Density for material flow equation when a calculation is performed in combination with groundflow. In case an element is above the phreatic level, as specified by groundflow_phreaticlevel_bounda, the density_dry will be used. In case an element is below the phreatic level, the density_wet will be used.
In case the groundflow_phreaticlevel_bounda is not specified, the density_wet will be used.
The index specifies the element_group, see element_group.