In geography, lag time refers to the amount of time it will take for a river to flood after a period of heavy precipitation. Lag time is determined by the length of time that the precipitation falls and the degree of the rainfall's interception or infiltration. Although it is usually measured in days, shorter lag times can be an indicator that the area around a river can suffer dangerous flash floods if a heavy rainfall takes place during a short length of time.
The degree of rainwater infiltration into the soil and the remaining amount of runoff that reaches rivers and streams can be determined by a variety of geographic characteristics. These include the soil type, the climate in the area, vegetation, frozen ground conditions or a preexisting state of soil or water table saturation. Human construction and deforestation can significantly decrease the degree of infiltration, shorten the lag time and lead to potential flash flood conditions.
The other factor affecting lag time is interception. This refers to anything that stops precipitation from adding to the water runoff that can drain into a river. Interception can include rainfall being stored in leaves and plants or becoming trapped in man-made retention ponds.
The measurements and calculations made with regard to a river's lag time and its related factors are often charted in a hydrograph or issued in a hydrology report. This information can enable infrastructure engineers and environmental protection planners to take steps to better protect residents and properties from the effects of flooding.