Development of an Empirical Lag Time Equation
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Volume 134, Issue 4
Abstract
A regression analysis was performed on measured lag times from gauged watersheds to develop a lag time equation. The watersheds are part of the Agricultural Research Service’s database. They are located in several states and are comprised of varying terrain. The goal of the analysis was to develop a lag time equation that is useful in hydrologic modeling. The study included measurements from approximately 10,000 direct runoff events from 52 watersheds to determine which watershed parameters are best for predicting lag time. The lag time was found to correlate strongly with the longest hydraulic length of the watershed. Therefore an equation was developed that used only this parameter. The inclusion of any other watershed characteristics in the equation did not improve its ability to predict the lag time. Finally, the National Resource Conservation Service procedures for calculating watershed lag time were used to determine the lag times of the watersheds. These estimated lag times were then compared with the measured lag time of the watershed. It was found that the use of these methods generally underpredicted the true lag time of a watershed.
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References
Dripchak, M. N., and Hawkins, R. H. (1992). GETPQ manual, Watershed Resources Program, University of Arizona Press, Tucson, Ariz.
Folmar, N. D., Miller, A. C., and Woodward, D. E. (2007). “History and development of the NRCS lag equation.” J. Am. Water Resour. Assoc., 43(3), 829–838.
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Natural Resources Conservation Service (NRCS). (1972). “Hydrology, Section 4.” National engineering handbook (NEH-4), U.S. Department of Agriculture, Washington, D.C.
Natural Resources Conservation Service (NRCS). (1986). “Urban hydrology for small watersheds.” Technical release–55, U.S. Department of Agriculture, Washington, D.C.
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United States Department of Agriculture (USDA), Agricultural Research Service. (1956–1979 ). “Hydrological data for experimental agricultural watersheds in the United States.” Miscellaneous publications, Washington, D.C., 945–1469.
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© 2008 ASCE.
History
Received: Jun 5, 2007
Accepted: Oct 24, 2007
Published online: Aug 1, 2008
Published in print: Aug 2008
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