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Congress, AZ, United States

Stewart D.,Pima County Regional Flood Control District | Canfield E.,Pima County Regional Flood Control District | Hawkins R.,University of Arizona
Journal of Hydrologic Engineering | Year: 2012

Four curve number (CN) determination methods are evaluated from 16 watersheds in the southwestern U.S. using 1,284 events that satisfy rainfall and runoff criteria. The use of ordered pairs versus natural pairs of rainfall and runoff data has a larger effect on the CN, whereas the difference from using a partial duration series versus an annual series was not significant. The best-available USDA soil series data were obtained for 20 Arizona watersheds and 10 groups of New Mexico natural runoff plots. The hydrologic soil groups (HSG) were determined from either direct USDA assignment or textural properties and compared with the HSGs required by the CNs and the cover condition. This study showed a standard error of about one HSG, resulting in an error in CN of approximately seven units when using the best-available data. Compared with USDA handbook table values, the CNs found from rainfall and runoff data were higher for 21 of the 30 semiarid watersheds. © 2012 American Society of Civil Engineers. Source

Stewart D.,Pima County Regional Flood Control District | Canfield E.,Pima County Regional Flood Control District | Yitayew M.,University of Arizona | Nichols M.,U.S. Department of Agriculture
Journal of Hydrologic Engineering | Year: 2010

The time of concentration equation used in Pima County, Arizona, includes a hydrologic parameter representing the impedance to flow for peak discharge estimation on small [typically less than 2.59 km2 (1 mi2)] semiarid watersheds. The impedance-to-flow parameter is similar in function to the hydraulic Manning's n roughness coefficient in the kinematic wave time of concentration equation; however, the impedance to flow is a hydrologic parameter representing all portions of a watershed rather than a hydraulic parameter representing friction loss during uniform flow. To relate the impedance-to-flow parameter to physical watershed characteristics, impedance-to-flow values were calculated for return period and observed events on five undeveloped rangeland watersheds and correlated with Manning's n roughness coefficients determined from particle size analysis and simulated flow conditions. Impedance to flow displayed a positive trend with observed peak discharge on each watershed. The results indicate that local impedance-to-flow values can be developed for time of concentration equations using observed rainfall and runoff data, as well as measurable field characteristics. The impedance-to-flow parameter allows for a physical basis in time of concentration estimation without the additional detail of a physically based model. © 2010 ASCE. Source

Kimoto A.,Pima County Regional Flood Control District | Canfield H.E.,Pima County Regional Flood Control District | Stewart D.,Pima County Regional Flood Control District
Journal of Hydrologic Engineering | Year: 2011

Historical storms with a 24-h rainfall depth exceeding 50.8 mm (2 in.) recorded in southern Arizona were compared with design storms developed both locally and by the USDA Soil Conservation Service (SCS). This study aims to evaluate if those design storms can be representative to southern Arizona. These storm depths represent the return-period storms that pose hazards considered in design. The temporal rainfall distributions, pattern index (PI), and intensity-duration relationships of the observed and design storms were compared. The comparison of the distributions and PIs of 3- or 24-h storms showed that the observed storms were diverse, and it was not clear which design storm fits best with the observed storms. The comparison of the intensity-duration relationships showed that most of the observed storms were less intense than any design storms, especially for short duration. This suggests that the application of any of the 3- or 24-h design storms would result in overestimating rainfall intensities for relatively short duration. The SCS or locally developed design storm poorly represents the temporal distribution and intensities of the observed storms. © 2011 American Society of Civil Engineers. Source

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