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The modified rational method (MRM) is an extension of the rational method to develop triangular and trapezoidal runoff hydrographs. A trapezoidal unit hydrograph (UH) was developed from the MRM for the case when the duration of rainfall is less than the time of concentration of the watershed and is called the modified rational unit hydrograph (MRUH). The MRUH method was applied to 1400 rainfall-runoff events at 80 watersheds in Texas. Application of the MRUH method involved three steps: () determination of rainfall excess using the runoff coefficient; () determination of the MRUH using drainage area and time of concentration; and () simulating event runoff hydrographs. The MRUH performed as well as the Gamma function UH, Clark-HEC-1 UH and NRCS curvilinear UH methods when the same rainfall loss model was used. The MRUH method can be applied to time-variable rainfall distributions and at watersheds with drainage areas greater than typically used for the rational method (a few hundred acres). [ABSTRACT FROM AUTHOR]

The runoff coefficient, , of the rational method is an expression of rate proportionality between rainfall intensity and peak discharge. Values of were derived for 80 developed and undeveloped watersheds in Texas using two distinct methods. First, the rate-based runoff coefficient, , was estimated for each of about 1,500 rainfall-runoff events. Second, the frequency-matching approach was used to derive a runoff coefficient, , for each watershed. Published values, , 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 and the watershed-median , but for developed watersheds with more impervious cover, literature-based runoff coefficients are less than and . An equation applicable to many Texas watersheds is proposed to estimate as a function of impervious area. [ABSTRACT FROM AUTHOR]

The rational method for peak discharge () estimation was introduced in the 1880s. The runoff coefficient () is a key parameter for the rational method that has an implicit meaning of rate proportionality, and the has been declared a function of the annual return period by various researchers. Rate-based runoff coefficients as a function of the return period, , were determined for 36 undeveloped watersheds in Texas using peak discharge frequency from previously published regional regression equations and rainfall intensity frequency for return periods of 2, 5, 10, 25, 50, and 100 years. The values and return period adjustments determined in this study are most applicable to undeveloped watersheds. The return period adjustments determined for the Texas watersheds in this study and those extracted from prior studies of non-Texas data exceed values from well-known literature such as design manuals and textbooks. Most importantly, the return period adjustments exceed values currently recognized in Texas Department of Transportation design guidance when . [ABSTRACT FROM AUTHOR]

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 (Clit) are listed for different land-use/land cover (two words, no hyphen) (LULC) conditions in various design manuals and textbooks; however, these Clit values were developed with little basis on observed rainfall and runoff data. In this paper, Clit values were derived for 90 watersheds in Texas by using LULC data for 1992 and 2001; the Clit 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. [ABSTRACT FROM AUTHOR]