The urban water sensitivity analysis examines the impacts of climatic variability and population growth on water supply and demand in five urban areas of Arizona, and highlights the distinct sensitivities that each of these areas is likely to experience under one-, five-, and ten-year severe drought conditions.
The purpose of the urban water sensitivity analysis (conducted in 1998–1999) was to illustrate to water managers and policy makers the importance of factoring climatic variability into assessments and projections of water supply and demand variability. Our ultimate objective is to encourage the use of climate information, including historical and paleo data as well as forecasts, in water resource planning and decision making at time scales ranging from seasonal to annual, interannual, and multidecadal.
In order to examine the impacts of climatic variability on a variety of water management areas, we chose five sites that vary in population size, type of water demand, and available water resources. The study sites included the Phoenix AMA, the Tucson AMA, the Santa Cruz AMA, and the Benson and Sierra Vista subwatersheds of the San Pedro River.
The first step of urban water sensitivity analysis was to calculate the impacts of the most severe one-, five-, and ten-year droughts on record if they were to recur today, and at projected 2025 population levels, in order to better understand how increasing population pressure affects the urban water sector. We combined the water supply and demand budgets provided for each region by the Arizona Department of Water Resources with historical records from the Western Regional Climate Center.
The analysis also considered the possible impacts of changes to water supply and demand: supply was altered in some scenarios by eliminating Central Arizona Project (CAP) water, which is drawn from the Colorado River and subject to basin-wide drought; and demand was decreased by eliminating agricultural water use in other scenarios.
This component of the project has now been completed. Our calculations show what percentage of an area's water supply would have to be mined from groundwater reserves under the different climatic, water use, and population growth scenarios, and allowed us to evaluate the relative sustainability of water use in each situation.
For example, we found that under conditions equal to the worst one-year drought in the Phoenix AMA’s history, combined with increased water demand projected to meet the needs of the population anticipated by 2025, demand would outstrip the area's sustainable supply by 79 percent. A five-year drought could force the overdraft of 47 percent of the AMA's water supply, or 5.5 million acre-feet; and the ten-year drought scenario projected an overdraft of 9.2 million acre-feet would be necessary to meet the 39 percent of the water demand that could not be met by renewable sources.
The results of this study are included in CLIMAS Report #CL1-00: Assessing the Sensitivity of the Southwest's Urban Water Sector to Climate Variability: Case Studies in Arizona.