Final Determination on the Safe-yield Status of the Prescott AMA

The Prescott AMA is unique among the three safe-yield AMAs. During the early 1990s, when the Department was developing the Assured Water Supply (AWS) Rules, there was insufficient hydrologic data at that time to make a definite determination as to whether the Prescott AMA was at safe-yield or mining groundwater. Due to this uncertainty, the AWS Rules, adopted in 1995, established a monitoring requirement that specified that the Prescott AMA would be determined to be out of safe-yield after three consecutive years of data that demonstrated declining water levels and increasing demand for groundwater.

On August 28, 1998, the Director of the Department made a preliminary determination that the Prescott AMA was not at safe-yield and in an overdraft condition. This determination was based on data collected by the Department that demonstrated ongoing groundwater level declines and that existing groundwater pumping greatly exceeded the AMAs safe-yield goal. In the preceding five years, water levels in the Prescott AMA declined in more than 73% of monitored wells. Data demonstrated that the Prescott AMA had been out of safe-yield since at least 1990. After a public hearing and considering public comments and the analysis of an independent evaluation of the Department’s hydrologic studies, the Department made a final determination on January 12, 1999 that the Prescott AMA was no longer at safe-yield.

Prescott AMA Monitoring Program

Groundwater conditions are monitored in the Prescott AMA with one of the most comprehensive groundwater monitoring programs in the state. Hydrologic monitoring data and related information are compiled by ADWR and are presented in hydrologic monitoring reports. The measurement of water levels is an important data collection activity that provides information about changing groundwater storage conditions in the regional aquifer system. In general, rising water levels are indicators of increasing groundwater storage conditions, while declining water levels are indicators of decreasing groundwater storage.

The hydrologic monitoring reports include water level measurement data collected at 126 well sites. Decreasing groundwater storage trends were observed at a majority of the 105 wells that were measured in both 2003 and 2004 and that were used for statistical analysis. Increasing groundwater storage trends were observed in 10 of the 105 wells that were used for statistical analysis. Changes in measured water levels from 2003 to 2004 are shown on Figure 1 of the 2003-2004 Prescott Active Management Hydrologic Monitoring Report (906 KB). The report provides compilations of surface water, precipitation, pumpage and recharge data. The report also presents a conceptual water budget for the Prescott AMA for calendar year 2003.

The hydrologic monitoring reports provide ADWR with an excellent opportunity to keep water users posted on current hydrologic conditions and data collection and data analysis activities that support the water management goals of the AMA.

Prescott AMA Water Budget

A conceptual water budget (224 KB) was prepared from the assembled 2003 pumpage, recharge and surface water discharge data. Estimates of long-term natural recharge that have been developed from the Prescott model update are used for the water budget component. The 2003 conceptual water budget for the Prescott AMA, which is summarized in the 2003-2004 Prescott Active Management Hydrologic Monitoring Report (906 KB), indicates that groundwater outflows exceeded inflows, resulting in a 11,300 acre-foot overdraft for the year.

Prescott AMA Groundwater Flow Model

ADWR developed a regional groundwater flow model to quantify the impacts of groundwater pumpage and recharge in the Prescott Active Management Area. The model, Application of the Prescott Active Management Area Groundwater Flow Model Planning Scenario 1999-2025 (4.17 MB), was updated in 2002 with new hydrogeologic data and revised estimates of historical water use and recharge. The model was updated again in 2006 with new hydrogeologic data and an expanded model area: Prescott AMA Groundwater Flow Model Update Report October 31, 2006 (8.72 MB).