A 150-acre watershed includes two subareas: Subarea A, which includes 30% of the watershed, has a time of concentration of 20 min; subarea B has mild slope comprising 70% of the watershed with a time of concentration of 60 min. The abstraction can be taken as 1 in./h. Calculate the 25-year peak flow. Use the following IDF relation:

I = (30 × T 0.22)/(td + 18)0.75 where I = rainfall intensity, in./h; T = return period in years; and td = rainfall duration in minutes.

Assume a linear discharge at the watershed outlet. State any other assumptions used.