1. Determination of peak runoff In this case study the peak runoff of a small watershed in Southern Spain is determined using the Modified Rational Formula method, and the SCS Dimensionless Unit Hydrograph for an excess rainfall of 1 cm. Theoretical Background An important formula for determining the peak runoff rate is the Rational Formula. It is characterised by: consideration of the entire drainage area as a single unit, estimation of flow at the most downstream point only, the assumption that rainfall is uniformly distributed over the drainage area. The Rational Formula reads: Q p = 0.28*C*I*A where : Q p = Peak runoff rate [m³/sec] C =Runoff coefficient [-] I = Rainfall intensity [mm/hr] A = Drainage area [km²] The Rational Formula follows the assumption that: the predicted peak discharge has the same probability of occurence (return period) as the used rainfall intensity (I), the runoff coefficient (C) is constant during the rain storm, and the recession time is equal to the time of rise. In the modified version of the Rational Formula, a storage coefficient is included to account for a recession time larger than the time the hydrograph takes to rise. The Modified Rational Formula reads: Q p = 0.28 * C s * C * I * A
