Estimation of volumetric runoff coefficients for Texas watersheds using land-use and rainfall-runoff data

The rational method for peak discharge (Qp">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 (C">C) is a key parameter for the rational method and can be estimated in various ways. Literature-based C">C values (Clit">C_lit) are listed for different land-use/land cover (two words, no hyphen) (LULC) conditions in various design manuals and textbooks; however, these Clit">C_lit values were developed with little basis on observed rainfall and runoff data. In this paper, Clit">C_lit values were derived for 90 watersheds in Texas by using LULC data for 1992 and 2001; the Clit">C_lit values derived from the two data sets were essentially the same. Also for this study, volumetric runoff coefficients (Cv">C_v) 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">C_v values were computed, and both are consistent with data from the National Urban Runoff Program. In addition, Cv">C_v values were estimated by using rank-ordered pairs of rainfall and runoff depths (i.e., frequency matching). As anticipated, C">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">C_v versus percent impervious area were developed and combined into a single equation that can be used to rapidly estimate Cv">C_v values for similar Texas watersheds.