Climate, groundwater, and population growth

In many urban areas in the Southwest, rapid population growth plays a key role in determining the ability to strike a balance between groundwater withdrawals and aquifer recharge. Arizona is the second-fastest growing state in the nation, with a 6.4 percent growth rate between April 2000 and July 2002. New Mexico ranked 23rd for the same time period, but individual counties still had growth rates of more than 20 percent, as Table 1 illustrates (1).

The combination of climatic variability and population growth is likely to have more notable consequences on Arizona and New Mexico than would either factor alone. To see how these factors might interact, the CLIMAS Project (home of END InSight) examined the water budgets (supply and demand figures) of five Arizona cities under drought conditions of varying degrees to identify how much groundwater mining (see sidebar below) would be required to meet the needs of expanding populations. The full report is available at http:// www.ispe.arizona.edu/climas/pubs/ CL1-00.html (4).

The analysis started with the 1995 water budgets for five Arizona locations: the Phoenix Active Management Area (AMA), the Tucson AMA, the Santa Cruz AMA (where the border city of Nogales is located), the Sierra Vista subwatershed, and the subwatershed of Benson. We extrapolated how projected population increases might increase water demand, including plans for supplementing water supplies.

We then identified the most severe one-, five-, and ten-year droughts for the climate divisions in which the study locations were located. We used winter (November through April) rainfall totals for four of the five areas because hydrologists believe that in theseareas, most summer precipitation is lost to evaporation and runoff before it has a chance to infiltrate the aquifer. For the Nogales AMA, however, we used year-round rainfall, because the aquifer in this area is shallower, more sensitive, and responds directly to summer, as well as winter, rainfall.

In addition to varying the population figures and precipitation totals, we also examined the impacts of different water supply and use scenarios relevant to each location. For example, in each location we calculated the impacts of eliminating agriculture, because a reduction in agricultural activity is often cited as a means by which urban areas might cope with long-term drought.

We also calculated the impacts of Phoenix and Tucson having their Central Arizona Project (CAP) allocations of Colorado River water cut. This could occur in the event of a severe drought throughout the western United States, because these Arizona cities have junior rights to river water (see the CLIMAS report on Arizona water law and policy in response to climate at http://www.ispe.arizona. edu/climas/pubs/CL2-01.html [5] for a more detailed explanation).

In the Santa Cruz AMA, we included a scenario in which Mexico retains a greater share of the effluent it currently sends to the Nogales International Wastewater Treatment Plant. The treated water is discharged into the Santa Cruz River in Arizona and supports a rich riparian habitat, in addition to forming an important component of the area’s annual aquifer recharge.

In Sierra Vista and Benson, additional population projections, which some say more accurately reflects the area’s growth potential, were included to assess the impacts of population pressure beyond the official projections.

For all scenarios, we calculated a the percentage of the total water demand that could not be met by renewable supplies (including natural and inten-tional groundwater recharge). This bottom line amount would have to be compensated for through groundwater mining. Table 2 shows a sample of the results.

The results of these scenarios show that climatic variability could necessitate a great deal more groundwater pumping than is currently necessary, particularly when combined with population growth and changes in water supply and land use.

References

(1) U.S. Census Bureau. USA statistics in brief: state populations. Accessed at http://quickfacts.census.gov/qfd/ on Jan. 17, 2003.

(2) FedStats. Arizona MapStats. Accessed at http://www.fedstats.gov/qf/states/04000.html on Jan. 17, 2003.

(3) FedStats. New Mexico MapStats. Growth, continued Is Population Growth Really the Issue? Accessed at http://www.fedstats.gov/qf/states/35000.html on Jan. 17, 2003.

(4) Cater, R. H., P. Tschakert, and B. J. Morehouse. 2000. Assessing the sensitivity of the Southwest's urban water sector to climate variability: case studies in Arizona. CLIMAS Report CL1-00. Institute for the Study of Planet Earth, University of Arizona, Tucson. Available at http://www.climate.arizona.edu/library/publications/240.

(5) Cater, R. H., and B. J. Morehouse. 2001. An examination of Arizona water law and policy from the perspective of climate impacts. CLIMAS Report CL2-01. Institute for the Study of Planet Earth, University of Arizona, Tucson. Available at http://www.climate.arizona.edu/library/publications/221.

(6) Glennon, R. 2002. Water Follies: Groundwater Pumping and the Fate of America’s Fresh Waters. Island Press, Washington. p. 28.