Estimating an impedance-to-flow parameter for flood peak prediction in semiarid watersheds

Dave Stewart, Evan Canfield, Muluneh Yitayew, Mary Nichols

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish (US)
Article number005003QHE
Pages (from-to)182-190
Number of pages9
JournalJournal of Hydrologic Engineering
Issue number3
StatePublished - Mar 2010


  • Hydraulic roughness
  • Hydrologic models
  • Impedance to flow
  • Peak flow
  • Travel time

ASJC Scopus subject areas

  • Environmental Chemistry
  • Civil and Structural Engineering
  • Water Science and Technology
  • Environmental Science(all)


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