Runoff Coefficient

The OL Drainage is calculated from ponded water. It is not calculated from rainfall directly. However, the order of operations in the three UZ solvers (Richards, Gravity, and 2Layer Water Balance) is such that rainfall is added to existing ponded storage after interception and before ET and infiltration are calculated, and, thus, before the OL Drainage is calculated. Therefore, the OL Drainage effectively acts on net rainfall (i.e. net rainfall = rainfall minus leaf interception).

The amount of OL Drainage is calculated based on the available ponded water. That is, a specified fraction of the amount of available ponded water is routed directly to the OL Drainage system. The OL Drainage system immedi­ately adds this water into the OL Drainage Storage.

Water discharging from the OL Drainage Storage is instantly added to the destination. This outflow is analogous to a full-pipe of water. That is, for any inflow an equal amount of outflow is generated instantaneously.

Rainfall is added to the ponded depth and then the OL Drainage fraction is removed. However, if at the end of the time step, there is still ponded water in the cell, the Runoff Coefficient will be applied to the remainder again in the next time step. Thus, the ponded water will eventually drain away.

The OL Drainage is also linked to the detention storage. Only the available ponded water will be routed to the OL drainage network - that is the ponded depth above the detention storage. If you want to route all of the ponded water to the OL Drainage network, then you can use an Extra Parameter found in OL Drainage Options (V1 p. 746).

The OL Drainage function does not check to make sure that you do not cre­ate any physically impossible feedback loops. So, flood code cells, and over­bank spilling from MIKE Hydro River should not be directed to cells where OL Drainage is active. If this happens, you may encounter excessive feedback between MIKE SHE overland flow and MIKE Hydro River.

Note: The OL Drainage is calculated on the UZ time step, and stored in the UZ output file. Thus, the OL Drainage depends on the UZ and/or the OL time step length. For example, if the UZ time step is reduced by the precipitation controls, then the amount of inflow to the drain may change because the amount of rainfall in the time step changes. Thus, you may want to adjust your precipitation controls such that the UZ time step is not reduced by these parameters.