During storms, not all rainfall becomes runoff. The proportion that does varies from about 20% for parks and lawns up to 100% for roofs and paved areas. An overall average for a municipality might range between about 30-50% during fairly intense storms.

Consider a municipality with a population of 55,000 in an area of 15 square miles (like Corvallis) that uses water at an annual average rate of 105 gpcd. Average annual rainfall is 44 inches per year. A major rainfall event occurred on September 6, 2013, in which 2.94 inches of rain fell in a 24-hour period (data available at http://agsci.oregonstate.edu/farmunit/weather). (Major rainfall events are somewhat uncommon in this area, with upper-end 24-hour totals usually being in the 1 to 2-inch range.) Assuming that 40% of the rainfall during that event became runoff, compare the stormwater runoff with the average sanitary sewage flow (which does not include stormwater runoff) on (a) an annual basis and (b) during the September 6, 2013, event.