Correcting High & Low Flow Recession
Modelers working with predicting the flows at Yolo Bypass tried various adjustments to the model parameters. The most logical parameter adjustment given the FGM controls of the weir were to set the Lag and K flood routing parameters to account for the flood wave delay in the Yolo Bypass during high flows. It should be noted that Lag and K values are set according to various discharge or stage values. Often, these Lag and K values are reported in tables or graphs with the associated discharge or stage. In this case of the Yolo Bypass, higher and lower stages could accommodate adjustments to the Lag and K values.
The Lag parameter controls the timing of the inflow hydrograph, while the K parameter provides control on the timing of the attenuation of the outflow hydrograph. In this image, the better match between observed and modeled flow is compared with that shown earlier. For lower flows, the Lag and K correction also improved simulation results.
Image courtesy of NOAA