ADWR completes groundwater flow model update of North Santa Cruz AMA

5.10.2019 Santa_Cruz_River

Photo courtesy of University of Arizona Water Resources Research Center

Arizona Department of Water Resources hydrologists have completed an update  to the North Santa Cruz Active Management Area groundwater flow model, the first such updates since the release of modeling reports on the NSCAMA in 2007 and 2010.

The model is updated to include data for the approximately 14-year period beginning in 2002 through water year 2016. The calibrated, extended model will be used to provide model-simulated estimates of natural recharge and discharge components for the draft Fourth Management Plan for the Santa Cruz AMA.

Overall, the NSCAMA model update did not produce any big surprises or changes in the Department’s interpretation of the hydrology in  the area.

However, the groundwater flow model update did reinforce the importance to the NSCAMA aquifer of “episodic flood pulses” — episodes, usually lasting days or weeks, when runoff from large rainfall events flows into the Santa Cruz River and increases the amount of surface flow, often by orders of magnitude.

Water levels in the Santa Cruz AMA are largely dependent on stream recharge, which varies significantly from year to year in response to streamflow coming down the Santa Cruz River. That recharge mostly occurs as a result of those episodic flood pulses generated by substantial rain events.

The North Santa Cruz AMA aquifer is a narrow, shallow basin that provides less long-term storage capacity than wider, deeper aquifers such as that of the Tucson and Phoenix AMAs. Especially following major rain events, water flows through the aquifer quickly because the soil properties are such that the conductivities are very high.

The model update found that groundwater pumping in the southern Tucson AMA is continuing to impact water levels in the northern Santa Cruz AMA north of the town of Tubac.

From 1997 to 2016, water levels in this area have been steadily dropping – up to 45 feet, using 1997-1998 as the baseline.

The northern portion of the Santa Cruz AMA is simulated as a model separate from the southern portion due to the  distinct hydrologic regimes along the upper and lower reaches of the Santa Cruz River within the AMA.

For further information regarding the model update of the North Santa Cruz AMA, contact Sally Stewart Lee at ADWR. sslee@azwater.gov