Variable Lag & K

The Variable Lag & K method ("Figure: Reach Editor - Variable Lag & K Method") is a hydrologic routing method for use between flow points that is based on a graphical routing technique. This method was added to ResSim at the request of the National Weather Service. Lag & K is a flexible method of storage routing, where both Lag and K values can be either constant or variable. Lag & K works by delaying the inflow hydrograph in time causing a Lag in the graph. The K value is used to attenuate the newly shifted curve.

Constant Lag (hrs)—The travel time of the reach in hours.

Lag vs. Inflow—The Lag vs Inflow table is why the method is called "Variable". Use the Lag vs Inflow table to describe the relationship between Inflow and travel time.

Constant K (hrs)—the attenuation factor, in hours. The derivation of K is often given as a ratio of channel storage to discharge. It is related to the K value used in the Muskingum method.

K vs. Outflow—K can vary as a function of Outflow. Use the K vs Outflow table to enter the function; the Outflow values must be monotonically increasing. The thumbnail plot to the right of this table displays the data you entered in the table.