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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.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

Two catchments of diverse hydrologic character were chosen in which to test the hypothesis that tree-ring indices contain information about runoff that is of pertinent interest to the hydrologist. These two catchments, Bright Angel Creek basin in extreme north-central Arizona and Upper San Francisco River basin in extreme east-central Arizona and west-central New Mexico, are situated in different climatic regions. Although two semiannual maxima, summer and winter, occur in the annual precipitation regime, at Bright Angel Creek the winter maximum is dominant, resulting in large amounts of snow accumulation, and at Upper San Francisco River the summer maximum is dominant. These contrasting precipitation regimes in association with the annual temperature regimes create climatic conditions for which the growth response of the trees, specifically Douglas fir, the species used in the study, and the precipitation-runoff response are greatly different and comparably complex. The complexity of the climate-growth, climate-runoff relationships necessitated the use of multivariate methods in assessing their similarities and dissimilarities. The technique used here is that of principal components, with physical meaning attached to the components by comparison with the results of other statistical approaches such as autocorrelation, cross correlation, autospectra, and cross spectra, and such tree-ring statistics as the coefficient of -mean sensitivity. The orthogonality property of the principal components was used to develop prediction equations with a minimum of variables through use of multiple linear regression. The general approach was to relate ring-width indices to climate and develop a response function, to relate runoff to climatic variables and develop a response function, and finally to develop a prediction equation for predicting runoff from ring-width indices. Prediction equations and 214-year (1753-1966) synthetic runoff series were developed for both basins. The results for Bright Angel Creek basin are not impressive because the best prediction equation accounts for only 51% of the year-to-year variance in the annual runoff. However, this was not wholly unexpected, as it is shown that the nature of the annual runoff regime and the statistical nature of the ring-width index series from this basin are not conducive to maximum hydrologic information. Nevertheless, it is shown that an improved estimate of the mean annual runoff can be gained from the synthetic series. For Upper San Francisco River basin the results were more satisfactory: 72% to 79% of the annual variance in runoff can be accounted for using prediction equations based on ring-width indices, where one equation uses untransformed values of runoff (72%) and the other uses log-transformed values of runoff (79%). The synthetic series shows an improved estimate for the mean annual runoff but also offers the hydrologist a valuable tool in providing a series from which useful information can be obtained that could be valuable in decision-making processes concerning reservoir design and operation.

Almost all runoff-producing rainfall on small watersheds (100 square miles and less) in southeastern Arizona results from air-mass thunderstorms. On large watersheds (1,000 square miles and greater) frontal systems which may include thunderstorm activity or snowmelt produce the major flood peaks as well as much of the annual runoff. Air-mass thunderstorms are of short duration and limited areal extent, and generally occur in the late afternoons and early evenings in July, August, and September. Runoff-producing rainfall may occur from frontal-convective systems at any time although they are most common in southeastern Arizona in September. Rainfall and runoff records have been collected from the 58- square-mile Walnut Gulch rangeland watershed near Tombstone in southeastern Arizona by the Agricultural Research Service since 1954. These data represent the best information available on thunderstorm rainfall-runoff relationships in the Southwest. At present there are 95 recording rain gages and 22 permanent runoff-measuring stations on the Walnut Gulch watershed. Runoff-producing thunderstorm rainfall is extremely variable both in time and space, and is therefore difficult to measure accurately and define precisely. Isohyetal mapping for rainfall from individual thunderstorms both for total rainfall and shorter durations within the storm provides good qualitative information, and also provides some quantitative limits on storm movement, intensities and volumes, and areal extent. Runoff records from Walnut Gulch and other Arizona watersheds indicate that peak discharge and runoff volume from individual thunderstorms decrease with increasing watershed size because of the limited areal extent of runoff-producing thunderstorms and because cf the increasing channel abstractions with increasing watershed size. Channel abstractions greatly alter runoff hydrographs as flood surges move through the ephemeral channel system. Five major runoff-producing thunderstorms on Walnut Gulch between 1957 and 1967 were used to develop a model for the maximum expected rainfall in southeastern Arizona. The model was based on maximum 30-minute point rainfalls within the average 60-minute runoff-producing thunderstorm. Over 2.5 inches of rainfall has been recorded in 30 minutes on Walnut Gulch during 3 thunderstorms in 15 years of record (1955-1969). A thorough search of U.S. Weather Bureau and other records indicated that no storms of this combined intensity and magnitude have been recorded in Arizona. Therefore, for design purposes, the expected mean 30-minute rainfall for southeastern Arizona was estimated as 3 inches. Regression analysis was used to estimate peak discharges for major runoff events on Walnut Gulch and to develop a rainfall-runoff model for Walnut Gulch. Peak discharges were correlated with the maximum 30-minute rainfall, which was considered the core of runoff-producing rainfall for major runoff events. Antecedent channel conditions and distance between watershed outlet and runoff-producing rainfall had little effect on the correlation. The coefficients of determination for the regression equation correlating thunderstorm rainfall and peak runoff were 0.92 and o.84 for watershed 5 (8 square miles) and watershed 1 (58 square miles), respectively. With the model for maximum expected rainfall and the rainfall-runoff model for estimating peak discharge from maximum 30-minute rainfall, maximum discharge for the 58-square-mile Walnut Gulch watershed was 23,000 c.f.s. Assuming a normal distribution of errors, within 95 percent confidence limits, the limits were 19,000 and 27,000 c.f.s., and assuming the Chebyshev inequality, the limits were 15,000 and 31,000 c.f.s. Recurrence intervals for 20-, 50-, and 100-year storms and the maximum peak discharges were developed for small watersheds (100 square miles and less) from Walnut Gulch data. The curves were compared to a family of curves for Arizona watersheds up to several hundred thousand square miles. The family of curves based on Walnut Gulch data were much steeper, strongly suggesting that there are 2 families of curves, one steeper family for the small watersheds (100 square miles and less) which is based on runoff peaks from air-mass thunderstorms, and another flatter family of curves for the large watersheds (1,000 square miles and greater) which is based on runoff peaks from frontal-convective systems and snowmelt. The 2 families of curves probably intersect between 100 and 1,000 square miles.

In cities, storm runoff is usually collected in the streets and conveyed to underground storm drains through inlets located at street level. Construction and maintenance of storm drain systems is always expensive. In semiarid regions with a very low frequency of storm occurrence, most cities use streets as the drainage route even though their primary function is for the movement of traffic. As a result, the shape of a runoff hydrograph at the outlet of an urban watershed will vary with the pattern of street arrangement within the watershed. The objective of this study is to investigate tradeoffs between alternative street patterns with respect to urban drainage. The study area is confined to residential subdivisions where the subdivision boundary is assumed to be the watershed boundary. Three street patterns, namely, rectangular, curvilinear and dendritic, are considered in this study. The cost-effectiveness technique is employed to give an objective evaluation for these three alternative patterns. The effectiveness is measured by three kinds of evaluators. The first evaluator is the flow depth at certain points on the streets; the second evaluator is the total time during which the streets are occupied by a certain depth of water; the third evaluator is the total cost associated with the construction of a street pattern. A framework of a distributed system model has been constructed for simulating the runoff hydrographs and flow depths at certain points on the streets. A parcel of land near Willcox, Arizona, is selected to illustrate how the model could be used. Possible subdivisions of the land for each of the three different types of street patterns are shown, and the runoff hydrographs and flow depths from each of the patterns are examined. It is found that the peak flow rate resulting from the rectangular pattern is 13 percent higher than that from the curvilinear pattern and 29 percent higher than that from the dendritic pattern. The dendritic pattern has the smallest percentage of street intersections occupied by water during a storm. Time of occupation of high water stages at street intersections is much shorter in the dendritic pattern than that in the rectangular and curvilinear patterns. The development cost for the dendritic pattern is the lowest among these three patterns. Therefore, the dendritic pattern appears to be the best type of street arrangement in terms of urban drainage.

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.

This study is concerned with design forecasting of storm hydrographs with emphasis on runoff volume and peak discharge. The objective of the study was to develop, calibrate and test a method for forecasting storm runoff from small semi-arid watersheds using an available prediction model. In order to turn the selected prediction model into a forecasting model an objective procedure in terms of an API-type model was developed for evaluating the soil moisture deficit in the upper soil layer at the beginning of each storm. Distinction was made between the physically-based parameters and the other fitting parameters. The rainfall excess calculation was computed by solving the Green and Ampt equation for unsteady rainfall conditions using the physically-based parameters. For the physically-based parameters a geographic information system was developed in order to account for the variability in time and space of the input data and the watershed characteristics and to coregister parameters on a common basis. The fitting parameters were used to calibrate the model on one subwatershed in the Walnut Gulch Experimental Watershed while the physically-based parameters remained constant. Two objective functions were selected for the optimization procedure. These functions expressed the goodness of fit between the calculated hydrograph volume and peak discharge and the observed volume and peak discharge. Linear relationships between the effective matric potential parameter and the two objective functions obtained from the sensitivity analyses made it possible to develop a bilinear interpolation algorithm to minimize, simultaneously, the difference between the calculated and observed volume and peak discharge. The prediction mode of the model was tested both on different storm events on the same subwatershed and on another subwatershed with satisfactory results. In the prediction mode the effective matric potential parameter was allowed to vary from storm to storm, however, in the forecasting mode these values were obtained from the API model. Relatively poor results were obtained in testing the forecasting mode on another subwatershed. These errors were able to be corrected by changing the channel losses fitting parameters.

Southwestern mixed conifer forests cover approximately 2.5 million acres in Arizona and New Mexico, and provide a wide range of commercial and noncommercial products. The problem is to develop a management prescription which will benefit the greatest mix of resources. An alternatives analysis predicted that a prescription that included small patch clearcutting, in addition to other stand modifications, would meet this criteria. The two Thomas Creek watersheds, in eastern Arizona, were used to validate and test the responses of the forest resources to the preferred prescription, and to increase the understanding of the mixed conifer forest system. The actual harvest created 63 small patch clearcut and group selection openings, averaging 1-2 acres, over 13% of the South Fork watershed. Overall stand density was reduced 34% to 132 square feet per acre. The harvest resulted in significant hydrological changes. Average annual streamflow increased by about 45%, or 1.72 inches, mostly because of increased winter runoff. A greater proportion of the snowmelt generated streamflow occurred earlier in the spring, while annual peak flows were increased by an average of 66%, or about 2.60 cubic feet per second per square mile. The number of days without flow decreased. Average watershed maximum snow water equivalents remained unchanged. The primary causes of the increases were reduced evapotranspiration and increased snow accumulation in the openings; however, it appears that the partially cut stand also contributed to the increases. The treatment benefitted the timber resource. Diameter growth on the South Fork increased for most species compared to the unharvested stand on North Fork. Stand gross growth remained unchanged, but the same volume was being added to fewer trees. The stand, including most openings, is well stocked with adequate numbers of natural and advance regeneration. The Thomas Creek prescription, after 8 years of evaluation, has achieved its objectives of increasing water yields and stand growth while insuring adequate regeneration. It has also benefitted many wildlife species as well as livestock. A similar prescription should increase water yields, by about 15,000 acre-feet annually, from the Upper Black River Basin without adversely impacting other forest resources.