A simple method of establishing the synthetic design hyetograph is presented. The method is based on rainfall intensity-duration-frequency curves with simple computation. The method incorporates the rainfall duration, rainfall depth, and the advancing storm patterns. In addition, a method of computing the average runoff coefficient for the Rational Method is developed. This method is predicated on the principle of mass conservation using Horner's field data. Examples are given for the establishment of synthetic design hyetograph and its application to the determination of average runoff coefficient. The methods presented are useful tools for drainage analysis.