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

摘要

The rational method for peak discharge (Q_p) 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 © is a key parameter for the rational method and can be estimated in various ways. Literature-based C values (C_y) 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, Cut values were derived for 90 watersheds in Texas by using LULC data for 1992 and 2001; the C_iit 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 C_v values were computed, and both are consistent with data from the National Urban Runoff Program. In addition, C_v 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 C_v versus percent impervious area were developed and combined into a single equation that can be used to rapidly estimate C_v values for similar Texas watersheds.