The study highlights distinct sensitivities each study area is likely to experience under different climactic scenarios.

The five geographic study areas, each with varied population sizes and water supply and demand characteristics, represent a variety of water management areas and can be studied to determine disparate impacts of climactic variability. The central question is: What effect would one, five and 10-year drought periods have on water supply and demand of each area?

Researchers gathered climactic, demographic, water supply and demand data for each study area. One, five and 10-year precipitation scenarios that encompass maximum dry conditions in the Southwest at each time scale were assembled. The researchers also gathered demographic data for the year 2025, with the intention of examining possible effects of population growth on water supply and demand, under average and extreme precipitation scenarios. Water supply and demand data also was collected.

The renewable supplies figuring into the calculation included surface water, natural groundwater recharge, CAP deliveries and effluent supplies. The study areas each use different proportions of these supplies and analysis shows reliance on a greater or lesser proportion of each supply is an important variable contributing to an area's overall sensitivity to climactic changes.

The collected data was used to construct various scenarios for the five study areas. The scenarios show changes in supply, changes in demand, and changes in groundwater balance due to certain key variables.

The study also calculates groundwater impacts for the five study areas under the various 2025 scenarios. Groundwater overdraft reflects amount of groundwater not replaced by recharge under normal circumstances and is therefore unsustainably mined.

In calculating scenarios showing change in groundwater balance, variations in natural recharge due to increased or decreased precipitation and added effects of changes in human demands on water balance were explored. Also, these scenarios represent combinations of supply and net consumption changes shown in supply and demand scenarios.

By showing importance of climactic variability in evaluating water supply and demand, the study is of critical importance when reviewing public policy and statutes such as the Groundwater Management Act (GMA). The GMA established a regulatory framework to manage supply and demand in certain populated areas of the state to safeguard groundwater reserves. The management plans developed in response to GMA, however, do not give due consideration to effects of climactic variability when developing water budgets that reflect supply and demand.

For example, the study shows the effect a drought similar to one that occurred in the 1950s would have on Phoenix and Tucson water supplies in the year 2025, the year the GMA targets for the Phoenix and Tucson AMAs to achieve safe yield. Safe yield is long-term balance between the annual amount of groundwater withdrawn and amount of natural and artificial recharge.

Even without drought occurring, the Phoenix AMA is projecting a 24% groundwater overdraft and the Tucson AMA a 15% overdraft. With a 10-year drought, the Phoenix overdraft could reach 30% and Tucson 15%.

The study highlights differences in relative sensitivity of the study areas to climate impacts, an important factor to be taken into account when formulating and implementing water management policies. For example, existence of well-known and valued riparian areas in the Sierra Vista subwatershed and the Santa Cruz AMA suggests, under severe extended drought conditions, difficult choices may have to be made regarding allocation of scarce resources.

Titled "Assessing the Sensitivity of the Southwest's Urban Water Sector to Climactic Variability," the study was conducted by Rebecca H. Carter, Petra Tschakert and Barbara Morehouse from the University of Arizona's Climate Assessment Project for the Southwest. The agency is a National Oceanographic and Atmospheric Agency-funded program housed within the University's Institute for the Study of Planet Earth. For additional information, contact Barbara Morehouse, CLIMAS program manager, University of Arizona, 520-622-9018, or e-mail: morehoub@u.arizona.edu.

Reprinted with permission from Arizona Water Resource.