LESSONS LEARNED FROM A QUARTER CENTURY OF GROUNDWATER MANAGEMENT IN THE SANTA CRUZ ACTIVE MANAGEMENT AREA (Invited Presentation)

The Arizona Department of Water Resources develops and maintains groundwater flow models to better understand the past, present, and future of the state’s aquifers and to inform water resource management decisions, including the administration of the Assured Water Supply Program within the state’s five Active Management Areas (AMA).

The Department began work in the Upper Santa Cruz Basin shortly after the creation of the Santa Cruz AMA in 1994. Following a 10-year long preliminary field campaign, in 2007, the Department produced its first two models of the Santa Cruz AMA, originally simulating the 5-year period between 1997 and 2002. From the international border to the Nogales International Wastewater Treatment Plant (NIWWTP) at Rio Rico, the main stem of the Santa Cruz River and its major tributaries are captured by the Microbasins model. North of the NIWWTP and to the AMA boundary near the Santa Cruz/Pima County line, the aquifer is simulated by the North Santa Cruz AMA (NSCAMA) model. Subsequent efforts have refined the models based on new geophysical data and extended the calibration period. The next model release will feature a single model, capturing the AMA from the international border to its northern boundary, and an expanded domain that encompasses the Nogales (Potrero) sub-basin.

The results of more than two decades’ worth of the Department’s field data collection and modeling efforts continue to reinforce 1) the importance of episodic flood pulses for providing the bulk of groundwater recharge and temporarily ‘resetting’ water levels; 2) the limit imposed by high transmissivities and a narrow, shallow basin on long-term storage capacity; and 3) the impact of groundwater pumping in the southern portion of neighboring Tucson AMA on groundwater level decline in SCAMA north of Tubac.

Because the Upper Santa Cruz Basin’s hydrology is dominated by stream-aquifer interaction, the sustainability and future availability of the AMA’s water supply is tied to its most variable water budget component (streamflow). The development of more user-friendly statistical and risk-analysis outputs to better understand the uncertainty and manage water supplies, and the proper propagation of any new Assured Water Supply rules once the state-wide moratorium on rulemaking is lifted, remain a priority.