth through realloca-tion. As we discuss below, trans-boundary wastewater flows repre-sent a resource that Sonora will needto address directly. These flows arenot accounted for in the outflow com-ponent in Fig. 6 but are conveyed toArizona through a sewer line to theNogales International WastewaterTreatment Plant (NIWTP; see furtherdetails below).
Table 1 synthesizes current esti-mates of the water balances for theUSA (Santa Cruz AMA) and Mexi-can portions of the Santa Cruzaquifer. Both demonstrate positivebalances, i.e. withdrawals are cur-rently lower than recharge.
Based on these data, we considerclimate and human-demand impactson projected future water budgets.Methods are described in each sub-section below.
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Fig. 3. USA Santa Cruz water use, 2006. kAF: 1000 acre feet; MCM: million cubic meters; ET: evapotranspiration. Source: www.azwater.gov/DWR/
Water Management/ Content/AMAs/Assessment/Assessment4.htm
Fig. 4. US Santa Cruz water availability, 2006. kAF: 1000 acre feet; MCM: million cubic meters. Source: www.azwater. gov/Azdwr/WaterManagement/
Assessmenta/
Scott et al.: Assessment of Santa Cruz transboundary aquifier
2.1. Climate change and variability
Climate change can have marked impacts ongroundwater (Loaiciga 2003, Earman & Dettinger2011). In this region, variable aquifer recharge repre-sents a major source of water supply risk for depen-dent populations on both sides of the border. The cri-teria we applied in reviewing multiple global climatemodels were guided by consideration of model skillin simulating dominant regional climate (Brekke etal. 2008). For the region comprising the Santa Cruzbinational aquifer, the North American Monsoon(NAM) is the major source of annual precipitation(Lin et al. 2008), which averages 407 mm annually inAmbos Nogales. The NAM is considered to representthe most important source of water for recharge. Weselected the Hadley Centre Coupled Model Version3 (HadCM3) as the best for simulating precipitationfor the USA−Mexico border study area based on a personal communication with M. Montero (Instituto
Mexicano de Tecnología del Agua,February 23, 2009) who com-pared historical records with simulated precipitation for 1961to 1990. HadCM3 had the highestcorrelation coefficient (0.917) withthe lowest root mean square devi-ation (0.550 mm d−1) among all 15general circulation models evalu-ated). Fig. 7 presents Had CM3simulated precipitation during2000–2099 over the study area,extracted for the grid cell (32° N,
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High range / Good year (90th percentile)
Major TributaryRecharge 82%
(142.9 kAF, 176)
Underflow from Mexico 1%
(1.7 kAF, 2.1 MCM)
Incidental Recharge 2%(3.9 kAF, 4.8 MCM)
Mountain & Minor Tributary Recharge 7%(11.4 kAF, 14.0 MCM)
Effluent Recharge 8%(13.4 kAF, 16.5 MCM)
Low range / Poor year (10th percentile)
Major TributaryRecharge 57%
(39.6 kAF, 48.8 MCM)
Underflow from Mexico 1%
(0.44 kAF, 0.54 MCM)
Incidental Recharge 6%(3.9 kAF, 4.8 MCM)
Mountain & Minor Tributary Recharge 17%(11.8 kAF, 14.5 MCM)
Effluent Recharge 19%(13.2 kAF, 16.3 MCM)
Fig. 5. Variability in USA Santa Cruz water availability. kAF: 1000 acre feet; MCM: million cubic meters
Agriculture25% (6.1 kAF,
7.4 MCM)
Urban/Industrial70% (17.2 kAF,
20.9 MCM)
Outflow5% (1.4 kAF,
1.7 MCM)
Fig. 6. Mexican Santa Cruz water use. kAF: 1000 acre feet; MCM: million cubic meters
USA Mexico (MCM yr−1) (kAF yr−1) (MCM yr−1) (kAF yr−1)
Table 1. Santa Cruz aquifer estimated water balances for USA and Mexican por-tions. kAF: 1000 acre feet; MCM: million cubic meters; ET: evapotranspiration;
(–): no data
Clim Res 51: 159–170, 2012
110° W) centered on the Santa Cruz aquifer as 2-degree resolution data from the World ClimateResearch Programme’s Coupled Model Intercompar-ison Project using Domin guez et al. (2010). The pre-cipitation time-series covers the period of populationprojections for the Santa Cruz aquifer as detailedbelow.
Of the 3 Intergovernmental Panel on ClimateChange (IPCC 2007) 4th Assessment Report (AR4)scenarios used, A1B is considered conservative withlower carbon emissions than A2 and less optimistic(more carbon) than B1. However, since the AR4 waspublished, actual emissions for recent years haveexceeded levels for all these scenarios as well as the
most pessimistic (highest carbon)A1FI levels (Raupach et al. 2007).Fig. 7 indicates that precipitationwill change over the 21st centuryby −21 to +20% de pending onthe emissions scenario. A1B precip-itation is the least variable with anincreasing trend over the century;B1 and A2 are highly variable, withB1 simulating a decreasing trend.All scenarios exhibited in creas -ing temperatures (Scott 2011, Mag-aña et al. 2012), which combinedwith projected prolonged heatwaves (Diffenbaugh et al. 2005),are expected to raise de mand forgroundwater while diminishing re -charge.
2.2. Urban growth and increasing water demand
Human withdrawals have important impacts on thewater balances. Recent population data indicatesthat the border between the USA and Mexico isexperiencing rapid economic and population growth,de spite the 2008 to 2009 financial turmoil and realestate downturn (Norman et al. 2010). In this regionof limited surface water supplies, the water demandsof growing populations and increasing economicactivity are met using groundwater.
Arizona is one of the fastest growing states in theUSA, with population growth taking place mostly intowns and cities (Colby et al. 2007). In tandem with
the growth trends on the USA side ofthe border, Mexican border states andcities continue to ex pe ri ence growth inpopulation and econo mic activity thatoutpaces the national average. Bordersister cities have populations that aresignificantly greater on the Mexicanside than on the USA side (Varady &Morehouse 2004), e.g. Fig. 8. In SantaCruz County, Arizona, (where the SantaCruz aquifer is located), the populationis currently growing at 1.3% yr−1 and isprojected to continue growing into thenext century (Arizona Department ofCommerce [ADC] 2006). In the munici-palities of Nogales and Santa Cruz,Sonora, the population is larger and isgrowing more rapidly at 1.6% yr−1
(INEGI 2005) than in the Arizona por-
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Fig. 7. Projected precipitation (using HadCM3) for 32° N, 110° W, under differentcarbon emission scenarios. Percentage values in graph: change in annual precip-itation over the 21st century (2000 to 2099) as percent of annual mean for the first
decade (2000 to 2009)
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2061 Son. population = 346 800(61% increase over 2009)
Fig. 8. Projected population growth in Sonora (Nogales and Santa Cruzmunicipal district) and Arizona (Nogales and Santa Cruz County) portionsof the Santa Cruz aquifer. Solid line: projection; dotted: trend to stablepopulation. Gray: Sonora; black: Arizona. Source — Arizona Departmentof Commerce (ADC 2006); Sonora: Instituto Nacional de Estadística, Geo-
grafía e Informática (INEGI 2005)
Scott et al.: Assessment of Santa Cruz transboundary aquifier
tion; both municipalities rely on the Santa Cruzaquifer. The Sonora population is projected to stabi-lize by about 2060, earlier than in Arizona, as shownin Fig. 8. As the demographic models used for theseprojections differ, any multi-decadal projection issubject to considerable uncertainty.
For the Mexican side of the border, the municipio(roughly equivalent to a USA county) is the unit forpopulation analysis. The Mexican projections takeinto account regional immigration and emigrationfrom the municipio, as well as net internationalmigration (Partida Bush 2008). The Arizona projec-tion used the State of Arizona Composite EstimatesMethod. Age and sex are disaggregated at thecounty level, but international out-migration is notfactored against in-migration (ADC 2007). For bothsides of the border, data from each nation’s 2000 census is used as baseline and compared to 2005population estimates.
Rapid population growth that occurs primarily inurban centers has 2 principal implications for ground-water resources. First, ‘human consumption’ of water(consumo humano) has priority over other uses inMexican federal legislation, and as a result installedpumping capacity for urban water supply is expectedto continue to grow. Second, Nogales, Sonora, cur-rently has a relatively low per capita water supplyof 180 liters per capita per day (lpcd). This figure includes residents who receive piped water supply(Organismo Operador Municipal de Agua Potable,Alcan ta rillado y Saneamiento [OOMAPAS] & Alli -ance to Save Energy [ASE] 2008). In addition, there isa large and growing population living in informal settlements (colonias marginales) without access tohousehold-level water supply (Wilder et al. 2011),whose water consumption is less than half of 180 lpcd.Plans to extend supply to these under-served com -munities will undoubtedly continue to exert pressureon Santa Cruz aquifer resources. OOMAPAS is the municipal water utility in Nogales, Sonora. It uses 3pumping areas to supply the urban population. TheMascareñas well field, which is located on the SantaCruz River, currently produces 213 l s−1. The NogalesWash, a tributary of the Santa Cruz River, is also usedfor municipal supply, and produces 169 l s−1. TheLos Alisos Basin, located to the south of the city andpart of the Rio Magdalena basin, supplies 339 l s−1
(OOMAPAS & ASE 2008).Nevertheless, for groundwater resources that are
already under pressure (see previous section), thesegrowth projections have serious implications. Percapita water use in Arizona is diminishing but is notexpected to drop by a half to offset the approximate
doubling of population before it stabilizes. In theabsence of new sources of supply, this implies reallo-cation of water currently used in agriculture andreuse of effluent for human purposes (landscapingirrigation and/or, potentially, indirect potable usethrough recharge of effluent and ground-waterpumping for potable supplies). The water supply sit-uation in Sonora is more challenging given the largerincreases in absolute population numbers and thefact that close to half the water demand is alreadybeing met through inter-basin transfer from LosAlisos basin to the south.
2.3. Relative climate and human drivers of groundwater balance
To illustrate the implications for the Santa Cruzaquifer of climate variability coupled with growingwater demand, we undertook a sequential climateand human-demand forcing of the combined USA−Mexico aquifer balance, following Loaiciga (2003).Current recharge was held constant and the partialeffect of urban water demand was examined byincreasing this in proportion to population growth.Next, the partial effects of climate change scenarios(A1B, B1, and A2) were calculated by varying re -charge with projected precipitation (see justificationof this assumption below). Finally, the combinedeffects of population growth and precipitation-basedrecharge were calculated.
Our assumptions are as follows. The average ratioof annual recharge to precipitation over wet and dryyears for which data were available (1998 to 2002;from Erwin 2007) was applied to the modeled 2000 to2099 precipitation series, in order to estimate annualrecharge series. Even when precipitation is highlyseasonal, all other factors being constant, the result-ing annualized natural recharge (water flowingacross the water table from non-anthropogenicsources) approaches a constant percentage of annualprecipitation. It is therefore appropriate to estimatenatural recharge in this manner (Pool & Dickinson2006). Nelson (2007), and Flint & Flint (2007) esti-mated recharge as a percentage of precipitation.Nelson (2007) used constant values for both moun-tain front and tributary recharge in the Santa Cruz,and Flint & Flint (2007) used a recharge rate of 15%of mountain precipitation for their work on basinsthroughout the southwestern USA. Our analyses areintended to illustrate trends (Fig. 9) resulting fromvariability in aquifer balance recharge and with-drawal components.
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3. RESULTS AND DISCUSSION
Table 2 shows the simulation results of the waterbalance components that are influenced by projectedfuture changes, i.e. recharge varies with climatechange and urban/industrial demand for watervaries with population growth. As expected, the A1B
results with increasing precipitationtrends indicate the highest recharge,while those for B1 indicate the lowestrecharge. Although the results sug-gest that the aquifer will remain inpositive balance under projected fu -ture conditions (and would thus likelycontribute to surface baseflow), therelative USA and Mexico shares ofthe balance are not addressed.
For further analysis, we omit the A2scenario, given that its precipitationprojections to the year 2099 (+2.5%compared to the 2000 to 2009 meanprecipitation) are between those ofB1 (−20.9%) and A1B (+19.9%), asseen in Fig. 7. Fig. 9 shows the resultsof the projected urban and industrialwater supply sufficiency for Nogales,Sonora.
The implications of the scenarioanalysis displayed in Fig. 9 are 4-fold.(1) Water service expansion in So no rawill require additional investment ininfrastructure to meet the rising shareof demand that will need to comefrom non-aquifer sources and conser-vation. After 2060, population is pro-jected to stabilize in Nogales, Sonora,and the aquifer is expected to entera phase of heightened demand man-agement, conservation, and waterreuse. While the simulation for the
A1B scenario indicates that the period from 2060through to the end of the century may be character-ized by increased rainfall (and thus re charge), thesimulation for the B1 scenario indicates that the sec-ond part of the century will result in greater unmetdemand. (2) Water to supply growing urban de mandswill need to be accessed from agriculture, waste-
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Fig. 9. Scenario assessment (using HadCM3) of growth in water demand andrecharge under climate variability, Santa Cruz Aquifer, Sonora, Mexico. Precip-itation-based recharge: 2000–2009 mean = 100%; share of demand: demand
that must be met through non-aquifer sources or conservation
Component Partial effect Combined population growth Population Recharge & recharge growth A1B A2 B1 A1B A2 B1
Table 2. Simulated sequential climate and human-demand forcing of Santa Cruz water balance, 2099 (106 m3 yr–1).ET: evapotranspiration
Scott et al.: Assessment of Santa Cruz transboundary aquifier
water, and out-of-basin sources, such as currentlypumped from Los Alisos basin. We estimate that after2060 half or more of total demand will need to be metfrom non-aquifer sources. (3) Climate change, assimulated by the HadCM3 A1B and B1 scenarios, dri-ves significant variability in water availability. Thegenerally shallow Mexican portion of the aquifermeans that recharge variability translates into watersupply variability on an annual basis. (4) In theabsence of water importation and with agriculturalwater use and rights increasingly appropriated forurban demand, it is inevitable that wastewater willbe seen as a resource to meet urban demand. Thishas major implications for binational negotiationsover effluent treated and released on the USA sideby the Nogales International Wastewater TreatmentPlant (NIWTP, operated by the USA Section of theIBWC) and for the downstream riparian corridor inthe USA that depends on effluent flows.
What is particularly relevant is the transboundarycontrast in water balances and demand. Thus, weturn next to an ongoing set of binational assessmentactivities that seek to address these contrasts in thecontext of limited data on which to base managementdecision making.
4. AQUIFER ASSESSMENT PROCESS
The analyses reported in this paper were con-ducted as part of the USA−Mexico TAAP, with rele-vant background as follows. Growing de mand forgroundwater in the USA−Mexico border re gion hasraised the profile of transboundary groundwaterresources in the USA. As a result, the USA− MexicoTransboundary Aquifer Assessment Act (‘the Act’)gained approval of the 109th Congress of the USAand was signed into law by the President on Decem-ber 22, 2006. The purpose of the TAAP is to providestate, national, and local officials with informationto address pressing water resource challenges in theUSA−Mexico border region. As finalized, the Actauthorizes the Secretary of the Interior, through theUSA Geological Survey (USGS), to collaborate withthe states of Arizona, New Mexico and Texas, as wellas Mexico and others, to conduct hy dro logic charac-terization, mapping and assessments of prioritytransboundary aquifers. The TAAP has an appropria-tions ceiling of US$50 million over 10 yr, althoughfunds are appropriated annually and ex pected to besplit equally among the states of Arizona, New Mex-ico, and Texas. California determined it would notparticipate. Each of the 3 participating state’s funds
are to be split equally between the USGS and thatstate’s Water Resources Research Institute (WRRI),located at one of the state’s universities as stipulatedby the Water Resources Research Act. The WRRIshave active, ongoing partnerships with the USGS.This mechanism for TAAP implementation is aunique model of federal agency−university centerpartnership for program implementation.
For Arizona, the 2 priority transboundary aquifersestablished in the legislation are the Santa CruzRiver Valley and San Pedro aquifers. An ongoing andintensive process of water resources management,coordinated by the Upper San Pedro Partnership inthe USA, has generated scientific investigation andpolicy assessment for the San Pedro River andaquifer, even though the binational aquifer itself hasnot been the focus. As a result, for this paper, we de -termined that focusing on the Santa Cruz Aquiferwould contribute to broader understanding.
The process of binational consultation and prioritysetting for the Santa Cruz aquifer began in 2007.Reflecting the TAAP’s design of federal−state col -laboration on the USA side, the Water ResourcesResearch Center (WRRC, the state’s designatedwater re sources institute located at the University ofArizona) and USGS’s Tucson office form the defacto TAAP− Arizona executive committee. A techni-cal committee was identified, largely through self-selection of interested stakeholders, to review exist-ing studies, both hydrogeological and institutionalin nature, leading ultimately to the prioritization ofaquifer assessment activities, research, and addi-tional exchange of information among stakeholders.The timing and amount of TAAP financial resourcesalso played a determining role in the pace that couldbe maintained, i.e. initial funding during fiscal years2007 to 2008 and 2008 to 2009 were sufficient tocover a TAAP coordinator and modest support forfield trips (one each on the USA and Mexican sidesduring mid-2008). Increased funding in 2009 to 2010allowed additional work to be undertaken, under theprovision that USA funds could be used for TAAPactivities in Mexico on the condition that Mexico pro-vided a 50% cost match in cash or kind. However,lack of USA federal budget support in 2010 to 2011meant that activities are be ing continued using exist-ing funding. Future funding will be critical to the outcome of the program.
As a result of the consultation and technical ex -changes outlined here, the technical committeereport includes the following, reflecting priorities onboth sides of the border: ‘The consultation process forthe aquifer has identified water availability (implying
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concomitant water quality) to meet growing de -mands as the overarching goal. The implications ofclimate change and variability are also importantconcerns’. We summarize the priorities identified as:(1) water availability for urban areas particularly tomeet peak summer demands, (2) linked surface andgroundwater systems that are a primary mechanismof aquifer recharge, (3) water quality of aquifers usedfor potable supply, and (4) comparative assessment ofinstitutions for the management of water resources.
Institutional asymmetries in governmental function(Milman & Scott 2010, Megdal & Scott 2011) requirecareful consideration when developing a bi nationalplan of study. For example, under Mexican waterlaw, the federal government is responsible for sur-face and groundwater rights (Mumme 2000, Scott etal. 2010), whereas individual states in the USA over-see groundwater use and much surface water use. Bytreaty, the IBWC (USA section) and CILA (Mexicansection) serve as the binational coordination mecha-nism for surface water, including in some instancescross-border wastewater treatment. In the USA,however, the IBWC has very limited responsibility forgroundwater and the TAAP authorizing legislationestablished the USGS as the lead federal agency.
In this context, the binational assessment processin the Arizona−Sonora region has identified prioritytechnical and management studies that require addi-tional investment. Currently, TAAP 2009 to 2010funds are being used to generate new hydro-climato-logical and institutional data.
5. CONCLUSIONS
The HadCM3 model results assessed herein indi-cate that precipitation projections in the study areadepend on the emissions scenario used, and may beexpected to change by −21% to 20% over the 21stcentury. Precipitation projections for the A1B emis-sions scenario show increasing trends, while projec-tions for the B1 and A2 scenarios are highly variableand the B1 projection indicates a decreasing precipi-tation trend. All projections exhibited increasingtemperatures. These scenarios raise the specter oflonger, more se vere droughts (Magaña et al. 2012).Combined with the implications of increased tropicalstorms and intense precipitation (Karl et al. 2009), thelikelihood of future flooding is significant.
The availability of groundwater in the USA−Mexico border region is compounded by growth indemand driven by increasing population and cli-mate-induced variability in recharge. The Mexican
portion of the Santa Cruz aquifer already supportsmuch less agriculture and much more urban use thandoes the USA portion of the aquifer. As a result, thereis little remaining agricultural water to transfer tourban use for Nogales, Sonora. If growth continues asprojected, additional water will be required from out-side the Santa Cruz aquifer and basin; this is alreadyunderway from the Alisos Basin to the south ofNogales, Sonora. The USA side has slower popula-tion growth, but is projected to continue expandingfor a longer time. In the short- and medium-term,transferring water out of agriculture in the USA mayoffset growth-driven water demand. Also, on theUSA side of the border, the Arizona Department ofWater Re sources (ADWR) has prioritized maintainingsafe-yield conditions (where pumping equals esti-mated groundwater recharge) in the Santa CruzActive Management Area (SCAMA). Currently, theCity of Nogales in conjunction with the SCAMA andSanta Cruz Grandwater Users Advisory Committeeis in consultation with the US Bureau of Reclama -tion to explore potential enhancement of the munici -pal water supply through augmentation using watersources.
Wastewater is a critical resource to support overallwater demand; currently, it is largely generated inMexico and meets riparian evaporative demand onthe USA side. However, as the resource value ofwastewater increases, Mexico may opt to capture,treat, and reuse wastewater to meet its own growingwater demand. This is currently underway with theconstruction of the Los Alisos wastewater treatmentplant that takes a share of the wastewater fromNogales, Sonora, back over the watershed divide intothe Alisos basin where it will be treated and released.A significant share of the wastewater generated inMexico will continue to flow to the USA, requiringongoing technical and organizational collaboration.If Mexico opts to retain control over effluent cur-rently treated at the NIWTP, the most significantrepercussion may occur along the riparian area andassociated cottonwood/willow gallery forest directlydownstream from the treatment plant. There has notbeen extensive analysis of potential scenarios if thissource of surface flow is reduced.
In the context of differing institutional arrange-ments for water management in the USA andMexico, a binational consultation process to estab-lish priority assessment activities for the sharedSanta Cruz aquifer as part of the TAAP hasdemonstrated the value of technical exchanges, on -going consultation, and consensus building throughjoint field visits.
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The role of binational institutions, in this caseIBWC/ CILA and the Arizona−Mexico Commission, iscritical for the exchange of information; however,collaborative mechanisms initiated and pursuedmore locally, in this case between Arizona andSonora, offer greater possibilities for sustainedexchange of information. The successful exampleof the Santa Cruz priority-setting process byTAAP−Arizona is based on binational collaborationincorporating federal, state, and local agencies, uni-versities, and civil society. The role played by univer-sity and agency researchers in devising collaborativemechanisms complements regulatory functions andcivil society’s advocacy positions.
Acknowledgements. The binational aquifer priority-settingprocess analyzed in this paper was supported by the USA− Mexico TAAP. The authors would like to acknowledge theactive participation of the following agencies: ADWR, OOMA-PAS (Nogales Sonora), Friends of the Santa Cruz River, CILA,IBWC, US Bureau of Reclamation, and City of Nogales, Arizona. Additional support for coordination was provided bythe University of Arizona’s Water Sustainability Program.Groundwater and population scenario analysis was sup-ported by the National Science Foundation (Grant DEB-1010495) and by the Inter-American Institute for GlobalChange Research (Project SGP-HD #005, which is supportedby National Science Foundation Grant GEO-0642841). Fi -nally, the IPCC AR4 data were provided by SAHRA (Sustain-ability of semi-arid Hydrology and Riparian Areas) at the Uni-versity of Arizona (F. Dominguez, J. Cañon and J. Valdes).The original dataset comes from the World Climate ResearchProgramme’s (WCRP’s) Coupled Model Intercomparison Project phase 3 (CMIP3) multi-model dataset.
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Editorial responsibility: Bryson Bates, Wembley, Australia
Submitted: December 30, 2010; Accepted: September 30, 2011Proofs received from author(s): February 10, 2012
