Estimation of Rational Runoff Coefficients for Texas Watersheds

The rational method introduced in 1880s is often considered simplistic and still frequently used for estimating peak discharges for small drainage areas. The runoff coefficient (C) is a key parameter for the rational method, and typical C values are given and listed in various design manuals and textbooks for different land use conditions. Actually, these C values were developed without much support of observed data. This paper presents an in-depth literature review on the rational method and developed literature-based runoff coefficients estimated from land use data in 1992 and 2001 for 90 watersheds in central Texas. There is no statistically significant change on C values derived from two land use data. Runoff coefficients were also estimated from observed rainfall and runoff data of more than 1600 events in these watersheds. Distributions of runoff coefficients and watershed mean values are analyzed. Using watershed parameters (e.g., drainage area, slope, and channel length) and regression equations of discharge at different return periods we estimated runoff coefficients in these watersheds for the return periods of 2-, 5-, 10-, 25-, 50-, and 100-years. The rational runoff coefficients increase with the return period and the rate of increase is much larger than what typically recommended in design manuals.