Submitted to: Journal Hydrologic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/17/2009
Publication Date: N/A
Interpretive Summary: Mathematical models are commonly employed to simulate flood flows. In Pima County, Arizona, USA, the model that is used to estimate peak runoff rates requires input parameters, including a value representing impedance to flow. Research was conducted to evaluate the use of this parameter in simulating flows on several small rangeland watersheds on the Santa Rita Experimental Range. This research has shown that by using observed rainfall and runoff data in combination with measured watershed characteristics, it is possible to determine local impedance to flow values that can be used in the simulation model. This method of estimation is easier than determining values needed for more complex simulation models.
Technical Abstract: 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 (<10 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 suggest 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.