# iMOD User Manual version 4.4 (html)

#### 12.28SFR Surface water Flow Routing Package

The existing Surface Flow Routing package of MODFLOW2005 ((Prudic2004)) has been implemented in iMOD. With this package it is possible to route surface water through a network of segments. These segments are stored in an additional type of ISG file that contains more records than the conventional ISG. These additional records are specific for the SFR package and describe the connection and diversion between individual segments and how the water depth in the segments needs to be computed. The water depth is a function of precipitation, evaporation, external discharges and the computed hydraulic head. The cross-section of each segment can be specified by a rectangle, an eight-point cross-section and/or a relationship between discharge, width and depth. The SFR can discharge surface water in and from a lake as described by the LAK Package. The SFR is therefore a significant improvement in the interaction between surface- and groundwater.

There are a few limitations to the SFR package due to the iMOD implementation:

• • the input units for discharge are m$^\text {3}$/s;

• • it is not allowed to define the water depth by an empirical equation;

• • a connection to a lake (up- and/or downstream) needs to be specified by the negative lake identification, it is not necessary that the stream segment truly enters the lake, in fact any connection can be made to any lake, even if the lake isn’t in the neighbourhood of the segment;

• by default the output of each segment is written in an output file that is converted to an ISG file after the simulation is finished. For large networks, this file may become very large. The interaction between the SFR and the groundwater is saved in the BDGSFR file and is a per cell lumped quantity;

• inspect the *.LST-file for more detailed information on total water balances per lake and in- and/or outflow to streams that are connected;

• • the SFR may be subject to convergence problems; an increase of the riverbed resistance may avoid these problems.

• • the value used to set the tolerance level of stream depth used in computing leakage between each stream reach and active model cell is set to 0.0001;

• no solute transport is supported;

• a unsaturated zone underneath the segments is not supported.