This paper is digested from An Assessment of the Water Resources in the San Pedro River and Santa Cruz River Basins, Arizona and Sonora, 1997, by Diana Liverman, Robert Merideth, Andrew Holdsworth, Luis Cervera, and Francisco Lara, A Report to the Commission on Environmental Cooperation, Montreal, Quebec (published by the Latin American Area Center and Udall Center for Studies in Public Policy, The University of Arizona), 75p.

The southwestern United States and northwestern Mexico are vulnerable to climate variability and change because of their hot and dry climates, rapid population growth and economic development, and inflexible water management institutions.

Several projects at The University of Arizona use the transboundary basins of the San Pedro and Santa Cruz rivers to explore vulnerability through integrated assessments of hydrology, socioeconomic conditions, and institutional structures.

This paper focuses on the results of preliminary socioeconomic and institutional assessments in the upper portions of both basins (i.e., the areas adjacent to the U.S.-Mexico border).

Physical Geography, Climate, and Hydrology

The San Pedro and Santa Cruz rivers cross the U.S.-Mexico border between the states of Arizona and Sonora (Figure 1). The San Pedro River has its origins about 48 kilometers (km) south of the U.S.-Mexico border and flows about 240 km to the Gila River (Arizona Department of Water Resources, 1991; Melendez Torre, 1993). The surface water catchment for the San Pedro River drains an area of 11,620 square kilometers (km²), with about 16 percent, or 1,800 km², being in Mexico. The largest urban areas in the watershed are Sierra Vista, Arizona, and Cananea, Sonora, with 1990 populations of about 40,000 and 25,000 persons, respectively.

About 130 km to the west of the San Pedro valley lies the Santa Cruz River watershed. The river channel begins in Arizona, heads southward into Mexico for about 55 km, turning west and then north, crossing back into the United States about 10 km east of the twinned border communities of Nogales, Sonora, and Nogales, Arizona, or Ambos Nogales. The channel then passes through Santa Cruz County, past Tucson in Pima County, through Pinal County and ends up as a tributary to the Gila River at Maricopa County, for a total length of about 360 km. The Gila River eventually joins the Colorado River north of Yuma, Arizona.

In accordance with the Arizona Groundwater Management Act of 1980, the Santa Cruz River basin is divided into three units: the Santa Cruz, Tucson, and Pinal Active Management Areas (AMAs). The largest urban areas within the basin are Nogales, with a combined 1990 population of about 127,500 (108,000 in Sonora and 19,500 in Arizona), and the Tucson metropolitan area with a 1990 population of about 750,000.

The climate of the region is arid to semi-arid and characterized by warm summers and moderate winters. Climate varies largely due to elevation changes with cooler temperatures and higher rainfall at higher elevations. The precipitation for the area varies considerably from month-to-month and especially year-to-year. It is distributed bimodally over the year-occurring during the winter months as a result of large cyclonic storms arising in the north Pacific Ocean, and during the summer monsoonal season (July through September) as a result of convective thunderstorms which form as moisture is drawn into region from the Gulf of Mexico and Gulf of California (Sellers and Hill, 1974).

About 50 to 60 percent of the basins' rainfall occurs during the monsoon season, typically, the driest months in the basin are April, May, and June. Given this annual pattern for precipitation in the basins, surface water runoff into and within the stream channels is seasonally variable with minimum runoff and flow occurring during the driest months. As temperatures increase towards June evaporation also increases with associated increases in demand for irrigation and municipal uses. Because of the tendency for more rainfall to occur in the mountains and foothills than in the central valley floor, and because of the nature of convective thunderstorms (i.e., heavy downpours in some areas, no rainfall in others), there is also high spatial variation of precipitation across the two watersheds during any given year.

On average, the precipitation within the San Pedro River basin ranges from a little more than 290 millimeters (mm) per year in the valley to more than 760 mm per year in the Huachuca Mountains. Average annual precipitation in the Santa Cruz River basin ranges from 280 to 400 mm in the valley of the Santa Cruz and Tucson AMAs, to about 710 mm in the mountainous areas. Mean high temperatures are similar in all three AMAs with mean high temperatures ranging from 67 degrees F. in January to 103 degrees F. in July.

The surface water in the San Pedro River basin consists of runoff from precipitation events and melting snow in the mountain ranges, and baseflow from groundwater from the floodplain aquifer flowing into the river channel (Putnam et al., 1988). While most of the river flow is intermittent--a characteristic of many desert streams--several perennial reaches of the river occur where water losses are less than the supply, or where underlying bedrock forces the groundwater to the surface.

In 1990, for example, there were 16 perennial reaches along the river and its tributaries, ranging from less than one mile to about 45 km in length for a total length of about 150 km. (ADWR, 1991). Prior to the beginning of major irrigation diversions, there were more and longer such perennial flows.

Stream flows are highest (based on records for average monthly stream flows) between the months of July and September--during the summer monsoon season--and during the winter months, although less prominently. The lowest flows occur during the late autumn and late spring.

Most of the Santa Cruz River is intermittent or ephemeral. However, the river is mostly perennial from its point of origin in the San Rafael Valley in southern Arizona and along the approximately 56-km loop southward into Mexico and back into the United States near Nogales. The average annual flow of surface water from Mexico into the U.S. portion of the basin is 25.2 Mm³ per year, based on a 65-year gaging station record from 1913 to 1993 (ADWR, 1996).

Within the Santa Cruz Active Management Area (AMA), the subbasin located just north of the border, there are three perennial stretches on the Santa Cruz River ( 22 km from Nogales to Tubac) and its tributaries: Sonoita Creek (11 km) and the Nogales Wash (6 km). This stretch of river is fed by treated effluent from the NIWWTP, which during a twelve-month period for 1992/93 discharge 17.2 Mm^3. Average annual stream flow from the Santa Cruz AMA into the Tucson AMA to the north is about 26.2 Mm³ per year (based on a 44-year record from 1940-93).

Environmental History

The valley of the San Pedro River, and that of the Gila River, contain early evidence of prehistoric human transformation of nature, including mammoth hunting sites dating from 11,000 AD, and the massive Hohokham irrigation canals which diverted water from the San Pedro and Gila rivers (Sheridan, 1995). In 1540, a Spanish expedition led by Coronado crossed from Mexico into what is now the state of Arizona along the San Pedro River, but it was a number of years before the Spanish settled the region. In the U.S. frontier period, the San Pedro River was used for beaver trapping and, from 1881, for irrigated agriculture using the St. David canal (ADWR, 1991). The opening of the Southern Pacific railway station at Benson, located near the middle of the basin, encouraged economic development from about 1800.

The major transformation of the San Pedro River valley occurred when gold was discovered at Tombstone in 1878 and a copper smelter was opened. The boom associated with mining resulted in the deforestation of the hills surrounding the valley, which, in turn, may have affected the water balance of the basin by increasing runoff and decreasing infiltration into the soil. Substantial entrenchment of the San Pedro river channel occurred following large floods in the late 1800s. Although this also was a period of extreme rainfall, changes in land use--such as overgrazing, fire suppression, and the extensive timber harvest for the mines--are believed to have contributed to the deepening of the channel (Hereford, 1993). Cattle ranching expanded significantly with the formation of the Benson Cattle Company in 1908.

On the Mexican side of the San Pedro River basin, rapid growth of the town of Cananea began around 1900 when the Cananea Consolidated Copper Company was formed. By 1910 the population of Cananea was slightly more than 8,900 persons, growing to nearly 13,000 by 1930 and reaching a peak of about 19,700 in 1960 (Lorey, 1990).

There is also considerable evidence of human activity along the Santa Cruz between 500 and 1400 AD, including check dams used to irrigate 1500 acres near San Xavier. From the later 1600s, the Spanish priest, Padre Eusebio Kino established missions along the Santa Cruz, and, in 1752, a military Presidio was established at Tubac. In 1775, the Presidio of Tucson was established, and the Spanish began to construct aqueduct and canal systems to irrigate wheat and other crops on the Santa Cruz flood plain in the perennial reaches of the river to the south of Sentinel Peak.

In 1854, southern Arizona became part of the United States through the Gadsen Purchase. The Santa Cruz River, which at the time was flowing year-round in most sections, was dammed near Tucson in 1856 to create Silver Lake and to power a flour mill (Moorehouse, 1990). But subsequent attempts at controlling the river (such as a diversion ditch dug by Tucson property developer, Sam Hughes), combined with overgrazing and severe floods, led to the incision of the river and entrenchment below the flood plain by 1890. The first major groundwater extraction was initiated in 1890 when Tucson Water, the water company for the city of Tucson, began to pump subsurface water. By 1900, the Tucson had a population of about 7,500 persons; by 1990 the population had almost reached æ of a million.

Nearly one-hundred kilometers to the south of Tucson, the twinned border cities of Ambos Nogales began to grow with the establishment of the railway from Hermosillo to Tuscon in 1882. In 1900, Nogales, Sonora, had a population of some 2,700, growing to about 14,000 by 1930 and 52,100 by 1970 (Lorey, 1990). Nogales, Arizona, grew more slowly, from a size of about 1,800 in 1900 to nearly 9,000 by 1970.

Current Water Use

San Pedro River Basin

As in many semi-arid regions, agriculture is by far the major water user in southern Arizona. Based on 1990 data for the San Pedro River basin, 30 percent of total water depletion for the basin is used in agriculture, 44 percent to maintain natural ecosystems, 17 percent in industry, and 9 percent for municipalities and domestic use, with 93 percent of the demand met by groundwater. The water demand (depletion) in 1990 was 171.2 Mm³ (ADWR, 1991). Of this consumptive use, about 56 percent is used by humans, while 44 percent is consumed by natural processes (evapotranspiration from channels and riparian vegetation). Some 54 percent of the water used for human activity is for agriculture (irrigation and stock ponds), about 30 percent for mining and other industry, and 16 percent in private domestic and municipal water systems.

At the turn of the century, irrigated area in the San Pedro River basin totaled about 1,000 ha, but has since increased dramatically, driven by improved pump technology, to more than 8100 ha. in 1990. Stock ponds comprise 10 percent of total agricultural water use serving a portion of Cochise County's 100,000 livestock. Most of the supply is groundwater, and irrigation systems are mainly of low efficiency.

The three major crops in the watershed in 1992 included cotton, pecans, and alfalfa, as well as a mixture of double crops such as small grains, sorghum, and pinto beans together with pasture and alfalfa. Cochise County shows an overall decline in crop acreage in recent years, with about 46 percent of the 1992 acreage in cotton, 20 percent in pecans, and 4 percent in small grains. Total 1990 agricultural water consumption in the basin was 52.1 Mm³ of water of which 14.8 Mm³ is from surface water.

In the Mexico portion of the San Pedro 5100 acres of land have been developed for irrigation of cattle pastures using surface water from the San Pedro diverted from the river with a ditch and small reservoirs. Only 3460 acres is actually irrigated at 1.45 acre feet per acre for a total water use of 5000 acre feet. Stockponds and small reservoirs use an additional 1000 acre feet.

The Arizona Groundwater Code defines industrial use as "non-irrigation use of water not supplied by a city, town or private water company." This definition includes such activities as golf courses as well as mines, sand and gravel, and thermoelectric plants. Total industrial water demand in the San Pedro River basin is 17 percent of the total use, while that in the Santa Cruz River Basin is five percent.

There is only one major industrial water user in the U.S. portion of the San Pedro River basin, namely the Magma Copper Company in the Redington subbasin, employing a workforce of about 3,800 people and depleting 23.8 Mm³, or 14 percent of the total water (and more than 80 percent of the industrial water) consumed in the basin in 1990. Smaller industrial users include 5 sand and gravel operations, a chemical manufacturer and an aquaculture venture. In the San Pedro River basin, expansion of Magma's copper mine production, related to copper prices, could more than double industrial water use to 55.4 Mm³ per year, and could exacerbate a cone of groundwater depression which may be developing in the Redington subbasin.

In the Mexican portion of the San Pedro the major industrial water user is the Cananea copper mine and smelter which obtains water from both the San Pedro and Rio Sonora watersheds. The San Pedro use is estimated at 3000 acre feet per year. A small lime plant in Naco, Sonora, uses 100 acre feet. Recent plans to expand copper production at Cananea, and increase pumping from wells, has created concern among farmers on the ejidos in the Mexican part of the Santa Cruz, and for all water users in the Arizona portion.

Currently there are forty-four private water companies and municipal systems using water in the San Pedro River basin--about three percent of all water use-diverting 13.4 Mm³ and depleting 10.7 Mm³ per year. The town of Sierra Vista is the major user at 8.1 Mm³ per year with an average per capita demand of 164 gallons per day. About two percent of total human water use is by independent domestic users in the San Pedro River basin.

The most important influence on municipal water use in the basin is likely to be growth in and around the city of Sierra Vista. Between 1980 and 1990 the town grew 44 percent, and is projected to grow another 40 percent to more than 50,000 people by 2005. These developments will have more serious and immediate impact on river flows if they are pumping water from the very permeable floodplain. There have been proposals to build more than 12,500 residences using water from the floodplain aquifer.

The city of Sierra Vista has looked at several alternatives to groundwater pumping including extending the Central Arizona Project at a cost of 70-90 million dollars (Arizona Daily Star, May 23, 1994), and constructing an effluent recharge project at a cost of 5.3 million dollars (Arizona Daily Star, November 30, 1995).

Another major issue in the basin is the likely impact of the Federal Base Realignment and Closure Act of 1993. Fort Huachuca, located adjacent to Sierra Vista, is designated to receive reassigned units from closed bases elsewhere.--with a projected increase in water use of 20 percent and with off-base family housing, presumably in Sierra Vista. There were also plans to move the Defense Language Institute with 5,000 jobs to the Sierra Vista area, requiring an additional 3.1 Mm³ per year of water, but these plans have been shelved for the moment. In 1994, the Southwest Center for Biological Diversity brought a lawsuit against Fort Huachuca and the Department of Defense for failing to examine the cumulative environmental impacts of Fort Huachuca's growth.

A recent unpublished study by The Nature Consevancy examined the attitudes of 400 residents of Cochise County who were interviewed about the San Pedro River. Residents saw the primary value of the river as a source of water, and somewhat or very important for farming and ranching, recreational activities, wildlife and plant habitat and scenic beauty. Only about half of the residents thought the river was somewhat or seriously threatened by groundwater pumping, and about 3 out of 4 residents thought that Sierra Vista's effluent should be used to recharge groundwater. Opinions were split about the need to purchase farms and water rights to protect the river, and about mandatory water use restrictions, but most residents favored voluntary water conservation, said that they conserved in their home and thought it would save the river.

Urban use in the Mexican part of the San Pedro is associated with the towns of Naco and Cananea, Sonora. 30,000 people in the municipio of Cananea obtain most of their water from the Rio Sonora basin, but there are a few wells in the San Pedro floodplain which produce 2000 acre feet of water per year. In Naco, 5000 people use 300 acre feet, and domestic use in rural areas is about 300 acre feet.

Although the flow and evaporation of surface water is an important component of water supplies and was often considered as "wasted" water from a human use perspective the ecosystem and landscape use of water is increasingly recognized as an important and valued use sector. In allocating water to maintain river flows and riparian habitat water resource managers are supporting the use of water resources in fishing, birdwatching, hunting, recreation, and landscape appreciation as well as values such as biodiversity, natural cleansing, and climatic moderation.

These "in stream" uses have recently been recognized as a "beneficial" use of water under Arizona law. Agencies, environmental groups and individuals can now petition for a water permit and allocation to maintain these uses. Glennon and Maddock (1994) suggest 90 percent of rivers and riparian vegetation have been degraded and destroyed by diversions and groundwater pumping in Arizona. The natural or ecosystems water use in the San Pedro has been estimated at about 40 percent of overall water demand, but only at one percent in the Santa Cruz where there are hardly any perennial flows.

The Santa Cruz and the San Pedro include some of the most valued and important ecosystems in Southern Arizona. For example, there are nationally significant reserves, which attract thousands of bird watchers on the San Pedro, its Ramsey Canyon tributary, and near Patagonia on the Sonoita Creek tributary of the Santa Cruz. Ramsey Canyon and the Patagonia riparian area receive thousands of visitors annually bringing important tourist revenues. It has been estimated that birdwatchers bring in $3 million a year to the local economy.

The nature and importance of ecosystem use has been a high priority in the San Pedro, especially with the establishment of the San Pedro Riparian National Conservation Area in 1986. The Nature Conservancy declared the San Pedro as one of the world's "Last Great Places." The U.S. Bureau of Land Management has been mandated to manage this Conservation Area "in a manner that conserves, protects, and enhances the riparian, wildlife, archaeological, paleontological, scientific, cultural, educational and recreation resources" (Bureau of Land Management, 1988, p. 7). Agricultural land was retired from use and irrigation to allow more water for the maintenance of riparian vegetation. Willow and cottonwood require specific moisture regimes and perennial streamflow to provide shallow groundwater for roots and spring runoff for germination and establishment. Riparian consumptive use is higher than that of crops and has a less well defined economic value, but the non-market value of flood and erosion control, aquifer recharge, recreation, fish, wildlife and biodiversity protection have been seen as beneficial. However, data concerning water use and recharge in riparian areas of the San Pedro is inadequate and controversial.

Several applications have been filed to protect in stream flow for riparian, fish and wildlife habitat, including one to the The Nature Conservancy's nationally known preserve along Ramsey Creek, for Aravaipa Creek (also owned by The Nature Conservancy), and by the Bureau of Land Management and US Forest Service on the San Pedro. The total requested for ecosystem protection is more than 37,000 acre feet (45.5 Mm³) per year for the whole of the San Pedro and includes requests for constant annual minimum flow as well a monthly or seasonal streamflow variations. However the lack of data and regulations pertaining to instream uses, as well as conflicts with urban, Indian, and agricultural use, has created uncertainty about these water rights.

If groundwater declines because of changes in either demand or supply there could be serious impacts on ecosystems in the San Pedro River. Stromberg et al. (1996) suggest that a 0.3 meter drop in the floodplain aquifer could reduce key species such as rushes, and a 1 meter drop would eliminate them and reduce willow area by 51 percent. Mesquite and tamarisk could expand creating a "desertification" of the riparian vegetation.

Important legal questions have arisen because the Riparian Area is now managed by the Federal government and a legal precedent suggests that Federal water rights may supersede others. In order to claim these reserved rights an accurate assessment of the water needs to maintain the riparian ecosystems must be made.

The conflict between in stream uses and other uses is becoming intense in the San Pedro basin. In 1994, the San Pedro Riparian Technical Review committee was formed to examine and attempt to resolve scientific disagreements about issues such as links between groundwater pumping and river flows, and estimated requirements for maintenance of in-stream rights. In March 1994, a water issues group was created to try and resolve conflicts and develop a management plan. But tensions increased when the Southwest Center for Biological Diversity filed a lawsuit in July 1994 against the Federal government claiming that the Environmental Impact Assessment for Fort Huachuca was inadequate, however the court ruled that the Center has not filed its suit until after the allowed deadline. While the court dismissed the case, it did discuss at length the merits of the Center's arguments in the brief filed with the court (Judge Alfredo C. Marquez, 1995). (The Center subsequently filed a submission to the Commission on Environmental Cooperation in November 1996 under the provisions of the environmental side agreement of the North American Free Trade Agreement. However, the submission was withdrawn in June 1997; see details on the submission at the Commission's web site at www.cec.org). In 1995, the San Pedro Technical Review committee recommended that one solution to conflicts within the basin would be the establishment of an Active Management Area, but this met with intense local opposition. Additional questions and conflicts have recently been introduced with the designation of 4 new endangered species with habitat in the San Pedro basin.

Santa Cruz Active Management Area

The Santa Cruz AMA has a total water demand of 46.3 Mm³ per year for a population of almost 30,000 people. About one-third is used by agriculture, 18 percent by municipalities, four percent to industry, and most of the remainder is lost to evapotranspiration, especially from riparian systems. This AMA currently meets a safe yield criteria (no significant groundwater overdraft) and thus the AMA goal is to maintain this balance.

In 1991, a Mexican government report estimated total water use in the Mexican portion of the Santa Cruz at 25.1 Mm³ per year of which 13.7 Mm³ per year are used by the city of Nogales, 11.2 Mm³ per year are used in agriculture, and 0.2 Mm³ per year for cattle.

Sources: ADWR, 1996a; 1996b; 1996c.

Most of the agricultural water use in the Santa Cruz AMA occurs along the Santa Cruz River valley and uses fairly shallow groundwater. There are about 80 holders of grandfathered irrigation rights, about 75 percent of whom are active water users. Farm sizes range from less than 2 ha to more than 400 ha. The largest irrigator is Rio Rico Properties and uses more than half of the water used for agriculture. Although the valley once had more than 1,600 ha in cotton and other staple crops, now only about 800 ha are irrigated, mainly for growing forage (oats, barley, rye, alfalfa, pasture, sorghum). There are a few vegetable and grape producers. The agricultural census data for Santa Cruz County shows the predominance of alfalfa (to feed the county's 25,000 cattle) and grapes, and shows a decline in acreage over time. The consumptive use (water demand) of crops in the Santa Cruz AMA is slightly less (10 to 50 percent depending on the crop) than in the northern parts of the basin because higher elevations mean cooler temperatures, less evapotranspiration, and higher rainfall. The total agricultural water use averages about 15.3 Mm³ per year (with irrigation, mainly by flooding, at about 50 percent efficiency). Most of the water is supplied by wells, with a small amount of surface and effluent use.

Irrigators in the Santa Cruz AMA can accumulate flexible credits if they do not use their allotted groundwater. And, as in other regions where allocations were estimated during a period of high use and expanded acreage, only about half the allocated water is being used and credits have been accumulating quite rapidly to total more than five times the annual average needs. Future agricultural water use is likely to decline, especially as irrigation rights are retired in favor of municipal development.

The Mexican portion of the Santa Cruz River basin includes the municipios of Nogales and Santa Cruz, Sonora. Of the 1,132 ha of agricultural land in the Mexican section of the Santa Cruz River basin, a 1995 survey reported 892 ha as irrigated and 240 ha as rainfed. In 1991, 15 wells pumped a volume of 0.7 Mm³ for agricultural use in the municipio of Nogales.

Irrigation, potable water and cattle troughs within the Santa Cruz river basin.

*67 ejidal and 8 private wells **5 irrigation wells incapacitated due to lack of pumping equipment

Source: SAGAR, Centro de Apoyo al Desarrollo Rural 02-Nogales.

There are seven ejidos in the river basin irrigated by 86 wells along the river and providing water to their cattle with 60 cattle troughs. Although most of the land in these ejidos is used for grazing, each ejido has a small irrigated area, with the largest area, of 370 ha. in the ejido of Santa Cruz .

The 1990 Mexican agricultural census provides some insight into the types of crops grown on these ejidos. In the census year about 630 ha of land was irrigated in the Santa Cruz basin. Assuming an average consumptive use of crops this area would require 5.3 Mm³ of water with a pumping volume of 70.4 liters per second.

Source: SAGAR, Delegacion Estatal en Sonora. Distrito de Desarrollo Rural No. 140-Magdalena. Centro de Apoyo al Desarrollo Rural 01-Nogales.

More detailed estimates of water demand can be made separately for the winter and spring crop seasons based on agricultural plans for 1996. In summer, maize, beans, sorghum and lettuce occupy much of the irrigated area of 840 ha with a total estimated water demand of 6.9 Mm³ per year and 218.3 liters per second. In winter, the cultivated area is 340 ha., mostly in rye and barley, with a total water demand estimated at 3.0 Mm³ per year and 96.1 liters per second .

Two ejidos--Santa Cruz and Miguel Hidalgo--have perennial apple orchards totaling 91 ha irrigated by a groundwater pump. Apples have a consumptive use of about 120 cm, and the area therefore requires about 1.1 Mm³ per year of water.

Summing these estimates for summer, winter and perennial crops, the total agricultural water demand for 1996 is about 11 Mm³, equivalent to a pumping rate of 348 liters per second.

Estimate of irrigation volume for the agricultural cycle Spring-Summer 1996. Santa Cruz River basin.

Estimate of irrigation volume for the Fall-Winter agricultural cycle 1996. Santa Cruz River basin.

Industry uses about four percent of the water in the Santa Cruz AMA. Most of the current industrial water use in the Santa Cruz AMA is for irrigating four golf courses, with the remainder used by sand-and-gravel operations. Use peaked in 1989, which was a very dry year. However, water use on these golf courses has decreased by 40 percent between 1989 and 1994. There are limited plans to irrigate these golf courses using effluent although new incentives (credit in water allotments and per capita water use objectives) may convert new or existing facilities to recycled water if regional wastewater treatment expands. Other uses include sand and gravel associated with construction and some non-municipal industries. The projections are that industrial water use will increase slowly, and that some turfgrass use will shift to effluent. The Arizona Groundwater Management Code requires industrial water users to use the latest available technology to conserve as much water as possible with a reasonable economic return.

There are three large municipal water providers in the Santa Cruz AMA using about 6.2 Mm³ of water a year: the City of Nogales (which uses 30 percent of total), Rio Rico Utilities, and Valle Verde Water Company. Per capita water use has averaged about 190 gallons per day, with more than 200 gallons per person per day in Nogales. The City of Nogales has a population about 20,000, but water use is probably influenced by an effective population of 30,000 workers, shoppers, and transportation employees that pass through the city daily and use water on their way to and from Nogales, Sonora. Rio Rico and Valle Verde service a total of about 10,000 people.

Most of the municipal demand in the Mexican portion of the Santa Cruz River valley is associated with the city of Nogales, Sonora, and its rapid growth. The city is constantly faced with increasing overall water supplies as well as improving and expanding the spatial distribution and access. Prior to 1949, the public obtained water from several wells, including one community well with pumping equipment. In 1949, the current water system which has a filtration system, was installed with a flow of 105 liters per second. In 1989 the volume was increased to 450 liters per second, and again in 1991 to 512 liters per second.

However, by 1995 urban water demands had increased to 782.4 liters per second, although the available supply from the two major pumping plants was only 550 liters per second. This represented a deficit of almost 30 percent of total public demand. The southern pumping plant has a capacity of 300 liters per second and is actually brought by a 22 km pipe from Los Alisos outside the watershed. The eastern filtration plant has a capacity of 250 liters per second with water pumped from Mascarenas, Santa Barbara, and Paredes, all within the water shed.

There are some additional 22 wells within the city pumping about 125 liter per second. Assuming annual pumping to be constant, the eastern plant on the banks of the Santa Cruz extracts about 7.9 Mm³ per year from the river. The southern, Los Alisos plant supplies 9.5 Mm³ per year. City wells extract about 3.9 Mm³ per year. Thus total municipal demand is 21.3 Mm³ per year, of which about 45 percent is extracted from the Santa Cruz river basin.

There are five major types of users in Nogales--domestic, municipal, commercial, services and industrial--with domestic consumption the largest at 75 percent of total use. Currently drinking water is supplied to 85 percent of the city, but only 39 percent have water 24 hours a day. Most of the drinking water infrastructure is over 30 years old and desperately needs improvements. Only 75 percent of the city has adequate sanitation. Wastewater is treated at the binational plant "Dos Picos" in Nogales, Arizona. In 1995, 434 liters per second of Sonoran waste water was treated at this binational plant.

In the Santa Cruz AMA petitions have been approved for 11km of Sonoita Creek for 2648 acre feet (3.3 Mm³), and Lake Patagonia for 11420 acre feet (14.0 Mm³) capacity. Riparian zones are also maintained by effluent flows on 22 km of the river between Nogales and Tubac, The future of the latter zone depends on recharge and outflow from the International Wastewater Treatment Plant in Nogales and, in particular on the continued flow of effluent from the Mexican side of the border (to which the US has no legal right). Estimates for these in stream uses and allocations are highly uncertain because data is lacking on phreatophyte use and needs on the Santa Cruz.

Future Water Use

San Pedro River Basin

In the San Pedro River valley, population growth in the Sierra Vista area is likely to cause an increase in both municipal and domestic (individual wells) use. A key study on the future of the San Pedro River is a report which examines different scenarios for population growth, climate variation, and irrigation (Lord et al., 1991). The report suggests that changes in agricultural acreage and consumptive use are the most significant for river flows and ecosystem services.

Santa Cruz Active Management Area

Projections of future water use have been made using several scenarios for the Santa Cruz AMA. One scenario is usually the "Base Case" scenario which assumes the status quo in terms of growth in demand any drastic conservation measures or regulations. Another scenario, termed the "Best Case" or planned scenario, assumes much more aggressive conservation.

The future trends for the Santa Cruz AMA are associated with a significant growth in population which triples the municipal use in the "Base Case" scenario from 7.7 Mm³ per year in 1990 to 23.4 Mm³ per year in 2025, with a corresponding increase in total demand of 42 percent. This scenario foresees a slight decline in agricultural water use and a tripling in industrial use from 1.8 Mm³ to 5.6 Mm³ per year by 2025. The Best Case, the more conservation-oriented scenario, projects a total water use of about 52.1 Mm³ per year in 2025 with only a doubling of municipal and industrial use.

Scenarios prepared by the Arizona Department of Water Resources for the Santa Cruz AMA suggest that the water balance of this region of the Santa Cruz River could range from a 53.0 Mm³ per year surplus (if effluent use increases and agricultural use declines), to a 19.1 Mm³ deficit per year (if population and industry grows rapidly) by 2025. These estimates depend on assumptions about population growth, per capita consumption, effluent use, and the amount of wastewater flow into the Santa Cruz River from the Nogales, Sonora, area.

The future of agricultural water use in both the Santa Cruz and San Pedro basins will depend on a variety of factors, especially the potential for shifts in crop mix associated with price and subsidy variations, and the possibility of selling or legislating water rights transfers to other uses, especially to municipal and in stream ecosystem needs. For example, as agriculture shifts from basic grains such as barley and wheat to forage crops such as alfalfa and vegetables, the use of water increases because the new crops have a higher consumptive use (demand) for water.

The ability of Nogales, Arizona, to meet the assured water supply rule and continue economic and demographic expansion will depend on whether it can obtain, use, or sell CAP water, and whether it can get credit for using effluent from the Nogales International Water and Waste Treatment Plant for recharge. Currently, Nogales, Arizona, has a CAP water allocation but there is no infrastructure to bring it to the border area and the cost of the infrastructure is prohibitively expensive. However, if the SCAMA CAP allotments are sold to other users, the money could be used to buy and retire irrigation rights.

The future of water use in Nogales, Arizona, is heavily dependent on water management in Nogales, Sonora, especially Mexican groundwater pumping, quality management and effluent. Projections of NAFTA-related growth could increase demand on both sides of the border. There is no binational treaty concerning groundwater use and pumping on the Santa Cruz. Proposals to more than double pumpage on the Mexican portion of the Santa Cruz River valley which could make regions downstream in Arizona more vulnerable to drought, limit their development and degrade riparian areas. There is a need for data on the non-resident population demand and water use levels. There is concern about pollution from Nogales, Sonora, in the form of fecal and coliform pollution from Nogales Wash. There is also some contamination from agriculture (nitrates) and mining (heavy metals and sulfur) which could affect municipal supply and costs of treatment.

Nogales faces great challenges in supplying water in the immediate future, especially in improving efficiency and equity. The poor neighborhoods in Nogales are particularly lacking in access to reliable water and sanitation. A 1992 survey revealed that 16.5 percent of the population of Nogales (18,644 people) are not connected to the municipal system. Population growth is also a serious problem. The 1995 census estimates the population of Nogales, Sonora, at 133,359, more than 25 percent more than in 1990. Water plans had estimated a higher population. For example, the first stage of the Nogales Integrated Project for water management presented to the Border Environmental Cooperation Commission estimated the 1995 population at 215,300 living in 32,134 residences and demanding 19.618 Mm3 of water each year.

Future projections estimate that by 2015 there will be 385,478 people living in more than 50,000 houses in Nogales. Sonora. Assuming that per capita demand grows from 222 liters per day to 324 liters per day (85 gallons per day) this means a total municipal water demand of 1450 liters per second (45.7 Mm³ per year). Plans suggest that this demand would be covered by increasing the southern pumping plant to 550 liters per second and the eastern to 600 liters per day. This means an increased extraction from the Santa Cruz of 18.926 Mm3.

Assuming an annual growth rate of 4.7 percent, a minimum per capita water demand of 250 liters per day, and infrastructural efficiencies of 70 percent, future annual water demand for Nogales could reach 36.7 Mm³ per year by 2015. This is less than the Integrated Project estimates of 45.7 Mm³ per year because of different assumptions about per capita use and population projections but still poses a serious urban water problem and implies increased extraction of water in the Mexican portion of the Santa Cruz.

Drinking Water Use for Nogales, Sonora, for the year 1990.

Source: Subgerencia Estatal in Sonora, Comision Nacional del Agua, 1990.

Estimates of future demands for the city of Nogales, Sonora.

Social Vulnerability

A preliminary evaluation of census indicators which might show geographical variations and social vulnerability to water shortages in southern Arizona suggests that there are dramatic social differences which could make some people more vulnerable than others to drought or price increases. The graphs plot census data for tracts in the watersheds. The population of the Graham county portion of the San Pedro is very small and produces anomalous results. The environmental justice literature suggests that minority populations may be more vulnerable to environmental risks.

Social conditions in Southern Arizona

Source: Southeast Arizona Governments Organization (SEAGO). 1995. Overall Economic Development Plan. SEAGO, Bisbee, Arizona.

In the census tracts covering the San Pedro River and Santa Cruz River basins, the majority of the population is white and about 10 percent is Hispanic. Central Tucson and Nogales have higher levels of Hispanic populations (more than 20 percent). The San Pedro River basin has about a five percent African-American population (mostly due to personnel assigned to Fort Huachuca). The elderly and children are also often considered as more vulnerable to water shortages and quality problems. Between five percent and 15 percent of the population is under 9 years old (highest in the Nogales area), and a relatively high proportion of the population is over 65, reflecting southern Arizona's reputation as a retirement haven. Green Valley, marketed as an exclusive retirement community, has more than half its population being 65 or older.

The U.S.-Mexico border is one of the poorest regions of the state of Arizona, with the average per capita incomes in the border counties being only 75 percent of the state average, and house prices at 65 percent of the median for the state.

Poorer people may be less able to afford water or to purchase bottled water and home purifiers. Average incomes in the two watersheds are highest in northern Tucson and lowest in central Tucson, Santa Cruz County, and Cochise County. The income distribution is particularly noticeable in Santa Cruz County and central Tucson where 20 percent of the population earn under $10,000 a year and 55 percent earn below $25,000. In north Tucson, in contrast, 35 percent of the population earn above $50,000. This is paralleled by average housing prices which are highest in north Tucson and lowest in Central Tucson, Santa Cruz and the San Pedro.

Most people are on public or private water systems. More people rely on wells in southeast Tucson (30 percent), Santa Cruz and in the San Pedro. Most houses are served by public sanitation systems with a higher reliance on septic systems in Marana (60 percent), southeast and southwest Tucson (50 percent) and the San Pedro (30 percent).

Vulnerability on the Mexican side of the border is significant with many households without reliable access to safe drinking water and with industrial development increasing water demand and sometimes threatening water quality.

Institutional and Legal Framework

The institutional and legal framework for water resources in Arizona has a lengthy and controversial history and the present structure has significant implications for vulnerability and response to climate variability and change.

The overarching legal doctrine is that of prior appropriation, usually described simply as "first in time, first in right" with a "use it or lose it" basis. If you developed or diverted a water resource in Arizona prior to 1919, you have senior rights to that water so long as you use it. After 1919, a user could obtain water rights by applying for a permit for off-stream beneficial use. The State Water Code of 1919 states that "the water of all sources, flowing in streams, canyons, ravines or other natural channels, or in definite underground channels belong to the public and are subject to appropriation and beneficial use" (ADWR, 1994). Beneficial uses include domestic, municipal, irrigation, stockwatering, water power, recreation, wildlife including fish, artificial groundwater recharge, and mining . This philosophy of beneficial use and prior rights, whilst assuring that some water users have assured water in time of drought, encourages the inefficient use of water, and makes later users extremely vulnerable to climatic variability and change. For example, early irrigation diversions can obtain water in dry periods even if drinking water is scarce.

For many years it was not clear if the prior appropriation doctrine applied just to surface water or to groundwater as well. In 1953, the Arizona Supreme Court decided that the prior appropriation doctrine did not apply to groundwater, so that senior surface water rights could be negatively affected by more recent groundwater pumping with no legal recourse. As noted later, this artificial separation of surface and subsurface water rights has had enormous implications in the Santa Cruz and San Pedro watersheds (Glennon and Maddock, 1994). Drought can increase groundwater extraction's which then reduce in-instream flows.

Water rights on American Indian lands are another important issue in southern Arizona. Although for many years, Native Americans had great difficulty in gaining rights to water and were extremely drought vulnerable, there is some precedent to suggest that U.S. Federal lands, including Indian reservations, have senior water rights over others to take water for a "reserved" use. In Winters v. United States (1908)--now known as the Winters Doctrine--it was found that an Indian reservation may reserve enough water to irrigate all of the "practicably irrigable acreage" on the reservation with a priority dating from the treaty, act of Congress, or executive order that established the reservation. These "reserved rights" are based on water held, or "reserved," by the federal government. A court case involving the Gila River in 1935 recognized that Indian lands might hold prior appropriation rights, later defined as the right to use water for all practicable irrigable acreage (PIA) of land on a reservation. In 1974, the Salt River Valley Water Users Association requested the adjudication of water rights on the Gila River, and in 1981 this was consolidated into a General Adjudication of water rights for that basin.

The purpose of the Gila River Adjudication was to consolidate all diversely created and administered water rights into a single comprehensive determination of all rights. The adjudication resulted from the over-appropriation of surface water in the Gila River system, groundwater overdraft, and growing municipal water use. The Gila River Adjudication involves several thousand claimants (more than 8,000 just in the San Pedro River basin) and 12 Indian Reservations and could result almost 1,230 thousand cubic meters (Mm³) of water being assigned (ADWR, 1994). This would affect the water rights along the San Pedro River, which flows into the Gila River, as well as along the Santa Cruz River, if water transfers from the Colorado River are used in the Gila River settlement. This shift in rights and demand could reduce the vulnerability of Indian agriculture and communities to drought whilst dramatically increasing competition for water amongst non-Indian users.

The case Cappaert v.United States (1976) determined that federal lands can also have priority for water (reserved rights) on lands other than Indian reservations set aside for specific federal purposes such as National Parks and Forests. In this case, the federally reserved water rights consisted of enough water to maintain Devil's Hole National Monument in Nevada, an unusual underground pool that sustains a population of the endangered desert pup fish. The Cappaert case also established that when a dispute exists between state and federal interests, federal rights take precedence. On the other hand, the ruling implied that the federal right reserves only "that amount of water necessary to fulfill the purpose of the reservation, no more" (Glennon and Maddock, 1994). The Court also held that federal reserved rights can protect water from subsequent diversion, whether the diversion is of surface or groundwater. The Cappaert ruling is important with regard to the San Pedro River, in that the establishment of the federal San Pedro National Riparian Conservation Area may establish reserved rights, and may also be used to protect habitat of other endangered species in the two watersheds. If in-stream use has priority in the San Pedro, it creates greater drought vulnerability for other users.

Another set of important legal and institutional issues in southern Arizona is associated with the use of the Colorado River. In 1922, the Colorado River Compact divided the waters of the river at Lee's Ferry with 9,225 Mm³ (or, 7.5 million acre feet, maf) per year to the Upper Basin and 9,225 Mm³ (7.5 maf) per year to the Lower Basin. The 1928 Boulder Canyon Act specified that of the 9,225 Mm³ (7.5 maf) annually allocated to the Lower Basin, 5,412 Mm³ (4.4 maf) should go to California, 369 Mm³ (0.3 maf) to Nevada, and 3,444 Mm³ (2.8 maf) to Arizona, although at this time Arizona had little need or infrastructure to use its rights. The original compact had not taken account of Mexican needs, so, in 1945, the Mexican Water Treaty allocated 1,845 Mm³ (1.5 maf) to Mexico. California developed its use of Colorado River water much more rapidly than the other states, and has been consuming much of Arizona's unused share. Arizona then sued California and was assured its share in 1964. This lawsuit confirmed the division of the waters of the Colorado River among lower basin states, including Arizona's right to water it was not using but which was being used by California. As many have noted, the allocation of Colorado River water was made during a period of unusually high flows, and the 20,295 Mm³ (16.5 maf) per year currently allocated is more than the current average flows of 17,220Mm³ (14 maf) per year (ADWR, 1994). Recent dry periods have caused competition amongst states and users, and several researchers have suggested that the institutional arrangements for the Colorado may need restructuring to cope with climate change.

In order to transport and use its share of Colorado River water, Arizona persuaded the U. S. Congress to approve the funding of the Central Arizona Project (CAP) under the 1968 Colorado River Basin Project Act. The Central Arizona Project involved the construction of a canal to bring water to southern Arizona and was completed as far as Tucson in 1991. The water was originally targeted for municipal and industrial use, and to replace groundwater in existing irrigation districts. It is not supposed to be used for new irrigation development except on Indian lands (Glennon, 1995).

When Tucson Water, the major water provider to the City of Tucson, switched many of their customers to Central Arizona Project at the end of 1992, consumer complaints rose sharply. Although CAP water was extensively treated, CAP water had a higher level of total dissolved solids (double) and is more corrosive (lower pH and higher dissolved oxygen) than the groundwater in the Tucson basin. The change in the chemistry of the water and variations in flow dislodged material in pipes and caused chemical reactions in the system. Customers complained about the color, taste, and odor of the water, and that it corroded appliances (Woodard and Henderson, 1996).

In November 1995, Tucson voted in favor of Proposition 200, a ballot initiative that mandated that CAP water could not be delivered directly for drinking water unless it was treated to a quality equivalent to that of the high quality of Avra Valley groundwater. Since the only method to achieve this goal would be an expensive membrane or reverse osmosis process, the proposition effectively stalled municipal deliveries of CAP water and threw water planning in Tucson into chaos. The proposition also banned delivery of treated water from other areas (in an attempt to prevent delivery of water which had been treated after contamination by TCE) and required the recharge of the aquifer in central Tucson using CAP water. Unfortunately the ability to do this is limited by the large number of landfills and a hazardous waste site adjacent to the proposed recharge area. In order to try and achieve AMA goals, the current decision is to use CAP water to recharge the aquifer in the Avra valley (ADWR, 1996b).

The rejection of CAP water by Arizona voters has implications for vulnerability throughout the region in that Tucson is continuing to mine groundwater and is growing rapidly. However, if southern Arizona increases its dependency on the Colorado River it may become more sensitive to variations in the river, and as the last allocation, would be the first to lose water in time of sever drought. If Nevada and California can successfully argue that Arizona is not using CAP water beneficially, then Arizona could lose hard fought rights to the Colorado (Glennon).

Conclusions

This overview of water supply and use in the Santa Cruz and San Pedro River basins highlights a number of important research and policy issues which must be resolved if these rivers are to be managed in a sustainable bi-national context, especially in a highly varying and possibly changing climate.

For example, the basic hydrology and water balance of both river basins is not fully understood. More information is needed, for example, on the contribution of snowpack in the mountains that enclose the basins to recharge of groundwater in the regional aquifers and to streamflows. More data are needed to understand better the contributions of surface and groundwater flows from Mexico into the U.S. portions of the watersheds and a better understanding is needed of evapotranspiration rates, especially from riparian vegetation. More research is needed on the hydrological links between surface and groundwater flows (i.e., links between the streams and fluvial aquifers, and between fluvial aquifers and regional aquifers) and between different groundwater units, in particular to assess the extent to which pumping is capturing flows towards rivers in basins (such in the Sierra Vista/Ft. Huachuca area in the San Pedro River basin). Research is also needed about the existence and hydrological links to deep aquifers on both the U.S. and Mexican side of the border. These aquifers could provide important supplies during droughts if carefully managed.

Better estimates are needed of water needs and uses for riparian vegetation, habitat and other ecosystem uses, especially on a seasonal basis. This would allow better planning for in-stream use, and for management of highly-valued ecosystems during dry periods. Better information might also be important to help resolve some politicized debates about the relative importance of human and ecosystem impacts on the river flows.

There is great uncertainty (as in many regions) about the potential impacts of climate change and variation on water supply in these basins. Given that there already exists significant seasonal and year-to-year variations in precipitation in the region, improved understanding is needed both of the impacts of variations associated with El NiÒo as well as the potential impacts of global warming on river flow, soil moisture, and Colorado River allocations (as per the 1922 Colorado River Compact and its subsequent related interstate agreements). The recent drought caused serious problems in Nogales, and on the San Pedro.

Although some monitoring programs have been undertaken in recent years, water quality data continues to be inadequate, especially where the rivers cross the international boundary or in stretches of the river downstream of wastewater treatment plant effluent discharges. Given the evidence from previous monitoring efforts, there is need to maintain vigilance for potential chemical contaminants from industrial sources and biological contaminants from sewage and wastewater discharges, both in the U.S. and Mexico. Poor water quality increases drought vulnerability, especially for domestic users.

Any expansion in the world market for copper or other metals that are found in the region could mean that the water allocations held by mining interests--currently unused or underused in many cases--could be demanded by any increased mining activity, and as a result could affect other water users who have come to rely on these unclaimed mining allocations to meet their demands.

Southern Arizona and northern Mexico are among the areas with the highest population growth rates in the two respective nations. There is a need to understand better the short-term and long-term impacts of this rapid growth--primarily due to migration into the communities of Tucson, Green Valley, and Ambos Nogales in the Santa Cruz River basin and Sierra Vista and Cananea in the San Pedro River basin--on changes in water demand and patterns of water use in the basins. There is also a need to anticipate the potential changes in water use brought about by changes of the agricultural economy. Improved projections are needed on the impact of the likely retirement of agricultural water rights and their conversion for use in meeting growing urban development. The accumulation of flexible credits by agricultural users in many areas of the U.S., and specifically in the two basins studies here, suggests inefficiencies in the allocation of water between sectors. The future of these water credits is unsure, although may farmers hope to use them in future drought years.

A better understanding is needed regarding the relationships between crop types, irrigation technology, economics, and efficiency of water use, and what shifts in crop cultivation mean for water use. Unforeseen changes in agricultural, municipal, and industrial demand for water may occur due to the direct and indirect effects of implementing the North American Free Trade Agreement. If NAFTA results in increased demand for water, climate vulnerability could increase.

However, changes in water pricing and subsidies could dramatically affect the use and balance of demand for groundwater, CAP water, and treated effluent. An increase in market transfer of water from agriculture to other uses could increase water availability in the region, including during droughts, but at a price that some users could not afford.

References

Arizona Department of Water Resources (ADWR). 1991. Hydrographic Survey Report for the San Pedro River Watershed, Volune 1, General Assessment. Phoenix, AZ: ADWR.

Arizona Department of Water Resources (ADWR). 1994. Arizona Water Resources Assessment, Volume 1. Phoenix, AZ: ADWR.

Arizona Department of Water Resources (ADWR). 1996a. State of the AMA: Santa Cruz Active Management Area. Nogales, AZ: ADWR.

Arizona Department of Water Resources (ADWR). 1996b. Regional Recharge Committee Technical Report (final draft, July 19, 1996). Tucson, AZ: ADWR.

Bureau of Land Management, U.S. Department of the Interior. 1988. Assessment of Water Conditions and Management Opportunities in Support of Riparian Values: BLM San Pedro River Properties, Arizona (Project Completion Report). Phoenix, AZ: BLM-Arizona State Office.

Glennon, Robert J. 1995. "Coattails of the Past: Using and Financing the Central Arizona Project," Arizona State Law Journal, 27(2)677-756.

Glennon, Robert J., and Thomas Maddock III. 1994. "In Search of Subflow: Arizona's Futile Effort to Separate Groundwater from Surface Water," Arizona Law Review, 36(3):567-610.

Hereford, R. 1993. Entrenching and Widening of the Upper San Pedro River, Arizona. Geological Society of America Special Paper 282. Boulder, CO: GSA.

Lord, William B., and others. 1991. A Study of the Water Resources of the San Pedro Basin and Options for Efficient and Equitable Water Management Water Management. Tucson, AZ: Water Resources Research Center, The University of Arizona.

Lorey, David E. (ed.). 1990. United States-Mexico Border Statistics since 1900. Los Angeles, CA: UCLA Latin American Center.

Marquez, Judge Alfredo C. 1995. Memorandum Opinion, Southwest Center for Biological Diversity, et al., v. William J. Perry, et al., Filed August 30, 1995. Phoenix, AZ: U.S. District Court for the District of Arizona.

Melendez Torre, M. A. 1993. Propuesta de Recursos Naturales de la Sierra la Mariquita-Rio San Pedro, Municipios de Cananea, Naco, y Santa Cruz, Sonora, Mexico. Hermosillo, Sonora: Centro Ecologico de Sonora, Gobierno del Estado de Sonora.

Moorehouse, Barbara J. 1990. Landscape as Text: A Sociogeographic Study of the Santa Cruz River within the Vicinity of Tucson, Arizona. Unpublished M.S. Thesis. Tucson, AZ: Department of Geography, The University of Arizona.

Putnam, Frank, Kim Mitchell, and Greg Bushner. 1988. Water Resources of the Upper San Pedro Basin, Arizona. Phoenix, AZ: ADWR.

Sellers, W. D., and R. H. Hill (eds.). 1974. Arizona Climate, 1931-1972. Tucson, AZ: University of Arizona Press.

Sheridan, Thomas. 1995. Arizona: A History. Tucson, AZ: University of Arizona Press.

Stromberg, J., R. Tiller, and B. Richter. 1996. "Effects of Groundwater Decline on Riparian Vegetation of Semiarid Regions: The San Pedro, Arizona," Ecological Applications.

Woodard, G. C., and J. L. Henderson. 1996. When Water Quaility Divides a Community: Economic, Geographic and Political Impacts of Tucson's Use of Colorado River Water. Tucson, AZ: Water Resources Research Center, The University of Arizona.

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