Estimation of Volumetric Runoff Coefficients for Texas Watersheds Using Land-Use and Rainfall-Runoff Data

by Nirajan Dhakal, Research Assistant; Dept. of Civil Engineering, Auburn Univ., Auburn, AL 36849-5337.,,
Xing Fang, (corresponding author), P.E., (M.ASCE), Associate Professor; Dept. of Civil Engineering, Auburn Univ., Auburn, AL 36849-5337.,,
Theodore G. Cleveland, P.E., (M.ASCE), Associate Professor; Dept. of Civil and Environmental Engineering, Texas Tech Univ., Lubbock, TX 79409-1023.,,
David B. Thompson, P.E., Director of Engineering; R.O. Anderson Engineering, Inc., Minden, NV 89423.,,
William H. Asquith, P.G., Research Hydrologist; U.S. Geological Survey, Texas Tech Univ., Lubbock, TX 79409.,,
Luke J. Marzen, Professor; Dept. of Geology and Geography, Auburn Univ., Auburn, AL 36849-5337.,,

Serial Information: Issue 1, Pg. 43-54

Document Type: Journal Paper

Abstract: The rational method for peak discharge (Qp) estimation was introduced in the 1880s. Although the rational method is considered simplistic, it remains an effective method for estimating peak discharge for small watersheds. The runoff coefficient (C) is a key parameter for the rational method and can be estimated in various ways. Literature-based C values (C lit) are listed for different land-use/land cover (two words, no hyphen) (LULC) conditions in various design manuals and textbooks; however, these C lit values were developed with little basis on observed rainfall and runoff data. In this paper, C lit values were derived for 90 watersheds in Texas by using LULC data for 1992 and 2001; the C lit values derived from the two data sets were essentially the same. Also for this study, volumetric runoff coefficients (Cv) were estimated by using observed rainfall and runoff depths from more than 1,600 events observed in the watersheds. Watershed-median and watershed-average Cv values were computed, and both are consistent with data from the National Urban Runoff Program. In addition, Cv values were estimated by using rank-ordered pairs of rainfall and runoff depths (i.e., frequency matching). As anticipated, C values derived by all three methods (literature based, event totals, and frequency matching) consistently had larger values for developed watersheds than for undeveloped watersheds. Two regression equations of Cv versus percent impervious area were developed and combined into a single equation that can be used to rapidly estimate Cv values for similar Texas watersheds.

Subject Headings: Watersheds | Runoff | Rainfall-runoff relationships | Land use | Parameters (statistics) | Rain water | Water discharge | Professional societies | Hydrologic data | North America | Texas | United States

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