What is a 100 Year Storm?
- To understand how a 100-year storm event is calculated, you need to first understand the concept of a watershed. A watershed is an area in which water drains to one location or point of accumulation. On land, water that does not evaporate or soak into the soil usually drains into ditches, streams, washes or lakes. The land area from which the water drains is called a watershed. When you were a small child, you may have had a favorite mud puddle in which you liked to play. The part of the yard from which the water drained into the puddle was its watershed.
- For a 100-year storm to occur, all portions of a watershed must receive 3.4 inches of rainfall within six hours.
- In central Arizona, there are generally two times each year when heavy rainstorms occur. The summer monsoon seasons are typically stronger and can produce large amounts of rainfall in short periods of time. The summer thunderstorms are smaller storm systems but could produce 100-year storm events within watersheds of one to 10 square miles. The winter storms, on the other hand, are larger in area and longer in duration. If enough rainfall occurred, these winter storms could produce 100-year storm events within larger watersheds.