The time interval between the center of mass of rainfall and the peak of the resulting runoff hydrograph. It reflects how quickly a watershed responds to precipitation.
Lag time is the temporal delay between the centroid (center of mass) of effective rainfall and the peak discharge of the resulting storm hydrograph at a watershed outlet. It is a key parameter characterizing the speed of watershed response and is influenced by catchment properties including area, shape, slope, channel length, land use, soil type, and drainage density. Shorter lag times indicate a rapid response, typical of small, steep, urbanized, or impervious catchments, while longer lag times are associated with larger, flatter, forested, or permeable catchments. Lag time is used in several rainfall-runoff models, including the SCS (NRCS) curve number method and the unit hydrograph method, where it is related to the time of concentration. Various empirical formulas have been developed to estimate lag time from measurable watershed characteristics, such as the Snyder method and the SCS lag equation. Urbanization typically reduces lag time by increasing impervious surfaces and improving drainage efficiency, leading to higher and earlier peak flows. Accurate estimation of lag time is essential for flood forecasting, stormwater management design, and emergency response planning.