In 1951, following a decade of "unprecedented development" in the city of Phoenix, a report addressing future water production and transmission facilities projected that the city's population would reach 420,000 by the year 2000. Almost fifty years later, the U.S. Census for 2000 established Phoenix's population at 1.3 million - a threefold increase over that seemingly robust 1951 projection.
Uncertainty regarding the rate of population growth is but one of many challenges inherent in assessing future municipal water needs. An effective water resource plan must consider not only the traditional "consensus opinion" of what is likely to happen over time, but a range of alternative scenarios which could occur. This 2005 update to the city's Water Resources plan attempts to do just that, and in so doing, better prepares the city for a variety of potential outcomes. A strategic planning approach which considers a variety of future conditions becomes particularly important in an era of over-allocated watersheds, concerns over long-term drought, more stringent environmental regulations, increasing water quality issues, litigation and tough competition for supplies.
The Phoenix Water Resources plan is typically updated every five years to reflect updated water demand projections and water supply conditions. Prior plans have concluded that Phoenix's historic and anticipated water supplies were sufficient to meet the long term needs of current customers and a significant level of growth. Though the city maintains a sound and well-diversified portfolio of water supplies, the region's recent experience with relatively severe and lengthy drought conditions in its major source watersheds has reinforced the need to further assess Phoenix's vulnerability to long-term drought conditions. In addition, Phoenix's growth patterns - and thus water demand - may be significantly affected by changing economic or demographic trends. For these reasons, the city's 2005 Water Resources plan update concentrates on these key uncertainties and identifying relevant strategies to more effectively prepare the city for what may lie ahead.
A key objective of this plan is to determine the optimal level of water supply and infrastructure redundancy to maintain sufficient deliveries to the city's growing population under a variety of conditions. The plan also provides general guidance for capital program planning, and serves as a basis for the Water Resources Acquisition Fee (a one-time charge for newly developed lots which helps to offset the city's water acquisition costs). In embarking on this exercise, several questions were raised. These questions helped provide insights and direction that will ultimately reduce the risk associated with future conditions. Examples of the questions addressed in the city's evaluation are:
- Is the past 100 years of record an appropriate indicator of future droughts (both length and severity) or should we be planning for potentially deeper and longer-term droughts as reflected in tree ring analyses covering several centuries?
- How soon are we likely to encounter shortages on the Colorado River and how severe are these shortages likely to be?
- How will Colorado River and shortages affect CAP supplies allocated to Phoenix?
- What is the probability that SRP will need to reduce allocations beyond the roughly 1/3 cutback implemented in 2003 and 2004?
- What is the likelihood of drought on both major surface water systems at the same time?
- To what degree can we depend on groundwater to mitigate drought?
- Considering costs and relative probabilities, what is the optimal level of water supply and infrastructure redundancy needed to avoid adverse consequences to residents, businesses and the local economy?
- What mechanisms are most appropriate for recovering these costs?
- How will environmental regulations (e.g. Clean Water Act, Endangered Species Act and Safe Drinking Water Act compliance) affect future water supply availability?
- To what degree can local water suppliers and wholesalers better collaborate to address challenges at a regional level?
- How will climate change affect water supply availability?
- How could higher density development in Central Phoenix and elsewhere (over and above that described in the General Plan) affect water demands?
- How will changes in the commercial/industrial makeup of the local economy affect water demands?
- How much further can customers conserve without adversely impacting lifestyle, the economy, and the overall "quality of life" in the community?
- Under what conditions is it appropriate for the city enact mandatory customer water use reductions?
Though many of these questions, and several others, were difficult to address, the process of seeking out the answers provided valuable insights in our long-range planning effort. The evaluation process included the identification of key variables, developing and running scenarios through models which are spatially and seasonally sensitive and developing water supply and demand strategies based on the outcomes. The updated plan concludes that Phoenix's portfolio of current and future supplies is capable of handling a wide variety of growth and drought conditions. However, significant infrastructure-related capital expenditures will be necessary to accommodate growth and to better "drought-proof" the portfolio.
