Hydrological and Environmental Controls on Water Management in Semiarid Urban Areas -- Phase II

Abstract

Rainfall and runoff studies initiated by the University of Arizona provide data for three small urban watersheds from 1968 and one rural watershed from 1957 to 1969. These watersheds typify various land use patterns in Tucson, Arizona. Annual precipitation of about 11 inches produces annual runoff, as measured at outflow flumes, ranging from 0.44 inches in depth for the rural watershed and 1.10 to 2.10 inches for the urban watersheds. The runoff is produced by as few as 5 runoff events per year in the rural watershed and 16 to 22 events per year for the urban watersheds. About 60 percent of the rural and 50 to 58 percent of the urban annual runoff events occur in the summer season, as does 55 to 65 percent of the annual volume of measured runoff for both. There is about a four to five-fold increase in average yearly storm runoff volume with urbanization in the Tucson area. 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 storm runoff, reducing average pollutant concentrations to 62 mg/1 of turbidity, total coliform of 70-3,200 organisms per 100 ml, 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. Multi-purpose urban storm runoff management systems can be developed to control floods while at the same time maintaining water-based linear parks along minor stream channels in semiarid regions. Multi-purpose systems are more economical than the single-purpose systems required to accomplish the same purposes. Further studies are needed to characterize the quality of storm runoff from selected urban land use areas with a view toward on-site control and disposal.