Time of concentration

Time of concentration is a concept used in hydrology to measure the response of a watershed to a rain event.

Time of concentration is useful in predicting flow rates that would result from hypothetical storms, which are based on statistically derived return periods through IDF curves.

[4][5] For many (often economic) reasons, it is important for engineers and hydrologists to be able to accurately predict the response of a watershed to a given rain event.

In this simplified example, the watershed outlet is located at the bottom of the picture with a stream flowing through it.

Moving up the map, we can say that rainfall which lands on all of the places along the first yellow line will reach the watershed outlet at exactly the same time.