Lag time (Lag factor)

In hydrology, catchment lag or delay has a general meaning, as the time taken for runoff in general to flow off a catchment to its outlet. It relates to the time difference between the centroids of the rainfall hyetograph and the runoff hydrograph. This is implemented in the times of concentration used in the Horton (ILSAX) hydrological model and rational method, and in routing factors applied in storage routing models.

However, there is also a specific lag time used to provide more flexibility in dealing with land-uses within a sub-catchment. This is entered into the Sub-Catchment property sheet as a time in minutes, and is used to delay the grassed area hydrograph by this time. It represents the time of flow across the paved area between the outlet to the grassed area and the outlet of the overall sub-catchment.

By applying this lag time, allowance can be made by different patterns of the paved, supplementary and grassed land-uses over a sub-catchment.

In older DRAINS models, it was possible to also apply a lag factor instead of a lag time, when the comprehensive data option was chosen in the Sub-Catchment property sheet. This feature has been removed, mainly because it was confusing to users. While it cannot be implemented in new DRAINS models, it is still available in previously-created models.

The lag factor takes a value from 0.0 to 1.0. The grassed area hydrograph is lagged by this number multiplied by the paved area flow time (minus the paved area property time). A value of 0.0 implies that the grassed and paved areas are more or less side by side, while a value of 1.0 assumes that all of the grassed area is located above the paved area and runoff from the grassed area has to run over the paved area. Values of the factor intermediate between 0.0 and 1.0 imply different spatial distributions of paved and grassed areas.