Rate-based estimation of the runoff coefficients for selected watersheds in Texas

The runoff coefficient, C, of the rational method is an expression of rate proportionality between rainfall intensity and peak discharge. Values of C were derived for 80 developed and undeveloped watersheds in Texas using two distinct methods. First, the ratebased runoff coefficient, C_rate, was estimated for each of about 1,500 rainfall-runoff events. Second, the frequency-matching approach was used to derive a runoff coefficient, C_r, for each watershed. Published C values, C_lit, or literature-based runoff coefficients were compared to those obtained from the methods investigated here. Using the 80 Texas watersheds, comparison of the two methods shows that about 75% of literature-based runoff coefficients are greater than C_r and the watershed-median C_rate, but for developed watersheds with more impervious cover, literature-based runoff coefficients are less than C_r and C_rate. An equation applicable to many Texas watersheds is proposed to estimate C as a function of impervious area.