The existing Unsaturated Zone Flow package of MODFLOW2005 ((Niswronger2006)) has been implemented in iMOD. Percolation of precipitation through unsaturated zones is important for recharge of ground water. Rain and snow melt at land surface are partitioned into different pathways including runoff, infiltration, evapotranspiration, unsaturated-zone storage, and recharge. The package was developed to simulate water flow and storage in the unsaturated zone and to partition flow into evapotranspiration and recharge. A kinematic wave approximation to Richards’ equation is solved by the method of characteristics to simulate vertical unsaturated flow. The approach assumes that unsaturated flow occurs in response to gravity potential gradients only and ignores negative potential gradients (upward capillary flow); the approach further assumes uniform hydraulic properties in the unsaturated zone for each vertical column of model cells.
The iMOD implementation of the UZF package has the following limitation and/or assumptions:
• the UZF can be added to the model in a spatially distributed way and therefore for user-specified areas. The pointer value for this needs to be negative to ignore any UZF at that particular location, the absolute value in this IDF file is the model layer for which the upper elevation will be used to specify the surface level;
• the package also accounts for land surface runoff to streams and lakes, however this is not supported by iMOD.