Hydrological and Environmental Controls on Water Management in Semiarid Urban Areas

Abstract

Rainfall and runoff studies initiated in 1968 by the University of Arizona provide data for three small urban watersheds with different land use patterns in Tucson, Arizona. Annual precipitation of about 11 inches produces annual runoff, as measured at outflow flumes, ranging from 1.30 to 3.95 inches, produced by 15 to 23 runoff events per year. About 60 to 70 percent of the annual runoff events occur in the summer season, as does 65 to 75 percent of the annual volume of measured runoff. Water samples collected on a lumped basis show generally high concentrations of suspended sediment, bacterial loading, and dissolved organics. Initial field treatment and exploratory laboratory studies of treatment methods indicate that three days is an optimal length of time for detention storage of runoff, reducing average pollutant concentrations to 62 mg /1 of turbidity, total coliform of 70 -3200 organisms per 100 mg /1, and 7 mg /1 of chemical oxygen demand. Simple laboratory treatment with alum and polyelectrolyte yielded an 80 percent reduction in COD, 90 percent reduction in bacterial loading, and appreciable clarification of the runoff samples. Continuing research should be conducted to utilize a longer data record for improving understanding of rainfall- runoff relations; to use distributed sampling within individual watershed areas to define specific pollutant source areas; and to incorporate economic and legal questions involved in the utilization of urban runoff in an arid area.