FGM and Lag / K Model Routing Parameters
Routing at RFCs requires lag parameter adjustment to delay the inflow hydrograph and better model the timing of observed flood waves. The K parameter is adjusted to attenuate the lagged inflow hydrograph and create a well sized and timed outflow hydrograph matching the downstream observation.
Lag and K attenuation can be applied as a constant factor or as a variable factor, changing with inflow or outflow discharge, respectively. Caution must be given to ensure that with use of variable lag values the hydrograph does not have two discharges for a single time, a volume corruption, or a missed peak value.
K is given units of time, and its derivation is often given as a ratio of channel storage to discharge. It is related to the K value used in the Muskingum flood wave model.
Image courtesy of NOAA