Infiltration of effluent into the Santa Cruz River channel from water-treatment plants near Tucson, Arizona was studied from March 23, 1990, to September 30, 1993. The study reach extended along a 23-mile stream reach from the water-treatment plants, about 5 miles northwest of central Tucson, downstream to Trico Road, about 5 miles west of Marana, Arizona. Data indicate that 88.4 to 90.2 percent of the effluent discharged from the two water-treatment plants infiltrated the Santa Cruz River channel. During 1991 93, the volume of effluent discharge that flowed out of the study area was 2,880, 4,120, and 3,320 acre-feet per year, respectively, and the volume of infiltration was 41,890, 43,640, and 45,670 acre-feet per year, respectively. Intermittent rainstorms resulted in high flows that altered the composition, structure, and geometry of the channel bed and may have caused the infiltration to increase to nearly 100 percent. In comparison, variations in evapotranspiration and open-channel evaporation had a minimal effect on the water budget. In the study reach, 3.2 to 3.9 percent of the effluent was lost to evapotranspiration and open-channel evaporation; 6.2 to 8.3 percent flowed through the reach.
