An Office of Arid Lands Studies Report to the City of Tucson, Real Estate Division, Contract No. 0255-83 / "This report represents one of two parts of the project "Evaluation, Monitoring and Operation of Existing City Water Harvesting System and Expansion Plan for Future Development of Rainfall Utilization," funded by the City of Tucson during the period November 1982 to June 1983." / Introduction: This report deals with the potential harvesting of rainfall and runoff in the Tucson urban area, as distinguished from the rural (farmland) setting that is discussed separately. The principal differences are that 1) rainfall catchment surfaces like rooftops and pavement already exist in the urban area, and 2) harvested rainwater in the urbanized area can be put to a variety of beneficial uses.

With the population of urban areas rapidly increasing, a much greater demand is being placed on existing water supplies. The arid southwestern region of the United States, in particular, is experiencing large population increases while possessing limited water resources. Tucson is a representative city in the region facing problems of providing an adequate water supply to the public in the future. Presently, Tucson is being supplied entirely with groundwater. Increases in population and industrial activities, however, have caused a steady decline of the groundwater table in the Tucson Basin. The reclamation of wastewater and the importation of water have been studied as alternatives in alleviating the annual decline of the groundwater table. Problems still exist, however, preventing the immediate use of both aforementioned supplies of water. In developing the water resources of an area, every possible source of water must be evaluated. A source which has not received much attention, but which merits much attention, is the water occurring as urban runoff following intense storms. In order to evaluate the potential of urban runoff as a water supply, the study includes investigations of water quality, water treatment through storage and coagulation, and problems involved with the utilization of storm water. Samples of runoff from three diversified urban watersheds in the Tucson area were analyzed for bacterial, mineral, pesticide, solids, and chemical oxygen demand concentrations. The watersheds were characterized according to the percentage of the total area devoted to a particular land use. In addition, the hydrologic characteristics of each storm were tabulated. Correlation coefficients were determined between the quality parameters and the watershed and hydrological characteristics. Development of regression equations equating quality parameters as a function of both watershed and hydrological characteristics was also undertaken. The final analysis of the quality study involved the determination of relationships between quality parameters of chemical oxygen demand, total coliforms and suspended solids and the point of time on the hydrograph at which runoff was sampled. Prior to beneficial use of the urban runoff, treatment to varying degrees will be required. In the second phase of this study, the efficiency of treatment by the simple methods of storage and alum coagulations was studied. Five gallon samples were collected from randomly selected storms and used either in the storage or coagulation study. Changes in chemical oxygen demand, solids and bacterial concentrations were evaluated at selected intervals during storage for a period of a week. Jar test studies utilizing varying doses of alum were undertaken on water collected from each of the watersheds, determining the efficiency of chemical oxygen demand, turbidity, and total coliform removals. The final phase of the study involved discussion of the problems attendant with the planning and design of treatment facilities. Included in this phase were sections involving water quality standards and the related treatment processes, waste sludge production and treatment methods, and costs pertaining to treatment. Legal aspects of appropriating the urban runoff were considered and the possible conflicts between upstream and downstream interests noted. The study concluded with a demonstration of the application of dynamic programming for optimally planning the location and capacity of storage treatment facilities at urban sites.

A study was undertaken to determine the effect of urbanization on the rainfall-runoff relationship of a semiarid region. A concentrated network of rain gages was installed on the urban Tucson Arroyo-Arroyo Chico Watershed located in the city of Tucson, Arizona. Rainfall data from this watershed were compared with that of the non-urban Atterbury Experimental Watershed located to the southeast of the city of Tucson in order to determine if there were significant differences in the runoff from these two watersheds. In addition, test plots were constructed by the researcher for the study of the effects of intensity of precipitation, season, antecedent moisture, and percent of impervious cover on the rainfall-runoff relationship. Data from the test plots were compared with that of the Tucson Arroyo-Arroyo Chico Watershed. A prediction model was developed for the Tucson Arroyo-Arroyo Chico Watershed. The feasibility of retaining urban runoff for more beneficial uses was investigated. It was determined that the runoff from the urban Tucson Arroyo- Arroyo Chico Watershed was 4.75 times greater per square unit of area than that from the non-urban Atterbury Experimental Watershed. Significant factors in the rainfall-runoff relationships of the test plot data were found to be intensity of precipitation and amount of impervious cover. Season was found not to be significant. Antecedent moisture was a significant factor in the analysis of variance of the data. However, it was not significant in the regression analysis. In the comparison of the test plot data and the data from the Tucson Arroyo-Arroyo Chico Watershed, it was found that intensity of precipitation was the predominant factor in the rainfall-runoff relationship. In plotting the effects of intensity of precipitation versus runoff, the resulting graph indicated an increasing effect of impervious cover and intensity of precipitation on runoff from the test plots. A regression model was developed from the data of the Tucson Arroyo-Arroyo Chico Watershed. The factors of average precipitation and intensity of precipitation accounted for 82 percent of the variation in the analysis. Of these two factors, intensity of precipitation accounted for 68 percent of the variation. Duration of precipitation and the antecedent moisture index did not significantly increase the correlation coefficient of the regression analysis when they were included. The projected runoff from the city of Tucson would amount to less than 10 percent of the present yearly use. The treatment necessary for the domestic use of urban runoff would be greater than the present cost of producing municipal water from well-sites. Recharging this water into the groundwater supply appears to be the most feasible method of utilizing urban runoff at this time.

The variably saturated flow model Unsat 2 was used for three case study simulations of dry well recharge in the Tucson Basin, Arizona. Dry well design, and rainfall/runoff and vadose zone conditions representative of the locality were assumed in the simulations to address travel time to the regional aquifer, rates and extent of radial flow, and relative degree of solute attenuation by sorption and dilution with regional groundwater. Soil specific surface was used to estimate relative degree of sorption among the three cases. One case of uniform soil composition and two cases of layered soil composition were simulated. Clay content had the greatest influence on specific surface. Hydraulic conductivity had the greatest influence on soil water velocities and degree of radial flow. The presence of layered subsurface conditions that included strata of low hydraulic conductivity enhanced the degree of subsurface solute attenuation by sorption and dilution.

The U.S. Geological Survey and the City of Tucson, Arizona, have been collecting rainfall and runoff data on several watersheds in the Tucson area for several years. Among the purposes of this project is to use the data to test rainfall-runoff models in an effort to find one to successfully simulate flood flows in Tucson. One such model, the Distributed Routing Rainfall-Runoff Model (DR3M), was tested using data collected on Rob Wash in Tucson. It was found DR3M performs about as well as it does in other parts of the United States, although it tends to underestimate flood flows for large storms and overestimate flows for smaller storms. Unique features with regard to the hydrology of urban Tucson require special attention when using DR3M; these features are associated with the nature of dry washes and summer rainfall in Tucson. Experience indicates DR3M is not truly a deterministic model.

Project Completion Report, OWRT Project No. B-012-ARIZ / Agreement No. 14-31-0001-3056 / Period of Operation: July 1969 to June 1972 / Acknowledgement: The work upon which this report is based was supported by funds provided by the United States Department of the Interior, Office of Water Resources Research, as authorized under the Water Resources Research Act of 1964. / 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.

Project Completion Report, OWRT Project No. A-049-ARIZ / Agreement No. 14-31-0001-4003 / Project Dates: July 1, 1973 - June 30, 1974. / Acknowledgement: The work upon which this report is based was supported by funds provided by the United States Department of the Interior, Office of Water Research and Technology, as authorized under the Water Resources Research Act of 1964. / A non-linear reservoir model is used to represent the rainfall-runoff relationships for thunderstorms on the urban watersheds of Tucson, Arizona. Two types of computer programs are developed: a calibration program to obtain a best -fit calculated hydrograph; and a verification program to generate storm hydrographs given the watershed characteristics and a hyetograph. Calibration reveals the relationship of the model parameters, namely, (f) the inflow coefficient, (a) the constant coefficient, and (TL) the time lag, to the total rainfall, drainage area, channel length, and infiltration capacity of the watershed. The average discrepancy between the predicted hydrograph and the actual hydrograph for Tucson urban watersheds is 20 -25 percent.

Research Project Technical Completion Report (B-023-ARIZ) For: United States Department of the Interior, Project Dates: 1971-1973. / The work upon which this report is based was supported by federal funds provided by the United States Department of the Interior, as authorized under the Water Research and Development Act of 1978, through Agreement No. 14-31-0001-3556. / 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.